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diff --git a/old/60043-0.txt b/old/60043-0.txt deleted file mode 100644 index 172d2cf..0000000 --- a/old/60043-0.txt +++ /dev/null @@ -1,12566 +0,0 @@ -Project Gutenberg's Tunneling: A Practical Treatise., by Charles Prelini - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org/license - - -Title: Tunneling: A Practical Treatise. - -Author: Charles Prelini - -Release Date: August 3, 2019 [EBook #60043] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK TUNNELING: A PRACTICAL TREATISE. *** - - - - -Produced by deaurider, Harry Lamé and the Online Distributed -Proofreading Team at http://www.pgdp.net (This file was -produced from images generously made available by The -Internet Archive) - - - - - - - - Transcriber’s Notes - - Text between _underscores_, =equal signs= and ~tildes~ represents text - printed in italics, bold face and sans-serif, respectively. Small - capitals have been replaced with ALL CAPITALS. - - More Transcriber’s Notes may be found at the end of this text. - - - - - TUNNELING: - A PRACTICAL TREATISE - - BY - - CHARLES PRELINI, C. E. - - AUTHOR OF “EARTH AND ROCK EXCAVATION,” “DREDGES AND DREDGING,” - “EARTH SLOPES, RETAINING WALLS AND DAMS,” ETC. PROFESSOR - OF CIVIL ENGINEERING IN MANHATTAN COLLEGE, - NEW YORK - - _167 ILLUSTRATIONS_ - - SIXTH EDITION, REVISED AND ENLARGED - - [Illustration] - - NEW YORK - D. VAN NOSTRAND COMPANY - TWENTY-FIVE PARK PLACE - 1912 - - - COPYRIGHT, 1912, - BY - D. VAN NOSTRAND COMPANY - NEW YORK - - Stanhope Press - F. H. GILSON COMPANY - BOSTON, U.S.A. - - - - -PREFACE TO THE SIXTH EDITION - - -During the few years that have elapsed since the publication of the -first edition of this work, the art of tunneling through different soils -and especially under large bodies of water, has made considerable -progress. During the last ten years, no less than eight subaqueous -tunnels involving the construction of sixteen tubes have been -constructed for the service of the city of New York alone. The reader -will, no doubt, also recall the tunnels under the Boston Harbor, the St. -Clair, the Charles and Detroit Rivers in our own country as well as the -tunnels under the Thames and the Seine in Europe. Engineers, contractors -and workmen have acquired such experience in these difficult underground -and under-river construction that the work is now undertaken without any -of the fear and hesitation that were associated with the earlier -enterprises. - -As entirely new methods have been introduced by professional men, it was -found necessary to arrange the presentation of the subject in this sixth -edition so as to give due prominence to these recent methods. - -Besides this, other changes have been made in order to give greater -attention to American method of excavating tunnels through rock and -loose soil. This will explain the treatment of the crown-bar and also -the extensive illustration of the heading and bench method as well as -the drift method of driving tunnels which is followed in the United -States. - -Space has also been given to important tunnels recently built mainly for -the purpose of illustrating the various methods discussed in the text -and also to bring out more clearly the characteristics of the different -methods of tunnel excavation. - -The author hopes that these added features will meet the present -requirements of engineers and students. - - CHARLES PRELINI. - - MANHATTAN COLLEGE, - NEW YORK CITY. - - - - -CONTENTS - - - PAGE - - INTRODUCTORY--THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING xiii - - CHAPTER - - I. PRELIMINARY CONSIDERATIONS; CHOICE BETWEEN A TUNNEL AND AN - OPEN CUT; GEOLOGICAL SURVEYS 1 - - II. METHODS OF DETERMINING THE CENTER LINE AND FORMS AND - DIMENSIONS OF CROSS-SECTION 9 - - III. EXCAVATING MACHINES AND ROCK DRILLS; EXPLOSIVES AND - BLASTING 22 - - IV. GENERAL METHODS OF EXCAVATION; SHAFTS; CLASSIFICATION OF - TUNNELS 36 - - V. METHODS OF TIMBERING OR STRUTTING TUNNELS 47 - - VI. METHODS OF HAULING IN TUNNELS 59 - - VII. TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL - LININGS OF MASONRY 66 - - VIII. METHODS OF LINING TUNNELS 72 - - IX. TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; - REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK 84 - - X. TUNNELS THROUGH HARD ROCK (_continued_); EXCAVATION BY - DRIFTS; THE SIMPLON AND MURRAY HILL TUNNELS 102 - - XI. TUNNELS THROUGH HARD ROCK (_continued_); EXCAVATION BY - HEADINGS 130 - - XII. EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL - DISCUSSION; THE BELGIAN METHOD 143 - - XIII. THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND - (_continued_); BALTIMORE BELT LINE TUNNEL 155 - - XIV. THE FULL SECTION METHOD OF TUNNELING; ENGLISH METHOD; - AMERICAN METHOD; AUSTRIAN METHOD 166 - - XV. SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; - QUICKSAND TUNNELING; PILOT METHOD 182 - - XVI. OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; - BOSTON SUBWAY AND NEW YORK RAPID TRANSIT 195 - - XVII. SUBMARINE TUNNELING; GENERAL DISCUSSION; THE SEVERN TUNNEL 218 - - XVIII. SUBMARINE TUNNELING (_continued_); THE COMPRESSED AIR - METHOD; THE MILWAUKEE WATER-WORKS TUNNEL 225 - - XIX. SUBMARINE TUNNELING (_continued_); THE SHIELD SYSTEM 238 - - XX. SUBMARINE TUNNELING (_continued_); THE SHIELD AND - COMPRESSED AIR METHOD; THE HUDSON RIVER TUNNEL OF THE - PENNSYLVANIA RAILROAD 263 - - XXI. SUBMARINE TUNNELING (_continued_); TUNNELS AT VERY SHALLOW - DEPTH; THE COFFERDAM METHOD; THE PNEUMATIC CAISSON METHOD; - THE JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND 281 - - XXII. ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER - CONSTRUCTION 301 - - XXIII. RELINING TIMBER-LINED TUNNELS WITH MASONRY 315 - - XXIV. THE VENTILATION AND LIGHTING OF TUNNELS DURING - CONSTRUCTION 325 - - XXV. THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR - WORK 336 - - - - -LIST OF ILLUSTRATIONS - - - FIGURE PAGE - - 1. Diagram Showing Manner of Lining in Rectilinear Tunnels 10 - - 2. B. R. Value’s Device for Locating the Center Line Inside of - a Tunnel 11 - - 3. Triangulation System for Establishing the Center Line of the - St. Gothard Tunnel 12 - - 4. Method of Transferring the Center Line down Center Shafts 13 - - 5. Method of Transferring the Center Line down the Side Shafts 14 - - 6. Method of Laying out the Center Line of Curvilinear Tunnels 15 - - 7. Diagram of Polycentric Sectional Profile 19 - - 8, 9 and 10. Typical Sectional Profiles for Tunnel 20 - - 11. Soft Ground Bucket Excavating Machine; Central London - Underground Railway 22 - - 12. Column Mounting for Percussion Drill; Ingersoll Sargent Drill - Co. 26 - - 13. Sketch of Diamond Drill Bit 27 - - 14. Diagram Showing Sequence of Excavation for St. Gothard - Tunnel 36 - - 15. Diagram Showing Manner of Determining Correspondence of - Excavation to Sectional Profile 38 - - 16. Polar Protractor for Determining Profile of Excavated Cross- - Section 39 - - 17. Joining Tunnel Struts by Halving 48 - - 18. Round Timber Post and Cap Bearing 48 - - 19. Ceiling Strutting for Tunnel Roofs 49 - - 20. Ceiling Strutting with Side Post Supports 49 - - 21. Sill, Side Post and Cap Cross Frame Strutting 49 - - 22. Reinforced Cross Frame Strutting for Treacherous Materials 49 - - 23. Longitudinal Poling-Board System of Roof Strutting 50 - - 24. Transverse Poling-Board System of Roof Strutting 50 - - 25. Shaft with Single Transverse Strutting 52 - - 26. Rectangular Frame Strutting for Shafts 53 - - 27. Reinforced Rectangular Frame Strutting for Shafts in - Treacherous Materials 53 - - 28. Strutting of Timber Posts and Railway Rail Caps 56 - - 29. Strutting Made Entirely of Railway Rails 56 - - 30. Rziha’s Combined Strutting and Centering of Cast Iron 57 - - 31. Cast-Iron Segment of Rziha’s Strutting and Centering 57 - - 32. Cast-Iron Segmental Strutting for Shafts 58 - - 33. Platform Car for Tunnel Work 59 - - 34. Iron Dump-Car for Tunnel Work 60 - - 35. Wooden Dump-Car for Tunnel Work 60 - - 36. Box-Car for Tunnel Work 61 - - 37. Elevator Car for Tunnel Shafts 65 - - 38. Ground Mold for Constructing Tunnel Invert Masonry 67 - - 39. Combined Ground Mold and Leading Frame for Invert and Side - Wall Masonry 67 - - 40. Leading Frame for Constructing Side Wall Masonry 68 - - 41. Plank Center for Constructing the Roof Arch 69 - - 42. Trussed Center for Constructing the Roof Arch 70 - - 43 and 44. A Typical Form of Timber Lining for Tunnels 73 - - 45. Diagram Showing Forms adopted for Side-Wall Foundations 76 - - 46 and 47. Transverse Sections of Tunnels Showing Methods for - Increasing the Thickness of the Lining at Different Points 79 - - 48. Refuge Niche in St. Gothard Tunnel 81 - - 49. East Portal of Hoosac Tunnel 82 - - 50, 51 and 52. Arrangement of Drill Holes in the Heading of - Turchino Tunnel 91 - - 53 and 54. Arrangement of Drill Holes in the Heading of the Fort - George Tunnel 91 - - 55. Diagram Showing Sequence of Excavations in Drift Method of - Tunneling Rock 102 - - 56. Sketches Showing Sequence of Work in Excavating and Lining - the Simplon Tunnel 111 - - 57. General Details of the Brandt Rotary Drills Employed at the - Simplon Tunnel 112 - - 58. Sequence of Excavation in the Murray Hill Tunnel 124 - - 59. Traveling Platform for the Excavation of the Upper Side of - the Murray Hill Tunnel 125 - - 60. Timbering Used in the Murray Hill Tunnel 126 - - 61. Diagram Showing Sequence of Excavation in Heading Method of - Tunneling Rock 132 - - 62. Method of Strutting Roof, St. Gothard Tunnel 135 - - 63. Sketch Showing Arrangement of Tracks, St. Gothard Tunnel 135 - - 64. Arrangement of Drill Holes in the Fort George Tunnel 137 - - 65. Longitudinal Section of the Heading and Bench Excavation at - the Fort George Tunnel 137 - - 66. Diagram Showing the Arrangement of Drill Holes in the - Heading and Bench of the Gallitsin Tunnel 140 - - 67. Diagram Showing Modification of the Heading and Bench Method 140 - - 68 and 68A. Diagrams Showing Sequence of Excavation in the - Belgian Method 145 - - 69. Sketch Showing Radial Roof Strutting, Belgian Method 147 - - 70. Sketch Showing Roof Arch Center, Belgian Method 147 - - 71. Sketch Showing Method of Underpinning Roof Arch with the - Side Wall Masonry 149 - - 72. Longitudinal Section Showing Construction by the Belgian - Method 149 - - 73. Diagram Showing Sequence of Excavation in Modified Belgian - Method 152 - - 74. Sketch Showing Failure of Roof Arch by Opening at Crown 153 - - 75. Sketch Showing Methods of Repairing Roof Arch Failures 154 - - 76. Diagrams Showing Sequence of Excavation in German Method of - Tunneling 155 - - 77. Diagram Showing Sequence of Excavation in Water Bearing - Material, German Method 156 - - 78. Sketch Showing Work of Excavating and Timbering Drifts and - Headings 157 - - 79. Sketch Showing Method of Roof Strutting 157 - - 80. Sketch Showing Roof Arch Centers and Arch Construction 158 - - 81. Sketch Showing Method of Excavating and Strutting Baltimore - Belt Line Tunnel 162 - - 82. Roof Arch Construction with Timber Centers, Baltimore Belt - Line Tunnel 163 - - 83. Roof Arch Construction with Iron Centers, Baltimore Belt - Line Tunnel 164 - - 84. Diagram Showing Sequence of Excavation in English Method of - Tunneling 167 - - 85. Sketches Showing Construction of Strutting, English Method 168 - - 86 and 87. Sketches of Typical Timber Roof-Arch Centers, English - Method 169 - - 88. Sequence of Excavation in the American Method 172 - - 89. Strutting the Heading in the American Method 172 - - 90. Temporary Timbering of the Roof in the American Method 173 - - 91. Showing Crown Bars Supported by Segmental Arches 173 - - 92. Transversal and Longitudinal Section of a Tunnel Excavated - and Strutted According to the American Method 174 - - 93 and 94. Diagrams Showing Sequence of Excavation in Austrian - Method of Tunneling 177 - - 95, 96 and 97. Sketches Showing Construction of Strutting, - Austrian Method 178 - - 98. Sketch Showing Manner of Constructing the Lining Masonry, - Austrian Method 179 - - 99. Diagram Showing Sequence of Excavation in Italian Method of - Tunneling 183 - - 100. Sketch Showing Strutting for Lower Part of Section 183 - - 101 and 101A. Sketches Showing Construction of Centers, Italian - Method 184 - - 102. Sketch Showing Invert and Foundation Masonry, Italian Method. 185 - - 103. Sketch Showing Longitudinal Section of a Tunnel under - Construction, Italian Method 186 - - 104. Sketch Showing Sequence of Excavation, Stazza Tunnel 186 - - 105. Sketch Showing Method of Strutting First Drift, Stazza - Tunnel 187 - - 106 and 107. Sketches Showing Temporary Strutting Arch - Construction, Stazza Tunnel 187 - - 108. Sketch Showing Preliminary Drainage Galleries, Quicksand - Method 190 - - 109. Sketch Showing Construction of Roof Strutting, Quicksand - Method 190 - - 110. Sketch Showing Construction of Masonry Lining, Quicksand - Method 191 - - 111. Sketch Showing Pilot Method of Tunneling 193 - - 112. Diagram Showing Sequence of Construction in Open-Cut Tunnels 197 - - 113. Sketch Showing Method of Timbering Open-Cut Tunnels, Double - Parallel Trench Method 198 - - 114. Side-Wall Foundation Construction Open-Cut Tunnels 198 - - 115. Wide-Arch Section, Boston Subway 204 - - 116. Double-Barrel Section, Boston Subway 205 - - 117. Four-Track Rectangular Section, Boston Subway 206 - - 118. Section Showing Slice Method of Construction, Boston Subway 206 - - 119. Double-Track Section, New York Rapid Transit Railway 212 - - 120. Park Avenue Deep Tunnel Construction, New York Rapid Transit - Railway 214 - - 121. Harlem River Tunnel, New York Rapid Transit Railway 215 - - 122. Sketch Showing Underground Stream, Milwaukee Water-Works - Tunnel 229 - - 123. Sketch Showing Methods of Lining, Milwaukee Water-Works - Tunnel 232 - - 124. Longitudinal Section of Brunel’s Shield, First Thames Tunnel 241 - - 125. First Shield Invented by Barlow 242 - - 126. Second Shield Invented by Barlow 243 - - 127. Shield Suggested by Greathead for the Proposed North and - South Woolwich Subway 245 - - 128. Beach’s Shield Used on Broadway Pneumatic Railway Tunnel 245 - - 129. Shield for City and South London Railway 246 - - 130. Shield for St. Clair River Tunnel 247 - - 131. Shield for Blackwall Tunnel 248 - - 132. Elliptical Shield for Clichy Sewer Tunnel, Paris 249 - - 133. Semi-Elliptical Shield for Clichy Sewer Tunnel 250 - - 134. Roof Shield for Boston Subway 251 - - 135. Transversal and Longitudinal Section of Prelini’s Shield 252 - - 136. Elevation and Section of Hydraulic Jack, East River Gas - Tunnel 260 - - 137. Cast-Iron Lining, St. Clair River Tunnel 262 - - 138. General Elevations and Sections of Shields 270 - - 139. Plan and Elevation of First Bulkhead Wall in South Tube, - Manhattan 273 - - 140. Typical Cross-Sections of One Tube of Pennsylvania Railroad - Tunnel under the Hudson River 278 - - 141. Sections of Cofferdam, Van Buren St. Tunnel, Chicago 283 - - 142. Showing Working Platforms and Piles Sunk in Dredged Channel 286 - - 143. Showing Sheeting-Piles for the Sides of the Caisson and - Trussed Beam for the Roof 287 - - 144. Showing the Caisson with the Working-Chamber 287 - - 145. Showing the Tunnel Constructed within the Caisson 289 - - 146. Showing Sides of the Caisson and Supports for the Roof 290 - - 147. Showing the Roof of the Caisson Formed by the Upper Half of - the Tunnel 291 - - 148. Showing the Tunnel Completed by Building the Lower Half - within the Caisson 292 - - 149. Transversal Section of the Caissons for the Tunnel under the - Seine River 294 - - 150. Showing the Joining of the Caissons at the Pont Mirabeau - Tunnel under the Seine River 295 - - 151. Cross-Sections and Plans of the Detroit River Tunnel 298 - - 152. Tunneling through Caved Material by Heading 306 - - 153. Tunneling through Caved Material by Drifts 307 - - 154 and 155. Filling in Roof Cavity Formed by Falling Material 307 - - 156. Timbering to Prevent Landslides at Portal 308 - - 157. Shortening Tunnel Crushed by Landslide at Portal 308 - - 158. Extending Tunnel through Landslide at Portal 309 - - 159 and 160. Relining Timber-Lined Tunnel 316 - - 161. Relining Timber-Lined Tunnel, Great Northern Ry 317 - - 162. Relining Timber-Lined Tunnel, Great Northern Ry 318 - - 163. Relining Timber-Lined Tunnel, Great Northern Ry 319 - - 164. Construction of Centering Mullan Tunnel 320 - - 165. Centering Mullan Tunnel 321 - - 166. Relining Timber-Lined Tunnel, Norfolk & Western Ry 322 - - 167. Relining Timber-Lined Tunnel, Norfolk & Western Ry 323 - - - - -INTRODUCTION - -THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING. - - -A tunnel, defined as an engineering structure, is an artificial gallery, -passage, or roadway beneath the ground, under the bed of a stream, or -through a hill or mountain. The art of tunneling has been known to man -since very ancient times. A Theban king on ascending the throne began at -once to drive the long, narrow passage or tunnel leading to the inner -chamber or sepulcher of the rock-cut tomb which was to form his final -resting-place. Some of these rock-cut galleries of the ancient Egyptian -kings were over 750 ft. long. Similar rock-cut tunneling work was -performed by the Nubians and Indians in building their temples, by the -Aztecs in America, and in fact by most of the ancient civilized peoples. - -The first built-up tunnels of which there are any existing records were -those constructed by the Assyrians. The vaulted drain or passage under -the southeast palace of Nimrud, built by Shalmaneser II. (860-824 B.C.), -is in all essentials a true soft-ground tunnel, with a masonry lining. A -much better example, however, is the tunnel under the Euphrates River, -which may quite accurately be claimed as the first submarine tunnel of -which there exists any record. It was, however, built under the dry bed -of the river, the waters of which were temporarily diverted, and then -turned back into their normal channel after the tunnel work was -completed, thus making it a true submarine tunnel only when finished. -The Euphrates River tunnel was built through soft ground, and was lined -with brick masonry, having interior dimensions of 12 ft. in width and -15 ft. in height. - -Only hand labor was employed by these ancient peoples in their tunnel -work. In soft ground the tools used were the pick and shovels, or -scoops. For rock work they possessed a greater range of appliances. -Research has shown that among the Egyptians, by whom the art of -quarrying was highly developed, use was made of tube drills and saws -provided with cutting edges of corundum or other hard, gritty material. -The usual tools for rock work were, however, the hammer, the chisel, and -wedges; and the excellence and magnitude of the works accomplished by -these limited appliances attest the unlimited time and labor which must -have been available for their accomplishment. - -The Romans should doubtless rank as the greatest tunnel builders of -antiquity, in the number, magnitude, and useful character of their -works, and in the improvements which they devised in the methods of -tunnel building. They introduced fire as an agent for hastening the -breaking down of the rock, and also developed the familiar principle of -prosecuting the work at several points at once by means of shafts. In -their use of fire the Romans simply took practical advantage of the -familiar fact that when a heated rock is suddenly cooled it cracks and -breaks so that its excavation becomes comparatively easy. Their method -of operation was simply to build large fires in front of the rock to be -broken down, and when it had reached a high temperature to cool it -suddenly by throwing water upon the hot surface. The Romans were also -aware that vinegar affected calcareous rock, and in excavating tunnels -through this material it was a common practice with them to substitute -vinegar for water as the cooling agent, and thus to attack the rock both -chemically and mechanically. It is hardly necessary to say that this -method of excavation was very severe on the workmen because of the heat -and foul gases generated. This was, however, a matter of small concern -to the builders, since the work was usually performed by slaves and -prisoners of war, who perished by thousands. To be sentenced to labor on -Roman tunnel works was thus one of the severest penalties to which a -slave or prisoner could be condemned. They were places of suffering and -death as are to-day the Spanish mercury mines. - -Besides their use of fire as an excavating agent, the Romans possessed a -very perfect knowledge of the use of vertical shafts in order to -prosecute the excavation at several different points simultaneously. -Pliny is authority[1] for the statement that in the excavation of the -tunnel for the drainage of Lake Fucino forty shafts and a number of -inclined galleries were sunk along its length of 3¹⁄₂ miles, some of the -shafts being 400 ft. in depth. The spoil was hoisted out of these shafts -in copper pails of about ten gallons’ capacity by windlasses. - - [1] “Tunneling,” Encly. Brit., 1889, vol. xxiii., p. 623. - -The Roman tunnels were designed for public utility. Among those which -are most notable in this respect, as well as for being fine examples of -tunnel work, may be mentioned the numerous conduits driven through the -calcareous rock between Subiaco and Tivoli to carry to Rome the pure -water from the mountains above Subiaco. This work was done under the -Consul Marcius. The longest of the Roman tunnels is the one built to -drain Lake Fucino, as mentioned above. This tunnel was designed to have -a section of 6 ft. × 10 ft.; but its actual dimensions are not uniform. -It was driven through calcareous rock, and it is stated that 30,000 men -were employed for eleven years in its construction. The tunnels which -have been mentioned, being designed for conduits, were of small section; -but the Romans also built tunnels of larger sections at numerous points -along their magnificent roads. One of the most notable of these is that -which gives the road between Naples and Pozzuoli passage through the -Posilipo hills. It is excavated through volcanic tufa, and is about 3000 -ft. long and 25 ft. wide, with a section of the form of a pointed arch. -In order to facilitate the illumination of this tunnel, its floor and -roof were made gradually converging from the ends toward the middle; at -the entrances the section was 75 ft. high, while at the center it was -only 22 ft. high. This double funnel-like construction caused the rays -of light entering the tunnel to concentrate as they approached the -center, and thus to improve the natural illumination. The tunnel is on a -grade. It was probably excavated during the time of Augustus, although -some authorities place its construction at an earlier date. - -During the Middle Ages the art of tunnel building was practiced for -military purposes, but seldom for the public need and comfort. Mention -is made of the fact that in 1450 Anne of Lusignan commenced the -construction of a road tunnel under the Col di Tenda in the Piedmontese -Alps to afford better communication between Nice and Genoa; but on -account of its many difficulties the work was never completed, although -it was several times abandoned and resumed. For the most part, -therefore, the tunnel work of the Middle Ages was intended for the -purposes and necessities of war. Every castle had its private -underground passage from the central tower or keep to some distant -concealed place to permit the escape of the family and its retainers in -case of the victory of the enemy, and, during the defense, to allow of -sorties and the entrance of supplies. - -The tunnel builders of the Middle Ages added little to the knowledge of -their art. Indeed, until the 17th century and the invention of gunpowder -no practical improvement was made in the tunneling methods of the -Romans. Engravings of mining operations in that century show that -underground excavation was accomplished by the pick or the hammer and -chisel, and that wood fires were lighted at the ends of the headings to -split and soften the rocks in advance. Although gunpowder had been -previously employed in mining, the first important use of it in tunnel -work was at Malpas, France, in 1679-81, in the tunnel for the Languedoc -Canal. This tunnel was 510 ft. long, 22 ft. wide, and 29 ft. high, and -was excavated through tufa. It was left unlined for seven years, and -then was lined with masonry. - -With the advent of gunpowder and canal building the first strong impetus -was given to tunnel building, in its modern sense, as a commercial and -public utilitarian construction, since the days of the Roman Empire. -Canal tunnels of notable size were excavated in France and England -during the last half of the 17th century. These were all rock or -hard-ground tunnels. Indeed, previous to 1800 the soft-ground tunnel was -beyond the courage of engineer except in sections of such small size -that the work better deserves to be called a drift or heading than a -tunnel. In 1803, however, a tunnel 24 ft. wide was excavated through -soft soil for the St. Quentin Canal in France. Timbering or strutting -was employed to support the walls and roof of the excavation as fast as -the earth was removed, and the masonry lining was built closely -following it. From the experience gained in this tunnel were developed -the various systems of soft-ground subterranean tunneling since -employed. - -It was by the development of the steam railway, however, that the art of -tunneling was to be brought into its present prominence. In 1820-26 two -tunnels were built on the Liverpool & Manchester Ry. in England. This -was the beginning of the rapid development which has made the tunnel one -of the most familiar of engineering structures. The first railway tunnel -in the United States was built on the Alleghany & Portage R. R. in -Pennsylvania in 1831-33; and the first canal tunnel had been completed -about 13 years previously (1818-21) by the Schuylkill Navigation Co., -near Auburn, Pa. It would be interesting and instructive in many -respects to follow the rise and progress of tunnel construction in -detail since the construction of these earlier examples, but all that -may be said here is that it was identical with that of the railway. - -The art of tunneling entered its last and greatest phase with the -construction of the Mont Cenis tunnel in Europe and the Hoosac tunnel in -America, which works established the utility of machine rock-drills and -high explosives. The Mont Cenis tunnel was built to facilitate railway -communication between Italy and France, or more properly between -Piedmont and Savoy, the two parts of the kingdom of Victor Emmanuel II., -separated by the Alps. It is 7.6 miles long, and passes under the Col di -Fréjus near Mont Cenis. Sommeiller, Grattoni, and Grandis were the -engineers of this great undertaking, which was begun in 1857, and -finished in 1872. It was from the close study of the various -difficulties, the great length of the tunnel, and the desire of the -engineers to finish it quickly, that all the different improvements were -developed which marked this work as a notable step in the advance of the -art of tunneling. Thus the first power-drill ever used in tunnel work -was devised by Sommeiller. In addition, compressed air as a motive power -for drills, aspirators to suck the foul air from the excavation, air -compressors, turbines, etc., found at Mont Cenis their first application -to tunnel construction. This important rôle played by the Mont Cenis -tunnel in Europe in introducing modern methods had its counterpart in -America in the Hoosac tunnel completed in 1875. In this work there were -used for the first time in America power rock-drills, air compressors, -nitro-glycerine, electricity for firing blasts, etc. - -There remains now to be noted only the final development in the art of -soft-ground submarine tunneling, namely, the use of the shield and metal -lining. The shield was invented and first used by Sir Isambard Brunel in -excavating the tunnel under the River Thames at London, which was begun -in 1825, and finished in 1841. In 1869 Peter William Barlow used an iron -lining in connection with a shield in driving the second tunnel under -the Thames at London. From these inventions has grown up one of the most -notable systems of tunneling now practiced, which is commonly known as -the shield system. - -In closing this brief review of the development of modern methods of -tunneling, to the presentation of which the remainder of this book is -devoted, mention should be made of a form of motive power which promises -many opportunities for development in tunnel construction. Electricity -has long been employed for blasting and illuminating purposes in tunnel -work. It remains to be extended to other uses. For hauling and for -operating certain classes of hoisting and excavating machinery it is one -of the most convenient forms of power available to the engineer. Its -successful application to rock-drills is another promising field. For -operating ventilating fans it promises unusual usefulness. - - - - -TUNNELING - - - - -CHAPTER I. - -PRELIMINARY CONSIDERATIONS. CHOICE BETWEEN A TUNNEL AND OPEN CUT. -GEOLOGICAL SURVEYS. - - -CHOICE BETWEEN A TUNNEL AND AN OPEN CUT. - -When a railway line is to be carried across a range of mountains or -hills, the first question which arises is whether it is better to -construct a tunnel or to make such a détour as will enable the -obstruction to be passed with ordinary surface construction. The answer -to this question depends upon the comparative cost of construction and -maintenance, and upon the relative commercial and structural advantages -and disadvantages of the two methods. In favor of the open road there -are its smaller cost and the decreased time required in its -construction. These mean that less capital will be required, and that -the road will sooner be able to earn something for its builders. Against -the open road there are: its greater length and consequently its heavier -running expenses; the greater amount of rolling-stock required to -operate it; the heavy expense of maintaining a mountain road; and the -necessity of employing larger locomotives, with the increased expenses -which they entail. In favor of the tunnel there are: the shortening of -the road, with the consequent decrease in the operating expenses and -amount of rolling-stock required; the smaller cost of maintenance, -owing to the protection of the track from snow and rain and other -natural influences causing deterioration; and the decreased cost of -hauling due to the lighter grades. Against the tunnel, there are its -enormous cost as compared with an open road and the great length of time -required to construct it. - -To determine in any particular case whether a tunnel or an open road is -best, requires a careful integration of all the factors mentioned. It -may be asserted in a general way, however, that the enormous advance -made in the art of tunnel building has done much to lessen the strength -of the principal objections to tunnels, namely, their great cost and the -length of time required for their construction. Where the choice lies -between a tunnel or a long détour with heavy grades it is sooner or -later almost always decided in favor of a tunnel. When, however, the -conditions are such that the choice lies between a tunnel or a heavy -open cut with the same grades the problem of deciding between the two -solutions is a more difficult one. - -It is generally assumed that when the cut required will have a vertical -depth exceeding 60 ft. it is less expensive to build a tunnel unless the -excavated material is needed for a nearby embankment or fill. This rule -is not absolute, but varies according to local conditions. For instance, -in materials of rigid and unyielding character, such as rock, the -practical limit to the depth of a cut goes far beyond that point at -which a tunnel would be more economical according to the above rule. In -soils of a yielding character, on the other hand, the very flat slope -required for stability adds greatly to the cost of making a cut. - -It may be noted in closing that the same rule may be employed in -determining the location of the ends of the tunnel, for assuming that it -is more convenient to excavate a tunnel than an open cut when the depth -exceeds 60 ft., then the open cut approaches should extend into the -mountain- or hill-sides only to the points where the surface is 60 ft. -above grade, and there the tunnel should begin. If, therefore, we draw -on the longitudinal profile of the tunnel a line parallel to the plane -of the tracks, and 60 ft. above it, this line will cut the surface at -the points where the open-cut approaches should cease and the tunnel -begin. This is a rule-of-thumb determination at the best, and requires -judgment in its use. Should the ground surface, for example, rise only a -few feet above the 60 ft. line for any distance, it is obviously better -to continue the open cut than to tunnel. - - -THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS. - -When it has been decided to build a tunnel, the first duty of the -engineer is to make an accurate geological survey of the locality. From -this survey the material penetrated, the form of section and kind of -strutting to be used, the best form of lining to be adopted, the cost of -excavation, and various other facts, are to be deduced. In small tunnels -the geological knowledge of the engineer should enable him to construct -a geological map of the locality, or this knowledge may be had in many -cases by consulting the geological maps issued by the State or general -government surveys. When, however, the tunnel is to be of great length, -it may be necessary to call in the assistance of a professional -geologist in order to reconstruct accurately the interior of the -mountain and thereby to ascertain beforehand the different strata and -materials to be excavated, thus obtaining the data for calculating both -the time and cost of excavating the tunnel. - -The geological survey should enable the engineer to determine, (1) the -character of the material and its force of cohesion, (2) the inclination -of the different strata, and (3) the presence of water. - - -=Character of Material.=--The character of the material through which -the proposed tunnel will penetrate is best ascertained by means of -diamond rock-drills. These machines bore an annular hole, and take away -a core for the whole depth of the boring, thus giving a perfect -geological section showing the character, succession, and exact -thickness of the strata. By making such borings at different points -along the center line of the projected tunnel, and comparing the -relative sequence and thickness of the different strata shown by the -cores, the geological formation of the mountain may be determined quite -exactly. Where it is difficult or impracticable to make diamond drill -borings on account of the depth of the mountain above the tunnel, or -because of its inaccessibility, the engineer must resort to other -methods of observation. - -The present forms of mountains or hills are due to weathering, or the -action of the destructive atmospheric influences upon the original -material. From the manner in which the mountain or hill has resisted -weathering, therefore, may be deduced in a general way both the nature -and consistency of the materials of which it is composed. Thus we shall -generally find mountains or hills of rounded outlines to consist of soft -rocks or loose soils, while under very steep and crested mountains hard -rock usually exists. To the general knowledge of the nature of its -interior thus afforded by the exterior form of the mountain, the -engineer must add such information as the surface outcroppings and other -local evidences permit. - -For the purposes of the tunnel builder we may first classify all -materials as either, (1) hard rock, (2) soft rock, or (3) soft soil. - -Hard rocks are those having sufficient cohesion to stand vertically when -cut to any depth. Many of the primary rocks, like granite, gneiss, -feldspar, and basalt, belong to this class, but others of the same group -are affected by the atmosphere, moisture, and frost, which gradually -disintegrate them. They are also often found interspersed with pyrites, -whose well-known tendency to disintegrate upon exposure to air -introduces another destructive agency. For these reasons we may divide -hard rocks into two sub-classes; viz., hard rocks unaffected by the -atmosphere, and those affected by it. This distinction is chiefly -important in tunneling as determining whether or not a lining will be -required. - -Soft rocks, as the term implies, are those in which the force of -cohesion is less than in hard rocks, and which in consequence offer less -resistance to attacks tending to break down their original structure. -They are always affected by the atmosphere. Sandstones, laminated clay -shales, mica-schists, and all schistose stones, chalk and some volcanic -rocks, can be classified in this group. Soft rocks require to be -supported by timbering during excavation, and need to be protected by a -strong lining to exclude the air, and to support the vertical pressures, -and prevent the fall of fragments. - -Soft soils are composed of detrital materials, having so little cohesion -that they may be excavated without the use of explosives. Tunnels -excavated through these soils must be strongly timbered during -excavation to support the vertical pressure and prevent caving; and they -also always require a strong lining. Gravel, sand, shale, clay, -quicksand, and peat are the soft soils generally encountered in the -excavation of tunnels. Gravels and dry sand are the strongest and -firmest; shales are very firm, but they possess the great defect of -being liable to swell in the presence of water or merely by exposure to -the air, to such an extent that they have been known to crush the -timbering built to support them. Quicksand and peat are proverbially -treacherous materials. Clays are sometimes firm and tenacious, but when -laminated and in the presence of water are among the most treacherous -soils. Laminated clays may be described as ordinary clays altered by -chemical and mechanical agencies, and several modifications of the same -structure are often found in the same locality. They are composed of -laminæ of lenticular form separated by smooth surfaces and easily -detached from each other. Laminated clays generally have a dark color, -red, ocher or greenish blue, and are very often found alternating with -strata of stiatites or calcareous material. For purposes of construction -they have been divided into three varieties. - -Laminated clays of the first variety are those which alternate with -calcareous strata and are not so greatly altered as to lose their -original stratification. Laminated clays of the second variety are those -in which the calcareous strata are broken and reduced to small pieces, -but in which the former structure is not completely destroyed; the clay -is not reduced to a humid state. Laminated clays of the third variety -are those in which the clay by the force of continued disturbance, and -in the presence of water, has become plastic. Laminated clays are very -treacherous soils; quicksand and peat may be classed, as regards their -treacherous nature, among the laminated clays of the third variety. - - -=Inclination of Strata.=--Knowing the inclination of the strata, or the -angle which they make with the horizon, it is easy to determine where -they intersect the vertical plane of the tunnel passing through the -center line, thus giving to a certain extent a knowledge of the -different strata which will be met in the excavation. On the inclination -of the strata depend: (1) The cost of the excavation; the blasting, for -instance, will be more efficient if the rocks are attacked perpendicular -to the stratification; (2) The character of the timbering or strutting; -the tendency of the rock to fall is greater if the strata are horizontal -than if they are vertical; (3) The character and thickness of the -lining; horizontal strata are in the weakest position to resist the -vertical pressure from the load above when deprived of the supporting -rock below, while vertical strata, when penetrated, act as a sort of -arch to support the pressure of the load above. The foregoing remarks -apply only to hard or soft rock materials. - -In detrital formations the inclination of the strata is an important -consideration, because of the unsymmetrical pressures developed. In -excavating a tunnel through soft soil whose strata are inclined at 30° -to the horizon, for instance, the tunnel will cut these strata at an -angle of 30°. By the excavation the natural equilibrium of the soil is -disturbed, and while the earth tends to fall and settle on both sides at -an angle depending upon the friction and cohesion of the material, this -angle will be much greater on one side than on the other because of the -inclination of the strata; and hence the prism of falling earth on one -side is greater than on the other, and consequently the pressures are -different, or in other words, they are unsymmetrical. These -unsymmetrical pressures are usually easily taken care of as far as the -lining is concerned, but they may cause serious cave-ins and badly -distort the strutting. Caving-in during excavation may be prevented by -cutting the materials according to their natural slope; but the -distortion of the strutting is a more serious problem to handle, and one -which oftentimes requires the utmost vigilance and care to prevent -serious trouble. - - -=Presence of Water.=--An idea of the likelihood of finding water in the -tunnel may be obtained by studying the hydrographic basin of the -locality. From it the source and direction of the springs, creeks, -ravines, etc., can be traced, and from the geological map it can be seen -where the strata bearing these waters meet the center line. Not only -ought the surface water to be attentively studied, but underground -springs, which are frequently encountered in the excavation of tunnels, -require careful attention. Both the surface and underground waters -follow the pervious strata, and are diverted by impervious strata. Rocks -generally may be classed as impervious; but they contain crevices and -faults, which often allow water to pass through them; and it is, -therefore, not uncommon to encounter large quantities of water in -excavating tunnels through rock. As a rule, water will be found under -high mountains, which comes from the melted ice and snow percolating -through the rock crevices. - -Some detrital soils, like gravel and sand, are pervious, and others, -like clay and shale, are impervious. Detrital soils lying above clay are -almost certain to carry water just above the clay stratum. In tunnel -work, therefore, when the excavation keeps well within the clay stratum, -little trouble is likely to be had from water; should, however, the -excavation cut the clay surface and enter the pervious material above, -water is quite certain to be encountered. The quantity of water -encountered in any case depends upon the presence of high mountains near -by, and upon other circumstances which will attract the attention of the -engineer. - -A knowledge of the pressure of the water is desirable. This may be -obtained by observing closely its source and the character of the strata -through which it passes. Water coming to the excavation through rock -crevices will lose little of its pressure by friction, while that which -has passed some distance through sand will have lost a great deal of its -pressure by friction. Water bearing sand, and, in fact, any water -bearing detrital material, has its fluidity increased by water pressure; -and when this reaches the point where flow results, trouble ensues. The -streams of water met in the construction of the St. Gothard tunnel had -sufficient pressure to carry away timber and materials. - - - - -CHAPTER II. - -METHODS OF DETERMINING THE CENTER LINE AND FORMS AND DIMENSIONS OF -CROSS-SECTION. - - -DETERMINING THE CENTER LINE. - -Tunnels may be either curvilinear or rectilinear, but the latter form is -the more common. In either case the first task of the engineer, after -the ends of the tunnel have been definitely fixed, is to locate the -center line exactly. This is done on the surface of the ground; and its -purpose is to find the exact length of the tunnel, and to furnish a -reference line by which the excavation is directed. - - -=Rectilinear Tunnels.=--In short tunnels the center line may be -accurately enough located for all practical purposes by means of a -common theodolite. The work is performed on a calm, clear day, so as to -have the instrument and observations subjected to as little atmospheric -disturbance as possible. Wooden stakes are employed to mark the various -located points of the center line temporarily. The observations are -usually repeated once at least to check the errors, and the stakes are -altered as the corrections dictate; and after the line is finally -decided to be correctly fixed, they are replaced by permanent monuments -of stone accurately marked. The method of checking the observations is -described by Mr. W. D. Haskoll[2] as follows: - - “Let the theodolite be carefully set up over one of the stakes, with - the nail driven into it, selecting one that will command the best - position so as to range backwards and forwards over the whole length - of line, and also obtain a view of the two distant points that range - with the center line; this being done, let the centers of every stake - ... be carefully verified. If this be carefully done, and the centers - be found correct, and thoroughly in one visual line as seen through - the telescope, there will be no fear but that a perfectly straight - line has been obtained.” - - [2] “Practical Tunneling,” by F. W. Simms. - -[Illustration: FIG. 1.--Diagram Showing Manner of Lining in Rectilinear -Tunnels.] - -The center line which has thus been located on the ground surface has to -be transposed to the inside of the tunnel to direct the excavation. To -do this let _A_ and _B_ be the entrances and _a_ and _b_ be the two -distinct fixed points which have been ranged in with the center line -located on the ground surface over the hill _A f B_, Fig. 1. The -instrument is set up at _V_, any point on the line _A a_ produced, and a -bearing secured by observation on the center line marked on the surface. -This bearing is then carried into the tunnel by plunging the telescope, -and setting pegs in the roof of the heading. Lamps hung from these pegs -furnish the necessary sighting points. This same operation is repeated -on the opposite side of the hill to direct the excavation from that end -of the tunnel. These operations serve to locate only the first few -points inside the tunnel. As the excavation penetrates farther into the -hill, it becomes impossible to continue to locate the line from the -outside point, and the line has to be run from the points marked on the -roof of the heading. Great accuracy is required in all these -observations, since a very small error at the beginning becomes greater -and greater as the excavation advances. To facilitate the accurate -location of points on the roof of the tunnel, a simple device was -designed by Mr. Beverley R. Value, shown in Fig. 2. Two iron spikes, -each having a small hole in the flat end, are driven into the rock about -9 ins. apart. A brass bar, 1 in. high, ¹⁄₄ in. thick and 10 ins. long, -having a hole near one end and a 1 in. slot at the other, is screwed -tightly into the head of the spikes. The middle part of the bar is -divided into inches and tenths of an inch. A separate brass hanger is -fitted to the bar, having a vernier with its zero at the middle of the -hanger and corresponding to a plumb line attached below. The hanger is -moved back and forth until it coincides with the line of sight of the -transit, and then the readings of the vernier are recorded. Any time -that the hanger is placed on the bar and the vernier marks the same -reading, the plumb line will indicate the center line of the tunnel. -When, instead of one bar, two are inserted at a distance of 20 or 30 ft. -apart, the plumb lines suspended from the hangers will represent the -vertical plane passing through the axis of the tunnel in coincidence -with the one staked out on the surface ground. - -[Illustration: FIG. 2.--B. R. Value’s Device for Locating the Center -Line Inside of a Tunnel.] - -The location of the center line of a long tunnel, which is to be -excavated under high mountains, is a very difficult operation, and the -engineers usually leave this part of the work to astronomers, who fix -the stations from which the engineers direct the work of construction. -The center lines of all the great Alpine tunnels were located by -astronomers who used instruments of large size. Thus, in ranging the -center line of the St. Gothard tunnel, the theodolite used had an object -glass eight inches in diameter.[3] Instead of the ordinary mounting a -masonry pedestal with a perfectly level top is employed to support the -instrument during the observations. The location is made by means of -triangulation. The various operations must be performed with the -greatest accuracy, and repeated several times in such a way as to reduce -the errors to a minimum, since the final meeting of the headings -depends upon their elimination. - - [3] See also the Simplon Tunnel, Chapter X. - -[Illustration: FIG. 3.--Triangulation System for Establishing the Center -Line of the St. Gothard Tunnel.] - -The St. Gothard tunnel furnishes perhaps the best illustration of -careful work in locating the center line of long rectilinear tunnels of -any tunnel ever built. The length of this tunnel is 9.25 miles, and the -height of the mountain above it is very great. The center line was -located by triangulation by two different astronomers using different -sets of triangles, and working at different times. The set or system of -triangles used by Dr. Koppe, one of the observers, is shown by Fig. 3; -it consists of very large and quite small triangles combined, the latter -being required because the entrances both at Airolo and Goeschenen were -so low as to permit only of a short sight being taken. The apices of the -triangles were located by means of the contour maps of the Swiss Alpine -Club. Each angle was read ten times, the instrument was collimated four -times for each reading, and was afterwards turned off 5° or 10° to avoid -errors of graduation. The average of the errors in reading was about one -second of arc. The triangulation was compensated according to the method -of least squares. The probable error in the fixed direction was -calculated to be 0.8″ of arc at Goeschenen and 0.7″ of arc at Airolo. -From this it was assumed that the probable deviation from the true -center would be about two inches at the middle of the tunnel, but when -the headings finally met this deviation was found to reach eleven -inches. - -Comparatively few tunnels are driven by working from the entrances -alone, the excavation being usually prosecuted at several points at once -by means of shafts. In these cases, in order to direct the excavation -correctly, it is necessary to fix the center line on the bottom of the -shaft. This is accomplished in two ways,--one being employed when the -shaft is located directly over the center line, and the other when the -shaft is located to one side of the center line. - -When the shaft is located on the center line two small pillars are -placed on opposite edges of the shaft and collimating with the center -line as shown by Fig. 4. On these two pillars the points corresponding -to the center line are correctly marked, and connected by a wire -stretched between them. To this wire two plumb bobs are fastened as far -apart as possible. These plumb bobs mark two points on the center line -at the bottom of the shaft, and from them the line is extended into the -headings as the work advances. In these operations, heavy plumb bobs are -used. In the New York subway plumb bobs of steel, weighing 25 lbs. each, -were used, and to prevent rotation they were made with cross-sections, -in the shape of a Greek cross, and were sunk in buckets filled with -water. Owing to the difference between the temperature at the top and -that at the bottom of the shaft, strong currents of air are produced, -which keep in constant oscillation the wires to which the bobs are -suspended. - -[Illustration: FIG. 4.--Method of Transferring the Center Line down -Center Shafts.] - -To determine the center line at the bottom of the shaft, the headings -are first driven from both sides of the shaft, after which a transit is -set up on the same alignment with the two wires, and this will indicate -the vertical plane passing through the axis of construction. Two points -are then fixed on the roof of the tunnel in continuation of this -vertical plane. When the plumb bobs are removed from the shaft and two -small plumb bobs are suspended to the two points mentioned, they will -always give the same vertical plane passing through the axis of -construction transferred from the surface. - -Because of the continuous moving of the wires, the fixing of the points -on the roof of the tunnel is very troublesome, and the operation should -be repeated by different men at different times before the points are -permanently fixed. - -[Illustration: FIG. 5.--Method of Transferring the Center Line down Side -Shafts.] - -When the shaft is placed at one side of the tunnel the pillars or bench -marks are placed normal to the center line on the edges of the shaft as -shown by Fig. 5. Between the points _A_ and _B_ a wire is stretched, and -from it two plumb bobs are suspended, as described in the preceding -case; these plumb bobs establish a vertical plane normal to the axis of -the tunnel. The excavation of the side tunnel is carried along the line -_BW_ until it intersects the line of the main tunnel, whose center line -is determined by measuring off underground a distance equal to the -distance _BO_ on the surface. By setting the instrument over the -underground point _O_, and turning off a right angle from the line _BO_, -the center line of the tunnel is extended into the headings. - - -=Curvilinear Tunnels.=--There are various methods of locating the center -lines of curvilinear tunnels, but the method of tangent offsets is the -one most commonly employed. - -At the beginning the excavation is conducted as closely as may be to -the line of the curve, and as soon as it has progressed far enough the -tangent _AT_, Fig. 6, is ranged out. At _B_ a point is located over -which to set the instrument, and the distance _AB_ is measured for the -purpose of finding the ordinate of the right angle triangle _OAB_. Now -_OA_ = _r_, _AB_ = _d_, and φ = angle _ABO_. Then: - - _r_ - Tang. φ = ---. - _d_ - -[Illustration: FIG. 6.--Method of Laying Out the Center Line of -Curvilinear Tunnels.] - -Doubling the value of φ and making the angle _ABC_ = 2 φ, the line _BC_ -will be fixed and the point _C_ located by taking _AB_ = _BC_. On _BC_ -the ordinates are laid off to locate the curve. Prolong _CB_ so that -_CD_ = _CB_. Then the portion of the curve _CF_ is symmetrical with -_CE_, and the ordinates used to locate _EC_ may be employed to locate -_CF_, by laying them off in the reverse order. - -In curvilinear tunnels several cases may be considered. - -(1) When the tunnel for almost its entire length is driven on a tangent -with a curve at each end. - -(2) When the tunnel begins with a curve and ends with a straight line. - -(3) When the whole tunnel is in curve from portal to portal. - -(4) The helicoidal or corkscrew tunnel. - - * * * * * - -(1) The axis of every one of the great Alpine tunnels is a straight -line, with a curve at each end. To range out the center line of one of -these long tunnels from a curve, no matter how accurately laid out, will -certainly cause an error, which, magnified with the distance, may -produce serious results. To avoid these inconveniences, the -determination of the axis of the tunnel should be made from a straight -line. This means that the tunnel is at first excavated on a straight -line for its entire length and after the headings driven from both -portals have met, the two portions of the tunnel or curve are excavated -and constructed. The portions of the tunnel excavated on straight lines -for conveniences of construction may then be abandoned or used in cases -of accidents or repairs. - -When the axis of a short tunnel has a curve at each end and a straight -line in the middle, it is driven directly from the entrances; first, -however, excavating the curvilinear portions of the tunnel. In such a -case it would be advisable to proceed in the following manner. Drive the -headings on the curvilinear portions of the tunnel, staking out the -center line by means of the offsets from the tangents. At the ends of -the curves lay out from both fronts the rectilineal portion of the -tunnel. Only very narrow headings should be excavated at first while the -whole section could be enlarged near the entrances. The excavation of -the headings at the front should advance very rapidly, in order that the -headings may meet in the shortest possible time. When communication is -established, it is comparatively easy to correct an error resulting from -driving the tunnel from the curves. - -(2) When a tunnel begins with a curve and ends with a straight line, the -work of excavation should proceed from both ends. From the straight end -of the tunnel only the heading should be driven, while from the -curvilinear end the whole section could be opened at once. By this -arrangement the excavation progresses slowly from the curvilinear end -and rapidly from the straight end of the tunnel. Once communication has -been established and any error corrected, the work of enlarging the -profile of the tunnel may be pushed with the same activity from both -ends. - -(3) When the center line of the entire tunnel is a curve, there is more -probability of slight deviations from the true axis of the proposed -work. In such a case it would be advisable to first excavate a narrow -heading and to concentrate all the efforts in driving the headings as -rapidly as possible in order that they may meet in the shortest time. -The center line of these headings is staked out by the usual method of -the offsets from the tangent. The enlarging of the section of the tunnel -could be commenced at both portals and be driven slowly until the -headings have met and any errors corrected, when the work could be -pushed with the greatest activity all along the line. - -(4) In corkscrew or helicoidal tunnels the entire center line is on a -curve. In these tunnels, as a rule, there is a great difference of level -between the two portals, one being much higher than the other, so -careful attention should be paid to the tunnel grade. Working in the -limited spaces afforded by narrow headings it is very probable that -errors may be made in fixing both the alignment and the grade of the -tunnel. To prevent these almost unavoidable errors, it would be well to -excavate at first only the headings, to stake the center line in the -roof of these headings and then to lay the grade of the tunnel as -accurately as possible. The work on the headings should be pushed as -rapidly as possible in order that they may meet quickly, so that the -center line, as temporarily laid out, may be corrected and permanently -fixed for the direction of successive operations. In these tunnels the -headings should be excavated near the center of the tunnel cross-section -so that the sides and roof of the heading would be at some distance from -the sides and roof of the proposed tunnel. This arrangement will easily -permit corrections to be made in case any slight difference from the -true line was erroneously made during the excavation of the headings. - - -FORM AND DIMENSIONS OF CROSS-SECTION. - -In deciding upon the sectional profile of a tunnel two factors have to -be taken into consideration: (1) The form of section best suited to the -conditions, and (2) the interior dimensions of this section. - - -=Form of Section.=--The form of the sectional profile of a tunnel should -be such that the lining is of the best form to resist the pressures -exerted by the unsupported walls of the tunnel excavation, and these -vary with the character of the material penetrated. These pressures are -both vertical and lateral in direction; the roof, deprived of support by -the excavation, tends to fall, and the opposite sides for the same -reason tend to slide inward along a plane more or less inclined, -depending upon the friction and cohesion of the material. In some rocks -the cohesion is so great that they will stand vertically, while it may -be very small in loose earth which slides along a plane whose -inclination is directly proportional to the cohesion. - -From the theory of resistance of profiles we know that the resistance of -a line to exterior normal forces is directly proportional to its degree -of curvature, and consequently inversely proportional to the radius of -the curve. Hence the sectional profile of a tunnel excavated through -hard rock, where there are no lateral pressures owing to the great -cohesion of the material, and having to resist only the vertical -pressure, should be designed to offer the greatest resistance at its -highest point, and the curve must, therefore, be sharper there, and may -decrease toward the base. In quicksand, mud, or other material -practically without cohesion, the pressures will all be normal to the -line of the profile, and a circular section is the one best suited to -resist them. These theoretical considerations have been proved correct -by actual experience, and they may be employed to determine in a general -way the form of section to be adopted. Applying them to very hard rock, -they give us a section with an arched roof and vertical side walls. In -softer materials they give us an elliptical section with its major axis -vertical, and in very soft quicksands and mud they give us the circular -section. These three forms of cross-section and their modifications are -the ones commonly employed for tunnels. An important exception to this -general practice, however, is met with in some of the city underground -rapid-transit railways built of late years, where a rectangular or box -section is employed. These tunnels are usually of small depth, so that -the vertical pressures are comparatively light, and the bending strains, -which they exert upon the flat roof, are provided for by employing steel -girders to form the roof lining. - -[Illustration: FIG. 7.--Diagram of Polycentric Sectional Profile.] - -From what has been said it will be seen that it is impossible to -establish a standard sectional profile to suit all conditions. The best -one for the majority of conditions, and the one most commonly employed, -is a polycentric figure in which the number of centers and the length of -the radii are fixed by the engineer to meet the particular conditions -which exist. In a general way this form of center may be considered as -composed of two parts symmetrical in respect to the vertical axis. Fig. -7 shows such a profile, in which _DH_ is the vertical axis. The section -is unsymmetrical in respect to the horizontal axis _GE_. The upper part -forming the roof arch is usually a semi-circle or semi-oval, while the -lower part, comprising the side walls and invert of floor, varies -greatly in outline. Sometimes the side walls are vertical and the invert -is omitted, as shown by Fig. 8; and sometimes the side walls are -inclined, with their bottoms braced apart by the invert, as shown by -Fig. 9. In more treacherous soils the side walls are curved, and are -connected by small curved sections to the invert, as shown by Fig. 10. -In the last example the side walls are commonly called skewbacks, and -the lower part of the section is a polycentric figure like the upper -part, but dissimilar in form. - -In a tunnel section whose profile is composed entirely of arcs the -following conditions are essential: The centers of the springer arcs -_Ga_ and _Ea′_, Fig. 7, must be located on the line _GE_; the center of -the roof arc _bDb′_ must be located on the axis _HD_; the total number -of centers must be an odd number; the radii of the succeeding arcs from -_G_ toward _D_ and _E_ toward _D_ must decrease in length, and finally -the sum of the angles subtended by the several arcs must equal 180°. - -[Illustration: - -~Fig. 8~ - -~Fig. 9~ - -~Fig. 10~ - -FIGS. 8 to 10.--Typical Sectional Profiles for Tunnel.] - - -=Dimensions of Section.=--The dimensions to be given to the -cross-section of a tunnel depend upon the purpose for which it is to be -used. Whatever the purpose of the tunnel, the following three points -have to be considered in determining the size of its cross-section: (1) -The size of clear opening required; (2) the thickness of lining masonry -necessary; and (3) the decrease in the clear opening from the -deformation of the lining. - -Railway tunnels may be built either to accommodate one or two, three and -four tracks. In single-track tunnels a clear space of at least 2¹⁄₂ ft. -on each side should be allowed for between the tunnel wall and the side -of the largest standard locomotive or car, and a clear space of at least -3 ft. should be allowed for between the roof and the top of the same -locomotive or car. Since the roof of the tunnel is arch-shaped, to -secure a clearance of 3 ft. at every point will necessitate making the -clearance at the center greater than this amount. In double-track -tunnels the same amounts of side and roof clearances have to be provided -for, and, in addition, there has to be a clearance of at least 2 ft. -between trains. On the three- and four-track tunnels only the width -varies while the height remains almost equal to the two track. Referring -to Fig. 7, and assuming the line _AB_ to represent the level of the -tracks, then the ordinary dimensions in feet required for both single- -and double-track tunnels are as follows:-- - - +--------------+--------+----------+----------+---------------+ - | | HEIGHT, | WIDTH, | HEIGHT, | HEIGHT, | - | | D. F. | G. E. | C. F. | C. H. | - | | FEET. | FEET. | FEET. | FEET. | - +------------+----------+----------+----------+---------------+ - |Single track|17.6 to 18|16.5 to 18|6 to 7.4|¹⁄₄ to ¹⁄₈ _AB_| - |Double track|26.6 to 28|26.6 to 28|6.3 to 6.9|¹⁄₄ to ¹⁄₈ _AB_| - +------------+----------+----------+----------+---------------+ - -The dimensions of tunnels built for aqueduct purposes are determined so -as to have an area of cross-section equal to the required waterway. In -the Croton Aqueduct two different types of cross-sections were used, the -circular one for tunnels through rock and the horseshoe section for -tunnels through loose materials. In the Catskill aqueduct three -different cross-sections have been selected, the circular one for -tunnels under pressure and the horseshoe for tunnels at the hydraulic -gradient. These, however, through rock have a cross-section formed of a -semi-circular arch and vertical side walls, while through earth the -semi-circular arch is supported by skewback walls. - -In tunnels built for railroad aqueduct sewer and any other purpose the -thickness of the masonry lining to be allowed for varies with the -material penetrated, as will be explained in a succeeding chapter where -the dimensions for various ordinary conditions are given in tabular -form. The lining masonry is subject to deformation in three ways: by the -sinking of the whole masonry structure, by the squeezing together of the -side walls by the lateral pressures, and by the settling of the -roof-arch. The whole masonry structure never sinks more than three or -four inches, and merits little attention. The movement of the side walls -towards each other, which may amount to three or four inches for each -wall without endangering their stability, has, however, to be allowed -for; and similar allowance must be made for the settling of the -roof-arch, which may amount to from nine inches to two feet, when the -arch is built first as in the Belgian system and rests for some time -upon the loose soil. - - - - -CHAPTER III. - -EXCAVATING MACHINES AND ROCK DRILLS: EXPLOSIVES AND BLASTING. - - -=Earth-Excavating Machines.=--Comparatively few of the labor-saving -machines employed for breaking up and removing loose soil in ordinary -surface excavation are used in tunnel excavation through the same -material. Several forms of tunnel excavating machines have been tried at -various times, but only a few of them have attained any measure of -success, and these have seldom been employed in more than a single work. -In the Central London underground railway work through clay a continuous -bucket excavator (Fig. 11) was employed with considerable saving in time -and labor over hand work. In some recent tunnel work in America the -contractors made quite successful use of a modified form of steam -shovel. These are the most recent attempts to use excavating machines in -soft ground, and they, like all previous attempts, must be classed as -experiments rather than as examples of common practice. The Thomson -machine,[4] however, can be employed in any tunnel driven through loose -soil. It occupies a comparatively small space and may easily work when -the timbering is used to support the roof of the tunnel. Steam shovel -instead may give efficient result only in the case that the whole -section of the tunnel is open at once and there are no timbers to -prevent the free swinging of the dipper handle and boom. But in tunnels -through loose soils it is almost impossible to open the whole section at -once without the necessity of supporting the roof. Consequently the use -of steam shovel in loose tunnels is very limited. The shovel, the spade, -and the pick, wielded by hand, are the standard tools now, as in the -past, for excavating soft-ground tunnels. - - [4] The machine was designed by Mr. Thomas Thomson, Engineer for - Messrs. Walter Scott & Co. - -[Illustration: FIG. 11.--Soft Ground Bucket Excavating Machine: Central -London Underground Railway.] - - -=Rock-Excavating Machines.=--At one period during the work of -constructing the Hoosac tunnel considerable attention was devoted to the -development of a rock excavating, boring, or tunneling machine. This -device was designed to cut a groove around the circumference of the -tunnel thirteen inches wide and twenty-four feet in diameter by means of -revolving cutters. It proved a failure, as did one of smaller size, -eight feet in diameter, tried subsequently. During and before the Hoosac -tunnel work a number of boring-machines of similar character were -experimented with at the Mont Cenis tunnel and elsewhere in Europe; but, -like the American devices, they were finally abandoned as impracticable. - - -=Hand Drills.=--Briefly described, a drill is a bar of steel having a -chisel-shaped end or cutting-edge. The simplest form of hand drill is -worked by one man, who holds the drill in one hand, and drives it with a -hammer wielded by his other hand. A more efficient method of hand-drill -work is, however, where one man holds the drill, and another swings the -hammer or sledge. Another form of hand drill, called a churn drill, -consists of a long, heavy bar of steel, which is alternately raised and -dropped by the workman, thus cutting a hole by repeated impacts. - -In drilling by hand the workman holding the drill gives it a partial -turn on its axis at every stroke in order to prevent wedging and to -offer a fresh surface to the cutting-edge. For the same reason the chips -and dust which accumulate in the drill-hole are frequently removed. The -instruments used for this purpose are called scrapers or dippers, and -are usually very simple in construction. A common form is a strong wire -having its end bent at right angles, and flattened so as to make a sort -of scoop by which the drillings may be scraped or hoisted out of the -hole. It is generally advantageous to pour water into the drill-hole -while drilling to keep the drill from heating. - - -=Power Drills.=--When the conditions are such that use can be made of -them, it is nearly always preferable to use power drills, on account of -their greater speed of penetration and greater economy of work. Power -drills are worked by direct steam pressure, or by compressed air -generated by steam or water power, and stored in receivers from which it -is led to the drills through iron pipes. A great variety of forms of -power drills are available for tunnel work in rock, but they can nearly -all be grouped in one of two classes: (1) Percussion drills, and (2) -Rotary drills. - - -_Percussion Drills._--The first American percussion drill was patented -by Mr. J. J. Couch of Philadelphia, Penn., in March, 1849. In May of the -same year, Mr. Joseph W. Fowle, who had assisted Mr. Couch in developing -his drill, patented a percussion drill of his own invention. The Fowle -drill was taken up and improved by Mr. Charles Burleigh, and was first -used on the Hoosac tunnel. In Europe Mr. Cavé patented a percussion -drill in France in October, 1851. This invention was soon followed by -several others; but it was not until Sommeiller’s drill, patented in -1857 and perfected in 1861, was used on the Mont Cenis tunnel, that the -problem of the percussion drill was practically solved abroad. Since -this time numerous percussion drill patents have been taken out in both -America and Europe. - -A percussion drill consists of a cylinder, in which works a piston -carrying a long piston rod, and which is supported in such a manner that -the drill clamped to the end of the piston rod alternately strikes and -is withdrawn from the rock as the piston reciprocates back and forth in -the cylinder. Means are devised by which the piston rod and drill turn -slightly on their axis after each stroke, and also by which the drill is -fed forward or advanced as the depth of the drill-hole increases. The -drills of this type which are in most common use in America are the -Ingersoll-Sergeant and the Rand. There are various other makes in common -use, however, which differ from the two named and from each other -chiefly in the methods by which the valve is operated. All of these -drills work either with direct steam pressure or with compressed air. -Workable percussion drills operated by electricity are built, but so far -they do not seem to have been able to compete commercially with the -older forms. No attempt will be made here to make a selection between -the various forms of percussion drills for tunnel work, and for the -differences in construction and the merits claimed for each the reader -is referred to the makers of these machines. All of the leading makes -will give efficient service. It goes almost without saying that a good -percussion drill should operate with little waste of pressure, and -should be composed of but few parts, which can be easily removed and -changed. - - -_Drill Mountings._--For tunnel work the general European practice is to -mount power drills upon a carriage moving on tracks in order that they -be easily withdrawn during the firing of blasts. Connection is made with -the steam or compressed air pipes by means of flexible hose which can -easily be attached or detached as the drill advances or when it is moved -for repairs or during blasts. Two, four, and sometimes more drills are -mounted and work simultaneously on a single carriage. In America it has -been found that column mountings have been more successful for tunnel -work than any other form. The column mounting made by the -Ingersoll-Sergeant Drill Co. is shown in Fig. 12. In using this form of -mounting no tracks or other special apparatus is required; it is not -necessary, as is the case with the carriage mounting, to remove the -débris before resuming operations, but as soon as the blasting has been -finished and the smoke has sufficiently disappeared the column can be -set up and drilling resumed. - -[Illustration: FIG. 12.--Column Mounting for Percussion Drill: -Ingersoll-Sergeant Drill Co.] - - -_Rotary Drills._--Rotary drilling machines, or more simply rotary -drills, were first used in 1857 in the Mont Cenis tunnel. The advantages -claimed for rotary drills in comparison with percussion drills are: (1) -That less power is required to drive the drill, and the power is better -utilized; (2) once the machines work easily they do not require -continual repairs, and (3) in driving holes of large size the interior -nucleus is taken away intact, thus reducing work and increasing the -speed of drilling. Rotary drills are extensively used for geological, -mining, well-driving, and prospecting purposes; but they are very seldom -employed in tunnels in America, although successfully used for this -purpose in Europe. The reason they have not gained more favor among -American tunnel builders is due to some extent perhaps to prejudice, but -chiefly to the great cost of the machine as compared with percussion -drills, and to the expense of diamonds for repairs. Those who advocate -these machines for tunnel work point out, however, that under ordinary -usage the diamonds have a very long life,--borings of 700 lin. ft. being -recorded without repairs to the diamonds. - -[Illustration: FIG. 13.--Sketch of Diamond Drill Bit.] - -The form of rotary drill used chiefly for prospecting purposes is the -diamond drill. This machine consists of a hollow cylindrical bit having -a cutting-edge of diamonds, which is revolved at the rate of from two -hundred to four hundred revolutions per minute by suitable machinery -operated by steam or compressed air. The diamonds are set in the -cutting-edge of the bit so as to project outward from its annular face -and also slightly inside and outside of its cylindrical sides (Fig. 13). -When the drill rod with the bit attached is rotated and fed forward the -bit cuts an annular hole into the rock; the drillings being removed from -the hole by a constant stream of water which is forced down through the -hollow drill rod and emerges, carrying the débris with it, up through -the narrow space between the outside of the bit and the walls of the -hole. There are various makes of diamond drills, but they all operate in -essentially the same manner. - -The rotary drill principally employed in Europe in tunneling is the -Brandt. The cutting-edge of the Brandt drill consists of hardened steel -teeth. The bit is pressed against the rock by hydraulic pressure, and -usually makes from seven to eight revolutions per minute. Some of the -water when freed goes through the hollow bit, keeping it cool, and -cleaning the hole of débris. A water pressure of from 300 to 450 lbs. -per square inch is required to operate these drills. Rotary rock-drills -may be mounted either on carriages or on columns for tunnel work. -Several machines have recently been constructed for the purpose of -breaking the rock in tunnels without blasting, but they did not meet the -approval of tunnel engineers. One of these machines is provided with -numerous electric torches, which are applied to the rock at the front. -By suddenly chilling the rock with a stream of cold water the stone will -crumble away. Another machine was tested, with little success, in the -excavation for the New Grand Central Depot in New York. On the face of -this machine there is a multitude of chipping drills revolving on four -arms and driven by air pressure. They attack the rock and chip it into -fragments, which are carried away by an endless band. - - -EXPLOSIVES AND BLASTING. - -When the holes are once drilled, either by hand or power drills, they -are charged with explosives. The principal explosives employed in -tunneling are gunpowder, nitroglycerine, and dynamite. - - -=Gunpowder.=--Gunpowder is composed of charcoal, sulphur, and saltpeter -in proportions varying according to the quality of the powder. For -mining purposes the composition employed is 65% saltpeter, 15% sulphur, -and 20% charcoal. It is a black granulated powder having a specific -gravity of 1.5; the black color is given by the charcoal; and the -grains have an angular form, and vary in size from ¹⁄₈ in. to ³⁄₈ in. -Good blasting powder should contain no fine grains, which may be -detected by pouring some of the powder upon a sheet of white paper. The -force developed by the explosion of gunpowder is not accurately known; -it depends upon the space in which it is confined. Different authorities -estimate the pressure at from 15,000 lbs. per sq. in. in loose blasts to -200,000 lbs. per sq. in. in gunnery. Authorities also differ in opinion -as to the character of the gases developed by the explosion of -gunpowder, a matter of vital concern to the tunnel engineer, since they -are likely to affect the health and comfort of his workmen. It may be -assumed in a general way, however, that the oxygen of the saltpeter -converts nearly all of the carbon of the charcoal into carbon dioxide, a -portion of which combines with the potash of the saltpeter to form -carbonate of potash, the remainder continuing in the form of gas. The -sulphur is converted into sulphuric acid, and forms a sulphate of -potash, which by reaction is decomposed into hyposulphite and sulphide. -The nitrogen of the saltpeter is almost entirely evolved in a free -state; and the carbon not having been wholly burnt into carbonic acid, -there is a proportion of carbonic oxide. - - -=Nitroglycerine.=--Nitroglycerine is one of the modern explosives used -as a substitute for gunpowder. It is a fluid produced by mixing -glycerine with nitric and sulphuric acids; it freezes at +41° F., and -burns very quietly, developing carbonic acid, nitrogen, oxygen, and -water. By percussion or by the explosion of some substances, such as -capsules of gunpowder or fulminate of mercury, nitroglycerine produces a -sudden explosion in which about 1250 volumes of gases are produced. The -pressure of these gases has been calculated at 26,000 atmospheres, or -324,000 lbs. per sq. in. Nitroglycerine explodes very easily by -percussion in its normal state, but with great difficulty when frozen; -hence, in America, at the beginning of its use, it was transported only -in a frozen state. When dirty, nitroglycerine undergoes a spontaneous -decomposition accompanied by the development of gases and the evolution -of heat, which, reaching 388° F., causes it to explode. Notwithstanding -the enormous pressures which nitroglycerine develops, it is very seldom -used in its liquid state, but is mixed with a granular absorbent earth -composed of the shells of diatoms. The fluid undergoes no chemical -change by being absorbed, and explodes, freezes, and burns under the -same conditions as in the fluid state. - - -=Dynamite.=--The credit of rendering nitroglycerine available for the -purposes of the engineer by mixing it with a granular absorbent is due -to Albert Nobel of Stockholm, Sweden, who named the new material -dynamite. The nitroglycerine in dynamite loses very little of its -original explosive power, but is very much less easily exploded by -percussion, and can be employed in horizontal as well as vertical holes, -which was, of course, not possible in its liquid state. Dynamite must -contain at least 50% of nitroglycerine. Some manufacturers, instead of -using diatomaceous earth, use other absorbents which develop gases upon -explosion and increase the force of the explosion. These mixtures are -classed under the general name of false dynamites. A great many -varieties of dynamite are manufactured, and each manufacturer usually -makes a number of grades to which he gives special names. Dynamite for -railway work, tunneling, and mining contains about 50% of -nitroglycerine; for quarrying about 35%, and for blasting soft rocks -about 30%. It is sold in cylindrical cartridges covered with paper. - - -=Storage of Explosives.=--In driving tunnels through rock large -quantities of explosives must be used, and it is necessary to have some -safe place for storing them. In many States there are special laws -governing the transportation and storage of explosives; where there is -no regulation by law the engineer should take suitable precautions of -his own devising. It is best to build a special house or hut in one of -the most concealed portions of the work and away from the tunnel, and -protect it with a lightning-rod and from fire. Strict orders should be -given to the watchman in charge not to allow persons inside with lamps -or fire in any form, and smoking should be prohibited. The use of -hammers for opening the boxes should be prohibited; and dynamite, -gunpowder, and fulminate of mercury should not be stored together in the -same room. A quantity of dynamite for two or three days’ consumption may -be stored near the entrance of the tunnel in a locked box, the keys of -which are kept by the foreman of the work. When dynamite has been frozen -the engineer should provide some arrangement by which it may be heated -to a temperature not exceeding 120° F., and absolutely forbid it being -thawed out on a stove or by an open fire. - - -=Fuses.=--When gunpowder is used in tunneling it is ignited by the -Blickford match. This match, or fuse as it is more commonly called, -consists of a small rope of yarn or cotton having as a core a small -continuous thread of fine gunpowder. To protect the outside of the fuse -from moisture it is coated with tar or some other impervious substance. -These fuses are so well made that they burn very uniformly at the rate -of about 1 ft. in 20 seconds, hence the moment of explosion can be -pretty accurately fixed beforehand. Blickford matches have the objection -for tunnel work of burning with a bad odor, especially when they are -coated with tar, and to remedy this many others have been invented. -Those of Rzika and Franzl are the best known of these. The former has -many advantages, but it burns too quickly, about 3 ft. per second, and -is expensive; the latter consists of a small hollow rope filled with -dynamite. - -Blickford matches cannot be used to explode dynamite, the use of a -cartridge being required. These cartridges are small copper cylinders -containing fulminate of mercury. They may be attached to the end of the -Blickford match, which being ignited the spark travels along its length -until it reaches the copper cylinder, where it explodes the fulminate of -mercury, which in turn explodes the dynamite. Blasts may also be fired -by electricity, which, in fact, is the most common and the preferable -method, because several blasts can be fired simultaneously, and because -the current is turned on at a great distance, thus affording greater -safety to the workmen. - -The method of electric firing generally employed in America is known as -the connecting series method, and consists in firing several mines -simultaneously. The ends of the wires are scraped bare, and the wire of -the first hole of the series is twisted together with the wire of the -second hole, and so on; finally the two odd wires of the first and last -holes are connected to two wires of a single cable or to two separate -cables extending to some safe place to which the men can retreat. Here -the two cable wires are connected by binding screws to the poles of a -battery, or sometimes to a frictional electric machine. The current -passes through the wires, making a spark at each break, and so fires the -fulminate of mercury, which explodes the dynamite. - -Simultaneous firing by electricity by utilizing the united strength of -the blasts at the same instant secures about 10% greater efficiency from -the explosives. Another advantage of electric firing is that in case of -a missfire of any one of the holes there is slight possibility of -explosion afterwards, and the place can be approached at once to -discover the cause. - - -=Tamping.=--Tamping is the material placed in the hole above the -explosive to prevent the gases of explosion from escaping into the air. -Tamping generally consists of clay. When gunpowder is used the clay must -be well rammed with a wooden tool, and paper, cotton, or some other dry -material must be placed between the moist clay and the powder. When -dynamite is used it is not necessary to ram the tamping, since the -suddenness of the explosion shatters the rock before the clay can be -driven from the hole. - -A few experienced men should be appointed to fire the blasts. These men -should give ample warning previous to the blast in order that all -machinery and tools which might be injured by flying fragments may be -removed out of danger, and so that the workmen may seek safety. When -all is ready they should fire the blasts, keeping accurate count of the -explosions to ensure that no holes have missed fire, and should call the -workmen back when all danger is over. In case any hole has missed fire -it should be marked by a red lamp or flag. - - -=Nature of Explosions.=--When the explosives are ignited a sudden -development of gases results, producing a sudden and violent increase of -pressure, usually accompanied by a loud report. The energy of the -explosion is exerted in all directions in the form of a sphere having -its center at the point of explosion, and the waves of energy lose their -force as the distance from this central point increases. The energy of -the explosion at any point in the sphere of energy is, therefore, -inversely proportional to the distance of this point from the center of -explosion. In the vicinity of the center of explosion the gases have -sufficient power to destroy the force of cohesion and shatter the rock; -further on, as they lose strength, they only destroy the elasticity of -the material and produce cracks; and still further away they only -produce a shock, and do not affect the material. Within the sphere of -energy there are, therefore, three other concentric spheres: the first -one being where cohesion is destroyed, the second where elasticity is -overcome, and the third where the shock is transmitted by elasticity. -When the latter sphere comes below the surface, the gases remain inside -the rock; but when the surface intersects either of the other two -spheres, the gases blow up the rock, forming a cone or crater, whose -apex is at the point of explosion, and which is called the -blasting-cone. The larger the blasting-cone is, the greater is the -amount of rock broken up; and the object of the engineer should, -therefore, always be so to regulate the depth of the hole and the -quantity of explosive as to secure the largest possible blasting cone in -each case. Experiments are required to determine the most efficient -depth of hole, and quantity of explosive to be employed, since these -differ in different kinds of rock, with the position of the rock -strata, etc.; but in ordinary practice, the depths of the holes are -made from 2 to 3 ft. in the heading and upper portion of the tunnel, -when drilled by hand; and from 6 to 8 ft. when drilled by power drills. -In the lower portion of the profile, the holes are made deeper, from 3 -ft. to 4 ft. when drilled by hand, and exceeding 6 ft. when drilled by -power. The distance of the holes apart should be about equal to the -diameter of the blasting-cone; as a general rule it is assumed that the -base of the blasting-cone has a diameter equal to twice the depth of the -hole. The following table gives the average number of holes required in -each part of the excavation for the St. Gothard tunnel in which the -heading was excavated by machine drills while the other parts were -excavated by hand drills: - - NO. OF PART.[5] NAME OF PART. NO. OF HOLES. - 1. Heading 6 to 9 - 2. Right wing of heading 3 to 5 - 3. Left wing of heading 3 to 5 - 4. Shallow trench with core 2 - 5. Deepening of trench to floor 6 to 9 - 6. Narrow mass of core to left 3 - 7. Greater mass of core to left 6 to 9 - 8. Culvert 1 - -------- - Total section 30 to 43 - - [5] The location of the parts numbered is shown by Fig. 14, p. 36. - -The quantity of explosives required for blasting depends upon the -quality of the rock, since the force of the explosives must overcome the -cohesion of the rock, which varies with its nature, and often differs -greatly in rocks of the same kind and composition. The quantity of -explosives required to secure the greatest efficiency in blasting any -particular rock may be determined experimentally, but in practice it is -usually deduced by the following rules: (1) The blasting force is -directly proportional to the weight of the explosives used, and (2) the -bulk of the blasted rock is proportional to the cube of the depth of the -holes. It is usually assumed, also, that the explosive should fill at -least one-fourth the depth of the hole. - -The following table gives the depth of holes and amount of dynamite used -at each advance in the Fort George Tunnel illustrated on page 135. - - +-------------------+------------+--------+-------+----------+ - | ORDER OF FIRING. | KINDS OF | DEPTH. |CHARGE.| KIND OF | - | | HOLES. | | | DYNAMITE.| - +-------------------+------------+--------+-------+----------+ - |Bench { 1st round|4 grading |3′ to 5′|50 lbs.|40% climax| - |Holes { |5 bench |9′ 6″ |45 „ |40% „ | - | { 2nd round|6 trimming |3′ to 9′|42 „ |40% „ | - | | | | | | - |Heading { 3d round |8 center cut|9′ |56 „ |60% „ | - | Holes { 4th round|8 side |8′ |48 „ |40% „ | - | { 5th round|6 dry |8′ |36 „ |40% „ | - +-------------------+------------+--------+-------+----------+ - - - - -CHAPTER IV. - -GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS. - - -A number of different modes of procedure are followed in excavating -tunnels, and each of the more important of these will be considered in a -separate chapter. There are, however, certain characteristics common to -all of these methods, and these will be noted briefly here. - -[Illustration: FIG. 14.--Diagram Showing Sequence of Excavation for St. -Gothard Tunnel.] - -[Illustration: FIG. 15.--Diagram Showing Manner of Determining -Correspondence of Excavation to Sectional Profile.] - - -=Division of Section.=--It may be asserted at the outset that the whole -area of the tunnel section is not ordinarily excavated at one time, but -that it is removed in sections, and as each section is excavated it is -thoroughly timbered or strutted. The order in which these different -sections are excavated varies with the method of excavation, and it is -clearly shown for each method in succeeding chapters. As a single -example to illustrate the proposition just made, the division of the -section and the sequence of excavation adopted at the St. Gothard tunnel -is selected (Fig. 14). The different parts of the section were excavated -in the order numbered; the names given to each part, and the number of -holes employed in breaking it down, are given by the table on page 35. -Whatever method is employed, the work always begins by driving a -heading, which is the most difficult and expensive part of the -excavation. All the other operations required in breaking down the -remainder of the tunnel section are usually designated by the general -term of enlargement of the profile. The various operations of excavation -may, therefore, be classified either as excavation of the heading or -enlargement of the profile. - - -=Excavation of the Heading.=--There is considerable confusion among the -different authorities regarding the exact definition of the term -“heading” as it is used in tunnel work. Some authorities call a small -passage driven at the top of the profile a heading, and a similar -passage driven at the bottom of the profile a drift; others call any -passage driven parallel to the tunnel axis, whether at the top or at the -bottom of the profile, a drift; and still others give the name “heading” -to all such passages. For the sake of distinctness of terminology it -seems preferable to call the passage a heading when it is located at the -top of the profile, and a drift when it is located near the bottom. - -Headings and drifts are driven in advance of the general excavation for -the following purposes: (1) To fix correctly the axis of the tunnel; (2) -to allow the work to go on at different points without the gangs of -laborers interfering with each other; (3) to detect the nature of -material to be dealt with and to be ready in any contingency to overcome -any trouble caused by a change in the soil; and (4) to collect the -water. The dimensions of headings in actual practice vary according to -the nature of the soil through which they are driven. As a general rule -they should not be less than 7 ft. in height, so as to allow the men to -work standing, and have room left for the roof strutting. The width -should not be less than 6 ft., to allow two men to work at the front, -and to give room for the material cars without interfering with the wall -strutting. Usually headings are made 8 ft. wide. The length of headings -in practice varies according to circumstances. In very long tunnels -through hard rock the headings are sometimes excavated from 1000 ft. to -2000 ft. in advance, in order that they may meet as soon as possible and -the ranging of the center line be verified, and so that as great an -area of rock as possible may be attacked at the same time in the work of -enlarging the profile. In short tunnels, where the ranging of the center -line is less liable to error, shorter headings are employed, and in soft -soils they are made shorter and shorter as the cohesion of the soil -decreases. When the material has too little cohesion to stand alone, the -tops and sides of the heading require to be supported by strutting. To -prevent caving at the front of the heading, the face of the excavation -is made inclined, the inclination following as near as may be the -natural slope of the material. - - -=Enlargement of the Profile.=--The enlargement of the profile is -accomplished by excavating in succession several small prisms parallel -to the heading, and its full length, which are so located that as each -one is taken out the cross-section of the original heading is enlarged. -The number, location, and sequence of these prisms vary in different -methods of excavation, and are explained in succeeding chapters where -these methods are described. To direct the excavation so as to keep it -always within the boundaries of the adopted profile, the engineer first -marks the center line on the roof of the heading by wooden or metal -pegs, or by some other suitable means by which a plumb line may be -suspended. He next draws to a large scale a profile of the proposed -section; and beginning at the top of the vertical axis he draws -horizontal lines at regular intervals, as shown by Fig. 15, until they -intersect the boundary lines of the profile, and designates on each of -these lines the distance between the vertical axis and the point where -it intersects the profile. It is evident that if the foreman of -excavation divides his plumb line in a manner corresponding to the -engineer’s drawing, and then measures horizontally and at right angles -to the vertical center plane of the tunnel the distance designated on -the horizontal lines of the drawing, he will have located points on the -profile of the section, or in other words have established the limits of -excavation. - -[Illustration: FIG. 16.--Polar Protractor for Determining Profile of -Excavated Cross-Section.] - -In the excavation of the Croton Aqueduct for the water supply of New -York city, an instrument called a polar protractor was used for -determining the location of the sectional profile. It was invented by -Mr. Alfred Craven, division engineer of the work. This instrument -consists of a circular disk graduated to degrees, and mounted on a -tripod in such a manner that it may be leveled up, and also have a -vertical motion and a motion about the vertical axis. The construction -is shown clearly by Fig. 16. In use the device is mounted with its -center at the axis of the tunnel. A light wooden measuring-rod tapering -to a point, shod with brass and graduated to feet and hundredths of a -foot, lies upon the wooden arm or rest, which revolves upon the face of -the disk, and slides out to a contact with the surface of the -excavation at such points as are to be determined. If the only -information desired is whether or not the excavation is sufficient or -beyond the established lines, the rod is set to the proper radius, and -if it swings clear the fact is determined. If a true copy of the actual -cross-section is desired, the rod is brought into contact with the -significant points in the cross-section, and the angles and distances -are recorded. - -The general method of directing the excavation in enlarging the profile -by referring all points of the profile to the vertical axis is the one -usually employed in tunneling, and gives good results. It is considered -better in actual practice to have the excavation exceed the profile -somewhat than to have it fall short of it, since the voids can be more -easily filled in with riprap than the encroaching rock can be excavated -during the building of the masonry. In tunnels where strutting is -necessary the excavation must be made enough larger than the finished -section to provide the space for it. In soft-ground tunnels it is also -usual to enlarge the excavation to allow for the probable slight sinking -of the masonry. The proper allowance for strutting is usually left to -the judgment of the foreman of excavation, but the allowance for -settlement must be fixed by the engineer. - - -SHAFTS. - -Shafts are vertical walls or passages sunk along the line of the tunnel -at one or more points between the entrances, to permit the tunnel -excavation to be attacked at several different points at once, thus -greatly reducing the time required for excavation. Shafts may be located -directly over the center of the tunnel or to one side of it, and, while -usually vertical, are sometimes inclined. During the construction of the -tunnel the shafts serve the same purpose as the entrances; hence they -must afford a passageway for the excavated materials, which have to be -hoisted out, and also for the construction tools and materials which -have to be lowered down them. They must also afford a passageway for -workmen, draft animals, and for pipes for ventilation, water, compressed -air, etc. The character of this traffic indicates the dimensions -required, but these depend also upon the method of hoisting employed. -Thus, when a windlass or horse gin is used, and the materials are -hoisted in buckets of small dimensions, the dimensions of the shaft may -also be small; but when steam elevators are employed, and the material -is carried on cars run on to the platform of the elevator, large -dimensions must be given to the shaft. Generally the parts of the shaft -used for different purposes are separated by partitions. The elevator -for workmen and the various pipes are placed in one compartment, while -the elevator for hoisting the excavated material and lowering -construction material is placed in another. - -Shafts may be either temporary or permanent. They are temporary when -they are filled in after the tunnel is completed, and permanent when -they are left open to supply ventilation to the tunnel. Permanent shafts -are usually made circular, and lined with brick, unless excavated in -very hard and durable rock. When sunk for temporary use only, shafts are -usually made rectangular with the greater dimension transverse to the -tunnel. They are strutted with timber. A pump is generally located at -the bottom of the shaft to collect the water which seeps in from the -sides of the shaft and from the tunnel excavation. The dimensions of -this pump will of course vary with the amount of water encountered, as -will also the capacity of the pump for forcing it up and out of the -shaft, which has always to be kept dry. - -The majority of engineers prefer to sink shafts directly over the center -line of the tunnel. Side shafts are employed chiefly by French -engineers. The chief advantage of the former method is the great -facility which it affords for hoisting out the materials, while in favor -of the latter method is the non-interference of the shaft with the -operations inside the tunnel. Were it not that the side shaft requires -the introduction of a transverse gallery connecting it with the tunnel, -it would be on the whole superior to the center shaft; but the side -gallery necessitates turning the cars at right angles, and consequently -the use of a very sharp curve or a turntable to reach the shaft bottom, -which is a disadvantage that may outweigh its advantages in some other -respects. It is impossible to state absolutely which of these methods of -locating shafts is the best; both present advantages and disadvantages, -and the use of one or the other is usually determined more by the local -conditions than by any general superiority of either. - -When side shafts are employed they are sometimes made inclined instead -of vertical. This form is used when the depth of the shaft is small. By -it the hauling is greatly simplified, since the cars loaded at the front -with excavated material can be hauled directly out of the shaft and to -the dumping-place, surmounting the inclined shaft by means of continuous -cables. The short galleries connecting the side shafts with the tunnel -proper usually have a smaller section than the tunnel, but are excavated -in exactly the same manner. Another form of side shaft sometimes used is -one reaching to the surface when the tunnel runs close to the side of -cliff, as is the case with some of the Alpine railway tunnels. - - -CLASSIFICATION OF TUNNELS. - -Tunnels are classified in various ways, but the most logical method -would appear to be a grouping according to the quality of the material -through which they are driven; and this method will be adopted here. By -this method we have first the following general classification: (1) -Tunnels in hard rock; (2) tunnels in ordinary loose soil; (3) tunnels in -quicksand; (4) open-cut tunnels; and (5) submarine tunnels. It is hardly -necessary to say that this classification, like all others, is simply -an arbitrary arrangement adopted for the sake of order and convenience -in treating the subject. - - -=Tunnels in Hard Rock.=--With the numerous labor-saving methods and -machines now available, hard rock is perhaps the safest and easiest of -all materials through which to drive a tunnel. Tunnels through hard rock -may be excavated, either by a drift or by a heading. The difference -depends upon whether the advance gallery is located close to the floor -or near the soffit of the section. - - -=Tunnels in Loose Soils.=--In driving tunnels through loose soils many -different methods have been devised, which may be grouped as follows: -(1) Tunnels excavated at the soffit--Belgian method; (2) tunnels -excavated along the perimeter--German method; (3) tunnels excavated in -the whole section--English, Austrian and American methods; (4) tunnels -excavated in two halves independent of each other--Italian method. - -(1) Excavating the tunnel by beginning at the soffit of the section, or -by the Belgian method, is the method of tunneling in loose soils most -commonly employed in Europe at the present time. It consists in -excavating the soffit of the section first; then building the arch, -which is supported upon the unexcavated ground; and finally in -excavating the lower portion of the section, and building the side walls -and invert. - -(2) In excavating tunnels along the perimeter an annular excavation is -made, following closely the outline of the sectional profile in which -the lining masonry is built, after which the center core is excavated. -In the German method two drifts are opened at each side of the tunnel -near the bottom. Other drifts are excavated, one above the other, on -each side to extend or heighten the first two until all the perimeter is -open except across the bottom. The masonry lining is then built from the -bottom upwards on each side to the crown of the arch, and then the -center core is removed and the invert is built. - -(3) This method, as its name implies, consists in taking out short -lengths of the whole sectional profile before beginning the building of -the masonry. In the English method the invert is built first, then the -side walls, and finally the arch. The excavators and masons work -alternately. The Austrian method differs in two particulars from the -English: the length of section opened is made great enough to allow the -excavators to continue work ahead of the masons, and the side walls and -roof are built before the invert. In the American method the whole -section of the tunnel is open at once: excavators and masons work -simultaneously, but a very large quantity of timbering is required. - -(4) The Italian method is very seldom employed on account of its -expensiveness, but it can often be used where the other methods fail. It -consists in excavating the lower half of the section, and building the -invert and side walls, and then filling the space between the walls in -again except for a narrow passageway for the cars; next the upper part -of the section is excavated, as in the Belgian method, and the arch is -built; and finally the soil in the lower part is permanently removed. - - -=Tunnels in Quicksand.=--Tunnels through quicksand are driven by one of -the ordinary soft-ground methods after draining away the water, or else -as submarine tunnels. - - -=Open-Cut Tunnels.=--Open-cut tunnels are those driven at such a small -depth under the surface that it is more convenient to excavate an open -cut, build the tunnel masonry inside it, and then refill the open -spaces, than it is to carry on the work entirely underground. In firm -soils the usual mode of operation is to excavate first two parallel -trenches for the side walls, then remove the core, and build the arch -and the invert. In unstable soils, since the invert must be built first, -it is usual to open up a single wide trench. In infrequent cases where a -tunnel is desired in a place which is to be filled in, the masonry is -built as a surface structure, which in due time is covered. - - -=Submarine Tunnels.=--The mode of procedure followed in excavating -submarine tunnels depends upon whether the material penetrated is -pervious or impervious to water. In impervious material any of the -ordinary methods of tunneling found suitable may be employed. In -pervious material the excavation may be accomplished either by means of -compressed air to keep the water out of the excavation, or by means of a -shield closing the front of the excavation, or by a combination of these -two methods. Tunnels on the river bed are built by means of coffer dams -which inclose alternate portions of the work, by sinking a continuous -series of pneumatic caissons and opening communication between them, and -by sinking the tunnel in sections constructed on land. - - {_In hard rock._ {By drifts. - { {By a heading. - { {_By upper half:_ } - { { the arch is built }Belgian method. - { { before the side walls. } - { { - { {_By the perimeter:_ } - { { excavated and lined } - { { before the central }German method. - { { nucleus is removed. } - { { - {_In loose soil._{_By whole section:_ {English method. - { { the lining begins after{Austrian method. - { { the whole section is {American method. - { { excavated. { - { { - { {_By halves:_ } - { { the lower half is } - { { excavated and lined, }Italian method. - { { followed by the work } - { { of the upper half. } - METHODS OF{ - EXCAVATING{_In quicksand._ - TUNNELS. { - { {In resistant soils. {By two lateral - { { {narrow trenches. - {_Open-cut_ { - {_tunnels._ {In loose soils. {By one very large - { { {trench. - { { - { {Built up. By slices. - { - { {At great depths under }By any method. - { {the river bed. } - { { - { { {By shield. - { {At small depths {By compressed - { {under the river {air. - {_Submarine_ {bed. {By shield and - {_tunnels._ { {compressed air. - { { - { { {By coffer dams. - { { {By pneumatic - { {On the river bed. {caissons. - { { {By built-up - { { {sections. - -The above diagram gives in compact form the classification of tunnels -according to materials penetrated and methods of excavation adopted, -which have been described more fully in the succeeding paragraphs. It -may be noted here again that this is a purely arbitrary classification, -and serves mostly as a convenience in discussing the different classes -of tunnels without confusion. - - - - -CHAPTER V. - -METHODS OF TIMBERING OR STRUTTING TUNNELS. - - -The purpose of timbering or strutting in tunnel work is to prevent the -caving-in of the roof and side walls of the excavation previous to the -construction of the lining. As the strutting has to resist all the -pressures developed in the roof and side walls, which may be exceedingly -troublesome and of great intensity in loose soils, its design and -erection call for particular care. The method of strutting adopted -depends upon the method of excavation employed; but in every case the -problem is not only to build it strong enough to withstand the pressures -developed, but to do this as economically as possible, and with as -little hindrance as may be to the work which is going on simultaneously -and which will come later. Only the latter general problems of strutting -peculiar to all methods of tunnel work will be considered here. For this -consideration strutting may be classified according to the material of -which it is built, under the heads of timber structures and iron -structures. - -[Illustration: FIG. 17.--Joining Tunnel Struts by Halving.] - -[Illustration: FIG. 18.--Round Timber Post and Cap Bearing.] - - -TIMBER STRUTTING. - -Timber is nearly always employed for strutting in tunnel work. So long -as it has the requisite strength, any kind of timber is suitable for -strutting, since, it being only temporarily employed, its durability is -a matter of slight importance. Timber with good elastic properties, like -pine or spruce, is preferably chosen, since it yields gradually under -stress, thus warning the engineer of the approach of danger; while oak -and other strong timbers resist until the last moment, and then yield -suddenly under the breaking load. Soft woods, moreover, are usually -lighter in weight than hard woods, which is a considerable advantage -where so much handling is required in a restricted space. Round timbers -are generally employed, since they are less expensive, and quite as -satisfactory in other respects as sawed timbers. In the English and -Austrian methods of strutting, which are described further on, a few of -the principal struts are of sawed timbers. - -The various timbers of the strutting are seldom attached by framed -joints, but wedges are used to give them the necessary bearing against -each other. Where framed joints are employed they are made of the -simplest form usually by halving the joining timbers, as shown by Fig. -17. Fig. 18 shows a form of joint used where round posts carry beams of -similar shape. The reason why it is possible to do away with jointed -connections to such a great extent, is that the strains which the -timbers have to resist are either compressive or bending strains, and -because the timbers are so short that they do not require to be spliced. - - -=Strutting of Headings.=--The method of strutting the heading that is -employed depends upon the material through which the heading is driven. -In solid rock strutting may not be required at all, or only for the -purpose of preventing the fall of loose blocks from the roof, then -vertical props are erected where required, or horizontal beams are -inserted into the side walls, as shown by Fig. 19. These horizontal -beams may be used singly at dangerous places, or they may be placed from -2 ft. to 3 ft. apart all along the heading. In the latter case they -usually carry a lagging of planks, which may be placed at intervals or -close together, and filled above with stone in case the roof of the -excavation is very unstable. Planks used in this manner are usually -called poling-boards. Where the side walls as well as the roof require -support, vertical side posts are employed to carry the roof beams, as -shown by Fig. 20; and, when necessary, poling-boards are inserted -between these posts and the walls of the excavation. - -[Illustration: FIG. 19.--Ceiling Strutting for Tunnel Roofs.] - -[Illustration: FIG. 20.--Ceiling Strutting with Side Post Supports.] - -[Illustration: FIG. 21.--Sill, Side Post and Cap Cross Frame Strutting.] - -[Illustration: FIG. 22.--Reinforced Cross Frame Strutting for -Treacherous Materials.] - - -_Frame Strutting._--In very loose soils not only the roof and side -walls, but also the floor of the heading require strutting. In these -cases frame strutting is employed, as shown by Fig. 21. It consists -simply of a rectangular frame; at the top there is a crown bar supported -by two vertical side posts setting on a sill laid across the bottom of -the heading. These frames are spaced at close intervals, and carry -longitudinal planks or poling-boards. The sill of the frame is sometimes -omitted when the soil is stable enough to permit it, and in its place -wooden footing blocks are substituted to carry the side posts. In soils -where the pressures are great enough to bend the crown bar, a secondary -frame is employed, as shown by Fig. 22, the two inclined roof members, -or rafters, of which support the crown bar at the center. - -[Illustration: FIG. 23.--Longitudinal Poling-Board System of Roof -Strutting.] - -[Illustration: FIG. 24.--Transverse Poling-Board System of Roof -Strutting.] - -It is the more common practice in driving headings through soft soils to -use inclined poling-boards to support the roof. Fig. 23 shows one method -of doing this. The method of operation is as follows: Assuming the -poling-boards _a_ and _b_ to be in place, and supported by the frames -_A_, _B_, _C_, as shown, the first step in continuation of the work is -to insert the poling-board _c_ over the crown bar of frame _C_, and -under the block _m_. Excavation is then begun at the top, and as fast as -the soil is removed ahead of it the poling-board _c_ is driven ahead -until its rear end only slightly overhangs the crown bar of frame _C_. -The remainder of the face of the heading is then excavated nearly to the -front end of the poling-board _c_, and another frame is set up. By a -succession of these operations the heading is advanced. The -poling-boards at the sides of the heading are placed in a similar manner -to the roof poling-boards. A second method of using inclined -poling-boards is shown by Fig. 24. Here the poling-boards run -transversely, and are supported by the arrangement of timbering shown. -The chief advantage of using these inclined poling-boards, particularly -in the manner shown by Fig. 23, is that the excavators work under cover -at all times, and are thus safe from falling fragments or sudden -cavings. - - -_Box Strutting._--In very treacherous soils, such as quicksand, peat, -and laminated clay, box strutting is commonly employed. The method of -building this strutting is to set up at the face of the work a -rectangular frame, and use it as a guide in driving a lagging or boxing -of horizontal planks into the soft soil ahead. These planks have sharp -edges, and are driven to a distance of 2 ft. or 3 ft. into the face of -the heading, so as to inclose a rectangular body of earth. This earth is -excavated nearly to the ends of the planks, and then another frame is -inserted close up against the new face of the excavation, which supports -the planks so that the remainder of the earth included by them may be -removed. These two frames, with their plank lagging, constitute a “box;” -and a series of these boxes, one succeeding another, form the strutting -of the heading. - - -=Strutting the Face.=--In some cases it is found necessary to strut the -face of the heading in order to prevent it from caving in. This is -generally done by setting plank vertically, and bracing them up by means -of inclined props whose feet abut against the sill of the nearest cross -frame. This strutting is erected while the workmen are placing the side -and roof strutting, and is removed to permit excavation. - - -=Full Section Timber Strutting.=--For strutting the full section two -forms of timbering are employed, known as the polygonal system and the -longitudinal system. - -Longitudinal strutting consists of a timber structure so arranged as to -have all the principal members supporting the poling-boards parallel to -the axis of the tunnel. This system of strutting is peculiar to the -English method of tunneling. The longitudinal timbers rest on this -finished masonry at one end, and are carried on a cross frame or by -props at the other end. At intermediate points the longitudinals are -braced apart by struts in planes transverse to the tunnel axis. This -construction makes a very strong strutting framework, since the -transverse struts act as arch ribs to stiffen the longitudinals; but the -use of transverse poling-boards requires the excavation of a larger -cross-section than is necessary when longitudinal poling-boards are -employed, and this increases the cost both for the amount of earth -excavated and the greater quantity of filling required. - -In polygonal strutting the main members are in a plane normal to the -axis of the tunnel. They form a polygon whose sides follow closely the -sectional profile of the excavation. These polygonal frames are placed -at more or less short intervals apart, and are braced together by short -longitudinal struts lying close to the sides of the excavation, and -running from one frame to the next, and also by longer longitudinal -members which extend over several frames. The polygonal system of -strutting is peculiar to the Austrian method of tunneling, and is fully -described in a succeeding chapter. One of its distinctive -characteristics is that the poling-boards are inserted parallel to the -tunnel axis. Polygonal strutting is generally held to be stronger than -longitudinal strutting under uniform loads, but it is more liable to -distortion when the loads are unsymmetrical. - -[Illustration: FIG. 25.--Shaft with Single Transverse Strutting.] - -[Illustration: FIG. 26.--Rectangular Frame Strutting for Shafts.] - -[Illustration: FIG. 27.--Reinforced Rectangular Frame Strutting for -Shafts in Treacherous Materials.] - - -=Strutting of Shafts.=--Tunnel shafts are strutted both to prevent the -caving-in of the sides and to divide them into compartments. When the -material penetrated is very compact, and caving is not likely, a single -series of transverse struts, one above the other, running from the top -to the bottom of the shaft, as shown by Fig. 25, is used to divide it -into two compartments. In softer material, where the sides of the shaft -require support, Fig. 26 shows a form of strutting commonly employed. It -consists of vertical corner posts braced apart at intervals by four -horizontal struts placed close to the walls of the shaft. The longer -side struts are also braced apart at the center by a middle strut which -divides the shaft into two compartments. A lagging of vertical plank is -placed between the walls of the shaft and the horizontal side struts. In -very loose soils the form of strutting shown by Fig. 27 is employed. -This is practically the same construction as is shown by Fig. 26, with -the addition of an interior polygonal horizontal bracing in each half of -the shaft. Referring to Fig. 27, the timbers _a_, _a_, etc., are -vertical and continuous from the top to the bottom of the shaft; and the -horizontal timbers, _b_, _b_, etc., are spaced at more or less close -intervals vertically. The lagging planks may be laid with spaces between -them, or close together, or, in case of very loose material, with their -edges overlapping. The manner of constructing the strutting is also -governed by the stability of the soil. In firm soils it is possible to -sink the shaft quite a depth without timbering, and the timbering can -be erected in sections of considerable length, which is always an -advantage, but in loose soils the timbering has to follow closely the -excavation. - -The solid wall shaft struttings which have been described are -discontinued at the point where the shaft intersects the tunnel -excavation; and from this point to the floor of the tunnel an open -timbering is employed, whose only duty is to support the weight of the -solid strutting above. This timbering is made in various forms, but the -most common is a timber truss or arch construction which spans the -tunnel section. - - -=Quantity of Timber.=--The quantity of timber employed in strutting a -tunnel varies with the character of the material through which the -tunnel is excavated: it is small for solid-rock tunnels, and large for -soft-ground tunnels. In the Belgian method of excavation a smaller -quantity of timber is used than in any of the other ordinary methods. -For single-track tunnels excavated by this method there will be needed -on an average about 3 to 3¹⁄₃ cu. yds. of timber per lineal foot of -tunnel. Practical experience shows that about four-fifths of the timber -once used can be employed for the second time. In any of the methods in -which the whole tunnel section is excavated at once, the average amount -of timber required per lineal foot is about 8.7 cu. yds. Of this amount -about two-thirds can be used a second time. In the Italian method, in -which the upper half and the lower half are excavated separately, about -5 cu. yds. of timber are required per lineal foot of tunnel, about -one-half of which can be employed a second time. For quicksand tunnels -the amount of timbering required per lineal foot varies from 3 to 5 -cubic yds. Shaft strutting requires from 1 to 1¹⁄₂ cu. yds. of timber -per lineal foot. - - -=Dimensions of Timber.=--The dimensions of the principal members -composing the strutting of headings, full section, and shafts, are given -in Table I. The planks used for lagging or the poling-boards are usually -from 4 ins. to 6 ins. wide, with a length depending upon the method of -strutting employed. - -TABLE I. - -Showing Sizes of Various Timbers Used in Strutting Tunnels Driven -Through Different Materials. - - +---------------------------------+-----------+----------------------+ - | | ROCK. | SOFT SOILS. | - | +-----+-----+--------+------+------+ - | |Hard.|Soft.|Compact.|Loose.| Very | - | | | | | |loose.| - | +-----+-----+--------+------+------+ - | | ins.| ins.| ins. | ins. | ins. | - |Headings: | | | | | | - | Cap-pieces and vertical struts | 6 | 8 | 10 | 12 | 14 | - | Sills | | | 8 | 10 | 12 | - | Struts | 5 | 5 | 6 | 7 | 8 | - | Distance apart of the frames in | | | | | | - | feet | 6 | 4.5| 3 | 2.6 | 2.6 | - | | | | | | | - |Strutting of the tunnel, | | | | | | - |longitudinal strutting: | | | | | | - | Crown bars | 12 | 14 | 14 | | | - | Props vertical or inclined | | | | | | - | supporting the crown bars | 10 | 12 | 14 | | | - | Sills | 8 | 8 | 10 | | | - | Cap-pieces or saddles | 10 | 12 | 14 | | | - | Struts to stiffen the structure | 6 | 8 | 10 | | | - | Distance apart of the frames (in| | | | | | - | feet) | 4.5 | 4 | 3 | | | - | | | | | | | - |Polygonal strutting: | | | | | | - | Cap-pieces and contour pieces | 8 | 10 | 12 | 14 | 16 | - | Vertical struts on top | 10 | 12 | 14 | 16 | 18 | - | Vertical struts below | 12 | 14 | 16 | 20 | 24 | - | Intermediate sills | 12 | 14 | 16 | 20 | 24 | - | Lower sills | | | 12 | 16 | 18 | - | Raking props | 10 | 10 | 10 | 12 | 12 | - | Distance apart of the frames (in| | | | | | - | feet) | 6 | 4.5 | 4 | 3 | 3 | - | | | | | | | - |Shafts: | | | | | | - | Horizontal beams forming the | | | | | | - | frame | 8 | 8 | 10 | 12 | 14 | - | Transverse beams | 8 | 8 | 8 | 10 | 12 | - | Vertical struts between the | | | | | | - | frames | 8 | 8 | 10 | 12 | 12 | - | Struts to reënforce the frame | | 6 | 8 | 8 | 8 | - | Distance apart of the strutting | | | | | | - | (in feet) | 6 | 4.5 | 4 | 3 | 2.6 | - +---------------------------------+-----+-----+--------+------+------+ - - -IRON STRUTTING. - -In 1862 Mr. Rziha employed old iron railway rails for strutting the -Naensen tunnel, and his example was successfully followed in several -tunnels built later where timber was scarce and expensive. The -advantages which iron strutting is claimed to possess over the more -common wooden structure are: its greater strength; the smaller amount of -space which it takes up; and the fact that it does not wear out, and -may, therefore, be used over and over again. - -[Illustration: FIG. 28.--Strutting of Timber Posts and Railway Rail -Caps.] - -[Illustration: FIG. 29.--Strutting made entirely of Railway Rails.] - - -=Iron Strutting in Headings.=--In strutting the headings the cross -frames have a crown bar consisting of a section of old railway rail -carried either by wood or iron side posts. When wooden side posts are -used their upper ends have a dovetail mortise, and are bound with an -iron band, as shown by Fig. 28. The base of the rail crown bar is set -into the dovetail mortise and fastened by wedges. When iron side posts -are employed they usually consist of sections of railway rails, and the -crown bar is attached to them by fish-plate connections, as shown by -Fig. 29. The iron cross frames are set up as the heading advances, and -carry the plank lagging or poling-boards, exactly in the same manner as -the timber cross frames previously described. - -[Illustration: FIG. 30.--Rziha’s Combined Strutting and Centering of -Cast Iron.] - -[Illustration: FIG. 31.--Cast-Iron Segment of Rziha’s Strutting and -Centering.] - - -=Full Section Iron Strutting.=--The iron strutting devised by Mr. Rziha -for full section work is shown by Fig. 30. Briefly described, it -consists of voussoir-shaped cast-iron segments, which are built up in -arch form. Fig. 31 shows the construction of one of the segments, all of -which are alike, with the exception of the crown segment, which has a -mortise and tenon joint which is kept open by filling the mortise with -sand. The segments are bolted together by means of suitable bolt-holes -in the vertical flanges, and when fully connected form an arch rib of -cast iron. This arch rib, A, Fig. 30, carries a series of angle or -T-iron frames bent into approximately voussoir shape, as shown at B, -Fig. 30. Above these frames are inserted the poling-boards, running -longitudinally, and spanning the distance between consecutive arch ribs. -By removing the bent iron frames the cast-iron rib forms a center upon -which to construct the masonry. Finally, to remove the cast-iron rib -itself, the sand is drawn out of the mortise and tenon joint in the -crown segment, which allows the joint to close, and loosen the segments -so that they are easily unbutted. - -The illustration, Fig. 30, shows longitudinal poling-boards; more often -longitudinal crown bars of railway rails span the space between -connective arch ribs, and support transverse poling-boards. In building -the masonry, work is begun at the bottom on each side, the bent iron -frames being removed one after another to give room for the masonry. As -each frame is removed, it is replaced with a sort of screw-jack to -support the poling-boards until the masonry is sufficiently completed to -allow their removal. The interior bracing of the arch rib shown at _a a_ -and _b b_ consists of railway rails carried by brackets cast on to the -segments. A similar bracing of rails connects the successive arch ribs. -These lines of bracing serve to carry the scaffolding upon which the -masons work in building the lining. - -[Illustration: FIG. 32.--Cast-Iron Segmental Strutting for Shafts.] - - -=Iron Shaft Strutting.=--In soft-ground shaft work, the use of an iron -strutting, consisting of consecutive cast-iron rings, has sometimes -been employed to advantage. Fig. 32 shows the construction of one of -these rings, which, it will be seen, is composed of four segments -connected to each other by means of bolted flanges. The holes shown in -the circumferential web of the ring are to allow for the seepage from -the earth side walls. The method of placing this cylindrical strutting -is to start with a ring having a cutting-edge. By means of excavation -inside the ring, and by ramming, the ring is sunk into the ground a -distance equal to its height. Another ring is then fastened by special -hooks on top of the first one, and the sinking continued until the -second ring is down flush with the surface. A third ring is then added, -and so on until the entire shaft is excavated and strutted. As in timber -shaft strutting, the solid iron ring strutting is carried down only to -the top of the tunnel section, and below this point there is an open -timber or iron supporting framework. - - - - -CHAPTER VI. - -METHODS OF HAULING IN TUNNELS. - - -The transportation from one point to another within the tunnel and its -shafts of any material, whether it is excavated spoil or construction -material, is defined as hauling. In all engineering construction, the -transportation of excavated materials, and materials for construction, -constitutes a very important part of the expense of the work; but -hauling in tunnels where the room is very limited, and where work is -constantly in progress over and at the sides of the track, is a -particularly expensive process. Hauling in tunnels may be done either by -way of the entrances, or by way of the shafts, or by way of both the -entrances and shafts. - -[Illustration: FIG. 33.--Platform Car for Tunnel Work.] - - -=Hauling by Way of Entrances.=--When the hauling is done by the way of -the entrances, the materials to be hauled are taken directly from the -point of construction to the entrances, or in the opposite direction, -by means of special cars of different patterns. For general purposes, -these different patterns of cars may be grouped into three -classes,--platform-cars, dump-cars, and box-cars. Representative -examples of these several classes of cars are shown in Figs. 33 to 36[6] -inclusive, but it will be readily understood that there are many other -forms. - - [6] Reproduced from catalogue of Arthur Koppel, New York. - -Briefly described, platform-cars (Fig. 33) consist of a wooden platform -mounted on tracks, and they are usually employed for the transportation -of timber, ties, etc. Dump-cars are used in greater numbers in tunnel -work than any other form. Fig. 34 shows a dump-car of metal -construction, and Fig. 35 one constructed with a metal under-frame and -wooden box. These cars are made to run on narrow-gauge tracks, and -usually have a capacity of about one to one and one-half cubic yards. -Box-cars are more extensively employed in Europe for tunnel work than in -America. Fig. 36 shows a typical European box-car for tunnel work. It is -made either to run on narrow-gauge or standard-gauge tracks. - -[Illustration: FIG. 34.--Iron Dump-Car for Tunnel Work.] - -[Illustration: FIG. 35.--Wooden Dump-Car for Tunnel Work.] - -[Illustration: FIG. 36.--Box-Car for Tunnel Work.] - -It is usually desirable in tunnel work to employ cars of different forms -for different parts of the work. In rock tunnels it is a common practice -to use narrow-gauge cars of small size in the headings, and larger, -broad-gauge cars for the enlargement of the profile. Where narrow-gauge -cars are employed for all purposes, it will also be found more -convenient to use platform-cars for handling the construction material, -and dump-cars for removing the spoil. The extent to which it is -desirable to use cars of different forms will depend upon the character -and conditions of the work, and particularly upon how far it is possible -to install the permanent track. - -As a general ride, it is considered preferable to lay the permanent -tracks at once, and do all the hauling upon them, so that as soon as the -tunnel is completed, trains may pass through without delay. To what -extent this may be done, or whether it can be done at all or not, -depends upon the method of excavation and other local conditions. In -soft-ground tunnels excavated by the English or Austrian methods, it is -quite possible to lay the permanent tracks at first, since the whole -section is excavated at once, and the excavation is kept but a little -ahead of the completed tunnel. In rock tunnels, where the heading is -driven far ahead of the completed section, it is, of course, impossible -to keep the permanent track close to the advance work, and narrow-gauge -tracks must be laid in the heading. The same thing is true in -soft-ground tunnels driven by successive headings and drifts. In these -cases, therefore, where narrow-gauge tracks have to be used for some -portions of the work anyway, the question comes up whether it is -preferable to use temporary narrow-gauge tracks throughout, or to lay -the permanent track as far ahead as possible, and then extend -narrow-gauge tracks to the advance excavation. In the latter case it -will, of course, be necessary to trans-ship each load from the -narrow-gauge to the standard-gauge cars, or _vice versa_, which means -extra cost and trouble. To avoid this, the method is sometimes adopted -of laying a third rail between the standard-gauge rails, so that either -standard- or narrow-gauge cars may be transported over the line. -Whatever form the local conditions may require the system of -construction tracks to assume, it may be set down as a general rule that -the permanent tracks should be kept as far advanced as possible, and -temporary tracks employed only where the permanent tracks are -impracticable. - -The motive power employed for hauling in tunnels may be furnished by -animals or by mechanical motors. Animal power is generally employed in -short tunnels and in the advance headings and galleries. In long -tunnels, or where the excavated material has to be transported some -distance away from the tunnel, mechanical power is preferable, for -obvious reasons. The motors most used are small steam locomotives, -special compressed-air locomotives, and electric motors. Compressed air -and electric locomotives are built in various forms, and are -particularly well adapted for tunnel work because of their small -dimensions, and freedom from smoke and heat. - - -=Hauling by Way of Shafts.=--When the excavated material and materials -of construction are handled through shafts, the operation of hauling may -be divided into three processes: the transportation of the materials -along the floor of the tunnel, the hoisting of them through the shaft, -and the surface transportation from and to the mouth of the shaft. These -three operations should be arranged to work in harmony with each other, -so as to avoid waste of time and unnecessary handling of the materials. -An endeavor should be made to avoid, if possible, breaking or -trans-shipping the load from the time it starts at the heading until it -is dumped at the spoil bank. This can be accomplished in two ways. One -way is to hoist the boxes of the cars from their trucks at the bottom of -the shaft, and place them on similar trucks running on the surface -tracks. The other way is to run the loaded cars on to the elevator -platform at the bottom, hoist them, and then run them on to the surface -tracks. If the first method is employed, the car box is usually made of -metal, and is provided at its top edges with hooks or ears to which to -attach the hoisting cables. When the second method is used, the elevator -platform has tracks laid on it which connect with the tracks on the -tunnel floor, and also with those on the surface. - - -=Hoisting Machinery.=--The machines most commonly employed for hoisting -purposes in tunnel shafts are steam hoisting engines, horse gins, and -windlasses operated by hand. Windlasses and horse gins are rather crude -machines for hoisting loads, and are used only in special -circumstances, where the shaft is of small depth, when the amount of -material to be hoisted is small, or where for any reason the use of -hoisting engines is precluded. The steam hoisting engine is the standard -machine for the rapid lifting of heavy vertical loads. Recently oil -engines and electric hoists have also come to be used to some extent, -and under certain conditions these machines possess notable advantages. - -The construction of hand windlasses is familiar to every one. In tunnel -work this device is located directly over the shaft, with its axis a -little more than half a man’s height, so that the crank handle does not -rise above the shoulder line. To develop its greatest efficiency the -hoisting rope is passed around the windlass drum so that the two ends -hang down the shaft, and as one end descends the other ascends. A skip, -or bucket, is attached to each of the rope ends; and by loading the -descending skip with construction materials and the ascending skip with -spoil, the two skip loads tend to balance each other, thus increasing -the capacity of the windlass, and decreasing the manual labor required -to operate it. Skips varying from 0.3 cu. yd. to 0.5 cu. yd. are used. -The horse gin consists of a vertical cylinder or drum provided with -radial arms to which the horses are hitched, which revolve the cylinder -by walking around it in a circle. The hoisting rope is rove around the -drum so that the two ends extend down the shaft with skips attached, as -described in speaking of the hand windlass. The power of the horse gin -is, of course, much greater than that of a windlass operated by hand, -skips of 1 cu. yd. capacity being commonly used. Horse gins are no -longer economical hoisting machines, according to one prominent -authority, when V(H + 20) > 5000, where V equals the volume of material -to be hoisted, and H equals the height of the hoist, the weight of the -excavated material being 2100 lbs. per cu. yd. As a general rule, -however, it is assumed that it is not economical to employ horse gins -with a depth of shaft exceeding 150 ft. - -As already stated, the most efficient and most commonly used device for -hoisting at tunnel shafts is the steam hoisting engine. There are -numerous builders of hoisting engines, each of which manufactures -several patterns and sizes of engines. In each case, however, the -apparatus consists of a boiler supplying steam to a horizontal engine -which operates one or more rope drums. The engines are always -reversible. They may be employed to hoist the skips directly, or to -operate elevators upon which the skips or cars are loaded. In either -case the hoisting ropes pass from the engine drum to and around vertical -sheaves situated directly over the shaft so as to secure the necessary -vertical travel of the ropes down the shaft. Where the shaft is divided -into two compartments, each having an elevator or hoist, double-drum -engines are employed, one drum being used for the operations in one -compartment, and the other for the operations in the other compartment. -Where the work is to be of considerable duration, or when it is done in -cold weather, more or less elaborate shelters or engine houses are built -to cover and protect the machinery. - -Choice between the method of hoisting the skips directly, and the method -of using elevators, depends upon the extent and character of the work. -Where large quantities of material are to be hoisted rapidly, it is -generally considered preferable to employ elevators instead of hoisting -the skips directly. In direct hoisting at high speed, oscillations are -likely to be produced which may dash the skips against the sides of the -shaft and cause accidents. The loads which can be carried in single -skips are also smaller than those possible where elevators are used; and -this, combined with the slower hoisting speed required, reduces the -capacity of this method, as compared with the use of elevators. Where -elevators are employed, however, the plant required is much more -extensive and costly; it comprising not only the elevator cars with -their safety devices, etc., but the construction of a guiding framework -for these cars in the tunnel shaft. For these various reasons the -elevator becomes the preferable hoisting device where the quantity of -material to be handled is large, where the shafts are deep, and where -the work will extend over a long period of time; but when the contrary -conditions are the case, direct hoisting of the skips is generally the -cheaper. The engineer has to integrate the various factors in each -individual case, and determine which method will best fulfill his -purpose, which is to handle the material at the least cost within the -given time and conditions. - -[Illustration: FIG. 37.--Elevator Car for Tunnel Shafts.] - -The construction of elevators for tunnel work is simple. The elevator -car consists usually of an open framework box of timber and iron, having -a plank floor on which car tracks are laid, and its roof arranged for -connecting the hoisting cable (Fig. 37[7]). Rigid construction is -necessary to resist the hoisting strains. The sides of the car are -usually designed to slide against timber guides on the shaft walls. Some -form of safety device, of which there are several kinds, should be -employed to prevent the fall of the elevator, in case the hoisting rope -breaks, or some mishap occurs to the hoisting machinery, which endangers -the fall of the car. Speaking tubes and electric-bell signals should -also be provided to secure communication between the top and bottom of -the shaft. - - [7] Reproduced from the catalogue of the Ledgerwood Manufacturing - Company, New York. - - - - -CHAPTER VII. - -TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF -MASONRY. - - -The masonry lining of a tunnel may be described as consisting of two or -more segments of circular arches combined so as to form a continuous -solid ring of masonry. To direct the operations of the masons in -constructing this masonry ring, templates or patterns are provided which -show the exact dimensions and form of the sectional profile which it is -desired to secure. These patterns or templates will vary in number and -construction with the form of lining and the method of excavation -adopted. Where the excavation is fully lined on all four sides, the -masonry work is usually divided into three parts,--the invert or floor -masonry, the side-wall masonry, and the roof-arch masonry. At least one -separate pattern has to be employed in constructing each of these parts -of the lining; and they are known respectively as ground molds, leading -frames, and arch centers, or simply centers. In the following paragraphs -the form and construction usually employed for each of these three kinds -of patterns is described. - - -=Ground Molds.=--Ground molds are employed in building the tunnel -invert. They are generally constructed of 3-inch plank cut exactly to -the form and dimensions of the invert masonry as shown in Fig. 38. To -permit of convenience of handling in a restricted space, they are -generally made in two parts, which are joined at the middle by means of -iron fish-plates and bolts. Either one or two ground molds may be -employed. Where two molds are used they are set up a short distance -apart, and cords are stretched from one to the other parallel to the -axis of the tunnel, by which the masons are guided in their work. -Extreme care has to be taken in setting the molds to ensure that they -are fixed at the proper grade, and are in a plane normal to the axis of -the tunnel. Where only one ground mold is employed, the finished masonry -is depended upon to supply the place of the second mold, cords being -stretched from it to the single mold placed the requisite distance -ahead. The leveling and centering of the molds is accomplished by means -of transit and level. - -[Illustration: FIG. 38.--Ground Mold for Constructing Tunnel Invert -Masonry.] - -[Illustration: FIG. 39.--Combined Ground Mold and Leading Frame for -Invert and Side Wall Masonry.] - -Two modifications of the form of ground mold shown by Fig. 39 are -employed. The first modification is peculiar to the English method of -excavation, and consists in combining the ground mold with the leading -frame for the lower part of the side walls, as shown by Fig. 39. The -second modification is employed where the two halves or sides of the -invert are built separately, and it consists simply in using one-half of -the mold shown by Fig. 38. When the last method of constructing the -invert masonry is resorted to, extreme care has to be observed in -setting the half-mold in order to avoid error. - -[Illustration: FIG. 40.--Leading Frame for Constructing Side Wall -Masonry.] - - -=Leading Frames.=--As already stated, leading frames are the patterns, -or molds, used in building the side walls of the lining. Like the ground -mold they are usually built of plank; one side being cut to the curve of -the profile, and the other being made parallel to the vertical axis of -the tunnel section. The vertical side usually has some arrangement by -which a plumb bob can be attached, as shown by Fig. 40, to guide the -workmen in erecting the frame. The combined leading frame and ground -mold shown in Fig. 39 has already been described. The use of this frame -is possible only where the masonry is begun at the invert and carried up -on each side at the same time. This mode of construction is peculiar to -the English method of tunneling; in all other methods the form of -separate ground frame shown by Fig. 40 is employed. The ground frames -are lined in and leveled up by transit and level; and, as in setting the -ground frames, care must be taken to secure accuracy in both direction -and elevation. - - -=Arch Centers.=--The template or form upon which the roof arch is built -is called a center. Unlike the ground molds and leading frames, the arch -centers have to support the weight of the masonry and the roof pressures -during the construction of the lining, and they, therefore, require to -be made strong. Owing to the fact that the pressures are indeterminate, -it is impossible to design a rational center, and resort is had to those -constructions which past experience has shown to work satisfactorily -under similar conditions. In a general way it can always be assumed that -the construction should be as simple as possible, that the center should -be so designed that it can be set up and removed with the least possible -labor, and that the different pieces of the framework and lagging should -be as short as possible, for convenience in handling. - -Tunnel centers are usually composed of two parts,--a mold or curved -surface upon which the masonry rests, and a framework which supports the -mold. The curved surface or mold consists of a lagging of narrow boards -running parallel to the tunnel axis, which rests upon the arched top -members of two or more consecutive supporting frames. The supporting -frame is built in the form of a truss, and must be made strong enough to -withstand the heavy superimposed loads, consisting of the arch masonry -during construction, and of the roof pressures which are transferred to -the center when the strutting is removed to allow the masonry to be -placed. The framework of the center is supported either by posts resting -upon the floor of the excavation, or upon the invert masonry when this -is built first, as in the English and Austrian methods, or it may be -supported directly upon the ground where the arch masonry is built -first, as in the Belgian method of tunneling. - -In describing the various methods of tunneling in succeeding chapters, -the center construction and method of supporting the center peculiar to -each will be fully explained, and only a few general remarks are -necessary here. Centers may be classified according to their -construction and composition into plank centers, truss centers, and iron -centers. - -[Illustration: FIG. 41.--Plank Center for Constructing the Roof Arch.] - -One of the most common forms of plank centers is shown by Fig. 41. It -consists of two half-polygons whose sides consist of 15 in. × 4 ft. -planks having radial end-joints. These two half-polygons are laid one -upon the other so that they break joints, as shown by the figure, and -the extrados of the frame is cut to the true curve of the roof arch. The -planks commonly used for making these centers are 4 ins. thick, making -the total thickness of the center 8 ins. Plank centers of the -construction described are suitable only for work where the pressures to -be resisted are small, as in tunnels through a fairly firm rock, -although there have been instances of their successful use in -soft-ground tunnels. - -Where heavy loads have to be carried, trussed centers are generally -employed, the trusses being composed of heavy square beams with scarfed -and tenoned joints, reinforced by iron plates. Different forms of -trusses are employed in each of the different methods of tunneling, and -each of these is described in succeeding chapters; but they are -generally either of the king-post or queen-post type, or some -modification of them. The king-post truss may be used alone, with or -without the tie-beam, as shown by Fig. 42; but generally a queen-post -truss is made to form the base of support for a smaller king-post truss -mounted on its top. This arrangement gives a trapezoidal form to the -center, which approaches closely to the arch profile. Owing to the -character of the pressures transmitted to the center, the usual tension -members can be made very light. - -[Illustration: FIG. 42.--Trussed Center for Constructing the Roof Arch.] - -The combined center and strutting system devised by Mr. Rziha has -already been described in a previous chapter. In recent European tunnel -work quite extensive use has also been made of iron centers consisting -of several segments of curved I-beams, connected by fish-plate joints so -as to form a semi-circular arch rib. The ends or feet of these I-beam -ribs have bearing-plates or shoes made by riveting angles to the -I-beams. Centers constructed in a similar manner, but made of sections -of old railway rail, were used in carrying out the tunnel work on the -Rhine River Railroad in Germany. The advantages claimed for iron centers -are that they take up less room, and that they can be used over and over -again. - - -_Setting Up Centers._--According to the method of excavation followed in -building the tunnel, the centers for building the roof arch may have to -be supported by posts resting on the tunnel floor; or where the arch is -built first, as in the Belgian and Italian methods, they may be carried -on blocking resting on the unexcavated earth below. Whichever method is -employed, an unyielding support is essential, and care must be taken -that the centers are erected and maintained in a plane normal to the -tunnel axis. To prevent deflection and twisting, the consecutive centers -are usually braced together by longitudinal struts or by braces running -to the adjacent strutting. Only skilled and experienced workmen should -be employed in erecting the centers; and they should work under the -immediate direction of the engineer, who must establish the axis and -level of each center by transit and level. - - -_Lagging._--By the lagging is meant the covering of narrow longitudinal -boards resting upon the upper curved chords of the centers, and spanning -the opening between consecutive centers. This lagging forms the curved -surface or mold upon which the arch masonry is laid. When the roof arch -is of ashlar masonry the strips of lagging are seldom placed nearer -together than the joints of the consecutive ring stones, but in brick -arches they are laid close together. Besides the weight of the arch -masonry, the lagging timbers support the short props which keep the -poling-boards in place after the strutting is removed and until the arch -masonry is completed. - - -_Striking the Centers._--The centers are usually brought to the proper -elevation by means of wooden wedges inserted between the sill of the -center and its support, or between the bottom of the posts carrying the -center and the tunnel floor. These wedges are usually made of hard wood, -and are about 6 ins. wide by 4 ins. thick by 18 ins. long. To strike the -center after the arch masonry is completed, these wedges are withdrawn, -thus allowing the center to fall clear of the masonry. Usually the -center is not removed immediately after striking, so that if the arch -masonry fails the ruins will remain upon the center. The method of -striking the iron center devised by Mr. Rziha has been described in the -previous chapter on strutting. - - - - -CHAPTER VIII. - -METHODS OF LINING TUNNELS. - - -Tunnels in soft soils and in loose rock, and rock liable to -disintegration, are always provided with a lining to hold the walls and -roof in place. This lining may cover the entire sectional profile of the -tunnel, or only a part of it, and it may be constructed of timber, iron, -iron and masonry, or, more commonly, of masonry alone. - - -=Timber Lining.=--Timber is seldom employed in lining tunnels except as -a temporary expedient, and is replaced by masonry as soon as -circumstances will permit. In the first construction of many American -railways, the necessity for extreme economy in construction, and of -getting the line open for traffic as soon as possible, caused the -engineers to line many tunnels with timber, which was plentiful and -cheap. Except for their small cost and the ease and rapidity with which -they can be constructed, however, these timber linings possess few -advantages. It is only the matter of a few years when the decay of the -timber makes it necessary to rebuild them, and there is always the -serious danger of fire. In several instances timber-lined tunnels in -America have been burned out, causing serious delays in traffic, and -necessitating complete reconstruction. Usually this reconstruction has -consisted in substituting masonry in place of the original timber -lining. In a succeeding chapter a description will be given of some of -the methods employed in replacing timber tunnel linings with masonry. -Various forms of timber lining are employed, of which Fig. 44 and the -illustrations in the chapter discussing the methods of relining -timber-lined tunnels with masonry are typical examples. - -[Illustration: ~Cross Section.~ - -~Longitudinal Section.~ - -FIGS. 43 and 44.--A Typical Form of Timber Lining for Tunnels.] - - -=Iron Lining.=--The use of iron lining for tunnels was introduced first -on a large scale by Mr. Peter William Barlow in 1869, for the second -tunnel under the River Thames at London, England, and it has greatly -extended since that time. The lining of the second Thames tunnel -consisted of cylindrical cast-iron rings 8 ft. in diameter, the abutting -edges of the successive rings being flanged and provided with holes for -bolt fastenings. Each ring was made up of four segments, three of which -were longer than quadrants, and one much smaller forming the “key-stone” -or closing piece. These segments were connected to each other by flanges -and bolts. To make the joints tight, strips of pine or cement and hemp -yarn were inserted between the flanges. Since the construction of the -second Thames tunnel, iron lining has been employed for a great many -submarine tunnels in England, Continental Europe, and America, some of -them having a section over 28 ft. in diameter. Where circular iron -lining is employed, the bottom part of the section is leveled up with -concrete or brick masonry to carry the tracks, and the whole interior -of the ring is covered with a cement plaster lining deep enough -thoroughly to embed the interior joint flanges. In the succeeding -chapter describing the methods of driving tunnels by shields several -forms of iron tunnel lining are fully described. - - -=Iron and Masonry Lining.=--During recent years a form of combined -masonry and iron lining has been extensively employed in constructing -city underground railways in both Europe and America. Generally this -form of lining is built with a rectangular section. Two types of -construction are employed. In the first, masonry side walls carry a flat -roof of girders and beams, which carry a trough flooring filled with -concrete, or between which are sprung concrete or brick arches. -Sometimes the roof framing consists of a series of parallel I-beams laid -transversely across the tunnel, and in other cases transverse plate -girders carry longitudinal I-beams. In the second type of construction -the roof girders are supported by columns embedded in the side walls. -Where the tunnel provides for two or four tracks, intermediate column -supports are in some cases introduced between the side columns. In this -construction the roofing consists of concrete filled troughs or of -concrete or brick arches, as in the construction first described. -Examples of combined masonry and iron tunnel lining are illustrated in -the succeeding chapter on tunneling under city streets. - - -=Masonry Lining.=--The form of tunnel lining most commonly employed is -brick or stone masonry. Concrete and reinforced concrete masonry lining -has been employed in several tunnels built in recent years. The masonry -lining may inclose the whole section or only a part of it. The floor or -invert is the part most commonly omitted; but sometimes also the side -walls and invert are both omitted, and the lining is confined simply to -an arch supporting the roof. The roof arch, the side walls, and the -invert compose the tunnel lining; and all three may consist of stone or -brick alone, or stone side walls may be employed with brick invert and -roof arch. Rubble-stone masonry is usually employed, except at the -entrances, where the masonry is exposed to view. Here ashlar masonry is -usually used. The stone selected for tunnel lining should be of a -durable quality which weathers well. Where bricks are used they should -be of good quality. Owing to the comparative ease with which brick -arches can be built, they are generally used to form the roof arch, even -where the side walls are of stone masonry. Masonry lining may be built -in the form of a series of separate rings, or in the form of a -continuous structure extending from one end of the tunnel to the other. -The latter method of construction produces a stronger structure; but in -case of failure by crushing, the damage done is likely to be more -widespread than where separate rings are employed, one or two of which -may fail without injury to the others adjacent to them. The construction -is also somewhat simpler where separate rings are employed, since no -provision has to be made for bonding the whole lining into a continuous -structure. Where a series of separate rings is employed, the length of -each ring runs from 5 ft. up to 20 ft., it depending upon the character -of the material penetrated, and the method of construction employed. For -the purpose of detailed discussion the construction of masonry lining -may be divided into four parts,--the side-wall foundations, the side -walls themselves, the roof arch, and the invert. - -Concrete and reinforced concrete linings are now extensively used on -account of cheapness and facility of handling, but they have the great -disadvantage of resisting pressure after they become hard, which is some -time after being placed. The strutting should, therefore, be left to -support the roof so as to prevent direct pressure on the fresh material. -The roof, as a rule, is supported by longitudinal planks held in -position by five or seven segments of arched frames placed across the -tunnel. A large quantity of timber and carpenter work is thus entirely -wasted and these costly items, in many cases, make the concrete lining -of a tunnel more expensive than the one built of brick and stone. To -avoid these inconveniences tunnels have been successfully lined with -concrete on the side walls and concrete blocks in the arches. These -blocks have been built by hand and molded in the shape of the arch -voussoirs. - - -=Foundations.=--In tunnels through rock of a hard and durable character -the foundations for the side walls are usually laid directly on the -rock. In loose rock, or rock liable to disintegration, this method of -construction is not generally a safe one, and the foundation excavation -should be sunk to a depth at which the atmospheric influences cannot -affect the foundation bed. In either case the foundation masonry is made -thicker than that of the side walls proper, so as to distribute the -pressure over a greater area, and to afford more room for adjusting the -side-wall masonry to the proper profile. In yielding soils a special -foundation bed has to be prepared for the foundation masonry. In some -instances it is found sufficient to lay a course of planks upon which -the masonry is constructed, but a more solid construction is usually -preferred. - -This is obtained by placing a concrete footing from 1 ft. to 2 ft. deep -all along the bottom of the foundation trench, or in some cases by -sinking wells at intervals along the trench and filling them with -concrete, so as to form a series of supporting pillars. - -[Illustration: FIG. 45.--Diagram Showing Forms Adopted for Side-Wall -Foundations.] - -The form given to the foundation courses and lower portions of the side -walls varies. Where a large bearing area is required, the back of the -wall is carried up vertically as shown by the line _AB_, Fig. 45, -otherwise the rear face of the wall follows the line of excavation _AC_. -For similar reasons the front face of the wall may be made vertical, as -at _FG_, or inclined, as at _FH_. The line _FE_ indicates the shelf -construction designed to support the feet of the posts used to carry the -arch centers during the construction of the roof arch. - - -=Side Walls.=--The construction of the side walls above the foundation -courses is carried out as any similar piece of masonry elsewhere would -be built. To direct the work and insure that the inner faces of the -walls follow accurately the curve of the chosen profile, leading frames -previously described are employed. - - -=Roof Arch.=--For the construction of the roof arch, the centers -previously described are employed. Beginning at the edges of the center -on each side, the masonry is carried up a course at a time, care being -taken to have it progress at the same rate on both sides, so that the -load brought onto the centering is symmetrical. As soon as the centers -are erected, the roof strutting is removed, and replaced by short props -which rest on the lagging of the centers and support the poling-boards. -These props are removed in succession as the arch masonry rises along -the curve of the center, and the space between the top of the arch -masonry and the ceiling of the excavation is filled with small stones -packed closely. The keystone section of the arch is built last, by -inserting the stones or bricks from the front edge of the arch ring, -there being no room to set them in from the top, as is the practice in -ordinary open-arch construction. The keying of the arch is an especially -difficult operation, and only experienced men skilled in the work should -be employed to perform it. The task becomes one of unusual difficulty -when it becomes necessary to join the arches coming from opposite -directions. - - -=Invert.=--In all but one or two methods of tunneling, the invert is the -last portion of the lining to be built. In the English method of -tunneling, the invert is the first portion of the lining to be built, -and the same practice is sometimes necessary in soft soils where there -is danger of the bottoms of the side walls being squeezed together by -the lateral pressures unless the invert masonry is in place to hold them -apart. The ground molds previously described are employed to direct the -construction of the invert masonry. - - -=General Observations.=--In describing the construction of the roof -arch, mention was made of the stone filling employed between the back of -the masonry ring and the ceiling of the excavation. The spaces behind -the side walls are filled in a similar manner. The object of this stone -filling, which should be closely packed, is to distribute the vertical -and lateral pressures in the walls of the excavation uniformly over the -lining masonry. As the masonry work progresses, the strutting employed -previously to support the walls of the excavation has to be removed. -This work requires care to prevent accident, and should be placed in -charge of experienced mechanics who are familiar with its construction, -and can remove it with the least damage to the timbers, so that they may -be used again, and without causing the fall of the roof or the caving of -the sides by removing too great a portion of the timbers at one time. - - -=Thickness of Lining Masonry.=--It is obvious, of course, that the -masonry lining must be thick enough to support the pressure of the earth -which it sustains; but, as it is impossible to estimate these pressures -at all accurately, it is difficult to say definitely just what thickness -is required in any individual case. Rankine gives the following formulas -for determining the depths of keystone required in different soils: - -For firm soils - - ( _r_²) - _d_ = √(0.12----), - ( _s_ ) - -and for soft soils, - - ( _r_²) - _d_ = √(0.48----), - ( _s_ ) - -where _d_ = the depth of the crown in feet, _r_ = the rise of the arch -in feet, and _s_ = the span of the arch in feet. Other writers, among -them Professor Curioni, attempt to give rational methods for calculating -the thickness of tunnel lining; but they are all open to objection -because of the amount of hypothesis required concerning pressures which -are of necessity indeterminate. Therefore, to avoid tedious and -uncertain calculations, the engineer adopts dimensions which experience -has proven to be ample under similar conditions in the past. Thus we -have all gradations in thickness, from hard-rock tunnels requiring no -lining, and tunnels through rocks which simply require a thin shell to -protect them from the atmosphere, to soft-ground tunnels where a masonry -lining 3 ft. or more in thickness is employed. Table II. shows the -thickness of masonry lining used in tunnels through soft soils of -various kinds. - -The thickness of the masonry lining is seldom uniform at all points, as -is indicated by Table II. Figs. 46 and 47 show common methods of varying -the thickness of lining at different points, and are self-explanatory. - -[Illustration: FIGS. 46 and 47.--Transverse Sections of Tunnels Showing -Methods of Increasing the Thickness of the Lining at Different Points.] - - -=Side Tunnels.=--When tunnels are excavated by shafts located at one -side of the center line, short side tunnels or galleries are built to -connect the bottoms of the shafts with the tunnel proper. These side -tunnels are usually from 30 ft. to 40 ft. long, and are generally made -from 12 ft. to 14 ft. high, and about 10 ft. wide. The excavation, -strutting, and lining of these side tunnels are carried on exactly as -they are in the main tunnel, with such exceptions as these short -lengths make possible. Table III. gives the thickness of lining used for -side tunnels, the figures being taken from European practice. - - -=Culverts.=--The purpose of culverts in tunnels is to collect the water -which seeps into the tunnel from the walls and shafts. The culvert is -usually located along the center line of the tunnel at the bottom. In -soft-ground tunnels it is built of masonry, and forms a part of the -invert, but in rock tunnels it is the common practice to cut a channel -in the rock floor of the excavation. Both box and arch sections are -employed for culverts. The dimensions of the section vary, of course, -with the amount of water which has to be carried away. The following are -the dimensions commonly employed: - - +----------------+--------+--------+----------+-----------+ - |KIND OF CULVERT.| HEIGHT | WIDTH |THICKNESS | THICKNESS | - | |IN FEET.|IN FEET.|OF WALLS |OF COVERING| - | | | | IN FEET. | IN FEET. | - +----------------+--------+--------+----------+-----------+ - |Box culvert |1 to 1.5|1 to 1.5|0.8 to 1.2| 0.3 | - |Arch culvert |1 to 1.5|1 to 1.5|0.8 to 1.2| 0.4 | - +----------------+--------+--------+----------+-----------+ - -It should be understood that the dimensions given in the table are those -for ordinary conditions of leakage; where larger quantities of water are -met with, the size of the culverts has, of course, to be enlarged. To -permit the water to enter the culvert, openings are provided at -intervals along its side; and these openings are usually provided with -screens of loose stones which check the current, and cause the suspended -material to be deposited before it enters the culvert. In cases where -springs are encountered in excavating the tunnel, it is necessary to -make special provisions for confining their outflow and conducting it to -the culvert. In all cases the culverts should be provided with catch -basins at intervals of from 150 ft. to 300 ft., in which such suspended -matter as enters the culverts is deposited, and removed through covered -openings over each basin. At the ends of the tunnel the culvert is -usually divided into two branches, one running to the drain on each -side of the track. - -[Illustration: FIG. 48.--Refuge Niche in St. Gothard Tunnel.] - - -=Niches.=--In short tunnels niches are employed simply as places of -refuge for trackmen and others during the passing of trains, and are of -small size. In long tunnels they are made larger, and are also employed -as places for storing small tools and supplies employed in the -maintenance of the tunnel. Niches are simply arched recesses built into -the sides of the tunnel, and lined with masonry; Fig. 48 shows this -construction quite clearly. Small refuge niches are usually built from 6 -ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. to 3 ft. -deep. Large niches designed for storing tools and supplies are made from -10 ft. to 12 ft. high, from 8 ft. to 10 ft. wide, and from 18 ft. to 24 -ft. deep, and are provided with doors. Refuge niches are usually spaced -from 60 ft. to 100 ft. apart, while the larger storage niches may be -located as far as 3000 ft. apart. The niche construction shown by Fig. -48 is that employed on the St. Gothard tunnel. - - -=Entrances.=--The entrances, or portals, of tunnels usually consist of -more or less elaborate masonry structures, depending upon the nature of -the material penetrated. In soft-ground tunnels extensive wing walls are -often required to support the earth above and at the sides of the -entrance; while in tunnels through rock, only a masonry portal is -required, to give a finish to the work. Often the engineer indulges -himself in an elaborate architectural design for the portal masonry. -There is danger of carrying such designs too far for good taste unless -care is employed; and on this matter the writer can do no better than to -quote the remarks of the late Mr. Frederick W. Simms in his well-known -“Practical Tunneling”: - - “The designs for such constructions should be massive to be suitable - as approaches to works presenting the appearance of gloom, solidity, - and strength. A light and highly decorated structure, however elegant - and well adapted for other purposes, would be very unsuitable in such - a situation; it is plainness combined with boldness, and massiveness - without heaviness, that in a tunnel entrance constitutes elegance, - and, at the same time, is the most economical.” - -[Illustration: FIG. 49.--East Portal of Hoosac Tunnel.] - -Fig. 49 is an engraving from a photograph of the east portal of the -Hoosac tunnel, which is an especially good design. The portals of the -Mount Cenis tunnel were built of samples of stone encountered all along -the line of excavation. The stones were cut and dressed and utilized for -walls and voussoirs. The only ornament that is usually allowed on the -portals is the date of the opening of the tunnel prominently cut in the -stone above the arch. - -TABLE II. - -Showing Thickness of Masonry Lining for Tunnels through Soft Ground. - - +------------------------------+------------+-----------+------------+ - | CHARACTER OF MATERIAL. | KEYSTONE. | SPRINGERS.| INVERT. | - +------------------------------+------------+-----------+------------+ - | | Ft. | Ft. | Ft. | - |Laminated clay, first variety |2.15 to 3 |2.75 to 3.5| 1.6 to 2.5| - |Laminated clay, second variety|3 to 4.5 |3.5 to 5.5| 2.5 to 4 | - |Laminated clay, third variety |4.5 to 6.5 |5.5 to 8.1| 4 to 4.5| - |Quicksand |2 to 3.28|2 to 4.1| 1.33 to 2.5| - +------------------------------+------------+-----------+------------+ - -TABLE III. - -Showing Thickness of Masonry Lining for Side Tunnels through Soft -Ground. - - +------------------------------+----------+----------+-----------+ - | CHARACTER OF MATERIAL. | KEYSTONE.|SPRINGERS.| INVERT. | - +------------------------------+----------+----------+-----------+ - | | Ft. | Ft. | Ft. | - |Laminated clay, first variety |1.6 to 2.3|1.8 to 3 |1.5 to 2 | - |Laminated clay, second variety|2.3 to 3 |3 to 4.1|2 to 2.6 | - |Laminated clay, third variety |3 to 4 |4.1 to 5 |2.6 to 3.29| - |Quicksand |1.6 to 2.5|1.3 to 2 |1.3 to 2 | - +------------------------------+----------+----------+-----------+ - - - - -CHAPTER IX. - -TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; REPRESENTATIVE MECHANICAL -INSTALLATIONS FOR TUNNEL WORK. - - -The present high development of labor-saving machinery for excavating -rock makes this material one of the safest and easiest to tunnel of any -with which the engineer ordinarily has to deal. To operate this -machinery requires, however, the development of a large amount of power, -its transmission to considerable distances, and, finally, its economical -application to the excavating tools. The standard rock excavating -machine is the power drill, which requires either air or hydraulic -pressure for its operation according to the special type employed. Under -present conditions, therefore, the engineer is limited either to air or -water under compression for the transmission of his power. Steam-power -may be employed directly to operate percussion rock drills; but owing to -the heat and humidity which it generates in the confined space where the -drills work, and because of other reasons, it is seldom employed -directly. Electric transmission, which offers so many advantages to the -tunnel builder, in most respects is largely excluded from use by the -failure which has so far followed all attempts to apply it to the -operation of rock drills. As matters stand, therefore, the tunnel -engineer is practically limited to steam and falling water for the -generation of power, and to compressed air and hydraulic pressure for -its transmission. - -Whether the engineer should adopt water-power or steam to generate the -power required for his excavating machinery depends upon their relative -availability, cost, and suitability to the conditions of work in each -particular case. Where fuel is plentiful and cheap, and where -water-power is not available at a comparatively reasonable cost, -steam-power will nearly always prove the more economical; where, -however, the reverse conditions exist, which is usually the case in a -mountainous country far from the coal regions, and inadequately supplied -with transportation facilities, but rich in mountain torrents, -water-power will generally be the more economical. In a succeeding -chapter the power generating and transmission plants for a number of -rock tunnels are described, and here only a general consideration of the -subject will be presented. - - -=Steam-Power Plant.=--A steam-power plant for tunnel work should be much -the same as a similar plant elsewhere, except that in designing it the -temporary character of its work must be taken into consideration. This -circumstance of its temporary employment prompts the omission of all -construction except that necessary to the economical working of the -plant during the period when its operation is required. The power-house, -the foundations for the machinery, and the general construction and -arrangement, should be the least expensive which will satisfy the -requirements of economical and safe operation for the time required. It -will often be found more economical as a whole to operate the machinery -with some loss of economy during the short time that it is in use than -to go to much greater expense to secure better economy from the -machinery by design and construction, which will be of no further use -after the tunnel is completed. The longer the plant is to be required, -the nearer the construction may economically approach that of a -permanent plant. As regards the machinery itself, whose further -usefulness is not limited by the duration of any single piece of work, -true economy always dictates the purchase of the best quality. Speaking -in a general way, a steam-power plant for tunnel work comprises a boiler -plant, a plant of air compressors with their receivers, and an electric -light dynamo. When hydraulic transmission of power is employed, the air -compressors are replaced by high-pressure pumps; and when electric -hauling is employed, one or more dynamos may be required to generate -electricity for power purposes, as well as for lighting. In addition to -the power generating machines proper, there must be the necessary piping -and wiring for transmitting this power, and, of course, the equipment of -drills and other machines for doing the actual excavating, hauling, etc. - - -=Reservoirs.=--When water-power is employed, a reservoir has to be -formed by damming some near-by mountain stream at a point as high as -practicable above the tunnel. The provision of a reservoir, instead of -drawing the water directly from the stream, serves two important -purposes. It insures a continuous supply and constant head of water in -case of drought, and also permits the water to deposit its sediment -before it is delivered to the turbines. The construction of these -reservoirs may be of a temporary character, or they may be made -permanent structures, and utilized after construction is completed to -supply power for ventilation and other necessary purposes. In the first -case they are usually destroyed after construction is finished. In -either case, it is almost unnecessary to say, they should be built amply -safe and strong according to good engineering practice in such works, -for the duration of time which they are expected to exist. - - -=Canals and Pipe Lines.=--For conveying the water from the reservoirs to -the turbines, canals or pipe lines are employed. The latter form of -conduit is generally preferable, it being both less expensive and more -easily constructed than the former. It is advisable also to have -duplicate lines of pipe to reduce the possibility of delay by accident -or while necessary repairs are being made to one of the pipes. The pipe -lines terminate in a penstock leading into the turbine chamber, and -provided with the necessary valves for controlling the admission of -water to the turbines. - - -=Turbines.=--There are numerous forms of turbines on the market, but -they may all be classed either as impulse turbines or as reaction -turbines. Impulse turbines are those in which the whole available energy -of the water is converted into kinetic energy before the water acts on -the moving part of the turbine. Reaction turbines are those in which -only a part of the available energy of the water is converted into -kinetic energy before the water acts on the moving vanes. Impulse -turbines give efficient results with any head and quantity of water, but -they give better results when the quantity of water varies and the head -remains constant. Reaction turbines, on the contrary, give better -results when the quantity of water remains constant and the head varies. -These observations indicate in a general way the form of turbine which -will best meet the particular conditions in each case. The number of -turbines required, and their dimensions, will be determined in each case -by the number of horse-power required and the quantity of water -available. The power of the turbines is transmitted to the air -compressors or pumps by shafting and gearing. - - -=Air Compressors.=--An air compressor is a machine--usually driven by -steam, although any other power may be used--by which air is compressed -into a receiver from which it may be piped for use. For a detailed -description of the various forms of air compressors the reader should -consult the catalogues of the several makers and the various text-books -relating to air compression and compressed air. Air compressors, like -other machines, suffer a loss of power by friction. The greatest loss of -power, however, results from the heat of compression. When air is -compressed, it is heated, and its relative volume is increased. -Therefore, a cubic foot of hot air in the compressor cylinder, at say, -60 lbs. pressure, does not make a cubic foot of air at 60 lbs. pressure -after cooling in the receiver. In other words, assuming pressure to be -constant, a loss of volume results due to the extraction of the heat of -compression after the air leaves the compressor cylinder. To reduce the -amount of this loss, air compressors are designed with means to extract -the heat from the air before it leaves the compressor cylinder. Air -compressors may first be divided into two classes, according to the -means employed for cooling the air, as follows: (1) Wet compressors, and -(2) dry compressors. A wet compressor is one which introduces water -directly into the cylinder during compression, and a dry compressor is -one which admits no water to the air during compression. Wet compressors -may be subdivided into two classes: (1) Those which inject water in the -form of spray into the cylinder during compression, and (2) those which -use a water piston for forcing the air into confinement. - -The following brief discussion of these various types of compressors is -based on the concise practical discussion of Mr. W. L. Saunders, M. Am. -Soc. C. E., in “Compressed Air Production.” The highest isothermal -results are obtained by the injection of water into the cylinders, since -it is plain that the injection of cold water, in the shape of a finely -divided spray, directly into the air during compression will lower the -temperature to a greater degree than simply to surround the cylinder and -parts by water jackets which is the means of cooling adopted with dry -compressors. A serious obstacle to water injection, and that which -condemns this type of compressor, is the influence of the injected water -upon the air cylinder and parts. Even when pure water is used, the -cylinders wear to such an extent as to produce leakage and to require -reboring. The limitation to the speed of a compressor is also an -important objection. The chief claim for the water piston compressor is -that its piston is also its cooling device, and that the heat of -compression is absorbed by the water. Water is so poor a conductor of -heat, however, that without the addition of sprays it is safe to say -that this compressor has scarcely any cooling advantages at all so far -as the cooling of the air during compression is concerned. The water -piston compressor operates at slow speed and is expensive. Its only -advantage is that it has no dead spaces. In the dry compressor a -sacrifice is made in the efficiency of the cooling device to obtain low -first cost, economy in space, light weight, higher speed, greater -durability, and greater general availability. - -Air compressors are also distinguished as double acting and simple -acting. They are simple acting when the cylinder is arranged to take in -air at one stroke and force it out at the next, and they are double -acting when they take in and force out air at each stroke. In form -compressors may be simple or duplex. They are simple when they have but -one cylinder, and duplex when they have two cylinders. A straight line -or direct acting compressor is one in which the steam and air cylinders -are set tandem. An indirect acting compressor is one in which the power -is applied indirectly to the piston rod of the air cylinder through the -medium of a crank. Mr. W. L. Saunders writes in regard to direct and -indirect compression as follows:-- - - “The experience of American manufacturers, which has been more - extensive than that of others, has proved the value of direct - compression as distinguished from indirect. By direct compression is - meant the application of power to resistance through a single straight - rod. The steam and air cylinders are placed tandem. Such machines - naturally show a low friction loss because of the direct application - of power to resistance. This friction loss has been recorded as low as - 5%, while the best practice is about 10% with the type which conveys - the power through the angle of a crank shaft to a cylinder connected - to the shaft through an additional rod.” - - -=Receivers.=--Compressed air is stored in receivers which are simply -iron tanks capable of withstanding a high internal pressure. The purpose -of these tanks is to provide a reservoir of compressed air, and also to -allow the air to deposit its moisture. From the receivers the air is -conveyed to the workings through iron pipes, which decrease gradually in -diameter from the receivers to the front. - - -=Rock Drills.=--The various forms of rock drills used in tunneling have -been described in Chapter III., and need not be considered in detail -here except to say that American engineers usually employ percussion -drills, while European engineers also use rotary drills extensively. A -comparison between these two types of drills was made in excavating the -Aarlberg tunnel in Austria, where the Brandt hydraulic rotary drill was -used at one end, and the Ferroux percussion drill was used at the other -end. The rock was a mica-schist. The average monthly progress was 412 -ft., with a maximum of 646 ft., with the rotary drills, and an average -of 454 ft. with the percussion drill. - - -=Excavation.=--Since considerable time is required to get the power -plant established, the excavation of rock tunnels is often begun by -hand, but hand work is usually continued for no longer a period than is -necessary to get the power plant in operation. Generally speaking, the -greatest difficulty is encountered in excavating the advanced drift or -heading. Based on the mode of blasting employed, there are two methods -of driving the advanced gallery, known as the circular cut and the -center cut methods. In the first method a set of holes is first drilled -near the center of the front in such a manner that they inclose a cone -of rock; the holes, starting at the perimeter of the base of the cone, -converge toward a junction at its apex. Seldom more than four to six -holes are comprised in this first set. Around these first holes are -driven a ring of holes which inclose a cylinder of rock, and if -necessary succeeding rings of holes are driven outside of the first -ring. These holes are blasted in the order in which they are driven, the -first set taking out a cone of rock, the second set enlarging this cone -to a cylinder, and the other sets enlarging this cylinder to the -required dimensions of the heading. The number of holes, however, varies -with the quality of rock and they are seldom driven deeper than 4 or 5 -ft. This method of excavating the heading, which is commonly followed by -European engineers, is illustrated in Figs. 50 to 52. In these figures -are indicated the number of holes in each round and the sequence of -rounds for the soft, medium and hard rock, as used in the Turchino -tunnel of the Genova Ovada Asti line of the Mediterranean Railway of -Italy. The heading was about 9 ft. square, and five sets of holes were -used in blasting, the depths being 3.91, 4.26 and 4.6 ft. for soft, -medium and hard rock, respectively, and the amount of dynamite consumed -was 2.38, 3.91 and 5.1 pounds per cubic yard for the three classes of -rock. - -[Illustration: ~in Soft Rock~ - -~in Medium Rock~ - -~in Hard Rock~ - -FIGS. 50 to 52.--Arrangement of Drill Holes in the Heading of Turchino -Tunnel.] - -[Illustration: FIGS. 53 and 54.--Arrangement of Drill Holes in the -Heading of the Fort George Tunnel.] - -In the center-cut method, which is the one commonly employed in America, -the holes are arranged in vertical rows, and are driven from 8 to 10 ft. -deep. Fig. 53 shows the arrangement of the holes, and the method of -blasting them, as used in the excavation of the heading for the Fort -George tunnel of the New York rapid transit. The two center rows of -holes converge toward each other so as to take out a wedge of rock; -others are bored straight, or parallel, with the vertical plane of the -tunnel. Those bored around the perimeter are driven either outward or -upward, according as they are located, close to the sides or roof of the -tunnel. In this case, the holes of the center cut were driven 9 ft. -deep, while all the other holes were bored to a depth of 8 ft. - -The width of the advanced gallery or heading depends upon the quality of -the rock. In hard rock American engineers give it the full width of the -tunnel section; but this cannot be done in loose or fissured rock, which -has to be supported, the headings here being usually made about 8 × 8 -ft. The wider heading is always preferable, where it is possible, since -more room is available for removing the rock, and deeper holes can be -bored and blasted. - -The important rôle played by the power plant and other mechanical -installations in constructing tunnels through rock has already been -mentioned. In some methods of soft-ground tunneling, and particularly in -soft-ground subaqueous tunneling, it is also often necessary to employ a -mechanical installation but slightly inferior in size and cost to those -used in tunneling rock. It is proposed to describe very briefly here a -few typical individual plants of this character, which will in some -respects give a better idea of this phase of tunnel work than the more -general descriptions. - - -=Rock Tunnels.=--The tunnels selected to illustrate the mechanical -installations employed in tunneling through rock are: The Mont Cenis, -Hoosac Tunnel, the Cascade Tunnel, the Niagara Falls Power Tunnel, the -Palisades Tunnel, the Croton Aqueduct Tunnel, the Strickler Tunnel in -America, and the Graveholz Tunnel and the Sonnstein Tunnel in Europe. In -addition there will be found in another chapter of this book a -description of the mechanical installations at the St. Gothard, -Pennsylvania and other tunnels. - - -_Mont Cenis Power Plant._--The mechanical installation consisted of the -Sommeilier air compressors built near the portals. The Sommeilier -compressors, Mr. W. L. Saunders says, were operated as a ram, utilizing -a natural head of water to force air at 80 lbs. pressure into a -receiver. The column of water contained in the long pipe on the side of -the hill was started and stopped automatically by valves controlled by -engines. The weight and momentum of the water forced a volume of air -with such a shock against the discharge valve that it was opened, and -the air was discharged into the tank; the valve was then closed, the -water checked; a portion of it was allowed to discharge, and the space -was filled with air, which was in turn forced into the tank. Only 73% of -the power of the water was available, 27% being lost by the friction of -the water in the pipes, valves, bends, etc. Of the 73% of net work, 49.4 -was consumed in the perforators, and 23.6 in a dummy engine for working -the valves of the compressors and for special ventilation. - -The compressed air was conveyed from each end through a cast-iron pipe -7⁵⁄₈ in. in diameter, up to the front of the excavation. The joints of -the pipes were made with turned faces, grooved to receive a ring of -oakum which was tightly screwed and compressed into the joint. To -ascertain the amount of leakage of the pipes, they and the tanks were -filled with air compressed to 6 atmospheres, and the machines stopped; -after 12 hours the pressure was reduced to 5.7 atmospheres, or to 95% of -the original pressure. - -Sommeilier’s percussion drilling machines were used in the excavation of -this tunnel. They were provided with 8 or 10 drills acting at the same -time, and mounted on carriages running on tracks. These were withdrawn -to a safe place during the blasting, and advanced again after the broken -rock was removed from the front and the new tracks laid. - -Machine shops were built at both ends of the tunnel for building and -repairing the drilling machines, bits, tools, etc. A gas factory was -built at each end for lighting purpose. - - -_Hoosac Tunnel._--The Hoosac tunnel on the Fitchburg R.R. in -Massachusetts is 25,000 ft. long, and the longest tunnel in America. The -material through which the tunnel was driven was chiefly hard granitic -gneiss, conglomerate, and mica-schist rock. The excavation was conducted -from the entrances and one shaft, the wide heading and single-bench -method being employed, with the center-cut system of blasting which was -here used for the first time. The tunnel was begun in 1854, and -continued by hand until 1866, when the mechanical plant was installed. -Most of the particular machines employed have now become obsolete, but -as they were the first machines used for rock tunneling in America they -deserve mention. The drills used were Burleigh percussion drills, -operated by compressed air. Six of these drills were mounted on a single -carriage, and two carriages were used at each front. The air to operate -these drills was supplied by air compressors operated by water-power at -the portals and steam-power at the shaft. The air compressors consisted -of four horizontal single-acting air cylinders with poppet valves and -water injection. The compressors were designed by Mr. Thomas Deane, the -chief engineer of the tunnel. - - -_Palisades Tunnel._--The Palisades tunnel was constructed to carry a -double track railway line through the ridge of rocks bordering the west -bank of the Hudson River and known as the Palisades. It was located -about opposite 116th St. in New York City. The material penetrated was a -hard trap rock very full of seams in places, which caused large -fragments to fall from the roof. The excavation was made by a single -wide heading and bench, employing the center-cut method of blasting with -eight center holes and 16 side holes for the 7 × 18 ft. heading. -Ingersoll-Sergeant 2¹⁄₂ in. drills were used, four in each heading and -six on each bench, and 30 ft. per 10 hours was considered good work for -one drill. - -The power-plant was situated at the west portal of the tunnel, and the -power was transmitted by electricity and compressed air to the middle -shaft and east portal workings. The plant consisted of eight 100 H. P. -boilers, furnishing steam to four Rand duplex 18 × 22 in. air -compressors, and an engine running a 30 arc light dynamo. The compressed -air was carried over the ridge by pipes, varying from 10 ins. to 5 ins. -in diameter, to the shaft and to the east portal, and was used for -operating the hoisting engines as well as the drills at these workings. -Inside the tunnel, specially designed derrick cars were employed to -handle large stones, they being also operated by compressed air. This -car ran on a center track, while the mucking cars ran on side tracks, -and it was employed to lift the bodies of the cars from the trucks, -place them close to the front, being worked where large stone could be -rolled into them, and return them to the trucks for removal. In addition -to handling the car bodies the derrick was used to lift heavy stones. -The hauling was done first by horse-power, and later by dummy -locomotives. - - -_Croton Aqueduct Tunnel._--In the construction of the Croton Aqueduct -for the water supply of New York City, a tunnel 31 miles long was built, -running from the Croton Dam to the Gate House at 135th St. in New York -City. The section of the tunnel varies in form, but is generally either -a circular or a horseshoe section. In all cases the section was designed -to have a capacity for the flow of water equal to a cylinder 14 ft. in -diameter. To drive the tunnel, 40 shafts were employed. The material -penetrated was of almost every character, from quicksand to granitic -rock, but the bulk of the work was in rock of some character. The -excavation in rock was conducted by the wide heading and bench method, -employing the center-cut method of blasting. Four air drills, mounted on -two double-arm columns were employed in the heading. The drills for the -bench work were mounted on tripods. Steam-power was used exclusively for -operating the compressors, hoisting engines, ventilating fans and pumps; -but the size and kind of boilers used, as well as the kind and capacity -of the machines which they operated, varied greatly, since a separate -power-plant was employed for each shaft with a few exceptions. A -description of the plant at one of the shafts will give an indication of -the size and character of those at the other shafts, and for this -purpose the plant at shaft 10 has been selected. - -At shaft 10 steam was provided by two Ingersoll boilers of 80 H. P. -each, and by a small upright boiler of 8 H. P. There were two 18 × 30 -in. Ingersoll air compressors pumping into two 42 in. × 10 ft. and two -42 in. × 12 ft. Ingersoll receivers. In the excavation there were twelve -3¹⁄₂ in. and six 3¹⁄₈ in. Ingersoll drills, four drills mounted on two -double arm columns being used on each heading, and the remainder mounted -on tripods being used on the bench. Two Dickson cages operated by one -12 × 12 in. Dickson reversible double hoisting engine provided -transportation for material and supplies up and down the shaft. A -Thomson-Houston ten-light dynamo operated by a Lidgerwood engine -provided light. Drainage was effected by means of two No. 9 and one No. -6 Cameron pumps. At this particular shaft the air exhausted from the -drills gave ample ventilation, especially when after each blast the -smoke was cleared away by a jet of compressed air. In other workings, -however, where this means of ventilation was not sufficient, Baker -blowers were generally employed. - - -_Strickler Tunnel._--The Strickler tunnel for the water supply of -Colorado Springs, Col., is 6441 ft. long with a section of 4 ft. × 7 ft. -It penetrates the ridge connecting Pike’s Peak and the Big Horn -Mountains, at an elevation of 11,540 ft. above sea level. The material -penetrated is a coarse porphyritic granite and morainal débris, the -portion through the latter material being lined. The mechanical -installation consisted of a water-power electric plant operating air -compressors. The water from Buxton Creek having a fall of 2400 ft. was -utilized to operate a 36 in. 220 H. P. Pelton water-wheel, which -operated a 150 K. W. three-phase generator. From this generator a 3500 -volt current was transmitted to the east portal of the tunnel, where a -step-down transformer reduced it to a 220 volt current to the motor. The -transmission line consisted of three No. 5 wires carried on cross-arm -poles and provided with lightning arresters at intervals. The plant at -the east portal of the tunnel consisted of a 75 H. P. electric motor, -driving a 75 H. P. air compressor, and of small motors to drive a -Sturtevant blower for ventilation, to run the blacksmith shop, and to -light the tunnel, shop, and yards. From the compressor air was piped -into the tunnel at the east end, and also over the mountain to the west -portal workings. Two drills were used at each end, and the air was also -used for operating derricks and other machinery. For removing the spoil -a trolley carrier system was employed. A longitudinal timber was -fastened to the tunnel roof, directly in the apex of the roof arch. This -timber carried by means of hangers a steel bar trolley rail on which the -carriages ran. Outside of the portal this rail formed a loop, so that -the carriage could pass around the loop and be taken back to the working -face. Each carriage carried a steel span of 1¹⁄₂ cu. ft. capacity, so -suspended that by means of a tripping device it was automatically dumped -when the proper point on the loop was reached. - - -_Niagara Falls Power Tunnel._--The tail-race tunnel built to carry away -the water discharged from the turbines of the Niagara Falls Power Co., -has a horse-shoe section 19 × 21 ft. and a length of 6700 ft. It was -driven through rock from three shafts by the center-cut method of -blasting. In sinking shaft No. 0 very little water was encountered, but -at shafts Nos. 1 and 2 an inflow of 800 gallons and 600 gallons per -minute, respectively, was encountered. The principal plant was located -at shaft No. 2, and consisted of eight 100 H.P. boilers, three 18 × 30 -in. Rand duplex air compressors, a Thomson-Houston electric-light plant, -and a sawmill with a capacity of 20,000 ft. B. M. per day. The shafts -were fitted with Otis automatic hoisting engines, with double cages at -shafts Nos. 1 and 2, and a single cage at shaft No. 0. The drills used -were 25 Rand drills and three Ingersoll-Sergeant drills. The pumping -plant at shaft No. 2 consisted of four No. 7 and one No. 9 Cameron -pumps, and that at shaft No. 2 consisted of two No. 7 and two No. 9 -Cameron pumps and three Snow pumps. An auxiliary boiler plant consisting -of two 60 H. P. boilers was located at shaft No. 1, and another, -consisting of one 75 H. P. boiler, was located at shaft No. 0. - - -_Cascade Tunnel._--The Cascade tunnel was built in 1886-88 to carry the -double tracks of the Northern Pacific Ry. through the Cascade Mountains -in Washington. It is 9850 ft. long with a cross-section 16¹⁄₂ ft. wide -and 22 ft. high, and is lined with masonry. The material penetrated was -a basaltic rock, with a dip of the strata of about 5°. The rock was -excavated by a wide heading and one bench, using the center-cut system -of blasting. A strutting consisting of five-segment timber arches -carried on side posts, spaced from 2 ft. to 4 ft. apart, and having a -roof lagging of 4 × 6 in. timbers packed above with cord-wood. The -mechanical plant of the tunnel is of particular interest, because of the -fact that all the machinery and supplies had to be hauled from 82 to 87 -miles by teams, over a road cut through the forests covering the -mountain slopes. This work required from Feb. 22 to July 15, 1886, to -perform. In many places the grades were so steep that the wagons had to -be hauled by block and tackle. The plant consisted of five engines, two -water-wheels, five air compressors, eight 70 H. P. steam-boilers, four -large exhaust fans, two complete electric arc-lighting plants, two fully -equipped machine-shop outfits, 36 air drills, two locomotives, 60 dump -cars, and two sawmill outfits, with the necessary accessories for these -various machines. This plant was divided about equally between the two -ends of the tunnel. The cost of the plant and of the work of getting it -into position was $125,000. - - -_Graveholz Tunnel._--The Graveholz tunnel on the Bergen Railway in -Norway is notable as being the longest tunnel in northern Europe, and -also as being built for a single-track narrow-gauge railway. This tunnel -is 17,400 feet long, and is located at an elevation of 2900 feet above -sea-level. Only about 3% of the length of the tunnel is lined. The -mechanical installation consists of a turbine plant operating the -various machines. There are two turbines of 100 H. P. and 120 H. P. -taking water from a reservoir on the mountain slope, and furnishing 220 -H. P., which is distributed about as follows: Boring-machines, 60 H. P.; -ventilation, 30 to 40 H. P.; electric locomotives, 15 H. P.; machine -shop, 15 H. P.; electric-lighting dynamo, 25 H. P.; electric drills, the -surplus, or some 40 H. P. The boring-machines and electric drills will -be operated by the smaller 100 H. P. turbine. - - -_Sonnstein Tunnel._--The Sonnstein tunnel in Germany is particularly -interesting because of the exclusive use of Brandt rotary drills. The -tunnel was driven through dolomite and hard limestone by means of a -drift and two side galleries. The dimensions of the drift were 7¹⁄₂ × -7¹⁄₂ ft. The power plant consisted of two steam pressure pumps, one -accumulator, and four drills. The steam-boiler plant, in addition to -operating the pumps, also supplied power for operating a rotary pump for -drainage and a blower for ventilation. The hydraulic pressure required -was 75 atmospheres in the dolomite, and from 85 to 100 atmospheres in -the limestone. The drift was excavated with five 3¹⁄₂ in. holes, one -being placed at the center and driven parallel to the axis of the -tunnel, and four being placed at the corners of a rectangle -corresponding to the sides of the drift, and driven at an angle -diverging from the center hole. The average depths of the holes were 4.3 -ft., and the efficiency of the drills was 1 in. per minute. One drill -was employed at each front, and was operated by a machinist and two -helpers, who worked eight-hour shifts, with a blast between shifts at -first, and later twelve-hour shifts, with a blast between shifts. The 24 -hours of the two shifts were divided as follows: boring the holes, 10.7 -hours; charging the holes, 1.1 hours; removing the spoil, 11.7 hours; -changing shifts, 0.5 hour. The average progress per day for each machine -was 6.7 ft. The total cost of the plant was $17,450. - - -_St. Clair River Tunnel._--The submarine double-track railway tunnel -under the St. Clair River for the Grand Trunk Ry. is 8500 ft. long, and -was driven through clay by means of a shield, as described in the -succeeding chapter on the shield system of tunneling. The mechanical -plant installed for prosecuting the work was very complete. To furnish -steam to the air compressors, pumps, electric-light engines, -hoisting-engines, etc., a steam-plant was provided on each side of the -river, consisting of three 70 H. P. and four 80 H. P. Scotch portable -boilers. The air-compressor plant at each end consisted of two 20 × 24 -in. Ingersoll air compressors. To furnish light to the workings, two 100 -candle-power Edison dynamos were installed on the American side, and two -Ball dynamos of the same size were installed on the Canadian side. The -dynamos on both sides were driven by Armington & Sims engines. These -dynamos furnished light to the tunnel workings and to the machine-shops -and power-plant at each end. Root blowers of 10,000 cu. ft. per minute -capacity provided ventilation. The pumping plant consisted of one set of -pumps installed for permanent drainage, and another set installed for -drainage during construction, and also to remain in place as a part of -the permanent plant. The latter set consisted of two 500 gallon -Worthington duplex pumps set first outside of each air lock, closing the -ends of the river portion of the tunnel. For permanent drainage, a -drainage shaft was sunk on the Canadian side of the river, and connected -with a pump at the bottom of the open-cut approach. In this shaft were -placed a vertical, direct-acting, compound-condensing pumping engine -with two 19¹⁄₂ in. high-pressure and two 33³⁄₈ in. low-pressure -cylinders of 24 in. stroke, connected to double-acting pumps with a -capacity of 3000 gallons per minute, and also two duplex pumps of 500 -gallons capacity per minute. For permanent drainage on the American -side, four Worthington pumps of 3000 gallons’ capacity were installed in -a pump-house set back into the slope of the open-cut approach. For the -permanent drainage of the tunnel proper two 400 gallon pumps were placed -at the lowest point of the tunnel grade. Spoil coming from the tunnel -proper was hoisted to the top of the open cut by derricks operated by -two 50 H. P. Lidgerwood hoisting-engines. The pressure pumping plant -for supplying water to the hydraulic shield-jacks at each end of the -tunnel consisted of duplex direct-acting engines with 12 in. steam -cylinders and 1 in. water cylinders, supplying water at a pressure of -2000 lbs. per sq. in. - - - - -CHAPTER X. - -TUNNELS THROUGH HARD ROCK (Continued). - - -EXCAVATION BY DRIFTS: THE SIMPLON AND MURRAY HILL TUNNELS. - -[Illustration: FIG. 55.--Diagram Showing Sequence of Excavations in -Drift Method of Tunneling Rock.] - - -=General Description.=--The method of tunneling through hard rock by -drifts is preferred by European engineers. All the great Alpine tunnels, -from the Mont Cenis tunnel to the Simplon, are examples of tunneling by -drifts. In this method the sequence of excavation is shown -diagrammatically by Fig. 55. The work begins by excavating a drift close -to the floor of the proposed tunnel (as shown in the center of the -figure) and far in advance of the excavation of any other part. The -section marked 2 is next removed and still later the portions marked 3. -Then with the removal of the parts marked 4 the whole section of the -tunnel will be open. - -The drift is usually strutted by means of side posts carrying a -cap-piece placed at intervals, and having a ceiling of longitudinal -planks resting on the successive caps. In hard rock the roof of the -section does not, as a rule, require regular strutting, occasional -supports being placed at intervals to prevent the fall of isolated -fragments: When the rock is disintegrated or full of seams, a regular -strutting may be necessary, and this may be either longitudinal or -polygonal in type. When longitudinal strutting is employed, a sill is -laid across the roof of the drift, and upon this are set up two struts -converging toward the top and supporting a cap-piece close to the roof. -On this cap-piece are placed the first longitudinal crown bars carrying -transverse poling-boards. Additional props standing on the sill and -radiating outward are inserted as parts No. 3 are excavated. These -radial props carry longitudinal bars which in turn support transverse -poling-boards. When polygonal strutting is used, it may take the form of -three or five segment arches of heavy timbers. - -In hard rock tunnels, as a rule, there is no danger of caving in because -of heavy pressures, and the whole section is left open for some time -before it is lined. The lining may be of concrete masonry, but in many -long tunnels, excavated through hard rock, the side walls are lined with -rubble masonry and the arch with brick, and, in some instances, even the -arch has been lined with rubble masonry. With skilful laborers at hand -the rubble masonry lining has proved most efficient and economical, -because the rock is utilized as it is excavated without any further -operation. Concrete, however, is more extensively employed for lining -tunnels than any other material. - -Tunnels excavated by drifts enable simple means of hauling to be -employed, and this is one of the reasons why the method finds so much -favor with European engineers. The tracks are laid along the floor of -the drift, and carry all the spoil from parts Nos. 2, 3, and 4, as well -as from the front of the drift itself. As fast as the full section is -completed, this single track in the drift is replaced by two tracks -running close to the sides of the tunnel, or by a broad-gauge track with -a third rail. - - -THE SIMPLON TUNNEL.[8] - -Before entering upon a description of the constructive details of this, -the longest railway tunnel in the world, it may be well to give a -general idea of the undertaking. Many schemes for the connection of -Italy and Switzerland by a railway near the Simplon Road Pass have been -devised, including one involving no great length of underground work, -the line mounting by steep gradients and sharp curves. The present -scheme, put forward in 1881 by the Jura-Simplon Ry. Co., consists -broadly of piercing the Alps between Brigue, the present railway -terminus in the Rhone Valley, and Iselle, in the gorge of the Diveria, -on the Italian side, from which village the railway will descend to the -existing southern terminus at Domo d’Ossola, a distance of about 11 -miles. - - [8] Abstract from a paper read before the Institution of Civil - Engineers by Charles B. Fox, Jan. 26, 1900. - -In conjunction with this scheme a second tunnel is proposed, to pierce -the Bernese Alps under the Lötschen Pass from Mittholz to a point near -Turtman in the Rhone Valley; and thus, instead of the long détour by -Lausanne and the Lake of Geneva, there will be an almost direct line -from Berne to Milan _via_ Thun, Brigue, and Domo d’Ossola. - -Starting from Brigue, the new line, running gently up the valley for -1¹⁄₄ miles, will, on account of the proximity of the Rhone, which has -already been slightly diverted, enter the tunnels on a curve to the -right of 1050 ft. radius. At a distance of 153 yards from the entrance, -the straight portion of the tunnel commences, and extends for 12 miles. -The line then curves to the left with a radius of 1311 ft. before -emerging on the left bank of the Diveria. Commencing at the northern -entrance, a gradient of 1 in 500 (the minimum for efficient drainage) -rises for a length of 5¹⁄₂ miles to a level length of 550 yards in the -center, and then a gradient of 1 in 143 descends to the Italian side. On -the way to Domo d’Ossola one helical tunnel will be necessary, as has -been carried out on the St. Gothard. There will be eventually two -parallel tunnels having their centers 56 ft. apart, each carrying one -line of way; but at the present time only one heading, that known as No. -1, is being excavated to full size, No. 2 being left, masonry lined -where necessary, for future developments. By means of cross headings -every 220 yds. the problems of transport and ventilation are greatly -facilitated, as will be seen later. As both entrances are on curves, a -small “gallery of direction” is necessary, to allow corrections of -alinement to be made direct from the two observatories on the axis of -the tunnel. - -The outside installations are as nearly in duplicate as circumstances -will allow, and consist of the necessary offices, workshops, -engine-sheds, power-houses, smithies, and the numerous buildings -entailed by an important engineering scheme. Great care is taken that -the miners and men working in the tunnel shall not suffer from the -sudden change from the warm headings to the cold Alpine air outside; and -for this purpose a large building is in course of erection, where they -will be able to take off their damp working clothes, have a hot and cold -douche, put on a warm dry suit, and obtain refreshments at a moderate -cost before returning to their homes. Instead of each man having a -locker in which to stow his clothes, a perfect forest of cords hangs -down from the wooden ceiling, 25 ft. above floor-level, each cord -passing over its own pulleys and down the wall to a numbered -belaying-pin. Each cord supports three hooks and a soap-dish, which, -when loaded with their owner’s property, are hauled up to the ceiling -out of the way. There are 2000 of these cords, spaced 1 ft. 6 ins. -apart, one to each man. The engineers and foremen are more privileged, -being provided with dressing-rooms and baths, partitioned off from the -two main halls. An extensive clothes washing and drying plant has been -laid down, and also a large restaurant and canteen. At Iselle, a -magazine holding 2200 lbs. of dynamite is surrounded and divided into -two separate parts by earth-banks, 16 ft. high. The two wooden houses, -in which the explosive is stored, are warmed by hot-water pipes to a -temperature between 61° F. and 77° F., and are watched by a military -patrol; but at Brigue a dynamite manufactory, started by an enterprising -company at the time of the commencement of the works, supplies this -commodity at frequent intervals, thereby avoiding the necessity of -storing in such large quantities. This dynamite factory has been -largely increased, and supplies dynamite to nearly all the mining and -tunneling enterprises in Switzerland. - - -=Geological Conditions.=--Before the Simplon tunnel was authorized, -expert evidence was taken as to the feasibility of the project. The -forecasts of the three engineers chosen, in reference to the rock to be -encountered and its probable temperature, have, as far as the galleries -have gone (an aggregate distance of nearly 2¹⁄₂ miles), generally been -found correct. At the north end, a dark argillaceous schist veined with -quartz was met with, and from time to time beds of gypsum and dolomite -have been traversed, the dip of the strata being on the whole favorable -to progress, though timbering is resorted to at dangerous places. Water -was plentiful at the commencement; in fact, one inrush has not been -stopped, and is still flowing down the heading. The total quantity of -water flowing from the tunnel mouth is 16 gallons per second, of which 2 -gallons per second are accounted for by the drilling machines. At -Iselle, however, a very hard antigorio gneiss obtains, and is likely to -extend for 4 miles. Very dry and very compact, it requires no timbering, -and represents no great difficulty to the powerful Brandt rock-drills, -which work under a head of 3280 ft. of water. - -The temperature of the rock depends not only on the depth from the -surface, but largely upon the general form of that surface combined with -the conductivity of the rock. Taking these points into consideration -with the experience gained from the construction of the St. Gothard -tunnel, 95° F. was estimated as the probable maximum temperature, owing -to the height of Monte Leone (11,660 ft.), which lies almost directly -over the tunnel axis. - - -=Survey.=--After having determined upon the general position of the -tunnels, taking into consideration the necessary gradients, the -temperature of the rock, and a large bed of troublesome gypsum on the -north side, two fixed points on the proposed center line were taken, -one at each entrance of tunnel No. 1, and the bearings of these two -points, with reference to a triangulation survey made in 1876, were -calculated sufficiently accurately to determine, for the time being, the -direction of the tunnel. In 1898, a new triangulation survey was made, -taking in eleven summits, Monte Leone holding the central position. This -survey was tied into that of the Wasenhorn and Faulhorn, made by the -Swiss Government, and the accuracy was such that the probable error in -the meeting of the two headings is only 6 cms. or 2¹⁄₂ ins. - -On the top of each summit is placed a signal, consisting of a small -pillar of masonry founded on rock, and capped with a sharp pointed cone -of zinc, 1 ft. 6 ins. high. An observatory was built at each end of the -tunnel in such a position that three of the summits could be seen, a -condition very difficult to fulfill on the south side owing to the depth -of the gorge, the mountains on either side being over 7000 ft. high. -Having taken the angles to and from each visible signal, and therefrom -having calculated the direction of the tunnel, it was necessary to fix, -with extreme accuracy, sighting-points on the axis of the tunnel, in -order to avoid sighting on to the surrounding peaks for each subsequent -correction of the alinement of the galleries. To do this, a theodolite -24 ins. long and 2³⁄₈ ins. in diameter, with a magnifying power of 40 -times, was set up in the observatory, and about 100 readings were taken -of the angles between the surrounding signals and the required -sighting-points. In this manner the error likely to occur was diminished -to less than 1′. Thus at the north end two points were found about 550 -yds. before and behind the observatory, while on the south side, owing -to the narrowness of the gorge, the points could only be placed at 82 -yds. and 126 yds. in front. One of these sighting-points consists of a -fine scratch ruled on a piece of glass fixed in an iron frame, behind -which is placed an acetylene lamp,--corrections of alinement are always -done by night,--the whole being rigidly fixed into a niche cut in the -rock and protected from climatic and other disturbing agencies by an -iron plate. - - -=Method of Checking Alinement.=--The direction of heading No. 1 is -checked by experts from the Government Survey Department at Lausanne -about three times a year, and for this purpose a transit instrument is -set up in the observatory. A number of three-legged iron tables are -placed at intervals of 1 mile or 2 miles along the axis of tunnel No. 1, -and upon each of these is placed a horizontal plane, movable by means of -an adjusting screw, in a direction at right angles to the axis, along a -graduated scale. On this plane are small sockets, into which the legs of -an acetylene lamp and screen, or of the transit instrument, can be -quickly and accurately placed. The screen has a vertical slit, 3 ins. in -height, and variable between ¹³⁄₁₆ in. and ³⁄₁₆ in. in breadth, -according to the state of the atmosphere, and at a distance shows a fine -thread of light. The instrument, having first been sighted on to the -illuminated scratch of the sighting-point, is directed up the tunnel, -where a thread of light is shown from the first table. With the aid of a -telephone this light is adjusted so that its image is exactly coincident -with the cross hairs, and the reading on the graduated scale is noted. -This is done four or five times, the average of these readings being -taken as correct, and the plane is clamped to that average. The -instrument is then taken to the first table and is placed quickly and -accurately over the point just found (by means of the sockets), and the -lamp is carried to the observatory. After first sighting back, a second -point is given on the second table, and so on. These points are marked -either temporarily in the roof of the heading by a short piece of cord -hanging down, or permanently by a brass point held by a small steel -cylinder, 8 ins. long and 3 ins. in diameter, embedded in concrete in -the rock floor, and protected by a circular casting, also sunk in cement -concrete, holding an iron cover resembling that of a small manhole. From -time to time the alinement is checked from these points by the -engineers, and after each blast the general direction is given by the -hand from the temporary points. To check the results of the -triangulation survey, astronomical observations have been taken -simultaneously at each end. With regard to the levels, those given on -the excellent Government surveys have been taken as correct, but they -have also been checked over the pass. - - -=Details of Tunnels.=--In cross-section, tunnel No. 1 is 13 ft. 7 ins. -wide at formation level, increasing to 16 ft. 5 ins., with a total -height of 18 ft. above rail-level, and a cross-sectional area of about -250 sq. ft. This large section will allow of small repairs being -executed in the roof without interruption of the traffic, and will also -allow of strengthening the walls by additional masonry on the inside. -The thickness of the lining, never wholly absent, and the material of -which it is composed, depend upon the pressure to be resisted, and only -in the worst case is an invert resorted to. The side drain, to which the -rock floor is made to slope, will be composed of half-pipes of 7 to 1 -cement concrete. The roof is constructed of radial stones. - -Tunnel No. 2, being left as a heading, is driven on that side nearest to -No. 1, to minimize the length of the cross-headings, and measures 10 ft. -2 ins. wide by 6 ft. 7 ins. high. Masonry is used only where necessary, -and in that case is so built as to form part of the lining of the tunnel -when eventually completed. Concrete is put in to form a foundation for -the side wall, and a water channel. The cross-headings, connecting the -two parallel headings, occur every 220 yds., and are placed at an angle -of 56° to the axis of the tunnel, to avoid sharp curves in the -contractors’ railway lines. They will eventually be used as much as -possible for refuges, chambers for storing the tools and equipment of -the platelayers, and signal-cabins. The refuges, 6 ft. 7 ins. wide by 6 -ft. 7 ins. high and 3 ft. 3 ins. deep, occur every 110 yards, every -tenth being enlarged to 9 ft. 10 ins. wide by 9 ft. 10 ins. deep and 10 -ft. 2 ins. high, still larger chambers being constructed at greater -intervals. - - -=Method of Excavation.=--The work at each end of the tunnel is carried -on quite independently, consequently, though similar in principle, the -methods vary in detail, apart from the fact that different geological -strata require different treatment. Broadly speaking, the two parallel -headings, each 59 sq. ft. in section, are first driven by means of -drilling-machines and the use of dynamite, this work being carried on -day and night, seven days in the week; No. 1 heading is then enlarged to -full size by hand-drilling and dynamite. On the Italian side, where the -rock is hard and compact, breakups are made at intervals of 50 yds., and -a top gallery is driven in both directions, but, for ventilation -reasons, is never allowed to get more than 4 yds. ahead of the break-up, -which is gradually lengthened and widened to the required section. No -timbering is required, except to facilitate the excavation and the -construction of the side walls. Steel centers are employed for the arch; -they entail fewer supports, give more room, and are capable of being -used over again more frequently without damage. They consist of two -I-beams bent to a template and riveted together at the crown, resting at -either side on scaffolding at intervals of 6 ft.; longitudinals 12 ft. -by 4 ins. by 4 ins. support the roof. Hand rock-drilling is carried out -in the ordinary way, one man holding the tool and a second striking; -measurements of excavation are taken every 2 or 3 yds., a plumb-line is -suspended from the center of the roof, and at every half-meter (20 ins.) -of height horizontal measurements are taken to each side. - -At the Brigue end a softer rock is encountered, necessitating at times -heavy timbering in the heading, and especially in the final excavation -to full size, Fig. 56. The bottom heading, 6 ft. 6 in. high, is driven -in the center, and the heading is then widened to the full extent and -timbered; the concrete forming the water channel and the foundation for -one side wall is put in; the side walls are built to a height of 6 ft. 6 -ins., and the tunnel is fully excavated to a further height of 6 ft. 6 -ins. from the first staging. The side walls are then continued up for -the second 6 ft. 6 ins., and from the second floor a third height of 6 -ft. 6 ins. is excavated and timbered. Finally the crown is cleared out, -heavy wooden centers are put in, the arch is turned and all timbers are -withdrawn except the top poling-boards, supporting the loose rock. - -[Illustration: FIG. 56.--Sketches Showing Sequence of Work in Excavating -and Lining the Simplon Tunnel. - -1 - -2 - -3 - -4 - -5 - -6 - -7 - -8] - -The masonry for the side walls is obtained either from the tunnel itself -or from a neighboring quarry, and varies in character according to the -pressure; but the face of the arch is always of cut or artificial -stones, the latter being 7 to 1 cement concrete. Where the alinement -heading, or the “gallery of direction,” joins the curving portion of -tunnel No. 1, the section is very much greater, and necessitates special -timbering. - - -=Transport (Italian Side).=--A small line of railway, 2 ft. 7¹⁄₂ ins. -gauge, with 40-lb. rails, enters all three portals; but since the -construction of a wooden bridge over the Diveria, the route through the -“gallery of direction,” across heading No. 2, to tunnel No. 1, is used -exclusively; this railway leads to the face in both headings, and, where -convenient, from one heading to the other by the cross-galleries. -Different types of wagons are in use; but in general they are -four-wheeled, non-tipping box wagons, supplied with brakes and holding 2 -cu. yds. of débris. A special type of locomotive is used, designed to -pass round curves of 50 ft. radius, and supplied with a specially large -boiler to avoid firing in the tunnel. - -[Illustration: FIG. 57.--General Details of the Brandt Rotary Drills -Employed at the Simplon Tunnel.] - - -=Method of Working.=--The drilling-machines employed are of the Brandt -type, Fig. 57, and are mounted in the following manner: A small -four-wheeled carriage supports at its center a beam, the shorter arm of -which carries the boring mechanism and the longer a counterpoise; near -its center is the distributor. In the short arm is a clamp holding the -rack-bar or butting column, which is a wrought-iron cylinder with a -plunger constituting a ram, and is jammed by hydraulic pressure between -the walls of the heading, thus forming a rigid support for the -boring-machine, and an efficient abutment against the reaction of the -drill. This rack-bar can be rotated on its clamp in a plane parallel to -the axis of the beam. Three or four separate boring-machines can be -mounted on the rack-bar, and can be adjusted in any reasonable position. - -The boring-machine performs the double function of continually pressing -the drill into the rock by means of a hollow ram (_I_) and of imparting -to the drill and ram a uniform rotary motion. This rotary motion is -given by a twin cylinder single-acting hydraulic motor (_E_), the two -pistons, of 2⁷⁄₈ ins. stroke, acting reciprocally as valves. The cranks -are fixed at an angle of 90° to each other on the shaft, which carries a -worm, gearing with a worm-wheel (_Q_) mounted upon the shell (_R_) of -the hollow ram (_I_), and this shell in turn engages the ram by a long -feather, leaving it free to slide axially to or from the face of the -rock. The average speed of the motor is 150 revolutions to 200 -revolutions per minute, the maximum speed being 300 revolutions per -minute. The loss of power between the worm and worm-wheel is only 15% at -the most; the worm being of hardened steel and the wheel of gun-metal, -the two surfaces in contact acquire a high degree of polish, resulting -in little wearing or heating. Taking into consideration all other -sources of loss, 70% of the total power is utilized. The pressure on the -drill is exerted by a cylinder and hollow ram (_I_), which revolves -about the differential piston (_S_), which is fixed to the envelope -holding the shell (_R_). This envelope is rigidly connected to the -bed-plate of the motor, and, by means of the vertical hinge and pin -(_T_), is held by the clamp (_V_) embracing the rack-bar. When water is -admitted to the space in front of the differential piston the ram -carrying the drilling-tool is thrust forward, and when admitted to the -annular space behind the piston, the ram recedes, withdrawing the tool -from the blast-hole. The drill proper is a hollow tube of tough steel -2³⁄₄ ins. in external diameter, armed with three or four sharp and -hardened teeth, and makes from five to ten revolutions per minute, -according to the nature of the rock. When the ram has reached the end of -its stroke of 2 ft. 2¹⁄₂ ins., the tool is quickly withdrawn from the -hole and unscrewed from the ram; an extension rod is then screwed into -the tool and into the ram, and the boring is continued, additional -lengths being added as the tool grinds forward; each change of tool or -rod takes about 15 secs. to 25 secs. to perform. The extension rods are -forged steel tubes, fitted with four-threaded screws, and having the -same external diameter as the drill. They are made in standard lengths -of 2 ft. 8 ins., 1 ft. 10 ins., and 11³⁄₄ ins. The total weight of the -drilling-machine is 264 lbs., and that of the rack-bar when full of -water is 308 lbs. The exhaust water from the two motor cylinders escapes -through a tube in the center of the ram and along the bore of the -extension rods and drill, thereby scouring away the débris and keeping -the drill cool; any superfluous water finds an exit through a hose below -the motors and thence away down the heading. The distributor, already -mentioned, supplies each boring-machine and the rack-bar with hydraulic -pressure from the mains, with which connection is effected by means of -flexible or articulated pipe connections, allowing freedom in all -directions. The area of the piston for advancing the tool is 15¹⁄₂ sq. -ins., which, under a pressure of 1470 lbs. per sq. in., gives a pressure -of over 10 tons on the tool, while for withdrawing the tool 2¹⁄₂ tons is -available. In the rock found at Iselle, namely, antigorio gneiss, a hole -2³⁄₄ ins. in diameter and 3 ft. 3 ins. in length is drilled, normally, -in 12 mins. to 25 mins.; a daily rate of advance of 18 ft. to 19 ft. 6 -ins. is made in a heading having a minimum cross-section of 59 sq. ft.; -the time taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 2¹⁄₂ -hrs. - -When the débris resulting from one operation has been sufficiently -cleared away, a steel flooring, which is provided near the face to -enable shoveling to be more easily done, and to give an even floor for -the wheels of the drilling-carriage, is laid bare at the head of the -line of rails, and the drilling-machines are brought up on their -carriage by eight or ten men. When advanced sufficiently close to the -face, the rack-bar is slewed round across the gallery and is wedged up -against the rock sides; connection is made between the distributor and -the hydraulic main, by means of the flexible pipe, and pressure is -supplied by a small copper tube to the rack-bar ram, thereby rigidly -holding the machine. Next, connections are made between the three -drilling-machines and the distributor, and in 20 mins. from the time the -machine was brought up all three drills are hard at work, water pouring -from the holes. - -The noise of the motors and grinding-tools is sufficient to drown all -but shouts; and where the extension rods do not fit tightly, small jets -of water play in all directions, necessitating the wearing of tarpaulins -by the men directing the tools. Lighting is done wholly by small -oil-lamps, provided with a hook to facilitate fixing in any crack in the -rock; electricity will probably be used to light that portion of the -tunnel which is completed. - -Two men are allotted to each drill, one to drive the motor, the other to -direct and replenish the tool, one foreman and two men in reserve -completing the gang. A small hammer is freely used to loosen the screw -joints of the extension rods and drill. A hole is usually commenced by a -two-edged flat-pointed tool, until a sufficient depth is reached to -prevent the circular tool from wandering over the face of the rock, but -in many instances the hole is commenced with a circular tool. The -exhaust water during this period flows away by the hose underneath the -motor. In the antigorio gneiss, ten to twelve holes are drilled for each -attack, three to four in the center to a depth of 3 ft. 3 ins., the -remainder, disposed round the outside of the face, having a depth of 4 -ft. 7 ins. The average time taken to complete the holes is 1³⁄₄ hr. to -2¹⁄₂ hrs. Instead of pulverizing the rock, as do the diamond drills, it -is found that the rock is crushed, and that headway is gained somewhat -in the manner of a circular saw through wood. The core of rock inside -the tool breaks up into small pieces, and can be taken out if necessary -when the drill requires lengthening. - -The lowest holes, inclined downwards, are full of water; consequently -two detonators and two fuses are inserted, but apart from this, water -has little effect on the charge. The fuses of the central holes are -brought together and cut off shorter than those of the outer holes, in -order that they may explode first to increase the effect of the outer -charges. All portable objects, such as drills, pipe connections, tools, -etc., have meanwhile been carried back; the steel flooring is covered -over with a layer of débris to prevent injury from falling rock, and to -the end of the hydraulic main is screwed a brass plug pierced by five -holes; and immediately the explosions occur a valve is opened in the -tunnel, and five jets of water play upon the rock, laying the dust and -clearing the air. The necessity for this was shown on one occasion when -this nozzle was broken by the explosion and the water had to be turned -off immediately to avoid useless waste; on reaching the face, the -atmosphere was found to be so highly charged with dust and smoke that it -was impossible to distinguish the stones at the feet, although a lamp -had been placed on the ground; and despite the fact that the air tube -was in full blast, the men experienced great difficulty in breathing. A -truck is now brought up, and four men clear a passage in front, through -the heap of débris, two with picks and two with shovels, while on either -side and behind are as many men as space will permit. The stone is -thrown either to the sides of the heading or into the wagon, shoveling -being greatly aided by the steel flooring, which, before the explosion, -had been laid over the rails for nearly 10 yds. down the tunnel to -receive the falling rock. These steel plates are taken up when cleared, -and the wagon is pushed forward until the drilling-machine can be -brought up again, leaving the remaining débris at the sides to be -handled at leisure during the next attack. The roof and side walls are, -of course, carefully examined with the pick, to discover and detach any -loose or hanging rock. The times taken for each portion of the attack in -this particular antigorio gneiss are as follows: Bringing up and -adjustment of drills, 20 mins.; drilling, between 1³⁄₄ hr. and 2¹⁄₂ -hrs.; charging and firing, 15 mins.; clearing away débris, 2 hrs.; or -for one whole attack, between 4¹⁄₂ hrs. and 5¹⁄₂ hrs., resulting in an -advance of 3 ft. 9 in., or a daily advance of nearly 18 ft. - -From this it appears that the time spent in clearing away the débris -equals that taken up in drilling, and it is in this clearing that a -saving of time is likely to be effected rather than in the process of -drilling. Many schemes have been tried, such as a mechanical plow for -making a passage; at Brigue, “marinage,” or clearing by means of -powerful high-pressure water-jets, directed down the tunnel, was tried, -but the idea is not yet sufficiently developed. - -Another series of experiments has been tried at Brigue with regard to -the utilization of liquid air as an explosive agent instead of dynamite; -and for this purpose a plant has been laid down, consisting of one -ammonia-compressor, two air-compressors, and two refrigerators, -furnishing ¹⁄₁₀ gallon of liquid air per hour at an expenditure of 17 H. -P. The system used is that of Professor Linde, who himself directs the -experiments. The great difficulty experienced is that of shortening the -interval of time that must elapse between the manufacture of the -cartridge and its explosion. The liquid oxygen, with which the -cartridge, containing kieselguhr (silicious earth) and paraffin, is -saturated, evaporates very readily, losing power every moment; hence the -effect of each cartridge cannot be guaranteed, and though it is an -exceedingly powerful explosive when used immediately after manufacture, -no practical result has yet been obtained. - - -=Power Station.=--Water is abundant at either end, and therefore -hydraulic power is the motive force employed. On the Italian side, a dam -5 ft. high has been thrown across the Diveria at a point near the Swiss -frontier, about 3 miles above the site of the installations. A portion -of the water thus held back enters, through regulating doors and -gratings, a masonry channel leading to two parallel settling tanks, each -111 ft. by 16 ft., whence, after dropping all its sand and solid matter, -the now pure water passes into the water-house, and, after flowing over -a dam, through a grating and past the admission doors, enters a metallic -conduit of 3-ft. pipes. Each of the settling tanks and the approach -canal are provided with doors at the lower end leading direct to the -river, through which all the sand and solid matter deposited can be -scoured naturally by allowing the river-water to rush freely through. -For this purpose the floor of the basins is on an average gradient of 1 -in 30. For a similar reason the river-bed just outside the entrance to -the approach canal is lined with wooden planks, from which the stones -collecting behind the dam can be scoured by allowing an iron flap, -hinged at the bottom, to change its position from the vertical to the -horizontal in a gap left purposely in the dam, so causing a rushing -torrent to sweep it clean. - -The chief levels are: - - Level of water at dam 794.00 meters above sea level. - „ in water-house 793.70 „ „ „ „ - „ at turbines 618.50 „ „ „ „ - -giving a total fall of 175.20 ms. or 570 ft., and a pressure of 17.52 -atmospheres. - -The quantity of water capable of being taken from the Diveria in winter, -when the rivers which are dependent upon the mountain snows for their -supply are at their lowest, is calculated to be 352 gallons per second. -Thus, taking the fall to be diminished by friction, etc., to 440 ft., -and the useful effect at 70%, there is obtained 2000 H. P. on the -turbine shaft. - -The metallic conduit varies in material according to the pressure; thus -cast-iron pipes 3 ft. in diameter and ¹³⁄₁₆ in. thick are used up to a -pressure of 2 atmospheres, from which point they are of wrought-iron. -The cast-iron portion has of late caused a good deal of trouble, owing -to settlement of the piers causing occasional bursts, consequently a -masonry pier has been placed under each joint of this portion. The -following table gives the thicknesses and diameters, varying with the -pressure: - - +---------+-------------+-------------+------+ - | WATER | THICKNESS. | DIAMETER. |WEIGHT| - |PRESSURE.| | | PER | - | | | | YARD.| - +---------+-------+-----+-----+-------+------+ - | Head | Milli-|Inch.|Feet.|Inches.| Lbs. | - | in Feet.|meters.| | | | | - +---------+-------+-----+-----+-------+------+ - | 246 | 6 | ¹⁄₄ | 3 | 0 | 326 | - | 311 | 7 | ... | 3 | 0 | 383 | - | 360 | 8 | ... | 3 | 0 | 431 | - | 393 | 9 | ... | 3 | 0 | 483 | - | 426 | 10 | ... | 3 | 0 | 556 | - | 476 | 12 | ... | 3 | 0 | 651 | - | 590 | 16 | ⁵⁄₈ | 3 | 3¹⁄₃ | 977 | - +---------+-------+-----+-----+-------+------+ - -This pipe is supported every 30 ft. on small masonry piers, on the top -of which is placed a block of wood hollowed out to receive the pipe, -thus allowing any movement due to the contraction and expansion of the -conduit. However, to prevent this movement becoming excessive, the pipe -is passed at intervals of 300 yds. to 500 yds. through a cubical block -of masonry of 13 ft. side, strengthened by longitudinal tie-bars. Five -bands of angle-bar riveted round the pipe, with their flanges embedded -in the masonry, constitute a rigid fixed point. Straw mats are thrown -over the pipe where it is exposed to the sun. The temperature of the -conduit is not, however, found to vary greatly, since the pipe is kept -full of water. To supply the rock-drills with water at a maximum -pressure of 100 atmospheres, or 1470 lbs. per sq. in., a plant of four -pairs of high-pressure pumps has been laid down, and a still larger -addition is in course of erection. At present, two Pelton turbines of -250 H.P. each, running at 170 revolutions per minute, drive the pumps, -by means of toothed gearing, at 63 revolutions per minute. These pumps -are of very simple but strong construction, single suction and double -delivery, entailing one suction and one delivery-valve, both heavy and -both of small lift. The larger portion of the plunger has exactly double -the cross-sectional area of the smaller portion, so that in the forward -stroke half of the water taken in at the last admission is pumped into -the high-pressure mains, and at the same time a fresh supply of water is -sucked in. During the backward stroke half of this new supply is pumped -into the mains, and the remainder enters the second chamber, to be -pumped during the next forward stroke. Thus the work done in the two -strokes is practically the same. The pumps are in pairs, and are set at -an angle of 90°, to insure uniform pressure and uniform delivery in the -mains. Their size varies; but at Iselle there are three pairs, with a -stroke of 2 ft. 2¹⁄₂ ins., and the plungers of 2¹¹⁄₁₆ in. and 1⁷⁄₈ ins. -(approximately) in diameter, supplying 1.32 gallons per second. - -To avoid injury to the valves, the water to be pumped is taken from a -stream up the mountain side, and is passed through filter screens. The -high-pressure water, after passing an accumulator, enters the tunnel in -solid drawn wrought-iron tubes, 3¹⁄₈ ins. in internal diameter, ³⁄₁₆ in. -thick, and in lengths of 26 ft. The diameter of these mains varies with -their length, so as to avoid loss of pressure. With the 1250 yds. of -tunnel now driven 10 atmospheres are lost. - -At Brigue the installations are, as far as possible, identical. The -Rhone water, however, before reaching the water-house, is carried from -the filter basins, a distance of 2 miles, in an armored canal built upon -the Hennebique system,[9] the walls and supporting beams, of cement -concrete, being strengthened by internal tie-bars of steel. The concrete -struts, resembling balks of timber at a distance, are occasionally 35 -ft. high and 1 ft. 7¹⁄₂ ins. square. The metallic conduit is 5 ft. in -diameter, with a minimum flow of 176 cu. ft. per second and a total fall -of 185 ft. In case water-power should be unavailable, three -semi-portable steam engines, two of 80 H.P. and one of 60 H.P., are -always kept in readiness at each end of the tunnel, and are geared by -belts to the turbine shaft. - - [9] Network of steel rods embedded in concrete. - - -=Ventilation.=--In tunneling, one of the most important problems to be -solved is that of ventilation, and it is for this reason that the -Simplon tunnel consists of two parallel headings with cross cuts at -intervals of 220 yds. At Brigue, a shaft 164 ft. deep was sunk through -the overlying rock until the “gallery of direction” was encountered. Up -this chimney the foul air is drawn by wood fires, the fresh air--a -volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per -minute--entering by heading No. 2, penetrating up to the last cross -gallery, and returning by tunnel No. 1. The entrances of No. 1 and the -“gallery of direction,” besides those of all the intermediate cross -galleries, are closed by doors. By this arrangement, however, fresh air -does not reach the working faces; therefore a pipe, 8 ins. in diameter, -is led from the fresh air in No. 2 to within 15 yds. of the face of each -heading, and up this pipe a draft of air is induced by means of a jet of -water, the volume to each face being 800 cu. ft. per minute. One single -jet of water from the high-pressure mains, with a diameter of ¹⁄₁₆ in., -is capable of supplying over 1000 cu. ft. of air per minute at the end -of 160 yds. of pipe, and during the attack the men at the drills are in -a constant breeze with the thermometer standing at 70° F. At Iselle, air -is blown into the entrance of heading No. 2 at the rate of 14,100 cu. -ft. per minute by two fans driven from the turbine shaft. This air -travels from the fans along a pipe 18 ins. in diameter, till a point 15 -yds. up the tunnel is reached, where beyond a door the pipe narrows to -form a nozzle 10 ins. in diameter. This door is kept open to allow the -outside air to be induced up the tunnel, as the headings are at present -only 2500 yds. long, giving a resistance of not quite sufficient power -to cause the air to return. The fresh air then travels up No. 2, -crossing over the top of the “gallery of direction,” from which it is -shut off by doors, to the last cross gallery, returning by No. 1, and -finally leaving either by the “gallery of direction” or by No. 1. A -system of cooling the air and driving it on by means of a large number -of water-jets will be installed in No. 2 where that heading crosses over -the “gallery of direction,” but at present there is no need for it. - -The average temperature at the face is 73° F. during the drilling -operation, 76° F. after firing the charges, and a maximum of 80° F., -lately attaining to 86° F. on the south side, with 80° F. and 85° F. -before and after firing. The temperature of the rock is taken at every -110 yds. in holes 5 ft. deep, and shows a gradual increase according to -the depth of over-laying rock, to the conductivity of the rock, and to -the form of the mountain surface. The maximum hitherto reached on the -north side is 68° F., while on the south side, although a smaller -distance has been traversed, it attains to 79° F., due to the more rapid -increase in depth. Moreover, the temperature of the rock is observed at -the permanent stations, 550 yds. from the entrances, in its relation to -that of the tunnel and outside air, and though on the north side that of -the rock varies almost as quickly as that of the tunnel air, on the -south it is influenced very much less. - -A few statistics may be of interest with regard to the progress of the -last three months (taken from the trimestrial report of January, 1900). -At Brigue, where there are three drilling-machines in No. 1 and two in -the parallel heading, the total length excavated was 995 yds. or 6409 -cu. yds. in 89 working days, the average cross-sectional area being 57 -sq. ft. This required 507 attacks and 3066 holes, which had a total -depth of 26,600 ft. and 14,700 re-sharpenings of the drilling-tool, with -44,000 lbs. of dynamite. - -The average time occupied in drilling was 2 hrs. 45 mins., while -charging, firing, and clearing away the débris took 6 hrs., 35 mins. At -Brigue 648 men and 29 horses were employed at one time in the tunnel. At -Iselle the numbers were 496 men and 16 horses, working in shifts of 8 -hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs. -to 11 hrs. per day, the total being 541 men at Brigue and 346 men at -Iselle. On the Italian side, where the rock is very much harder, there -were three drilling-machines in each heading; the total length -excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or -6700 cu. yds. in 91 working days. This required 61,293 re-sharpened -tools, 758 attacks, 7940 holes with a total depth of 33,000 ft., and -56,000 lbs. of dynamite. The average time spent in drilling was 2 hrs. -55 mins., and in charging and clearing 2 hrs. 36 mins. Thus, in the hard -gneiss, to excavate 1 cu. yd. of rock required 8¹⁄₂ lbs. of dynamite, -and each tool pierced 6¹⁄₂ ins. of rock before it required -re-sharpening. - - -THE MURRAY HILL TUNNEL. - -The drift method of excavating tunnels was followed in Section IV of the -New York Subway, under Park Avenue between 33rd and 41st Streets. At -this point the four tracks of the subway pass under a rocky elevation, -known as Murray Hill, in two double track parallel tunnels, 43 ft. -apart, center to center. Here already existed a double track tunnel -which was built many years ago by the New York Central and Hudson River -R.R., and is now used by the Madison Avenue surface cars. The two subway -tunnels were driven close below the existing tunnel and also very near -the foundations of expensive residences along Park Avenue, particularly -on Murray Hill, one of the best residential sections of the city. - - -=Material Penetrated.=--The material penetrated by the excavation -consisted chiefly of a surface outcrop of the mica-schist rock which -underlies Manhattan Island. The rock was for the most part in compact -strata, dipping at about 45° from East to West, but at intervals an -unstable stratum was encountered which when free slid on the underlying -stratum. Troubles from such slides were experienced during the -construction of the tunnel. - - -=Cross-Section.=--The cross-section selected for the tunnels had -vertical side walls and a three-centered roof arch with the flattest -curve at the crown. The interior dimensions were 25 ft. wide and 16 ft. -high. The selected cross-section was not the best suited for a tunnel to -be driven through rock, where the sharpest curve should be at the top, -but in this case the flattened curve was chosen because of local -conditions; chiefly, the presence of the existing tunnel and the -consequent necessity of leaving a certain thickness of rock between it -and the new tunnel, without depressing very much the grade of the -subway. - -[Illustration: FIG. 58.--Sequence of Excavation in the Murray Hill -Tunnel.] - - -=Excavation.=--The two parallel tunnels were driven exclusively from the -ends reached by shafts; thus the tunnels were attacked at four parts. It -was in these tunnels that a comparative test was made of the different -methods of driving tunnels through rock. The contractor applied the -heading and drift method at the southern ends of the tunnels, the -eastern tunnel being driven by means of a drift while in the western -tunnel the usual heading method was followed. This latter method is -illustrated in the chapter following and the eastern tunnel at 33rd -Street, excavated by means of a drift, is here considered. - -Fig. 58 shows the sequence of cuts adopted for this tunnel. It was begun -by a bottom drift, about 10 ft. high, 8 ft. wide and 7 ft. deep, which -was located at one side of the axis of the tunnel, as indicated in the -figure. This drift was immediately widened by removing the portions -marked 2. About 50 ft. in the rear the part marked 3 was taken away, -thus clearing the entire lower portion of the tunnel. Section 4, about -50 ft. to the rear of section 3, was then broken down and removed. - -The methods of drilling and blasting were as follows: In taking out the -original drift, a wedge-shaped center cut was made and then enlarged to -the full size of the drift by drilling parallel holes. The succeeding -sections, 2 and 3, were removed by driving parallel holes, while the top -section, 4, was taken away by a center cut and parallel holes. The -drills were mounted on columns, two drills to a column, and the holes -were usually drilled about 7 ft. deep, starting with a diameter of 2³⁄₄ -in. and ending with a diameter of 1³⁄₄ in. They were blasted with 40% -dynamite in light charges, only a few holes being fired at a time, -usually not more than three or four. - -[Illustration: FIG. 59.--Traveling Platform for the Excavation of the -Upper Side of the Murray Hill Tunnel.] - -To remove section 4, a traveling platform 10¹⁄₂ ft. long and 25 ft. wide -was used. This platform, as shown in Fig. 59, consisted of two -longitudinal beams mounted on four double flanged wheels which were -running on tracks laid 23 ft. apart. Resting on top of these beams were -four 12 in. × 12 in. uprights braced in every direction against the -framework of the platform. This frame was built of 12 in. × 12 in. beams -laid longitudinally, the transverse beams being 12 in. × 14 ins. The -platform proper was made of 3 in. planks, and was set 9 ft. above the -tunnel floor. The columns supporting the drills for the excavation of -the upper section 4, were set up above the platform which was then -reinforced by other vertical props, as indicated by the dotted lines in -the figure. These props, however, were placed so as to leave a clearance -beneath the platform for the cars to carry away the débris from the -front. During the blasting the platform was moved back so that the -blasted rock fell to the floor of the tunnel, whence it was loaded into -boxes on the cars. - - -=Strutting.=--When the rock was seamy and full of fissures, running in -every direction, it was necessary to support the roof of the excavation. -This was done in the following manner: After part 4 was removed the -timbers supporting the roof of the excavation were set up. In this case, -the polygonal strutting was used. This consisted of heavy timber frames -placed transversely to the axis of the tunnel and supporting the planks -or poling-boards which ran longitudinally against the roof of the -excavation. The seven-segment arch frame was used in the Murray Hill -tunnel. At the bottom of part 4 were placed longitudinally 12 × 16 in. -beams and upon them rested the inclined segments which, with a -horizontal one, formed the arch frame as shown in Fig. 60. When the -pressures were too heavy the crown segment was reinforced by a 6 × 12 -in. beam, kept in place by two 12 × 12 in. inclined props which rested -on the templates. As the tunnel was lined with concrete, the timbering -was left in place and it was built outside the line of the extrados of -the concrete lining. Timbering was only used for a short distance but it -necessitated a larger amount of rock excavation when it was required. - -[Illustration: FIG. 60.--Timbering Used in the Murray Hill Tunnel.] - - -=Hauling.=--Great efficiency was shown in the method of hauling away the -excavated materials. Three narrow-gauge parallel tracks were laid on the -floor of the tunnel and extended to the faces of the advance drifts. -Small flat cars were run on these tracks. They carried steel boxes, 5 -ft. square and 15 ins. deep, fitted with three lifting rings and chains. -When filled, the cars were run to the bottom of the shaft, the boxes -were hoisted by a stiff-legged derrick placed at the shaft head, and the -débris was dumped into storage bins of 300 cu. yds. capacity. These bins -were elevated 8 ft. above the street so that the wagons could be driven -under it to take loads of spoil by means of chutes. The broken rock was -loaded into the boxes by hand. - - -=Concrete Lining.=--The tunnel was lined with concrete which was -manufactured by a quite elaborate plant. A stone crushing plant, -consisting of bins for raw and crushed stone, was erected at the shaft -head and a mixing plant was suspended from the shaft. On the platform of -the shaft head were two bins side by side, one for crushed stone, the -other for sand; both of which communicated, by means of trap doors, with -a hopper chute. The materials from the hopper were delivered into a -measuring box where cement was laid on top of the other ingredients by -hand. They were then conveyed through a canvas chute into a cubical -mixer operated by an engine. The mixer discharged its contents into -skips set on cars at the bottom of the shaft and the concrete was hauled -inside the tunnel ready for use. - -The construction of the lining was accomplished by means of traveling -platforms. The footing courses were laid first. Because these projected -inward about 18 ins. from the faces of the finished sidewalks it was -possible to lay a track rail on their top inner edges on each side of -the tunnel. These track rails carried the traveling platforms. There -were three of these platforms; the forward one was used for building the -side walls; the center one, for carrying a derrick; the last one, for -building the roof arch. The side wall platform was mounted on six -wheels. On each side there was mounted an adjustable lagging which was -curved to conform to the inside profile of the side wall. In operation -this platform was run to the point where the side walls were to be -constructed and the lagging was adjusted to position and fastened. Skips -of concrete were then hoisted on its top, their contents were shoveled -into the space between the lagging and the wall of the excavation and -were there rammed into place until the finished concrete had reached the -top of the lagging. When the concrete had set, the wedges holding the -lagging in place were loosened and the platform was moved ahead and -adjusted for building a new section of wall. The derrick platform was -23¹⁄₂ ft. wide and 18 ft. long. Transversely, it had three bays, two of -which were floored over and one was left without flooring to allow -passage for the concrete skips to and from the cars, on the tunnel floor -beneath. At the center of the floored area was mounted a derrick to -handle the skips. In operation, the derrick platform came between the -side wall platform ahead and the roof platform behind. The construction -of the roof platform was practically the same as the side wall platform -with the addition of roof arch centers at each bent on which lagging -could be placed. The mode of procedure was to erect the form for a small -space between the side walls already built and the haunches of the -center, to shovel concrete from the skips and to run it into place. Then -the roof lagging, a part at a time, was placed upward from the haunches -and the concrete was filled and rammed behind it. The lining was built -from the haunches upward until the two sides approached within a -distance of about 5 ft. from each other at the crown. This 5 ft. crown -strip or key was built by working from the rear toward the front end of -the platform. - - -=Plant.=--The plant used by the contractors for Section IV. of the -subway comprised a central power plant located about 4000 ft. from the -work. This was on 42nd Street near the East River and furnished power -for the work on both Sections IV. and V. The buildings consisted of an -engine room 63 × 30 ft. and a boiler room, 42 × 28 ft. In the former -room was located one Rand-Corliss air compressor, 22 × 40 × 48 ins., -having a capacity of 5000 cu. ft. of free air per minute; in the latter -room there were two 200 H.P. water tube boilers. There were also the -necessary equipment of feed water pump, air condenser pump, etc. The -compressors discharged into a 20 × 5¹⁄₂ ft. receiver of riveted steel -through a 7 in. pipe. The air from the receiver was carried by a 10 in. -pipe 3.277 ft. to the corner of Park Avenue and 41st Street, and was -thence run south along Park Avenue in an 8 in. pipe, from which 3 in. -branches led to the four headings of the work. - - -=Ventilation.=--The ventilation of the tunnel caused very little -trouble. In cool weather the natural draft of the shafts and the air -discharged from the drills served to keep the atmosphere wholesome. In -warm weather, artificial means were necessary to clear the workings of -foul air, particularly after blasting. They comprised at each end a 4 -ft. American exhaust fan drawing air from a 12 in. riveted galvanized -iron pipe, which extended to the working faces. - - -=Illumination.=--The tunnel was lighted by electric lamps which extended -even to the working face. During the blasting, however, all the lamps -and wires within 100 ft. from the front were removed and gasoline -torches were used; they were also employed before the electric lamps and -wires could be replaced, to light the tunnel during the operation of -clearing the débris. - - - - -CHAPTER XI. - -TUNNELS THROUGH HARD ROCK (Continued).--EXCAVATION BY HEADINGS. - - -EUROPEAN AND AMERICAN METHODS. - -The more common method of tunneling through hard rock is to begin the -work by a heading, instead of by a drift. This heading may be of small -dimensions, and the remainder of the section may also be removed in -successive small parts, or it may be the full width of the section, and -the enlargement of the section be made in one other cut. - -[Illustration: FIG. 61.--Diagram Showing Sequence of Excavation in -Heading Method of Tunneling Rock.] - - -=General Discussion.=--When the tunnel is excavated by means of several -cuts, which is the method usually employed in Europe, the sequence of -work is as indicated by Fig. 61. Work is begun by driving the center top -heading No. 1, whose floor is at the level of the bottom of the roof -arch, and which is usually excavated by the circular cut method. This -heading is widened by removing parts Nos. 2 and 3 until the top part of -the section is removed, then the roof arch is built with its feet -resting on the unexcavated rock below. The lower portion of the section -or bench is removed by first sinking the trench No. 4, after which part -No. 5 is taken out, and then parts Nos. 6 and 7, and the side walls -built. Part No. 8 for the culvert is finally opened. The heading is, as -a rule, driven far in advance, but the excavation of each of the other -parts follows the preceding one at a distance behind of about 300 ft. - -The strutting, when any is required, is usually the typical radial -strutting of the Belgian method of tunneling. The masonry lining is -constructed practically the same as in tunnels excavated by a drift. The -hauling is done on a single track laid in the heading No. 1, which -separates into double tracks where the full top section has been -excavated by the removal of parts No. 2. These two tracks are again -combined and form a single track along the top of part No. 5, which has -been left wider than part No. 4 for this particular purpose. When part -No. 3 is excavated a standard-gauge track is laid on its floor; and as -the full section of the tunnel is completed by taking out parts Nos. 4 -and 5, this single track is replaced by two standard-gauge tracks, into -which it switches. Spoil is transferred from the narrow-gauge tracks on -the upper level, to the standard-gauge tracks on the tunnel floor, by -means of chutes, and building material is transferred in the opposite -direction by means of hoisting apparatus. - -When the excavation is made by a single wide heading, and a single other -cut for removing the bench, which is the method preferred by American -engineers, it is called the Heading and Bench method. The work begins by -removing a top heading the full width of the section; this heading is -usually made 7 ft. or 8 ft. high, and is excavated by the center cut -method. The method of strutting usually employed is to erect successive -three- or five-segment timber arches, whose feet rest on the top of the -bench; when the bench is removed, posts are inserted under the feet of -each arch. These arches are covered with a lagging of plank. In America -it has often been the practice to let this strutting serve as a -temporary lining, and to replace it only after some time, often after -years, with a permanent lining of masonry. In a succeeding chapter, some -of the methods adopted in relining timber-lined arches with masonry are -described. The hauling is done by either narrow or broad gauge tracks -laid on the floor of the completed section below. A device called a -bench carriage is often employed to enable the cars running on the -heading tracks to dump their loads into the cars below, without -interfering with the work on the bench front. This device consists of a -wide platform carried on trucks, running on rails at the sides of the -tunnel floor, so that it is level with the floor of the heading. The -front of this platform carries a hinged leaf which may be raised and -lowered, and which forms a sort of gang-plank reaching to the floor of -the heading. By running the heading cars out on to this traveling -platform, they can be dumped into the cars below entirely clear of the -work in progress on the bench front. - -For the purpose of illustrating the two methods of driving tunnels by a -heading, which have been briefly described, the St. Gothard and the Fort -George tunnels have been selected. The St. Gothard tunnel is selected, -as being one of the longest tunnels in the world, and because it was -excavated by a number of small parts; and the Fort George tunnel, as -being a double-track tunnel, driven by a heading, and bench, and having -a concrete lining. - - -ST. GOTHARD TUNNEL. - -The St. Gothard tunnel penetrates the Alps between Italy and France, and -is 9¹⁄₄ miles long. It was constructed in 1872-82. - - -=Material Penetrated.=--The St. Gothard tunnel was excavated through -rock, consisting chiefly of gneiss, mica-schist, serpentine, and -hornblende, the strata having an inclination of from 45° to 90°. At many -points the rock was fissured, and disintegrated easily, and water was -encountered in large quantities, causing much trouble. - - -=Excavation.=--The sequence of excavation is shown by Fig. 14, p. 36. -First the top center heading, No. 1, whose dimensions varied from 8.25 × -8.6 ft. to 8.5 × 9 ft., according to the quality of the rock, was driven -never less than 1000 ft. and sometimes over 3000 ft. in advance of parts -No. 2. The excavation of parts No. 2 opened up the full top section, and -parts Nos. 3, 4, 5, 6, and 7, were removed in the order numbered. - - -=Strutting.=--Where regular strutting was required, the construction -shown in Fig. 62 was adopted. - - -=Masonry.=--The St. Gothard tunnel is lined throughout with masonry. -After the upper portion of the section was fully excavated, the roof -arch was built with its feet resting upon short planks on the top of the -bench. Plank centers were used in constructing the arch. For the arch -brick masonry was employed, but the side walls were built of rubble -masonry. Shelter niches, about 3 ft. deep, were built into the side -walls at intervals, and about every 3,000 ft. storage niches about 10 -ft. deep, and closed with a door, were constructed. The culvert was of -brick masonry. - - -=Mechanical Installation.=--Water-power was used exclusively in driving -the St. Gothard tunnel. At the north end, the Reuss, and at the south -end, the Tessin and the Tremola, rivers or torrents were dammed, and -their waters conducted to turbine plants at the opposite ends of the -tunnel. The power thus furnished by the Reuss was about 1,500 H.P., and -the power furnished by the combined supply of the Tessin and Tremola was -1,220 H.P. The turbine plant at both ends at first consisted of four -horizontal impulse turbines, but later, two more turbines were added at -the south end. Each of the two sets of four turbines first installed -drove five groups of three compressors each, and the two supplementary -turbines drove two groups of four compressors each. The compressors were -of the Colladon type with water injection, and four groups of three -compressors each were capable of furnishing 1,000 cu. yds. of air -compressed to between seven and eight atmospheres every hour, or about -100 H.P. per hour, delivered to the drills at the front. This air when -exhausted provided about 8,000 cu. yds. of fresh air per hour for -ventilation. - -The compressors at each entrance discharged into a group of four -cylindrical receivers of wrought-iron each 5.3 ft. in diameter by 29.5 -ft. long, and having a capacity of 593 cu. ft. The cylinders were placed -horizontally, the first one receiving the air at one end and discharging -it at the other end into the next cylinder, and so on. By this -arrangement the air was drained of its moisture, and the discharge from -the end receiver into the tunnel delivery pipes was not affected by the -pulsations of the compressors. The delivery pipe decreased from 8 in. in -diameter at the receiver to 4 ins. in diameter, and finally to 2¹⁄₂ ins. -in diameter, at the front. - -The drills employed were of various patterns. The first one employed was -the Dubois & François “perforator,” in which the drill-bit was fed -forward by hand. This was replaced by Ferroux drills having an automatic -feed. Jules McKean’s “perforator” was employed at the north end of the -tunnel. All of these drills were of the percussion type, and were -mounted on carriages running on tracks. Their comparative efficiency was -officially tested in drilling granitic gneiss with an operating air -pressure of 5.5 atmospheres with the following results: - - NAME OF DRILL. PENETRATION - INS. PER MIN. - - Ferroux 1.6 - McKean 1.4 - Dubois & François 1.04 - Soummelier 0.85 - -The heading was excavated by the circular cut method, the holes being -driven as follows: Near the center of the heading three holes were first -drilled, converging so as to inclose a pyramid with a triangular base. -Around these center holes from 9 to 13 others were driven parallel to -the tunnel axis. The center holes were blasted first, and then the -surrounding holes. From 3 to 5 hours were required to drill the two sets -of holes, and from three to four hours were required to remove the -blasted rock. The number of holes drilled in removing each of the -various parts was as follows: - - Part No. 1 6 to 9 - Part No. 2 6 to 10 - Part No. 3 2 - Part No. 4 6 to 9 - Part No. 5 3 - Part No. 6 6 to 9 - Part No. 7 1 - -------- - Total for full section 36 to 40 - - -=Hauling.=--Two different systems were employed for hauling the spoil -and construction material in the St. Gothard tunnel. To remove the spoil -from parts Nos. 1 and 2 a narrow-gauge track was laid on the floor of -the heading, and the cars were hauled by horses, the grade being -descending from the fronts. These narrow-gauge cars were dumped into -larger broad-gauge cars running on the track laid on the floor of the -completed section and hauled by compressed air locomotives (Fig. 63). To -raise the incoming structural material from the broad-gauge cars to the -narrow-gauge cars running on the level above, hoisting devices were -employed. - -[Illustration: FIG. 62.--Method of Strutting Roof, St. Gothard Tunnel.] - -[Illustration: FIG. 63.--Sketch Showing Arrangement of Car Tracks, St. -Gothard Tunnel.] - - -FORT GEORGE TUNNEL.[10] - -From a point north of 157th Street and Broadway almost to Dyckman -Street, that is, a distance of nearly two miles, the New York Subway -passes under an elevation known as Fort Washington Heights, which almost -bounds Manhattan Island at its upper end near the Harlem Ship Canal. -Under this elevation the rapid transit railroad was constructed in -tunnel. The tunnel was driven from two intermediate shafts over 110 ft. -deep, located one at 169th Street and the other at 181st Street and -Broadway. Both shafts were sunk at one side of the center line of the -tunnel. After these shafts had been utilized for working purposes during -the construction of the tunnel, they were equipped with electric -elevators to carry passengers from the streets to the deep station. - - [10] Condensed from a paper by Stephen W. Hopkins in _Harvard - Engineering Journal_, April, ’08. - - -=Material.=--The material encountered in the excavation of the Fort -George tunnel was the usual mica schist met everywhere on Manhattan -Island. It was full of seams with strata running in every direction to -such an extent that at many points the roof of the tunnel had to be -supported by timbers; at other parts along the line the rock was so -disintegrated that it was considered a very loose and treacherous soil. -Two serious accidents, each accompanied by loss of life, occurred during -the construction of this tunnel. Both of them were caused by the sudden -fall of a large ledge of rock which, after the tunnel had been excavated -to the full section, remained hanging on the roof, deprived of any -support and held in place by the little cohesion of the material packing -the seams. - - -=Excavation.=--The tunnel was excavated by the heading method in only -two cuts, viz., the heading and bench as indicated in the Fig. 65. The -heading, almost as wide as the upper portion of the tunnel section, was -excavated in the manner explained on page 91. After the heading was -removed, the enlargement of the entire upper section of the tunnel was -accomplished by driving three inclined holes at each side of the -heading. They were driven at different depths and inclinations, as shown -in the figure and were called trimming holes. At the same time the bench -was removed by means of five holes--three vertical and two inclined. The -line of subgrade was reached by means of five grading holes driven -almost horizontal with a slight inclination downward. The air drills for -the heading were mounted on columns, all the others on tripods. The -blasting was done in the following order: the grading holes were blasted -in the first round, the bench and trimming in the second, the center cut -of the heading in the third, the sides in the fourth and the dry holes -in the last. Thus each advance of 7 ft. of the whole tunnel section was -made by means of forty holes fired in five rounds which consumed 277 -lbs. of dynamite with an average additional quantity of 76 lbs., making -a total of 353 lbs. With the exception of the center cut, where 60% -dynamite was used, all the other holes were discharged with 40% -dynamite. - -[Illustration: ~Cross Section.~ - -~Longitudinal Section.~ - -FIG. 64.--Arrangement of Drill Holes in the Fort George Tunnel. - -FIG. 65.--Longitudinal Section of the Heading and Bench Excavation at -the Fort George Tunnel.] - - -=Strutting.=--When the rock was of such a character as to be dangerous -and required permanent timber support, until the masonry lining was in -place, the method employed was as follows: a top heading was first -excavated about 10 ft. deep and from 10 ft. to 12 ft. wide for some -distance, 100 ft. to 500 ft., the dangerous rock being supported by 10 × -10 in. yellow pine plumb or raking posts and sometimes by timber bents -(“caps and legs”). The next process was to widen the heading to the full -width of 30 ft. for a length of about 20 ft., placing timber supports -under the dangerous rock as the widening-out progressed. The excavation -was deepened a little at the sides to 9.5 ft. below the roof grade -(ordered line of excavation) or about 11 ft. below the roof grade, which -was necessary when segmental timbering was to be used, to allow for -placing a 12 × 12 in. “wall plate” (timber sill) along each side. These -wall plates, generally 20 ft. long, were set to the correct elevation -and were leveled by blocking and wedging. As soon as the wall plates -were set, the work of erecting the segmental timber sets, one set at a -time, was begun by starting from the wall plates and supporting the -timber on scaffolding until keyed in, then it was blocked up to the rock -at each joint and at other necessary points. When two or more sets were -erected, lagging, made of boards 2 ins. thick by 6 to 10 ins. wide, was -placed over the segmental timber “sets” and the space above the timber -dry packed with small stone placed by hand. Sometimes there was enough -room between the timber and the rock to do all the dry packing after the -full number of sets, generally six, had been placed on the two wall -plates. The temporary timber posts and braces were taken out as the -segmental timber sets were erected. - -The seven timbers that made up a timber set were of yellow pine each -10 × 10 ins., 5 ft. 2 ins. long at the intrados and 5 ft. 6 ins. at the -extrados. The sets were spaced from 3 ft. to 5 ft. apart, but generally -3.5 ft. and braced to each other at each joint of the segmental timbers -by 6 × 8 in. spreaders which were wedged against the joint splices. - -When the timbers were all erected on a set of wall plates (20 ft.) and -the lagging and dry packing were completed the work of taking out the -bench, which had been partly drilled as the timber sets were erected, -was resumed. The face of the bench, which had been left about 4 ft. from -the end of the previous set of wall plates, was brought forward slowly -by placing 10 × 10 in. plumb posts which extended below subgrade under -the wall plates. These posts were generally spaced the same as the -timber sets above and directly under them. - -When the face of the bench had been brought to within 3 or 4 ft. of the -forward end of the wall plate, the process of widening out and timbering -another 20 ft. length of heading was begun. In some places the rock, -though needing permanent support, was such that the work of taking out -the bench and widening the heading was carried on simultaneously without -increasing the danger; but the greater portion of the work, when -strutting was required, was done as has been described. - -[Illustration: FIG. 66.--Diagram Showing the Arrangement of Drill Holes -in the Heading and Bench of the Gallitsin Tunnel.] - - -=Hauling.=--The excavated material was loaded at the foot of the bench -in dump cars which were run by mule power to the portal or the shaft -according to location, on 36 in. gauge-service tracks. Inclines at 159th -Street were graded from the portal at 158th Street to the street -surface. The cars were formed at this portal into a train and were taken -up the incline to the dump at 162nd Street and the North River by -construction locomotives. At the 168th Street and 181st Street shafts, -the cars were hoisted to the surface in cages (elevators). In the former -case, they were taken to the dump at 165th Street and the North River by -mules and gravity; in the latter case, to various dumps by teams. At -both shafts, stone crushers were located, therefore a great part of the -material did not have to be hauled to the dumps or even taken to the -surface as a great deal of stone was used in dry packing over the -concrete arch. The material from the portal at Fort George was hauled by -mules directly to the dump near by. - - -=Lining.=--The entire tunnel was lined with concrete, consisting of a -floor 4 ins. thick and vertical side walls 18 ins. thick and 25 ft. -apart, which carried a semicircular arch 18 ins. thick except in the -timbered portions where the thickness was increased to 21 ins. and to 24 -and 27 ins. in some places. The springing line of the arch is 6 ft. 2 -ins. above the concrete floor (5 ft. 6 ins. above the base of rail), -hence the maximum clearance above the base of rail is 18 ft. The side -walls and arch were built solid of rock to a height of 8 ft. above -springing line and the space above that point between the concrete and -the rock was packed by hand with small stones. The concrete of the arch -was laid on timber centers erected for that purpose. - -The heading and bench method of excavating rock tunnels is not always -followed in the manner just described but is employed with slight -modifications. There is a large variety of modifications but only the -two most commonly used in practical works are given here. The heading -and bench method illustrated in Fig. 66 was used, among others, on the -Gallitsin tunnel along the Pennsylvania R.R. at the summit of the -Alleghenies near Altoona, Pa., and more recently in the tunnels -constructed by the same company under Bergen Hill, N. J., for the -entrance to New York City. The shape of the cross-section of these -tunnels was semicircular arch on vertical side walls. The excavation was -made in three consecutive cuts, viz., the heading marked 1 in the -figure, the top bench 2, and the lower bench 3. A heading 7 ft. high and -10 ft. wide was attached near the crown of the arch and the rock was -removed by means of a center cut and parallel side holes, the number of -holes depending upon the consistency of the rock. The part No. 2 was -excavated by drilling holes at each side to different depths and at -different inclinations in order to reach the line of the profile as well -as the springing line of the proposed tunnel. The central part of the -top bench was excavated by means of holes driven vertically from the -floor of the heading. The bottom bench No. 3, included between the -springing line of the arch and subgrade, was removed by means of five -vertical holes driven from the floor of the top bench. The three -different working parts were kept nearly 10 ft. apart. Blasting was -effected in reversed order to the figures marked in the diagram, viz., -the bottom bench first and the heading last. - -[Illustration: FIG. 67.--Diagram Showing a Modification of the Heading -and Bench Method.] - -Still another modification of the heading and bench method, commonly -followed by American engineers, is the one shown in Fig. 67. This -consists in dividing the tunnel section in three parts by horizontal -lines. The resultant parts are first the heading excavated close to the -roof, and as wide as the whole section of the tunnel; second, the top -bench in the middle, and lastly the bottom bench excavated to the depth -of the proposed tunnel floor. The excavation proceeds in the numerical -order, beginning at the heading which was excavated, as usual, by means -of a center cut and side holes to the full width of the proposed tunnel. -First the top bench, then the bottom bench, are removed by means of -vertical holes driven from the floor of the heading and the floor of the -top bench, respectively. - - -COMPARISON OF METHODS. - -The differences between the drift and heading methods of excavating -tunnels through rock, consist chiefly in the excavations, strutting, and -hauling. When the drift method is employed an advanced gallery is opened -along the floor of the tunnel before the upper part of the section is -removed, and when the heading method is employed the upper part of the -section is completely excavated before any part of the section below is -excavated. When the drift method of driving is employed polygonal -strutting is usually used, and longitudinal strutting is employed with -the heading method of driving. In the drift method the hauling is done -by one system of tracks at the same level, while in the heading method -two systems of tracks are employed at different levels. - -It is, perhaps, impossible to state without qualification which method -is the better. European engineers who have been connected with both the -Mont Cenis and St. Gothard tunnels, driven by the drift and heading -methods respectively, had the opportunity to practically observe the -advantages and disadvantages of these two methods. Their conclusion was -that the drift method was more convenient for tunnels driven through -hard and compact rock, and that the heading method was better for -tunnels of fissured and disintegrated rocks. To prove this opinion, -experiments were made in one of the tunnels approaching the great St. -Gothard tunnel. On a short tunnel the excavation was made by the drift -method from one portal, while at the other, the heading method was -followed. Although the general rule was fully confirmed still the -conditions at the portals were not identical. More conclusive -experiments were made by Mr. Ira A. Shaler, the contractor for Section -IV., of New York Rapid Transit Railway. He had the opportunity of -driving two parallel tunnels under Murray Hill only 17 ft. apart. The -eastern tunnel was driven by the drift method, the western one by the -heading method. After the work had proceeded for a few months, Mr. -Shaler stated that in his case the drift method was more convenient. He -could spare drilling several holes at each advance, thus obtaining -economy in time, labor and material without considering the advantage of -a simpler transportation of the débris. He promised to publish his -results for the benefit of the profession, but, unfortunately, lost his -life in an accident in the tunnel before the completion of the work. - -An advantage that the drift method affords in long tunnels is, that the -water, which is usually found in large quantities under high mountains, -is easily collected in the drift and conveyed to the culvert, while in -the heading method the water from the advance gallery, before being -collected into the culvert built on the floor of the tunnel, must pass -through all the workings. This may be a serious inconvenience when water -is found in large quantities, as, for instance, was the case in the St. -Gothard tunnel, where the stream amounted to 57 gallons per second. - - - - -CHAPTER XII. - -EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL DISCUSSION; THE BELGIAN -METHOD. - - -GENERAL DISCUSSION. - -It may be set down as a general truth that the excavation of tunnels -through soft ground is the most difficult task which confronts the -tunnel engineer. Under the general term of soft ground, however, a great -variety of materials is included, beginning with stratified soft rock -and the most stable sands and clays, and ending with laminated clay of -the worst character. From this it is evident that certain kinds of -soft-ground tunneling may be less difficult than the tunneling of rock, -and that other kinds may present almost insurmountable difficulties. -Classing both the easy and the difficult materials together, however, -the accuracy of the statement first made holds good in a general way. -Whatever the opinion may be in regard to this point, however, there is -no chance for dispute in the statement that the difficulty of tunneling -the softer and more treacherous clays, peats, and sands is greater than -that of tunneling firm soils and rock; and if we describe the methods -which are used successfully in tunneling very unstable materials, no -difficulty need be experienced in modifying them to handle stable -materials. - - -=Characteristics of Soft-Ground Tunneling.=--The principal -characteristics which distinguish soft-ground tunneling are, first, that -the material is excavated without the use of explosives, and second, -that the excavation has to be strutted practically as fast as it is -completed. In treacherous soils the excavation also presents other -characteristic phenomena: The material forming the walls of the -excavation tends to cave and slide. This tendency may develop -immediately upon excavation, or it may be of slower growth, due to -weathering and other natural causes. In either case the roof of the -excavations tends to fall, the sides tend to cave inward and squeeze -together, and the bottom tends to bulge or swell upward. In materials of -very unstable character these movements exert enormous pressures upon -the timbering or strutting, and in especially bad cases may destroy and -crush the strutting completely. Outside the tunnel the surface of the -ground above sinks for a considerable distance on each side of the line -of the tunnel. - - -=Methods of Soft-Ground Tunneling.=--There are a variety of methods of -tunneling through soft ground. Some of these, like the quicksand method -and the shield method, differ in character entirely, while in others, -like the Belgian, German, English, Austrian, and Italian methods, the -difference consists simply in the different order in which the drifts -and headings are driven, in the difference in the number and size of -these advance galleries, and in the different forms of strutting -framework employed. In this book the shield method is considered -individually; but the description of the Belgian, German, English, -Austrian, Italian, and quicksand methods are grouped together in this -and the three succeeding chapters to permit of easy comparison. - - -THE BELGIAN METHOD OF TUNNELING THROUGH SOFT GROUND. - -[Illustration: FIGS. 68 and 68A.--Diagrams Showing Sequence of -Excavations in the Belgian Method.] - -The Belgian method of tunneling through soft ground was first employed -in 1828 in excavating the Charleroy tunnel of the Brussels-Charleroy -Canal in Belgium, and it takes its name from the country in which it -originated. The distinctive characteristic of the method is the -construction of the roof arch before the side walls and invert are -built. The excavation, therefore, begins with the driving of a top -center heading which is enlarged until the whole of the section above -the springing lines of the arch is opened. Various modifications of the -method have been developed, and some of the more important of these will -be described farther on, but we shall begin its consideration here by -describing first the original and usual mode of procedure. - - -=Excavation.=--Fig. 68 is the excavation diagram of the Belgian method -of tunneling. The excavation is begun by opening the center top heading -No. 1, which is carried ahead a greater or less distance, depending upon -the nature of the soil, and is immediately strutted. This heading is -then deepened by excavating part No. 2, to a depth corresponding to the -springing lines of the roof arch. The next step is to remove the two -side sections No. 3, by attacking them at the two fronts and at the -sides with four gangs of excavators. The regularity and efficiency of -the mode of procedure described consist in adopting such dimensions for -these several parts of the section that each will be excavated at the -same rate of speed. When the upper part of the section has been -excavated as described, the roof arch is built, with its feet supported -by the unexcavated earth below. This portion of the section is excavated -by taking out first the central trench No. 4 to the depth of the bottom -of the tunnel, and then by removing the two side parts No. 5. As these -side parts No. 5 have to support the arch, they have to be excavated in -such a way as not to endanger it. At intervals along the central trench -No. 4, transverse or side trenches about 2 ft. wide are excavated on -both sides, and struts are inserted to support the masonry previously -supported by the earth which has been removed. The next step is to widen -these side trenches, and insert struts until all of the material in -parts No. 5 is taken out. - -When the material penetrated is firm enough to permit, the plan of -excavation illustrated by the diagram, Fig. 68A, is substituted for the -more typical one just described. The only difference in the two methods -consists in the plan of excavating the upper part of the profile, which -in the second method consists in driving first the center top heading -No. 1, and then in taking out the remainder of the section above the -springing lines of the arch in one operation, while in the first method -it is done in two operations. The distance ahead of the masonry to which -the various parts can be driven varies from 10 ft. to, in some cases, -100 ft., being very short in treacherous ground, and longer the more -stable the material is. - - -=Strutting.=--The longitudinal method of strutting, with the -poling-boards running transversely of the tunnel, is always employed in -the Belgian method of tunneling. In driving the first center top -heading, pairs of vertical posts carrying a transverse cap-piece are -erected at intervals. On these cap-pieces are carried two longitudinal -bars, which in turn support the saddle planks. As fast as part No. 2, -Fig. 68, is excavated, the vertical posts are replaced by the batter -posts _A_ and _B_, Fig. 69. The excavation of parts No. 3 is begun at -the top, the poling-boards _a_ and _b_ being inserted as the work -progresses. To support the outer ends of these poling-boards, the -longitudinals _X_ and _Y_ are inserted and supported by the batter posts -_C_ and _D_. In exactly the same way the poling-boards _c_ and _d_, the -longitudinals _V_ and _W_, and the struts _E_ and _F_, are placed in -position; and this procedure is repeated until the whole top part of the -section is strutted, as shown by Fig. 69, the cross struts _x_, _y_, -_z_, etc., being inserted to hold the radial struts firmly in position. -The feet of the various radial props rest on the sill _M N_. These -fan-like timber structures are set up at intervals of from 3 ft. to 6 -ft., depending upon the quality of the soil penetrated. - -[Illustration: FIG. 69.--Sketch Showing Radial Roof Strutting, Belgian -Method.] - -[Illustration: FIG. 70.--Sketch Showing Roof Arch Center, Belgian -Method.] - - -=Centers.=--Either plank or trussed centers may be employed in laying -the roof arch in the Belgian method, but the form of center commonly -employed is a trussed center constructed as shown by Fig. 70. It may be -said to consist of a king-post truss carried on top of a modified form -of queen-post truss. The collar-beam and the tie-beam of the queen-post -truss are spaced about 7 ft. apart, and the posts themselves are left -far enough apart to allow the passage of workmen and cars between them. -The tie beam of the king-post truss is clamped to the collar-beam of the -queen-post truss by iron bands. On the rafters of the two trusses are -fastened timbers, with their outer edges cut to the curve of the roof -arch. These centers are set up midway between the fan-like strutting -frames previously described. They are usually built of square timbers. -The tie beams are usually 6 × 6 in., and the struts and posts 4 × 4 in. -timbers. The reason for giving the larger sectional dimensions to the -tie beams, contrary to the usual practice in constructing centers, is -that it has to serve as a sill for distributing the pressure to the -foundation of unexcavated soil which supports the center. Sometimes a -sub-sill is used to support the center upon the soil; and in any case -wedges are employed to carry it, which can be removed for the purpose of -striking the center. After the arch is completed, the centers may be -removed immediately, or may be left in position until the masonry has -thoroughly set. In either case the leading center over which the arch -masonry terminates temporarily is left in position until the next -section of the arch is built. - - -=Masonry.=--The masonry of the roof arch, which is the first part built, -is of necessity begun at the springing lines, and the first course rests -on short lengths of heavy planks. These planks, besides giving an even -surface upon which to begin the masonry, are essential in furnishing a -bearing to the struts inserted to support the arch while the earth below -them, part No. 5, Fig. 68, is being excavated. As the arch masonry -progresses from the springing lines upward, the radial posts of the -strutting are removed, and replaced by short struts resting on the -lagging of the centers, which support the crown bars or longitudinals -until the masonry is in place, when they and the poling-boards are -removed, and the space between the arch masonry and walls of the -excavation is filled with stone or well-rammed earth. - -Considering now the side wall masonry, it will be remembered that in -excavating the part No. 5, Fig. 68, of the section, frequent side -trenches were excavated, and struts inserted to take the weight of the -masonry. These struts are inserted on a batter, with their feet near the -center of the tunnel floor, so that the side wall masonry may be carried -up behind them to a height as near as possible to the springing lines of -the arch. When this is done the struts are removed, and the space -remaining between the top of the partly finished side wall and the arch -is filled in. This leaves the arch supported by alternate lengths or -pillars of unexcavated earth and completed side wall. The next step is -to remove the remaining sections of earth between the sections of side -wall, and fill in the space with masonry. Fig. 71 is a cross-section, -showing the masonry completed for one-half and the inclined props in -position for the other half; and Fig. 72 is a longitudinal section -showing the pillars of unexcavated earth between the consecutive sets of -inclined struts and several other details of the lining, strutting, and -excavating work. - -[Illustration: FIG. 71.--Sketch Showing Method of Underpinning Roof Arch -with the Side Wall Masonry.] - -[Illustration: FIG. 72.--Longitudinal Section Showing Construction by -the Belgian Method.] - -The invert masonry is built after the side walls are completed. This is -regarded as a defect of this method of tunneling, since the lateral -pressures may squeeze the side walls together and distort the arch -before the invert is in place to brace them apart. To prevent as much as -possible the distortion of the arch after the centers are removed, it is -considered good practice to shore the masonry with horizontal beams -having their ends abutting against plank, as shown by Fig. 71. These -horizontal beams should be placed at close intervals, and be supported -at intermediate points by vertical posts, as shown by the illustration. -Since the roof arch rests are for some time supported directly by the -unexcavated earth below, settlement is liable, particularly in working -through soft ground. This fact may not be very important so long as the -settlement is uniform, and is not enough to encroach on the space -necessary for the safe passage of travel. To prevent the latter -possibility the centers are placed from 9 ins. to 15 ins. higher than -their true positions, depending upon the nature of the soil, so that -considerable settlement is possible without any danger of the necessary -cross-section being infringed upon. In conclusion it may be noted that -the lining may be constructed in a series of consecutive rings, or as a -single cylindrical mass. - - -=Hauling.=--Since in this method of tunneling the upper part of the -section is excavated and lined before the excavation of the lower part -is begun, the upper portion is always more advanced than the lower. To -carry away the earth excavated at the front, therefore, an elevation has -to be surmounted; and this is usually done by constructing an inclined -plane rising from the floor of the tunnel to the floor of the heading, -as shown by Fig. 72. This inclined plane has, of course, to be moved -ahead as the work advances, and to permit of this movement with as -little interruption of the other work as possible, two planes are -employed. One is erected at the right-hand side of the section, and -serves to carry the traffic while the left-hand side of the lower -section is being removed some distance ahead and the other plane is -being erected. The inclination given to these planes depends upon the -size of the loads to be hauled, but they should always have as slight a -grade as practicable. Narrow-gauge tracks are laid on these planes and -along the floor of the upper part of the section passing through the -center opening mentioned before as being left in the centers and -strutting. - -In excavating the top center heading there is, of course, another rise -to its floor from the floor of the upper part of the section. Where, as -is usually the case in soft soils, this top heading is not driven very -far in advance, the earth from the front is usually conveyed to the rear -in wheelbarrows, and dumped into the cars standing on the tracks below. -In firm soils, where the heading is driven too far in advance to make -this method of conveyance adequate, tracks are also laid on the floor of -the heading, and an inclined plane is built connecting it with the -tracks on the next level below. In place of these inclined planes, and -also in place of those between the floor of the tunnel and the level -above, some form of hoisting device is sometimes employed to lift the -cars from one level to the other. There are some advantages to this -method in point of economy, but the hoisting-machines are not easily -worked in the darkness, and accidents are likely to occur. - -[Illustration: FIG. 73.--Diagram Showing Sequence of Excavation in -Modified Belgian Method.] - -In the advanced top heading and in the upper part of the section -narrow-gauge tracks are necessarily employed, and these may be continued -along the floor of the finished section, or the permanent broad-gauge -railway tracks may be laid as fast as the full section is completed. In -the former case the permanent tracks are not laid until the entire -tunnel is practically completed; and in the latter case, unless a third -rail is laid, the loads have to be transshipped from the broad- to the -narrow-gauge tracks or _vice versa_. It is the more general practice to -use a third rail rather than to transship every load. - - -=Modifications.=--Considering the extent to which the Belgian method of -tunneling has been employed, it is not surprising that many -modifications of the standard mode of procedure have been developed. The -modification which differs most from the standard form is, perhaps, that -adopted in excavating the Roosebeck tunnel in Germany. This method -preserves the principal characteristic of the Belgian method, which is -the construction of the upper part of the section first; but instead of -building the side walls from the bottom upward, they are built in small -sections from the top downward. The excavation begins by driving the -center top heading No. 1, Fig. 73, whose floor is at the level of the -springing lines of the roof arch, and then the two side parts No. 2 are -excavated, opening up the entire upper portion of the section in which -the roof arch is built, as in the regular Belgian method. The next step -is to excavate part No. 3, shoring up the arch at frequent intervals. -Between these sets of shoring the side walls are built, resting on -planks on the floor of part No. 3, and then the sets of shores are -removed and replaced by masonry. Next part No. 4 is excavated, shored, -and filled with masonry as was part No. 3. In exactly the same way parts -5, 6, 7, and 8 are constructed in the order numbered. To prevent the -distortion of the arch during the side-wall construction it is braced by -horizontal struts, as indicated above in Fig. 71. - - -=Advantages.=--The advantages of the Belgian method of tunneling may be -summarized as follows: (1) The excavation progresses simultaneously at -several points without the different gangs of excavators interfering -with each other, thus securing rapidity and efficiency of work; (2) the -excavation is done by driving a number of drifts or parts of small -section, which are immediately strutted, thus causing the minimum -disturbance of the surrounding material; (3) the roof of the tunnel, -which is the part of the lining exposed to the greatest pressures, is -built first. - -[Illustration: FIG. 74.--Sketch Showing Failure of Roof Arch by Opening -at Crown.] - - -=Disadvantages.=--The disadvantages of the Belgian method of tunneling -may be summarized as follows: (1) The roof arch which rests at first on -compressible soil is liable to sink; (2) before the invert is built -there is danger of the arch and side walls being distorted or sliding -under the lateral pressures; (3) the masonry of the side walls has to be -underpinned to the arch masonry. - - -=Accidents and Repairs.=--One of the most frequent accidents in the -Belgian method of tunneling is the sinking of the roof arch owing to -its unstable foundation on the unexcavated soil of the lower portion of -the section. The amount of settlement may vary from a few inches in firm -soil to over 2 ft. in loose soils. To counteract the effect of this -settlement it is the general practice to build the arch some inches -higher than its normal position. When the settlement is great enough to -infringe seriously upon the tunnel section, repairs have to be made; and -the only way of accomplishing them is to demolish the arch and rebuild -it from the side walls. It is usually considered best not to demolish -the arch until the invert has been placed, so that no further -disturbance is likely to occur once the lining is completed anew. - -The rotation of the arch about its keystone, or the opening of the arch -at the crown, by the squeezing inward of the haunches by the lateral -pressures, is another characteristic accident. Fig. 74 shows the nature -of the distortion produced; the segments of the arch move toward each -other by revolving on the intradosal edges of the keystone, which are -broken away and crushed together with the operation, while the -extradosal edges are opened. It is to prevent this occurrence that the -horizontal struts shown in Fig. 71 are employed. The manner of repairing -this accident differs, depending upon the extent of the injury. When the -intradosal edges of the keystone are but slightly crushed, the repairing -is done as directed by Fig. 75. When the keystone is completely crushed, -however, the indications are that the material of the keystone, usually -brick, is not strong enough to resist the pressures coming upon it, and -it is advisable to substitute a stronger material in the repairs, and a -stone keystone is constructed as shown by Fig. 75. The middle stone of -this keystone extends through the depth of the arch ring, and the two -side stones only half-way through, their purpose being merely to resist -the crushing forces which are greatest at the intrados. Sometimes, when -the pressures are unsymmetrical, the arch ring breaks at the haunches as -well as the crown, as shown by Fig. 75, which also indicates the mode of -repairing. This consists in demolishing the original arch, and -rebuilding it with stone voussoirs inserted in place of the brick in -which the rupture occurred. - -[Illustration: FIG. 75.--Sketches Showing Methods of Repairing Roof Arch -Failures.] - - - - -CHAPTER XIII. - -THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND (Continued); -BALTIMORE BELT LINE TUNNEL. - - -The German method of tunneling was first used in 1803 in constructing -the St. Quentin Canal. In 1837 the Königsdorf tunnel of the Cologne and -Aix la Chapelle R.R. was excavated by the same method. The success of -the method in these two difficult pieces of soft-ground tunneling led to -its extensive adoption throughout Germany, and for this reason it -gradually came to be designated as the German method. Briefly explained -the method consists in excavating first an annular gallery in which the -side walls and roof arch are built complete before taking out the center -core and building the invert. - -[Illustration: FIG. 76.--Diagrams Showing Sequence of Excavation in -German Method of Tunneling.] - - -=Excavation.=--The excavation of tunnels by the German method is begun -either by driving two bottom side drifts or by driving a center top -heading. Fig. 76 shows the mode of procedure when bottom side drifts are -used to start the work. The two side drifts No. 1 are made from 7 ft. to -8 ft. wide, and about one-third the total height of the full section; -the width of each heading has to be sufficient for the construction of -the masonry and strutting, and for the passage of narrow spoil cars -alongside them. These drifts are increased in height to the springing -line of the arch by taking out the two drifts No. 2. Next the top center -heading No. 3 is driven, and finally the two haunch headings No. 4 are -excavated. The center core No. 5 is utilized to support the strutting -until the side walls and roof arch are completed, when it is broken down -and removed. In case of very loose material, where the first side drifts -cannot be carried as high as one-third the height of the section, it is -the common practice to make them about one-fourth the height, and to -take out the side portions of the annular gallery in three parts, as -shown by Fig. 76. - -[Illustration: FIG. 77.--Diagram Showing Sequence of Excavations in -Water Bearing Material, German Method.] - -The top center heading plan of commencing the excavation is usually -employed in firm materials or when a vein of water is encountered in the -upper part of the section. In the latter contingency a small bottom -drift _A_, Fig. 77, is first driven to serve as a drain; but in any case -the excavation proper of the tunnel consists in first driving the center -top heading No. 1, and then by working both ways along the profile -parts, Nos. 2, 3, 4, and 5 are removed. Part No. 6 is left to support -the strutting until the side walls and roof arch are built, when it is -also excavated. - - -=Strutting.=--When the excavation is begun by bottom side drifts these -drifts are strutted by erecting vertical posts close against the sides -of the drift and placing a cap-piece transversely across the roof of the -drift. The side posts are usually supported by sills placed across the -bottom of the drift. These frameworks of posts, cap, and sill are -erected at short intervals, and the roof, and, if necessary, the sides -of the drift between them, are sustained by means of longitudinal -poling-boards extending from one frame to the next. The cap-pieces of -the strutting for the bottom drifts serve as sills for the exactly -similar strutting of the heading next above. To support the additional -weight, and to allow the construction of the side walls, the strutting -of the bottom drifts is strengthened by inserting an intermediate post -between the original side posts of each frame. These intermediate posts -are not inserted at the center of the frames or bents, but close to the -wall masonry line as shown by Fig. 78. This eccentric position of the -post avoids any interference with the hauling, and also allows the -removal of the adjacent side post when the masonry is constructed. - -[Illustration: FIG. 78.--Sketch Showing Work of Excavating and Timbering -Drifts and Headings.] - -[Illustration: FIG. 79.--Sketch Showing Method of Roof Strutting.] - -Two methods of strutting the soffit of the excavation are employed, one -being a modification of the longitudinal system employed in the English -method of tunneling described in a succeeding chapter, and the other a -modification of the Belgian system previously described. Fig. 79 shows -the method of employing the radial strutting of the Belgian system. At -the beginning the center top heading is strutted with rectangular bents -such as are employed for strutting the drifts. As this heading is -enlarged by taking out the haunch sections, radial posts are inserted, -as shown by Fig. 79, which also indicates the method of strutting the -side trenches when the excavation is carried downward from the center -top heading instead of upward from bottom side drifts. - - -=Masonry.=--Whatever plan of excavation or strutting is employed, the -construction of the masonry lining in the German method of tunneling -begins at the foundations of the side walls and is carried upward to the -roof arch. The invert, if one is required, is built after the center -core of earth is removed. - - -=Centering.=--Tunnel centers are generally employed in the German method -of tunneling, a common construction being shown by Fig. 80. It is -essentially a queen-post truss, the tie beam of which rests on a -transverse sill as shown by the illustration. The transverse sill is -supported along its central portion by the unexcavated center core of -earth, and at its ends either directly on the vertical posts or on -longitudinal beams resting on these posts. The diagonal members of the -queen-post truss form the bottom chords of small king-post trusses which -are employed to build out the exterior member of the center to a closer -approximation to the curve of the arch. - -[Illustration: FIG. 80.--Sketch Showing Roof Arch Centers and Arch -Construction.] - - -=Hauling.=--When the bottom side drift plan of excavation is employed, -the spoil from the front of the drift is removed in narrow-gauge cars -running on a track laid as close as practicable to the center core. -These same cars are also employed to take the spoil from the drifts -above, through holes left in the ceiling strutting of the bottom drifts. -The spoil from the soffit sections may be removed by the same car lines -used in excavating the drifts, or a narrow-gauge track may be laid on -the top of the center core for this special purpose. In the latter case -the soffit tracks are usually connected by means of inclined planes -with the tracks on the bottoms of the side drifts. Generally, however, -the separate soffit car line is not used unless the material is of such -a firm character that the headings and drifts can be carried a great -distance ahead of the masonry work. With the center top heading plan of -beginning the excavation, the car track has, of course, to be laid on -the top of the center core. The center core itself is removed by means -of car tracks along the floor of the completed tunnel. - - -=Advantages and Disadvantages.=--Like the Belgian method of tunneling, -the German method has its advantages and disadvantages. Since the -excavation consists at first of a narrow annular gallery only, the -equilibrium of the earth is not greatly disturbed, and the strutting -does not need to be so heavy as in methods where the opening is much -larger. The undisturbed center core also furnishes an excellent support -for the strutting, and for the centers upon which the roof arches are -built. Another important advantage of the method is that the -construction of the masonry lining is begun logically at the bottom, and -progresses upward, and a more homogeneous and stable construction is -possible. The great disadvantage of the method is the small space in -which the hauling has to be done. The spoil cars practically fill the -narrow drifts in passing to and from the front, and interfere greatly -with the work of the carpenters and masons. Another objection to the -method is that the invert is the very last portion of the lining to be -built. This may not be a serious objection in reasonably compact and -stable materials, but in very loose soils there is always the danger of -the side walls being squeezed together before the invert masonry is in -position to hold them apart. Altogether the difficulties are of a -character which tend to increase the expense of the method, and this is -the reason why to-day it is seldom used even in the country where it was -first developed, and for some time extensively employed. For repairing -accidents, such as the caving in of completed tunnels, the German method -of tunneling is frequently used, because of the ease with which the -timbering is accomplished. In such cases the cost of the method used -cuts a small figure, so long as it is safe and expeditious. - - -BALTIMORE BELT LINE TUNNEL. - -In the last few years a modification of the German method was used in -this country for the construction of several railroad tunnels. The -modification consists in excavating the two-side drifts up to the -springing line of the arch of the proposed tunnel. Then a central -heading, which is afterward enlarged to the whole section of the tunnel, -is excavated close to the crown. At the same time the masonry is -constructed from the foundation up in the side drifts. From the floor of -the upper section already excavated and strutted, the top of the masonry -of the drifts is reached by means of small side cuts; thus the lining is -made continuous up to the keystone. The central nucleus or bench is -removed after the tunnel has been lined. - -The most important tunnel excavated by this method was the Baltimore -Belt Line tunnel described as follows: - -The Baltimore Belt Ry. Co. was organized in 1890 by officials of the -Baltimore & Ohio, and Western Maryland railways, and Baltimore -Capitalists, to build 7 miles of double track railway, mostly within the -city limits of Baltimore. This railway was partly open cut and -embankment, and partly tunnel, and its object was to afford the -companies named facilities for reaching the center of the city with -their passengers and freight. To carry out the work the Maryland -Construction Co. was organized by the parties interested, and in -September, 1890, this company let the contract for construction to Ryan -& McDonald of Baltimore, Md. The chief difficulties of the work centered -in the construction of the Howard-street tunnel, 8350 ft. long, running -underneath the principal business section of the city. - - -=Material Penetrated.=--The soil penetrated by the tunnel was of almost -all kinds and consistencies, but was chiefly sand of varying degrees of -fineness penetrated by seams of loam, clay, and gravel. Some of the -clay was so hard and tough that it could not be removed except by -blasting. Rock was also found in a few places. For the most part, -however, the work was through soft ground, furnishing more or less -water, which necessitated unusual precautions to avoid the settling of -the street, and consequent damage to the buildings along the line. A -large quantity of water was encountered. Generally this water could be -removed by drainage and pumps, and the earth be prevented from washing -in by packing the space between the timbering with hay or other -materials. At points where the inflow was greatest, and the earth was -washed in despite the hay packing, the method was adopted of driving -6-in. perforated pipes into the sides of the excavation, and forcing -cement grout through them into the soil to solidify it. These pipes -penetrated the ground about 10 ft., and the method proved very efficient -in preventing the inflow of water. - - -=Excavation.=--The excavation was carried out according to the German -method of tunneling. Bottom side drifts were first driven, and then -heightened to the springing line of the roof arch. Next a center top -heading was driven, and the haunch sections taken out. The object of -beginning the excavations by bottom side drifts, was to drain the soil -of the upper part of the section. The center core was removed after the -side walls and roof arch were completed, its removal being kept from 50 -ft. to 75 ft. to the rear of the advanced heading. The dimensions of the -side drifts proper were about 8 × 8 ft., but they were often carried -down much below the floor level to secure a solid foundation bed for the -side walls. - - -=Strutting.=--The side drifts were strutted by means of frames composed -of two batter posts resting on boards, and having a cap-piece extending -transversely across the roof of the drift. These frames were spaced -about 4 ft. apart. The excavation was advanced in the usual way by -driving poling-boards at the top and sides, with a slight outward and -upward inclination, so that the next frame could be easily inserted -leaving space enough between it and the sheeting to permit the next set -of poling-boards to be inserted. These poling-boards were driven as -close together as practicable so as to prevent as much as possible the -inflow of water and earth. - -[Illustration: FIG. 81.--Sketch Showing Method of Excavating and -Strutting Baltimore Belt Line Tunnel.] - -The center top heading was strutted in the same manner as were the side -drifts. The arrangement of the strutting employed in enlarging the -center top heading is shown clearly by Fig. 81, which also shows the -manner of strutting the side drifts and face of the excavation, and of -building the masonry. - - -=Centers.=--Both wood and iron centers were employed in building the -roof arch. The timber centering was constructed of square timbers, as -shown by Fig. 82. This construction of the iron centers is shown by Fig. -83. Each of the iron centers consisted of two 6 × 6 in. angles butted -together, and bent into the form of an arch rib. Six of these ribs were -set up 4 ft. apart. They were made of two half ribs butted together at -the crown, and were held erect and the proper distance apart by spacing -rods. The rearmost rib was held fast to the completed arch masonry, and -in turn supported the forward ribs while the lagging was being placed. - -[Illustration: FIG. 82.--Roof Arch Construction with Timber Centers, -Baltimore Belt Line Tunnel.] - - -=Masonry.=--The side walls of the lining were built first in the bottom -side drifts, as shown by Fig. 81. They were generally placed on a -foundation of concrete, from 1 ft. to 2 ft. thick. As a rule the side -walls were not built more than 20 ft. in advance of the arch, but -occasionally this distance was increased to as much as 90 ft. The roof -arch consisted ordinarily of five rings of brick, but at some places in -especially unstable soil eight rings of brick were employed. The arch -was built in concentric sections about 18 ft. in length. All the timber -of the strutting above the arch and outside of the side walls was left -in place, and the voids were filled with rubble masonry laid in cement -mortar. It required about 125 mason hours to build an 18-ft. arch -section. Figs. 82 and 83 show various details of the masonry arch work. - -[Illustration: FIG. 83.--Roof Arch Construction with Iron Centers, -Baltimore Belt Line Tunnel.] - -Owing to the very unstable character of the soil, considerable -difficulty was experienced in building the masonry invert. The process -adopted was as follows: Two parallel 12 × 12 in. timbers were first -placed transversely across the tunnel, abutting against longitudinal -timbers or wedges resting against the side walls. Short sheet piles were -then driven into the tunnel bottom outside of these timbers, forming an -inclosure similar to a cofferdam, from which the earth could be -excavated without disturbing the surrounding ground. The earth being -excavated, a layer of concrete 8 ins. thick was placed, and the brick -masonry invert constructed on it. In less stable ground each of the -above described cofferdams was subdivided by transverse timbers and -sheet piling into three smaller cofferdams. Here the masonry of the -middle section was first constructed, and then the side sections built. -Where the ground was worst, still more care was necessary, and the -bottom had to be covered with a sheeting of 1¹⁄₄-in. plank held down by -struts abutting against the large transverse timbers. The invert masonry -was constructed on this sheeting. Refuge niches 9 ft. high, 3 ft. wide, -and 15 ins. deep were built in the side walls. - - -=Accidents.=--In this tunnel, owing to the quick striking of the -centers, it was found that the masonry lining flattened at the crown and -bulged at the sides. This was attributed to the insufficient time -allowed for the mortar to set in the rubble filling. Earth packing was -tried, but gave still worse results. Finally dry rubble filling was -adopted, with satisfactory results. There was necessarily some sinking -of the surface. This resulted partly from the necessity of changing and -removing of the timbers, and from the compression and springing of the -timbers under the great pressures. The crown of the arch also settled -from 2 ins. to 6 ins., due to the compression of the mortar in the -joints. The maximum sinking of the surface of the street over the tunnel -was about 18 ins.; it usually ran from 1 to 12 ins. Some damage was done -to the water and gas mains. This damage was not usually serious, but it -of course necessitated immediate repairs, and in some instances it was -found best to reconstruct the mains for some distance. At one point -along the tunnel where very treacherous material was found, the surface -settlement caused the collapse of an adjacent building, and necessitated -its reconstruction. - - - - -CHAPTER XIV. - -THE FULL SECTION METHOD OF TUNNELING: ENGLISH METHOD; AMERICAN METHOD; -AUSTRIAN METHOD. - - -ENGLISH METHOD. - -The English method of tunneling through soft ground, as its name -implies, originated in England, where, owing to the general prevalence -of comparatively firm chalks, clays, shales, and sandstones, it has -gained unusual popularity. The distinctive characteristics of the method -are the excavation of the full section of the tunnel at once, the use of -longitudinal strutting, and the alternate execution of the masonry work -and excavation. In America the method is generally designated as the -longitudinal bar method, owing to the mode of strutting, which has -gained particular favor in America, and is commonly employed here even -when the mode of excavation is distinctively German or Belgian in other -respects. - -[Illustration: FIG. 84.--Diagram Showing Sequence of Excavation in -English Method of Tunneling.] - - -=Excavation.=--Although, as stated above, the distinctive characteristic -of the English method is the excavation of the full section at once, the -digging is usually started by driving a small heading or drift to locate -and establish the axis of the tunnel, and to facilitate drainage in wet -ground. These advance galleries may be driven either in the upper or in -the lower part of the section, as the local conditions and choice of the -engineer dictate. Whether the advance gallery is located at the top or -at the bottom of the section makes no difference in the mode of -enlarging the profile. This work always begins at the upper part of the -section. A center top heading is driven and strutted by erecting posts -carrying longitudinal bars supporting transverse poling-boards. This -heading is immediately widened by digging away the earth at each side, -and by strutting the opening by temporary posts resting on blocking, and -carrying longitudinal bars supporting poling-boards. This process of -widening is continued in this manner until the full roof section, No. 1, -Fig. 84, is opened, when a heavy transverse sill is laid, and permanent -struts are erected from it to the longitudinal bars, the temporary posts -and blocking being removed. The excavation of part No. 2 then begins by -opening a center trench and widening it on each side, temporary posts -being erected to support the sill above. As soon as part No. 2 is fully -excavated, a second transverse sill is placed below the first, and -struts are placed between them. The excavation of part No. 3 is carried -out in exactly the same manner as was part No. 2. The lengths of the -various sections, Nos. 1, 2, and 3, generally run from 12 ft. to 20 ft., -depending upon the character of the soil. - - -=Strutting.=--The strutting in the English method of tunneling consists -of a transverse framework set close to the face of the excavation, which -supports one end of the longitudinal crown bars, the other ends of which -rest on the completed lining. The transverse framework is composed of -three horizontal sills arranged and supported as shown by Fig. 85. The -bottom sill _A_ is carried by vertical posts resting on blocking on the -floor of the excavation. From the bottom sill vertical struts rise to -support the middle sill _B_. The top sill, or miners’ sill _C_, is -carried by vertical posts or struts rising from the middle sill _B_. The -vertical struts are usually round timbers from 6 ins. to 8 ins. in -diameter; and the sills are square timbers of sufficient section to -carry the vertical loads, and generally made up of two posts -scarf-jointed and butted to permit them to be more easily handled. In -firm soils the struts between the sills are all set vertically, but -those at the extreme sides of the roof section are inclined. In loose -soils, however, where the sides of the excavation must be shored, the -V-bracing shown by Fig. 85 is employed between one or more pairs of -sills as the conditions necessitate. The manner of holding the -transverse framework upright is explained quite clearly by Fig. 85; -inclined props extending from the completed masonry to the sills of the -framework being employed. Two props are used to each sill. Sometimes, in -addition to the props shown, another nearly horizontal prop extends from -the crown of the arch masonry to the middle piece of the strutting. - -[Illustration: FIG. 85.--Sketches Showing Construction of Strutting, -English Method.] - -Referring to Fig. 85, it will be observed that the longitudinal crown -bars are above the extrados of the roof arch. When, therefore, the -lining masonry has been completed close up to the transverse framework, -the latter is removed, leaving the crown bars resting on the arch -masonry; and excavation, which has been stopped while the masonry was -being laid, is continued for another 12 ft. to 20 ft., and the -transverse framework is erected at the face, and braced or propped -against the completed lining as shown by Fig. 85. The next step is to -place the crown bars, and this is done by pulling them ahead from their -original position over the masonry of the completed section of the roof -arch. It will be understood that the crown bars are not pulled ahead -their full length at one operation, but are advanced by successive short -movements as the excavation progresses, their outer ends being supported -by temporary posts until the transverse framework is built at the face -of the excavation. - - -=Centers.=--Two standard forms of centers are employed in the English -method of tunneling, as shown by Figs. 86 and 87. Both consist of an -outer portion, constructed much like a typical plank center, which is -strengthened against distortion by an interior truss framework. The -elemental members of this truss framework take the form of a queen-post -truss, as is shown more particularly by Fig. 86. In Fig. 87 the -queen-post truss construction is less easily distinguished, owing to the -cutting of the bottom tie-beam and other modifications, but it can still -be observed. The possibility of cutting the tie-beam as shown in Fig. -87, without danger, is due to the fact that the lateral pressures on the -haunches of the center counteract the tendency of the center to flatten -under load, which is usually counteracted by the tie-beam alone. The -object of cutting the tie-beam is to afford room for the props running -from the completed masonry to the transverse framework of the strutting -as shown by Fig. 85. - -[Illustration: FIGS. 86 and 87.--Sketches of Typical Timber Roof-Arch -Centers, English Method.] - -Generally four or five centers are used for each length of arch built. -They are set up so that the tie-beams rest on double opposite wedges -carried by a transverse beam below. This transverse beam in turn rests -on another transverse beam which is supported by posts carried on -blocking on the invert masonry. It is usually made with a butted joint -at the middle to permit its removal, since it is so long that the -masonry has to be built around its extreme ends. The lagging is of the -usual form, and rests on the exterior edges of the curved upper member -of the centers. - - -=Masonry.=--In the English method of tunneling, the masonry begins with -the construction of the invert, and proceeds to the crown of the arch. -The lining is built in lengths, or successive rings, corresponding to -the length of excavation, which, as previously stated, is from 12 ft. to -20 ft. Each ring or length of lining terminates close to the transverse -strutting frame erected at the face of the excavation. Work is first -begun on the invert at the point where the preceding ring of masonry -ends, and is continued to the transverse strutting frame at the front of -the excavation. As fast as the invert is completed, work is begun on the -side walls. In very loose soils the longitudinal bars supporting the -sides of the excavation are removed after the side walls are built; but -in firmer soils they may be taken out one by one just ahead of the -masonry, or in very firm soils it may be possible to remove them -entirely before beginning the side walls. In all cases it is necessary -to fill the space between the masonry and the walls of the excavation -with riprap or earth. To build the roof arch the centers are first -erected as described above, and the crown bars are removed as previously -described by pulling them ahead after the arch ring is completed. As -with the side walls, the vacant space between the arch ring and the roof -of the excavation must be filled in. Usually earth or small stones are -used for filling; but in very loose soils it is sometimes the practice -not to remove the poling-boards, but to support them by short brick -pillars resting on the arch ring and then to fill around these pillars. - - -=Hauling.=--To haul away the material and take in supplies, tracks are -laid on the invert masonry. Generally the permanent tracks are laid as -fast as the lining is completed. A short section of temporary track is -used to extend this permanent track close to the work of the advanced -drift. - - -=Advantages and Disadvantages.=--The great advantage of the English -method of tunneling is that the masonry lining is built in one piece -from the foundations to the crown, making possible a strong, homogeneous -construction. It also possesses a decided advantage because of the -simple methods of hauling which are possible: there being no differences -of level to surmount, no hoisting of cars nor trans-shipments of loads -are necessary. The chief disadvantage of the method is that the -excavators and masons work alternately, thus making the progress of the -work slower perhaps than in any other method of tunneling commonly -employed under similar conditions. This disadvantage is overcome to a -considerable extent when the tunnel is excavated by shafts, and the work -at the different headings is so arranged that the masons or excavators -when freed from duty at one heading may be transferred to another where -excavation or lining is to be done as the case may be. Another -disadvantage of the English method arises from the excavation of the -full section at once, which in unstable soils necessitates strong and -careful strutting, and increases the danger of caving. The fact also -that the arch ring has to carry the weight of the crown bars, and their -loading at one end while the masonry is green, increases the chances of -the arch being distorted. - - -=Conclusion.=--The English method of tunneling in its entirety is -confined in actual practice pretty closely to the country from which it -receives its name. A possible extension of its use more generally is -considered by many as likely to follow the development of a successful -excavating machine for soft material. The space afforded by the opening -of the full section at once, especially adapts the method to the use of -excavators like, for example, the endless chain bucket excavator used -on the Central London Ry., and illustrated in Fig. 11. The method also -furnishes an excellent opportunity for electric hauling and lighting -during construction. - -The English method of tunneling has been used in building the Hoosac, -Musconetcong, Allegheny, Baltimore and Potomac, and other tunnels in -America. The names of the European tunnels built by this method are too -numerous to mention here. - - -AMERICAN METHOD. - -In this country tunnels through loose soils are excavated according to -the “Crown Bar” or American Method. This consists in opening the whole -section of the tunnel before the construction of the lining as in the -English Method. It differs from the English method, however, in that -many timber structures are erected for the support of the roof, and that -the excavation and construction of the lining are far apart, so allowing -the miners and the masons to work continuously and without interfering -with each other. - -[Illustration: FIG. 88.--Sequence of Excavation in the American Method.] - -[Illustration: ~Section A-B.~ - -FIG. 89.--Strutting the Heading in the American Method.] - -[Illustration: ~Section C-D.~ - -FIG. 90.--Temporary Timbering of the Roof in the American Method.] - -[Illustration: ~Section E-F.~ - -FIG. 91.--Showing Crown Bars Supported by Segmental Arches.] - - -=Excavation.=--The diagram in Fig. 88 shows the sequence of excavation. -The work begins by driving a central heading usually 7 × 8 ft., strutted -by means of vertical or batter posts and cap-piece. Fig. 89,[11] the -props resting on foot blocks. Between the cap-pieces of the consecutive -frames are placed planks driven upward at a slightly inclined angle. -After the heading has been excavated and strutted, the floor is lowered -by removing the part marked 2 in the figure. The two batter posts -supporting the cap-piece are now substituted by two longer ones resting -on the floor of part 2 and abutting against longitudinal beams which -are inserted underneath the cap-pieces. These longitudinal beams are -called crown bars. The new batter posts are resting either on foot -blocks or sills according to the quality of soil and they are strongly -wedged to the crown bars. On each side of these crown bars are inserted -poling-boards or planks close to each other, which are driven downward. -The part marked 3 in the figure is removed by enlarging the cut 1 × 2 on -both sides. The plank, inserted above the crown bar, is driven in either -preceding or following the excavation and another crown bar is inserted -at the end of this plank. This second crown bar is supported by a prop -whose other end abuts against the foot of the rafter strutting the -heading. Between this crown bar and the roof of the excavation, other -planks are placed transversally to the axis of the tunnel and are driven -in until they are supported by a new crown bar, etc. The various props -supporting the crown bars are placed radially or in a fan-like manner, -similar to the characteristic arrangement of the timbering in the -Belgian method. Bracers to strengthen the timbering and the roof of the -excavation are inserted longitudinally between the various posts and -transversally between the crown bars, Fig. 90. As a rule, only three or -four of these radial structures are temporarily erected. A trench is -excavated at the side of the part marked 3 in the figure to receive the -wall plate which is a heavy timber laid on the floor parallel to the -longitudinal axis of the tunnel. On the wall plates are erected the -arched timber sets composed of five or seven segments of hewn timbers so -as to form a polygonal frame which is wedged to the crown bars and -which will support the arch of the roof. After one of these segmental -timber sets is erected the temporary radial structure is removed and the -upper section of the tunnel is cleared of any obstruction as the -pressures are transferred to the wall plates, Fig. 91. The bench marked -4 in the figure is taken away and the vertical props inserted under the -wall plates, Fig. 92. - - [11] Figs. 89 to 91 are taken from a paper by S. W. Hopkins in - _Harvard Engineering Journal_, April, ’03, on the Fort George tunnel. - -[Illustration: ~Section G-H.~ - -~Longitudinal Section.~ - -FIG. 92.--Transversal and Longitudinal Section of a Tunnel Excavated and -Strutted According to the American Method.] - - -=Strutting.=--The longitudinal strutting is used in connection with the -American method of tunneling. In fact, the strutting consists of a -series of longitudinal bars supporting planks laid transversally to the -axis of the tunnel and abutting against the roof of the excavation. -These crown bars during the excavations and immediately after are -temporarily supported by radial timbers forming almost a fan-like -structure, but this is soon substituted by a permanent one composed of a -polygonal timber frame of five or seven segments which are cut to -dimensions. The batter posts of the heading, the radial posts of the -temporary timber structure and the crown bars are all round timbers from -10 to 12 ins. in diameter. All the other timbers are square edged, the -usual dimensions being 10 × 10 ins. or 12 × 12 ins. with the exception -of the wall plates which are 14 × 14 ins. The dimensions of the various -members of the strutting and the distance apart of the different frames -vary with the quality of the soil. For instance, in ordinary loose -soils the frames are placed between 4 to 6 ft., but in very soft soils -they are erected only 3 or 3¹⁄₂ ft. apart. - -Chiefly in the southwest, in tunnels excavated according to the American -method, the timbering has been left as regular lining and it was only -after many years when this temporary structure had decayed or was burned -down, that the tunnels were lined with masonry. But in many instances -the whole timber structure was left in place even when the tunnel was -lined with masonry immediately after the excavation had been made. This -was usually done when the tunnel was lined with concrete masonry. In -such a case the timbering was left to support the pressures of the roof -while the concrete was plastic and before it hardened. - - -=Centers.=--In the American method the whole section of the tunnel is -open before the construction of the lining, thus the masonry can be -built from the foundations up. The centers are designed so as to support -only the weight of the masonry during its construction and not the -pressures of the tunnel as in the other methods and consequently they -are of light construction. The centers described in the Murray Hill -tunnel, page 123, may be advantageously used in building the concrete -lining in tunnels through loose soils excavated by the American method. - - -=Hauling.=--The excavation of the heading and the upper section of the -tunnel is usually far ahead of the bench, consequently the hauling of -both the débris and the building materials is made at two different -levels, viz., on the bench and on the floor of the tunnel. When the face -of the heading and the excavation of the bench are not more than 50 ft. -apart, the hauling can be conveniently done on the tunnel floor, while -the materials and débris on the upper section of the tunnel are hauled -by wheelbarrows or light cars propelled by handpower. For a greater -distance, however, it is more convenient to use light cars running on -narrow-gauge tracks all through the tunnel. In this case the tracks on -the tunnel floor and on top of the bench are connected by means of an -inclined platform where the cars may ascend and descend without -interfering with the excavation of the bench. Here, as a rule, tunnels -have been excavated in soils considered good, generally through rock, -while loose soils have been encountered only in small sections. The same -method of excavation for whatever material is encountered is certainly -very convenient, as it affords a great regularity in the work; hence its -extensive use. A great disadvantage of this method is the double -strutting, viz., the polygonal and the longitudinal strutting succeeding -each other, whereas one of them could be easily spared. Another defect -is that it requires a larger amount of excavation, in case the strutting -is left in place. - - -AUSTRIAN METHOD. - -The Austrian full-section method of tunneling through soft ground was -first used in constructing the Oberau tunnel on the Leipsic and Dresden -R.R., in Austria in 1837. It consists in excavating the full section and -building up the lining masonry from the foundations as in the English, -but with the important exception that the invert is built last instead -of first in all cases except where the presence of very loose soil -requires its construction first. A still more important difference in -the two methods is that the excavation is carried out in smaller -sections and is continuous in the Austrian method instead of alternating -with the mason work as it does in the English method. - - -=Excavation.=--The excavation in the Austrian method begins by driving -the bottom center drift No. 1, Fig. 93, rising from the floor of the -tunnel section nearly to the height of the springing lines of the roof -arch. When this drift has been driven ahead a distance varying from 12 -ft. to 20 ft. or sometimes more, the excavation of the center top -heading No. 2 is driven for the same distance. The next operation is to -remove part No. 3, thus forming a central passage the full depth of the -tunnel section at the center. This trench is enlarged by removing parts -Nos. 4, 5, 6, 7, and 8 in the order named until the full section is -opened. A modification of this plan of excavation is shown by Fig. 94 -which is used in firm soils. - -[Illustration: FIGS. 93 and 94.--Diagrams Showing Sequence of Excavation -in Austrian Method of Tunneling.] - - -=Strutting.=--Each part of the section is strutted as fast as it is -excavated. The center bottom drift first excavated is strutted by laying -a transverse sill across the floor, raising two side posts from it, and -capping them with a transverse timber having its ends projecting beyond -the side posts and halved as shown by Fig. 95. The top center heading -No. 2, which is next excavated, is strutted by means of two side posts -resting on blocking and carrying a transverse cap as also shown by Fig. -95. Sometimes the side posts in the heading strutting-frames are also -carried on a transverse sill as are those of the bottom drift. This -construction is usually adopted in loose soils. When the sill is -employed, the middle part, No. 3, is strutted by inserting side posts -between the bottom of the top sill and the cap of the frame in the drift -below. When, however, the posts of the top heading frame are carried on -blocking, it is the practice to replace them with long posts rising from -the cap of the bottom drift frame to the cap of the top heading frame. -Further, when the intermediate sill is employed at the bottom level of -the top heading it projects beyond the side posts and has its ends -halved. - -[Illustration: FIGS. 95 to 97.--Sketches Showing Construction of -Strutting, Austrian Method.] - -After the completion of the center trench strutting the next task is to -strut parts Nos. 4 and 5. This is done by continuing the upper sill by -means of a timber having one end halved to join with the projecting end -of the sill in position. This extension timber is shown at _a_, Fig. 96. -The next operation is to place the timber _b_, having one end resting on -the cap-piece of the top heading frame and the other beveled and resting -on the top of the sill _a_ near the end. The timber _b_ is laid tangent -to the curve of the roof arch, and to support it against flexure the -strut _c_ is inserted as shown. To support the thrust of this strut the -additional post _d_ is inserted and the original bottom heading frame is -reinforced as shown. The next step is to insert the strut _e_, and when -this and the previous construction are duplicated on the opposite side -of the tunnel section we have the strutting of the parts Nos. 1 to 5; -inclusive, complete. Part No. 6 is then removed and strutted by -extending the bottom drift cap-piece by a timber similar to timber _a_ -above, and then by inserting a side strut between the outer ends of -these two timbers, as indicated by Fig. 97. As the final parts. Nos. 7 -and 8, are removed, the inclined prop _a_, Fig. 97, is inserted as -shown. When the soil is loose some of the members of the framework are -doubled and additional bracing is introduced as shown by Fig. 97. - -The frames just described are placed at intervals of about 4 ft. along -the excavation, and are braced apart by horizontal struts. Some of the -longitudinal bearing beams, as at _b_, Fig. 97, also extend through two -or three frames, and help to tie them together. Finally, the -longitudinal poling-boards extending from one frame to the next along -the walls of the excavation serve to connect them together. The short -transverse beam _c_, Fig. 90, located just above the floor of the -invert, serves to carry the planking upon which the train car tracks are -laid. Besides the timber strutting peculiar to the Austrian method, the -Rziha iron strutting described in a previous chapter is frequently used -in tunneling by the Austrian process. - -[Illustration: FIG. 98.--Sketch Showing Manner of Constructing the -Lining Masonry, Austrian Method.] - - -=Centers.=--The two forms of centers used in the English method of -tunneling are also used in the Austrian method. One of the methods of -supporting these centers is shown by Fig. 98. The tie-beam of the center -rests on longitudinal timbers carried by the strutting frames and -intermediate props. In single-track tunnels it is the frequent practice -also to carry the ends of the tie-beams in recesses left in the side -wall masonry, with intermediate props inserted to prevent flexure at -the center. When the Rziha iron strutting is employed, it also serves -for the centering upon which the arch masonry is built. - - -=Masonry.=--In the Austrian system of tunneling, the lining is built -from the foundations of the side walls upward to the crown of the roof -arch in lengths in consecutive rings equal to the lengths of the -consecutive openings of the full section, or from 12 ft. to 20 ft. long. -Except in infrequent cases in very loose materials the invert is the -last part of the masonry to be built, since to build it first requires -the removal of the strutting which cannot easily or safely be -accomplished until the side walls and roof arch are completed. As the -side wall foundations are built, however, their interior faces are left -inclined, as shown by Figs. 97 and 98, ready for the insertion of the -invert, and are meanwhile kept from sliding inward by the insertion of -blocking between them and the bottom of the strutting. Fig. 98 shows the -nature of this blocking, and also the manner in which the side wall and -roof arch masonry is carried upward. Finally when the roof arch is keyed -and the centers are struck, the strutting is taken down and the invert -is built. - - -=Advantages and Disadvantages.=--The principal advantages claimed for -the Austrian method of tunneling are: (1) The excavation being conducted -by driving a large number of consecutive small galleries, which are -immediately strutted, there is little disturbance of the surrounding -material; (2) the polygonal type of strutting adopted is easily erected -and of great strength against symmetrical pressures; (3) the masonry, -being built from the foundations up, is a single homogeneous structure, -and is thus better able to withstand dangerous pressures; (4) the -excavation is so conducted that the masons and excavators do not -interfere, and both can work at the same time. The disadvantages which -the method possesses are: (1) The strutting while very strong under -symmetrical pressures, either vertical or lateral, is distorted easily -by unsymmetrical vertical or lateral pressures, and by pressure in the -direction of the axis of the tunnel; (2) the construction of the invert -last exposes the side walls to the danger of being squeezed together, -causing a rotation of the arch of the nature discussed in describing the -Belgian method of tunneling. - - - - -CHAPTER XV. - -SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; QUICKSAND TUNNELING; -PILOT METHOD. - - -ITALIAN METHOD. - -The Italian method of tunneling was first employed in constructing the -Cristina tunnel on the Foggia & Benevento R.R. in Italy. This tunnel -penetrated a laminated clay of the most treacherous character, and after -various other soft-ground methods of tunneling had been tried and had -failed, Mr. Procke, the engineer, devised and used successfully the -method which is now known as the Italian or Cristina method. The Italian -method is essentially a treacherous soil method. It consists in -excavating the bottom half of the section by means of several successive -drifts, and building the invert and side walls; the space is then -refilled and the upper half of the section is excavated, and the -remainder of the side walls and the roof arch are built; finally, the -earth filling in the lower half of the section is re-excavated and the -tunnel completed. The method is an expensive one, but it has proved -remarkably successful in treacherous soils such as those of the Apennine -Mountains, in which some of the most notable Italian tunnels are -located. It is, moreover, a single-track tunnel method, since any soil -which is so treacherous as to warrant its use is too treacherous to -permit an opening to be excavated of sufficient size for a double-track -railway, except by the use of shields. - - -=Excavation.=--The plan of excavation in the Italian method is shown by -the diagram Fig. 99. Work is begun by driving the center bottom heading -No. 1, and this is widened by taking out parts No. 2. Finally part No. 3 -is removed, and the lower half of the section is open. As soon as the -invert and side wall masonry has been built in this excavation, parts -No. 2 are filled in again with earth. The excavation of the center top -heading No. 4 is then begun, and is enlarged by removing the earth of -part No. 5. The faces of this last part are inclined so as to reduce -their tendency to slide, and to permit of a greater number of radial -struts to be placed. Next, parts No. 6 are excavated, and when this is -done the entire section, except for the thin strip No. 7, has been -opened. At the ends of part No. 7 narrow trenches are sunk to reach the -tops of the side walls already constructed in the lower half of the -section. The masonry is then completed for the upper half of the -section, and part No. 7 and the filling in parts No. 2 are removed. The -various drifts and headings and the parts excavated to enlarge them are -seldom excavated more than from 6 ft. to 10 ft. ahead of the lining. - -[Illustration: FIG. 99.--Diagram Showing Sequence of Excavation in -Italian Method of Tunneling.] - -[Illustration: FIG. 100.--Sketch Showing Strutting for Lower Part of -Section.] - - -=Strutting.=--The bottom center drift, which is first driven, is -strutted by means of frames consisting of side posts resting on floor -blocks and carrying a cap-piece. Poling-boards are placed around the -walls, stretching from one frame to the next. As soon as the invert is -sufficiently completed to permit it, the side posts of the strutting -frames are replaced by short struts resting on the invert masonry as -shown by Fig. 100. To permit the old side posts to be removed and the -new shorter ones to be inserted, the cap-piece of the frame is -temporarily supported by inclined props arranged as shown by Fig. 103. -When parts No. 2 are excavated the roof is strutted by inserting the -transverse caps _a_, Fig. 100, the outer ends of which are carried by -the system of struts _b_, _c_, _d_, and _e_. The longitudinal -poling-boards supporting the ceiling and walls are held in place by the -cap _a_ and the side timber _e_. To stiffen the frames longitudinally of -the tunnel, horizontal longitudinal struts are inserted between them. - -The excavation of the upper half of the tunnel section is strutted as in -the Belgian method, with radial struts carrying longitudinal roof bars -and transverse poling-boards. On account of the enormous pressures -developed by the treacherous soils in which only is the Italian method -employed, the radial strutting frames and crown bars must be of great -strength, while the successive frames must be placed at frequent -intervals, usually not more than 3 ft. After the masonry side walls have -been built in the lower part of the excavation, longitudinal planks are -laid against the side posts of the center bottom drift frames, to form -an enclosure for the filling-in of parts No. 2. The object of this -filling is principally to prevent the squeezing-in of the side walls. - -[Illustration: FIGS. 101 and 101A.--Sketches Showing Construction of -Centers, Italian Method.] - - -=Centers.=--Owing to the great pressures to be resisted in the -treacherous soils in which the Italian method is used, the construction -of the centers has to be very strong and rigid. Figs. 101 and 101A show -two common types of center construction used with this method. The -construction shown in Fig. 101 is a strong one where only pressures -normal to the axis of the tunnel have to be withstood, but it is likely -to twist under pressures parallel to the axis of the tunnel. In the -construction shown by Fig. 101A, special provision is made to resist -pressures normal to the plane of the center or twisting pressures, by -the strength of the transverse bracing extending horizontally across the -center. - -[Illustration: FIG. 102.--Sketch Showing Invert and Foundation Masonry, -Italian Method.] - - -=Masonry.=--The construction of the masonry lining begins with the -invert, as indicated by Fig. 100, and is carried up to the roof of parts -No. 2, as already indicated, and is then discontinued until the upper -parts Nos. 4, 5, and 6 are excavated. The next step is to sink side -trenches at the ends of part No. 7, which reach to the top of the -completed side walls. This operation leaves the way clear to finish the -side walls and to construct the roof arch in the ordinary manner of such -work in tunneling. Since this method of tunneling is used only in very -soft ground which yields under load, the usual practice is to construct -the invert and side walls on a continuous foundation course of concrete -as indicated by Fig. 102. The lining is usually built in successive -rings, and the usual precautions are taken with respect to filling in -the voids behind the lining. The thickness of the lining is based upon -the figures for laminated clay of the third variety given in Table II. - - -=Hauling.=--The system of hauling adopted with this method of tunneling -is very simple, since the excavation of the various parts is driven only -from 6 ft. to 10 ft. ahead, and the work progresses slowly to allow for -the construction of the heavy strutting required. To take away the -material from the center bottom drift, narrow-gauge tracks carried by -cross-beams between the side posts above the floor line are employed. -This same narrow-gauge line is employed to take away a portion of parts -No. 2, the remaining portion being left and used for the refilling after -the bottom portion of the lining has been built, as previously -described. The upper half of the section being excavated, as in the -Belgian method, the system of hauling with inclined planes to the tunnel -floor below, which is a characteristic of that method, may be employed. -It is the more usual practice, however, since the excavation is carried -so little a distance ahead and progresses so slowly, to handle the spoil -from the upper part of the section by wheelbarrows which dump it into -the cars running on the tunnel floor below. Hand labor is also used to -raise the construction materials used in building the upper section. The -tracks on the tunnel floor, besides extending to the front of the -advanced bottom center drift, have right and left switches to be -employed in removing the refilling in parts No. 2, the spoil from the -upper part of the section, and the material of part No. 7. Fig. 103 is a -longitudinal section showing the plan of excavation and strutting -adopted with the Italian method. - -[Illustration: FIG. 103.--Sketch Showing Longitudinal Section of a -Tunnel under Construction, Italian Method.] - - -=Modifications.=--It often happens that the filling placed between the -side walls and the planking, which is practically the space comprised by -parts No. 2, is not sufficient to resist the inward pressure of the -walls, and they tip inward. In these cases a common expedient is to -substitute for the earth filling a temporary masonry arch sprung -between the side walls with its feet near the bottom of the walls, and -its crown just below the level of their tops, as shown by Fig. 107. This -construction was employed in the Stazza tunnel in Italy. In this tunnel -the excavation was begun by driving the center drift, No. 1, Fig. 104, -and immediately strutting it as shown by Fig. 105. The other parts, Nos. -2 and 3, completing the lower portion of the section, were then taken -out and strutted. While part No. 2 was being excavated at the bottom, -and the center part of the invert built, the longitudinal crown bars -carrying the roof of the excavation were carried temporarily by the -inclined props shown by Fig. 106. After completing the invert and the -side walls to a height of 2 or 3 ft., a thick masonry arch was sprung -between the side walls, as shown in transverse section by Fig. 107, and -in longitudinal section by Fig. 106. This arch braced the side walls -against tipping inward, and carried short struts to support the crown -bars. The haunches of the arch were also filled in with rammed earth. -The upper half of the section was excavated, strutted, and lined as in -the standard Italian method previously described. When the lining was -completed, the arch inserted between the side walls was broken down and -removed. - -[Illustration: FIG. 104.--Sketch Showing Sequence of Excavation, Stazza -Tunnel.] - -[Illustration: FIG. 105.--Sketch Showing Method of Strutting First -Drift, Stazza Tunnel.] - -[Illustration: FIGS. 106 and 107.--Sketches Showing Temporary Strutting -Arch Construction, Stazza Tunnel.] - - -=Advantages and Disadvantages.=--The great advantage claimed for the -Italian method of tunneling is that it is built in two separate parts, -each of which is separately excavated, strutted, and lined, and thus can -be employed successfully in very treacherous soils. Its chief -disadvantage is its excessive cost, which limits its use to tunnels -through treacherous soils where other methods of timbering cannot be -used. - - -QUICKSAND TUNNELING. - -When an underground stream of water passes with force through a bed of -sand it produces the phenomenon known as quicksand. This phenomenon is -due to the fineness of the particles of sand and to the force of the -water, and its activity is directly proportional to them. When sand is -confined it furnishes a good foundation bed, since it is practically -incompressible. To work successfully in quicksand, therefore, it is -necessary to drain it and to confine the particles of sand so that they -cannot flow away with the water. This observation suggests the mode of -procedure adopted in excavating tunnels through quicksand, which is to -drain the tunnel section by opening a gallery at its bottom to collect -and carry away the water, and to prevent the movement or flowing of the -sand by strutting the sides of the excavation with a tight planking. - -The sand having to be drained and confined as described, the ordinary -methods of soft-ground tunneling must be employed, with the following -modifications: - -(1) The first work to be performed is to open a bottom gallery to drain -the tunnel. This gallery should be lined with boards laid close and -braced sufficiently by interior frames to prevent distortion of the -lining. The interstices or seams between the lining boards should be -packed with straw so as to permit the percolation of water and yet -prevent the movement of the sand. - -(2) As fast as the excavation progresses its walls should be strutted -by planks laid close, and held in position by interior framework; the -seams between the plank should be packed with straw. - -(3) The masonry lining should be built in successive rings, and the work -so arranged that the water seeping in at the sides and roof is collected -and removed from the tunnel immediately. - - -=Excavation.=--The best and most commonly employed method of driving -tunnels through quicksand is a modification of the Belgian method. At -first sight it may appear a hazardous work to support the roof arch, as -is the characteristic of this method, on the unexcavated soil below, -when this soil is quicksand, but if the sand is well confined and -drained the risk is really not very great. Next to the Belgian method -the German method is perhaps the best for tunneling quicksand. In these -comparisons the shield system of tunneling is for the time being left -out of consideration. This method will be described in succeeding -chapters. Whenever any of the systems of tunneling previously described -are employed, the first task is always to open a drainage gallery at the -bottom of the section. - -Assuming the Belgian method is to be the one adopted, the first work is -to drive a center bottom drift, the floor of which is at the level of -the extrados of the invert. This drift is immediately strutted by -successive transverse frames made up of a sill, side posts, and a cap -which support a close plank strutting or lining, with its joints packed -with straw. Between the side posts of each cross-frame, at about the -height of the intrados of the invert, a cross-beam is placed; and on -these cross-beams a plank flooring is laid, which divides the drift -horizontally into two sections, as shown by Fig. 108; the lower section -forming a covered drain for the seepage water, and the upper providing a -passageway for workmen and cars. The bottom drift is driven as far ahead -as practicable, in order to drain the sand for as great a distance in -advance of the work as possible. After the construction of the bottom -drainage drift the excavation proper is begun, as it ordinarily is in -the Belgian method by driving a top center heading, as shown by Fig. -108. This heading is deepened and widened after the manner usual to the -Belgian method, until the top of the section is open down to the -springing lines of the roof arch. To collect the seepage water from the -center top heading it is provided with a center bottom drain constructed -like the drain in the bottom drift, as shown by Fig. 108. When the top -heading is deepened to the level of the springing lines of the roof -arch, its bottom drain is reconstructed at the new level, and serves to -drain the full top section opened for the construction of the roof arch. -This top drain is usually constructed to empty into the drain in the -bottom drift. - -[Illustration: FIG. 108.--Sketch Showing Preliminary Drainage Galleries, -Quicksand Method.] - -[Illustration: FIG. 109.--Sketch Showing Construction of Roof Strutting, -Quicksand Method.] - - -=Strutting.=--The method of strutting the bottom drift has already been -described. For the remainder of the excavation the regular Belgian -method of radial roof strutting-frames is employed, as shown by Fig. -109. Contrary to what might be expected, the number of radial struts -required is not usually greater than would be used in many other soils -besides quicksand. Single-track railway tunnels have been constructed -through quicksand in several instances where the number of radial props -required on each side of the center did not exceed four or five. It is -necessary, however, to place the poling-boards very close together, and -to pack the joints between them to prevent the inflow of the fine sand. -In strutting the lower part of the section it is also necessary to -support the sides with tight planking. This is usually held in place by -longitudinal bars braced by short struts against the inclined props -employed to carry the roof arch when the material on which they -originally rested is removed. This side strutting is shown at the right -hand of Fig. 110. - -[Illustration: FIG. 110.--Sketch Showing Construction of Masonry Lining, -Quicksand Method.] - - -=Masonry.=--As soon as the upper part of the section has been opened the -roof arch is built with its feet resting on planks laid on the -unexcavated material below. This arch is built exactly as in the regular -Belgian method previously described, using the same forms of centers and -the same methods throughout, except that the poling-boards of the -strutting are usually left remaining above the arch masonry. To prevent -the possibility of water percolating through the arch masonry, many -engineers also advise the plastering of the extrados of the arch with a -layer of cement mortar. This plastering is designed to lead the water -along the haunches of the arch and down behind the side walls. In -constructing the masonry below the roof arch the invert is built first, -contrary to the regular Belgian method, and the side walls are carried -up on each side from the invert masonry. Seepage holes are left in the -invert masonry, and also in the side walls just above the intrados of -the invert. At the center of the invert a culvert or drain is -constructed, as shown by Fig. 110, inside the invert masonry. This -culvert is commonly made with an elliptical section with its major axis -horizontal, and having openings at frequent intervals at its top. The -thickness of the lining masonry required in quicksand is shown by Table -II. - - -=Removing the Seepage Water.=--After the tunnel is completed the water -which seeps in through the weep-holes left in the masonry passes out of -the tunnel, following the direction of the descending grades. During -construction, however, special means will have to be provided for -removing the water from the excavation, their character depending upon -the method of excavation and upon the grades of the tunnel bottom. When -the excavation is carried on from the entrances only, unless the tunnel -has a descending grade from the center toward each end, the tunnel floor -in one heading will be below the level of the entrance, or, in other -words, the descending grade will be toward the point where work is going -on, while at the opposite entrance the grade will be descending from the -work. In the latter case the removal of the seepage water is easily -accomplished by means of a drainage channel along the bottom of the -excavation. In the former case the water which drains toward the front -is collected in a sump, and if there is not too great a difference in -level between this sump and the entrance, a siphon may be used to remove -it. Where the siphon cannot be used, pumps are installed to remove the -water. When the tunnel is excavated by shafts the condition of one high -and one low front, as compared with the level at the shaft, is had at -each shaft. Generally, therefore, a sump is constructed at the bottom of -the shaft; the culvert from the high front drains directly to the shaft -sump, while the water from the low-front sump is either siphoned or -pumped to the shaft sump. From the shaft sump the water is forced up the -shaft to the surface by pumps. - - -THE PILOT METHOD. - -The pilot system of tunneling has been successfully employed in -constructing soft-ground sewer tunnels in America by the firm of -Anderson & Barr, which controls the patents. The most important work on -which the system has been employed is the main relief sewer tunnel built -in Brooklyn, N.Y., in 1892. This work comprised 800 ft. of circular -tunnel 15 ft. in diameter, 4400 ft. 14 ft. in diameter, 3200 ft. 12 ft. -in diameter, and 1000 ft. 10 ft. in diameter, or 9400 ft. of tunnel -altogether. The method of construction by the pilot system is as -follows: - -Shafts large enough for the proper conveyance of materials from and into -the tunnel are sunk at such places on the line of work as are most -convenient for the purpose. From these shafts a small tunnel, -technically a pilot, about 6 ft. in diameter, composed of rolled boiler -iron plates riveted to light angle irons on four sides, perforated for -bolts, and bent to the required radius of the pilot, is built into the -central part of the excavation on the axis of the tunnel. This pilot is -generally kept about 30 ft. in advance of the completed excavation, as -shown by Fig. 111. The material around the exterior of the pilot is then -excavated, using the pilot as a support for braces which radiate from it -and secure in position the plates of the outside shell which holds the -sand, gravel, or other material in place until the concentric rings of -brick masonry are built. Ribs of T-iron bent to the radius of the -interior of the brick work, and supported by the braces radiating from -the pilot, are used as centering supports for the masonry. On these ribs -narrow lagging-boards are laid as the construction of the arch proceeds, -the braces holding the shell plates and the superincumbent mass being -removed as the masonry progresses. The key bricks of the arches are -placed in position on ingeniously contrived key-boards, about 12 ins. in -width, which are fitted into rabbeted lagging-boards one after another -as the key bricks are laid in place. After the masonry has been in place -at least twenty-four hours, allowing the cement mortar time to set, the -braces, ribs, and lagging which support it are removed. In the meantime -the excavation, bracing, pilot, and exterior shell have been carried -forward, preparing the way for more masonry. The top plates of the shell -are first placed in position, the material being excavated in advance -and supported by light poling-boards; then the side-plates are butted to -the top and the adjoining side-plates. In the pilot the plates are -united continuously around the perimeter of the circle, while in the -exterior shell the plates are used for about one-third of the perimeter -on top, unless treacherous material is encountered, when the plates are -continued down to the springing lines of the arch. This iron lining is -left in place. The bottom is excavated so as to conform to the exterior -lines of the masonry. The excavation follows so closely to the outer -lines of the normal section of the tunnel that very little loss occurs, -even in bad material; and there is no loss where sufficient bond exists -in the material to hold it in place until the poling-boards are in -position. - -[Illustration: ~Bracing.~ - -~Arch Construction.~ - -~Longitudinal Section.~ - -FIG. 111.--Sketch Showing Pilot Method of Tunneling.] - -In the Brooklyn sewer tunnel work, previously mentioned, the pilot was -built of steel plates ³⁄₈ in. thick, 12 ins. wide, and 37¹⁄₂ ins. long, -rolled to a radius of 3 ft. Steel angles 4 × 4¹⁄₂ ins. were riveted -along all four sides of each plate, and the plates were bolted together -by ³⁄₄-in. machine-bolts. The plates weighed 136 lbs. each, and six of -them were required to make one complete ring 6 ft. in diameter. In -bolting them together, iron shims were placed between the horizontal -joints to form a footing for the wooden braces for the shell, which -radiate from the pilot. The shell plates of the 15-ft. section of the -tunnel were of No. 10 steel 12 ins. wide and 37 ins. long, with steel -angles 2¹⁄₂ × 2¹⁄₂ × ³⁄₈ ins., riveted around the edges the same as for -the pilot, and put together with ⁵⁄₈-in. bolts. These plates weighed 61 -lbs. each, and eighteen of them were required to make one complete ring -15 ft. in diameter. The plates for the 12-ft. section were No. 12 steel -12 ins. wide with 2 × 2 × ¹⁄₄-in. angles. Seventeen plates were required -to make a complete ring. - - - - -CHAPTER XVI. - -OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; BOSTON SUBWAY -AND NEW YORK RAPID TRANSIT. - - -OPEN-CUT TUNNELING. - -When a tunnel or rapid-transit subway has to be constructed at a small -depth below the surface, the excavation is generally performed more -economically by making an open cut than by subterranean tunneling -proper. The necessary condition of small depth which makes open-cut -tunneling desirable is most generally found in constructing -rapid-transit subways or tunnels under city streets. This fact -introduces the chief difficulties encountered in such work, since the -surface traffic makes it necessary to obstruct the streets as little as -possible, and has led to the development of the several special methods -commonly employed in performing it. - -Subways are usually constructed under and along important streets where -electric cars are running. The engineers have taken advantage of the -presence of these lines to facilitate the construction of subways. In -New York, for instance, the tracks of the electric lines were supported -by cast-iron yokes 4 or 5 ft. apart and were surrounded by concrete, -leaving only a large hollow space in the middle for the wires and -trolleys. The rails from 40 to 60 ft. long formed almost a solid -concrete structure for their entire length. The tracks and the street -surface were supported by horizontal beams inserted underneath the -tracks. These were the caps of bents constructed underground whose -rafters were finally resting on the subgrade of the proposed subway. - -The various methods for constructing the subways may be classified as -follows: (1) The single wide trench method; (2) the single narrow -longitudinal trench method; (3) the parallel longitudinal trench method; -(4) the slice method. - - -=Single Longitudinal Trench.=--The simplest manner by which to construct -open-cut tunnels is to open a single cut or trench the full width of the -tunnel masonry. This trench is strutted by means of side sheetings of -vertical planks, held in place by transverse braces extending across the -trench and abutting against longitudinal timbers laid against the -sheeting plank. The lining is built in this trench, and is then filled -around and above with well-rammed earth, after which the surface of the -ground is restored. An especial merit of the single longitudinal trench -method of open-cut tunneling is that it permits the construction of the -lining in a single piece from the bottom up, thus enabling better -workmanship and stronger construction than when the separate parts are -built at different times. The great objection to the method when it is -used for building subways under city streets is, that it occupies so -much room that the street usually has to be closed to regular traffic. -For this reason the single longitudinal trench method is seldom -employed, except in those portions of city subways which pass under -public squares or parks where room is plenty. - -This method was followed in the construction of the New York subway, -Section 2, along Elm St., a new street to be opened to traffic after the -subway had been completed, and at other points where local conditions -allowed it. - -[Illustration: FIG. 112.--Diagram Showing Sequence of Construction in -Open-Cut Tunnels.] - -A modification of this method was used in Contract Section 6, on upper -Broadway. The street at this point is very wide, so by opening a trench -as wide as the proposed four-track line of the subway there still -remained room enough for ordinary traffic. The electric car tracks were -supported by means of trusses 60 or 70 ft. long, which were laid in -couples parallel to the tracks and which rested on firm soil. The soil -under the car tracks was removed, beginning with transversal cuts to -receive the needles which were tied to the lower chord of the trusses by -means of iron stirrups. After the excavation had reached the subgrade, -posts were erected to support the needles thus forming bents upon which -the tracks rested. The trusses were removed and advanced to another -section of the tunnel, and, in the clear space left, the subway was -built from foundation up. - - -=The Single Narrow Longitudinal Trench.=--This method was used on -Contract Section 5, of the New York subway in order to comply with the -peculiar conditions of the traffic along 42nd St. On this street, on -account of the New York Central Station, there is a constant heavy -traffic, while pedestrians use the northern sidewalks almost -exclusively. A single longitudinal trench was then opened along the -south side, and from this trench all the work of excavation and -construction was carried on. At first the steel structure of the subway -was erected in the trench and then a small heading was driven and -strutted under and across the surface-car tracks. Afterward heavy -I-beams were inserted, which rested with one end on top of the steel -bents and the other end blocked to the floor of the excavation. These -I-beams were located 5 ft. apart and they supported the surface of the -street by means of longitudinal planks. The soil was removed from the -wide space underneath the I-beams and the subway was constructed from -the foundation up. When the structure had been completed, the packing -was placed between the roof of the structure and the surface of the -street, the I-beams withdrawn and the voids filled in. - - -=Parallel Longitudinal Trenches.=--The parallel longitudinal trench -method of open-cut tunneling consists in excavating two narrow parallel -trenches for the side walls, leaving the center core to be removed after -the side walls have been built. The diagram, Fig. 112, shows the -sequence of operations in this method. The two trenches No. 1 are first -excavated a little wider than the side wall masonry, and strutted as -shown by Fig. 113. At the bottoms of these trenches a foundation course -of concrete is laid, as shown by Fig. 114, if the ground is soft; or the -masonry is started directly on the natural material, if it is rock. From -the foundations the walls are carried up to the level of the springing -lines of the roof arch, if an arch is used; or to the level of its -ceiling, if a flat roof is used. After the completion of the side walls, -the portion of the excavation shown at No. 2, Fig. 112, is removed a -sufficient depth to enable the roof arch to be built. When the arch is -completed, it is filled above with well-rammed earth, and the surface is -restored. The excavation of part No. 3 inclosed by the side walls and -roof arch is carried on from the entrances and from shafts left at -intervals along the line. - -[Illustration: FIG. 113.--Sketch Showing Method of Timbering Open-Cut -Tunnels, Double Parallel Trench Method.] - -[Illustration: FIG. 114.--Side-Wall Foundation Construction Open-Cut -Tunnels.] - -A modification of the method just described was employed in constructing -the Paris underground railways. It consists in excavating a single -longitudinal trench along one side of the street, and building the side -wall in it as previously described. When this side wall is completed to -the roof, the right half of part No. 2, Fig. 112, is excavated to the -line _AB_, and the right-hand half of the roof arch is built. The space -above the arch is then refilled and the surface of the street restored, -after which the left-hand trench is dug and the side wall and roof-arch -masonry is built just as in the opposite half. Generally the work is -prosecuted by opening up lengths of trench at considerable intervals -along the street and alternately on the left-and right-hand sides. By -this method one-half of the street width is everywhere open to traffic, -the travel simply passing from one side of the street to the other to -avoid the excavation. When the lining has been completed, the center -core of earth inclosed by it is removed from the entrances and shafts, -leaving the tunnel finished except for the invert and track -construction, etc. - -Another modification of the parallel longitudinal trenches method was -used in the construction of the New York subway. A narrow longitudinal -trench was excavated on one side of the street near the sidewalk. -Meanwhile the pavement of half of the street was removed and a wooden -platform of heavy planks, supported by longitudinal beams which were -buried in the ground, was substituted. Then small cuts underneath the -car tracks were directed from the side trench and heavy beams or needles -were placed in these cuts, which also reached the longitudinal beams of -the wooden platform. The needles were wedged and blocked to the car -track structure and the beams. They were temporarily supported by cribs -built from underneath as the excavation progressed. When the subgrade -was reached, vertical and batter posts were inserted to support the -needles, thus forming regular timber bents underground. In the space -thus left open the subway was constructed to the middle of the street. -While the work was going on as described, another longitudinal narrow -trench was excavated at some distance on the other side of the street. -From this trench, the work of constructing the other half of the subway -was carried on in the manner just described. After the work had been -completed, the timbers removed, the voids filled in and the pavement of -the street restored, another equal section was attacked on both sides of -the street. - - -=Transverse Trenches.=--The transverse trench or “slice” method of -open-cut tunneling has been employed in one work, the Boston Subway. -This method is described in the specifications for the work prepared by -the chief engineer, Mr. H. A. Carson, M. Am. Soc. C. E., as follows:-- - -“Trenches about 12 ft. wide shall be excavated across the street to as -great a distance and depth as is necessary for the construction of the -subway. The top of this excavation shall be bridged during the night by -strong beams and timbering, whose upper surface is flush with the -surface of the street. These beams shall be used to support the railway -tracks as well as the ordinary traffic. In each trench a small portion -or slice of the subway shall be constructed. Each slice of the subway -thus built is to be properly joined in due time to the contiguous -slices. The contractor shall at all times have as many slice-trenches in -process of excavation, in process of being filled with masonry, and in -process of being back-filled with earth above the completed masonry, as -is necessary for the even and steady progress of the work towards -completion at the time named in the contract.” - -In regard to the success of this method Mr. Carson, in his fourth annual -report on the Boston Subway work, says: - -“The method was such that the street railway tracks were not disturbed -at all, and the whole surface of the street, if desired, was left in -daytime wholly free for the normal traffic.” - - -=Tunnels on the Surface.=--It occasionally happens when filling-in is to -take place in the future, or where landslides are liable to bury the -tracks, that a railway tunnel has to be built on the surface of the -ground. In such cases the construction of the tunnel consists simply in -building the lining of the section on the ground surface with just -enough excavation to secure the proper grade and foundation. Generally -the lining is finished on the outside with a waterproof coating, and is -sometimes banked and partly covered with earth to protect the masonry -from falling stones and similar shocks from other causes. A recent -example of tunnel construction of this character was described in -“Engineering News” of Sept. 8, 1898. In constructing the Golden Circle -Railroad, in the Cripple Creek mining district of Colorado, the line had -to be carried across a valley used as a dumping-ground for the refuse of -the surrounding mines. To protect the line from this refuse, the -engineer constructed a tunnel lining consisting of successive steel -ribs, filled between with masonry. - - -=Concluding Remarks.=--From the fact that the open-cut method of -tunneling consists first in excavating a cut, and second in covering -this cut to form an underground passageway, it has been named the -“cut-and-cover” method of tunneling. The cut-and-cover method of -tunneling is almost never employed elsewhere than in cities, or where -the surface of the ground has to be restored for the accommodation of -traffic and business. When it is not necessary to restore the original -surface, as is usually the case with tunnels built in the ordinary -course of railway work, it would obviously be absurd to do so except in -extraordinary cases. In a general way, therefore, it may be said that -the cut-and-cover method of construction is confined to the building of -tunnels under city streets; and the discussion of this kind of tunnels -follows logically the general description of the open-cut method of -tunneling which has been given. - - -TUNNELS UNDER CITY STREETS. - -The three most common purposes of tunnels under city streets are: to -provide for the removal of railway tracks from the street surface, and -separate the street railway traffic from the vehicular and pedestrian -traffic; to provide for rapid transit railways from the business section -to the outlying residence districts of the city; and to provide conduits -for sewage or subways for water and gas mains, sewers, wires, etc. -Within recent years the greatest works of tunneling under city streets -have been designed and carried out to furnish improved transit -facilities. - - -=Conditions of Work.=--The construction of tunnels under city streets -may be divided into two classes, which may be briefly defined as shallow -tunnels and deep tunnels. Shallow tunnels, or those constructed at a -small depth beneath the surface, are usually built by one of the -cut-and-cover methods; deep tunnels, or those built at a great depth, -beneath the surface are constructed by any of the various methods of -tunneling described in this book, the choice of the method depending -upon the character of the material penetrated, and the local conditions. - -In building tunnels under city streets the first duty of the engineer is -to disturb as little as possible the various existing structures and the -activities for which these structures and the street are designed. The -character of the difficulties encountered in performing this duty will -depend upon the depth at which the tunnel is driven. In constructing -shallow tunnels by the cut-and-cover method care has to be taken first -of all not to disturb the street traffic any more than is absolutely -necessary. This condition precludes the single trench method of open cut -tunneling in all places where the street traffic is at all dense, and -compels the engineer to use the methods employed in Paris and New York, -as previously described, or else the transverse trench or slice method -employed in the Boston Subway. - -These methods have to be modified when the work is done on streets -having underground trolley and cable roads, and in which are located gas -and water pipes, conduits for wires, etc. Where underground trolley or -cable railways are encountered, a common mode of procedure is to -excavate parallel side trenches for the side walls, and turn the roof -arch until it reaches the conduit carrying the cables or wires. The -earth is then removed from beneath the conduit structure in small -sections, and the arch completed as each section is opened. As fast as -the arch is completed the conduit structure is supported on it. Where -pipes are encountered they may be supported by means of chains, -suspending them from heavy cross-beams, or by means of strutting, or -they may be removed and rebuilt at a new level. Generally the conditions -require a different solution of this problem at different points. - -Another serious difficulty of tunneling under city streets arises from -the danger of disturbing the foundations of the adjacent buildings. This -danger exists only where the depth of the tunnel excavation extends -below the depth of the building foundations, and where the material -penetrated is soft ground. Where the tunnel penetrates rock there is no -danger of disturbing the building foundations. To prevent trouble of -this character requires simply that the excavation of the tunnel be so -conducted that there is no inflow of the surrounding material, which -may, by causing a settlement of the neighboring material, allow the -foundations resting on it to sink. - -The Baltimore Belt tunnel, described in a preceding chapter, is an -example of the method of work adopted in constructing a tunnel under -city streets through very soft ground. This may be classed as a deep -tunnel. Another method of deep tunneling under city streets is the -shield method, examples of which are given in a succeeding chapter. Two -notable examples of cut-and-cover methods of tunneling are the Boston -Subway and the New York Rapid Transit Ry., a description of which -follows. - - -=Boston Subway.=--The Boston Subway may be defined as the underground -terminal system of the surface street railway system of the city, and as -such it comprises various branches, loops, and stations. The subway -begins at the Public Garden on Boylston St., near Charles St., and -passes with double tracks under Boylston St. to its intersection with -Tremont St., where it meets the other double-track branch, passing under -Tremont St. and beginning at its intersection with Shawmut Ave. From -their intersection at Tremont and Boylston streets the two double-track -branches proceed under Tremont St. with four tracks to Scollay Square. -At Scollay Square the subway divides again into two double-track -branches, one passing under Hanover St., and the other under Washington -St. At the intersection of Hanover and Washington streets the two -double-track branches combine again into a four-track line, which runs -under Washington St. to its terminus at Haymarket Square, where it comes -to the surface by means of an incline. The subway, therefore, has three -portals or entrances, located respectively at Boylston St., Shawmut -Ave., and Haymarket Square. It also has five stations and two loops, the -former being located at Boylston St., Park St., Scollay Square, Adams -Square, and Haymarket Square, and the latter at Park St. and Adams -Square. The total length of the subway is 10,810 ft. - - -_Material Penetrated._--The material met with in constructing the subway -was alluvial in character, the lower strata being generally composed of -blue clay and sand, and the upper strata of more loose soil, such as -loam, oyster shells, gravel, and peat. At many points the material was -so stable that the walls of the excavation would stand vertical for some -time after excavation. Surface water was encountered, but generally in -small quantities, except near the Boylston St. portal, where it was so -plentiful as to cause some trouble. - -[Illustration: FIG. 115.--Wide Arch Section, Boston Subway.] - - -_Cross-Section._--The subway being built for two tracks in some places -and for four tracks in other places, it was necessary to vary the form -and dimensions of the cross-section. The cross-sections actually adopted -are of three types. Fig. 115 shows the section known as the wide-arch -type, in which the lining is solid masonry. The second type was known as -the double-barrel section, and is shown by Fig. 116. The third type of -section is shown by Fig. 117. The lining consists of steel columns -carrying transverse roof girders, the roof girders being filled between -with arches, and the wall columns having concrete walls between them. -The wide-arch type and the double-barrel type of sections were employed -in some portions of the Tremont St. line, where the traffic was very -dense, since it was possible to construct them without opening the -street. Much of the wide-arch line was constructed by the use of the -roof shield, which is described in the succeeding chapter on the shield -system of tunneling. - -[Illustration: FIG. 116.--Double-Barrel Section, Boston Subway.] - - -_Methods of Construction._--Several different methods were employed in -constructing the subway. Where ample space was available, the single -wide trench method of cut-and-cover construction was employed, the earth -being removed as fast as excavated. In the streets, except where regular -tunneling was resorted to, the parallel trench or transverse trench -cut-and-cover methods were employed. - -In the transverse trench method, trenches about 12 ft. wide were -excavated across the street, their length being equal to the extreme -transverse width of the tunnel lining, and their depth being equal to -the depth of the tunnel floor. These trenches were begun during the -night, and immediately roofed over with a timber platform flush with the -street surface. Under these platforms the excavation was completed and -the lining built. As each trench or “slice” was completed, the street -above it was restored and the platform reconstructed over the succeeding -trench or slice. During the construction of each slice the street -traffic, including the street cars, was carried by the timber platform. - -[Illustration: FIG. 117.--Four-Track Rectangular Section, Boston -Subway.] - -[Illustration: FIG. 118.--Section Showing Slice Method of Construction, -Boston Subway.] - -In the parallel trench method, short parallel trenches were dug for the -opposite side walls, and also for the intermediate columns, and -completely roofed over during the night. Under this roofing the masonry -of the side walls and column foundations and the columns themselves were -erected. When the side walls and columns had been erected, the surface -of the street between them was removed, the roof beams laid, and a -platform covering erected, as shown by Fig. 118. This roofing work was -also done at night. The subsequent work of building the roof arches, -removing the remainder of the earth, and constructing the invert, was -carried on underneath the platform covering which carried the street -traffic in the meantime. The successive repetition of the processes -described constructed the subway. - -Where the traffic was very dense on the street above, tunneling was -resorted to. For small portions of this work the excavation was done in -the ordinary way, using timber strutting, but much the greater portion -of the tunnel work was performed by means of a roof shield. In the -latter case, the side walls were first built in small bottom side drifts -and were fitted with tracks on top to carry the roof shield. The -construction and operation of this shield are described fully in the -succeeding chapter on the shield system of tunneling. - - -_Masonry._--The masonry of the inclined approaches to the subway -consists simply of two parallel stone masonry retaining walls. In the -wide-arch and double-barrel tunnel sections, the side walls are of -concrete and the roof arches are of brick masonry. In the other parts of -the subway the masonry consists of brick jack arches sprung between the -roof beams and covered with concrete, of concrete walls embedding the -side columns, and of the concrete invert and foundations for the -columns. Figs. 115 to 118 inclusive show the general details of the -masonry work for each of the three sections. The inside of the lining -masonry is painted throughout with white paint. - - -_Stations._--The design and construction of the stations for the Boston -Subway were made the subjects of considerable thought. All the stations -consist of two island platforms of artificial stone having stairways -leading to the street above. The platforms are made 1 ft. higher than -the rails. The station structure itself is built of steel columns and -roof beams with brick roof arches and concrete side walls. Its interior -is lined with white enameled tiles. The intermediate columns are cased -with wood, and have circular wooden seats at their bottoms. Each -stairway is covered by a light housing, consisting of a steel framework -with a copper covering and an interior wood and tile finish. - - -_Ventilation._--The subway is ventilated by means of exhaust fans -located in seven fan chambers, some of which contain two fans, and -others only one fan. Each of the fans has a capacity of from 30,000 to -37,000 cu. ft. of air per minute, and is driven by electric motor, -taking current from the trolley wires. This system of ventilation has -worked satisfactorily. - - -_Disposal of Rain Water._--The rain water which enters the subway from -the inclined entrances, together with that from leakage, is lifted from -12 ft. to 18 ft. by automatic electric pumps to the city sewers. The -subway has pump-wells at the Public Garden, at Eliot St., Adams Square, -and Haymarket Square. In each of these wells are two vertical submerged -centrifugal pumps made entirely of composition metal. In each chamber -above, are two electric motors operating the pumps. Each motor is -started and stopped according to the height of water by means of a float -and an automatic release starting box. The floats are so placed that -only one pump is usually brought into use. The other, however, comes -into service in case the first pump is out of order or the water enters -more rapidly than one pump can dispose of it. In the latter case, both -motors continue to run until the same low level has been reached. - -Very little dampness except from atmospheric condensation is to be found -on the interior walls or roof of the subway, although numerous -discolored patches, caused by dampness and dust, may be seen on some -parts of the walls. Substantially all of the leakage comes through the -small drains in the invert leading from hollows left in the side walls. -Careful measurement was taken at the end of an unusually wet season to -determine the actual amount of leakage, and the total amount for the -entire subway was found to be about 81 gallons per minute. - - -_Estimated Quantities._--The estimated quantities of material used in -constructing the subway were as follows: - - Excavation 369,450 cu. yds. - Concrete 75,660 „ „ - Brick 11,105 „ „ - Steel 8,105 tons - Granite 2,285 cu. yds. - Piles 117,925 lin. ft. - Ribbed tiles 12,440 sq. yds. - Plaster 88,190 „ „ - Waterproofing (asphalt coating) 117,980 „ „ - Artificial stone 6,790 „ „ - Enameled brick 2,210 „ „ - Enameled tiles 2,855 „ „ - - -_Cost of the Subway._--The estimated cost of the subway made before the -work was begun was approximately $4,000,000, and the cost of -construction did not exceed $3,700,000. This includes ventilating and -pump chambers, changes of water and gas pipes, sewers and other -structures, administration, engineering, interest on bonds, and all cost -whatsoever. Dividing this number by the total length we obtain a cost -per linear foot of $342.30. - - -=New York Rapid Transit Railway.=--The project of an underground rapid -transit railway to run the entire length of Manhattan Island was -originated some years previous to 1890. In 1894, however, a Rapid -Transit Commission was appointed to prepare plans for such a road, and -after a large amount of trouble and delay this commission awarded the -contract for construction to Mr. John B. McDonald of New York City, on -Jan. 15, 1900. - - -_Route._--The road starts from a loop which encircles the City Hall -Park. Within this loop the tunnel construction is two-track; but where -the main line leaves the loop, all four tracks come to the same level, -and continue side by side thereafter except at the points which will be -noted as the description proceeds. Proceeding from the loop, the -four-track line passes under Center and Elm Streets. It continues under -Lafayette Place, across Astor Place and private property between Astor -Place and Ninth St. to Fourth Ave. The road then passes under Fourth and -Park Avenues until 42d St. is reached. At this point the line turns west -along 42d St., which it follows to Broadway. It turns northward again -under Broadway to the boulevard, crossing the Circle at 59th St. The -road then follows the boulevard until 97th St. is reached, where the -four-track line is separated into two double-track lines. - -At a suitable point north of 96th St. the outside tracks rise so as to -permit the inside tracks, on reaching a point near 103d St., to curve to -the right, passing under the north-bound track, and to continue thence -across and under private property to 104th St. From there the two-track -tunnel goes under 104th St. and Central Park to 110th St., near Lenox -Ave.; thence under Lenox Ave to a point near 142d St.; thence across and -under private property and the intervening streets to the Harlem River. -The road passes under the Harlem River and across and under private -property to 149th St., which street it follows to Third Ave., and then -passes under Westchester Ave., where, at a convenient point, the tracks -emerge from the tunnel and are carried on a viaduct along and over -Westchester Ave., Southern Boulevard, and Boston Road to Bronx Park. -This portion of the line, from 96th St. to Bronx Park, is known as the -East Side Line. - -From the northern side of 96th St. the outside tracks rise and after -crossing over the inside tracks they are brought together on a location -under the center line of the street and proceed along under the -boulevard to a point between 122d and 123d Streets. At this point the -tracks commence to emerge from the tunnel, and are carried on a viaduct -along and over the boulevard at a point between 134th and 135th Streets, -where they again pass into the tunnel under and along the boulevard and -Eleventh Ave. to a point about 1350 ft. north of the center line of -190th St. There the tracks again emerge from the tunnel, and are carried -on a viaduct across and over private property to Elwood St., and over -and along Elwood St. to Kingsbridge St. to Kingsbridge Ave., private -property, the Harlem Ship Canal and Spuyten Duyvil Creek, private -property, Riverdale Ave., and Broadway to a terminus near Van Cortland -Park. That portion of the line from 96th St. to the above-mentioned -terminus at Van Cortland Park is known as the West Side Line. - -The total length of the rapid transit road, including the parts above -and below the surface ground of the streets, as well as both the East -and West Side Lines, is about 22¹⁄₂ miles. - - -_Material Penetrated._--The soil through which the road was excavated -was a varied one. The lower portion of the road, or the part including -the loop up to nearly Fourth St., was excavated through loose soil, but -from Fourth St. to the ends it was excavated in rock. The loose soil -forming the southern part of Manhattan Island is chiefly composed of -clay, sand, and old rubbish--a soil very easy to excavate. Water was met -at some points, but not in such quantities as to be a serious -inconvenience. From Fourth St. to the ends of both the east and west -side lines, the soil was chiefly composed of rock of gneissoid and -mica-schistose character, these rocks prevailing nearly throughout the -whole of Manhattan Island. The rock, as a rule, was not compact, but -full of seams and fissures, and at many points it was found -disintegrated and alternated with strata of loose soils, and even -pockets of quicksand were met with along the line of the road. - - -_Cross-Sections._--The section of the underground road is of three -different types,--the rectangular, the barrel-vault, and the circular. -The rectangular section. Fig. 119, is used for the greater part of the -road, of which a portion is for four tracks and a portion for two -tracks. The dimensions adopted for the four tracks are 50 × 13 ft., and -for the double tracks 25 × 13 ft. The barrel-vault section, composed of -a polycentric arch, having the flattest curve at the crown, has been -adopted for the tunnels under Park Avenue--while the semicircular arch -is used for all the other portions of the road to be tunneled. The -circular section of 15-ft. diameter is used under the Harlem River, and -being for single track, two parallel tunnels were built side by side. - -[Illustration: FIG. 119.--Double-Track Section, New York Rapid Transit -Railway.] - -The main line from the City Hall loop to about 102d St. consists of four -tracks built side by side in one conduit, except for that portion under -the present Fourth Ave. tunnel where two parallel double-track tunnels -are employed. The West Side Line will consist of double tracks laid in -one conduit, except across Manhattan St. and beyond 190th St., where it -is carried on an elevated structure. The East Side Line consists of a -double-track tunnel driven from 102d St., and the boulevard under -Central Park to 110th St. and Lenox Ave., and two parallel circular -tunnels excavated under the Harlem River,--the other portions of the -road being double-track, subway and elevated structure. - - -_Methods of Excavation._--Both the double-and four-track subway were -built by using the different varieties of the cut-and-cover method. The -single wide-trench method was used for the construction of the -double-track line and also for the construction of the four-track line -where the local conditions allowed it. The single narrow-trench method -was used for the construction of the four-track subway at 42d St., to -meet with the peculiar conditions of the traffic. Almost the total -length of the four-track line of the subway was built by means of the -two parallel side trenches. The slice method, so successfully employed -in the Boston Subway, was used only on 42d St. west of 6th Avenue. - - -_Lining._--The lining of the subway is of concrete, carried by a -framework of steel. The floor consists of a foundation layer of concrete -at least eight inches thick on good foundation, but thicker, according -to conditions, where the foundation is bad. On top of this is placed -another layer of concrete, with a layer of waterproofing between the -two. In this top layer are set the stone pedestals for the steel -columns, and the members making up the tracks. - -In the four-track subway, the steel framework consists of transverse -bents of columns, and I-beams spaced about five feet apart along the -tunnel. The three interior columns of each bent are built-up bulb-angle -and plate columns of H-section. The wall columns are I-beams, as are -also the roof beams; between the I-beams, wall columns, and roof beams -there is a concrete filling, so that the roof of the subway will be made -up of concrete arches resting on the flanges of the I-beams of the roof. -The concrete used is of one part Portland cement, two parts sand, and -four parts broken stones. The double-track subway is built in the same -way, except that only one column is placed between the tracks for the -support of the roof. - -All the concrete masonry of the roof, foundations, and side walls -contains a layer of waterproofing, so as to keep perfectly dry the -underground road, and prevent the percolation of water. This -waterproofing is made up as follows: On the lowest stratum of concrete, -whose surface is made as smooth as possible, a layer of hot asphalt is -spread. On this asphalt are immediately laid sheets or rolls of felt; -another layer of hot asphalt is then spread over the felt, and then -another layer of felt laid, and so on, until no less than two, and no -more than six, layers of felt are laid, with the felt between layers of -asphalt. On top of the upper surface of asphalt the remainder of the -concrete is put in place so as to reach the required thickness of the -concrete wall. - -[Illustration: FIG. 120.--Park Avenue Deep Tunnel Construction, New York -Rapid Transit Railway.] - - -_Tunnels._--When the distance between the roof of the proposed structure -and the street was 20 ft. or over, the Standard Subway construction was -replaced by tunnels. Three important tunnels have been constructed along -the line of the New York Rapid Transit and these are located between 33d -and 42d Streets on Park Ave., under Central Park northeast of 104th St. -and under Broadway north of 152d St. The Park Ave. construction (Fig. -120) consists of two parallel double-track tunnels, located on each side -of the street, and about 10 ft. below the present tunnel. The soil being -composed of good rock, the tunnels were driven by a wide heading, and -one bench, since no strutting was required, and the masonry lining, even -of the roof, was left far behind the front of the excavation. The -masonry lining consists of concrete walls and brick arches. The tunnels -under Central Park and under Broadway being driven through a similar -rock, the same method of excavation and the same manner of lining was -used. - -The tunnel under the Harlem River was driven through soft ground; and it -was constructed as a submarine tunnel, according to the caisson process. -The tunnels were lined with iron made up of segments, with radial and -circumferential flanges. Concrete was placed inside and flush with the -flanges. - -[Illustration: FIG. 121.--Harlem River Tunnel, New York Rapid Transit -Railway.] - -The tracks, both in the subway and tunnels, are an intimate part of the -concrete flooring. The rail rests on a continuous bearing of wooden -blocks, laid with the grain running transversely with respect to the -line of the rail, and held in place by two channel iron guard rails. The -guard rails are bolted to metal cross-ties embedded in the concrete. - - -_Viaduct._--A considerable portion of the double-track branch lines -north of 103d St. is viaduct, or elevated structure. The viaduct -construction on the West Side Line extends, including approaches, from -122d St. to very near 135th St. Of this distance, 2018 ft. 8 ins. are -viaduct proper, consisting of plate girder spans carried by trestle -bents at the ends, and by trestle towers for the central portion. The -columns of the bents and towers are built-up bulb-angle and plate -columns of H-section of the same form as those used in the bents inside -the subway. The approaches proper are built of masonry. The elevated -line proper consists of plate girder spans, supported on plate cross -girders carried by columns. - - -_Stations._--Many stations are built along the line. These are located -on each side of the street. The entrances at the stations consist of -iron framework, with glass roofs covering the descending stairways. The -passageways leading down are walled with white enameled bricks and -wainscoted with slabs of marble. The stations for the local trains are -located on each side of the road close to the walls, since the outside -tracks are reserved for the local trains, while the middle ones are -reserved for the expresses. The few stations for the express trains are -located between the middle and outside tracks. Stations are provided -with all the conveniences required, having toilet rooms, news stands, -benches, etc., and are lighted day and night by numerous arc lamps. - - -_General._--The contractor completed the work in four years. No -difficulty was encountered in doing this, since the great extension of -the road and the great width of the avenues under which it runs allowed -work all along the line at the same time. The work, briefly summarized, -comprises the following items:-- - - Length of all sections, ft. 109,570 - Total excavation of earth, cu. yds. 1,700,228 - Earth to be filled back, cu. yds. 773,093 - Rock excavated, cu. yds. 921,128 - Rock tunneled, cu. yds. 368,606 - Steel used in structure, tons 65,044 - Cast iron used, tons 7,901 - Concrete, cu. yds. 489,122 - Brick, cu. yds. 18,519 - Waterproofing, sq. yds. 775,795 - Vault lights, sq. yds. 6,640 - Local stations, number 43 - Express stations, number 5 - Station elevators, number 10 - Track total, lin. ft. 305,380 - „ underground, lin. ft. 245,514 - „ elevated, lin. ft. 59,766 - -In addition to the construction of the railway itself, it was necessary -to construct or reconstruct certain sewers, and to adjust, readjust, -and maintain street railway lines, water pipes, subways, and other -surface and subsurface structures, and to relay street pavements. - -The total cost of the work, according to the contract signed by Mr. -McDonald, was $35,000,000. Dividing this amount by the total length of -the road, which is 109,570 lineal feet, we have the unit cost a lineal -foot $315, or a little less than unit of cost of the Boston Subway, -which was $342 per lineal foot. - -The road belongs to the city. The contractor acts as an agent for the -city in carrying out the work, and he is the leaser of the road for -fifty years. The work was paid for as soon as the various parts of the -road were completed, and the money was obtained from an issue of city -bonds. During the fifty years’ lease the contractor will pay the -interest plus 1% of the face value of the bonds. This 1% goes to the -sinking-fund, which within the fifty years at compound interest forms -the total sum required for the redemption of bonds. - -This first New York Subway has been extended to Brooklyn, and more lines -will be built so as to form a complete underground railway system to -accommodate the ever-increasing traveling crowd of the American -metropolis. No new method of construction has been devised yet. The only -variation from the illustrated methods has been where the subway is -built underneath the Elevated Road which had to be strongly supported -during the construction of the subway. This has been done in two -different ways, either by supporting the columns of the Elevated Road by -means of two wooden A-frames abutting at the top and leaving a large -space close to the foot of the column where a pit was excavated to the -required depth of the subway, or by attaching the columns to long iron -girders placed longitudinally and resting with both ends in firm soil. - - - - -CHAPTER XVII. - -SUBMARINE TUNNELING: GENERAL DISCUSSION.--THE SEVERN TUNNEL. - - -GENERAL DISCUSSION. - -Submarine tunnels, or tunnels excavated under the beds of rivers, lakes, -etc., have been constructed in large numbers during the last quarter of -a century, and the projects for such tunnels, which have not yet been -carried to completion, are still more numerous. Among the more notable -completed works of this character may be noted the tunnel under the -River Severn and those under the River Thames in England, the one under -the River Seine in France, those under the St. Clair, Detroit, Hudson, -Harlem and East Rivers, and the one under the Boston Harbor for -railways, that under the East River for gas mains, that under Dorchester -Bay, Boston, for sewage, and those under Lakes Michigan and Erie for the -water supply of Milwaukee, Chicago, Buffalo, and Cleveland in America. -For the details of the various projected submarine tunnels of note, -which include tunnels under the English and Irish Channels, under the -Straits of Gibraltar, under the sound between Copenhagen in Denmark and -Malmö in Sweden, under the Messina Straits between Italy and Sicily, and -under the Straits of Northumberland between New Brunswick and Prince -Edward Island, and those connecting the various islands of the Straits -of Behring, the reader is referred to the periodical literature of the -last few years. - -Previous to attempting the driving of a submarine tunnel it is necessary -to ascertain the character of the material it will penetrate. This fact -is generally determined by making diamond-drill borings along the line, -and the object of ascertaining it is to determine the method of -excavation to be adopted. If the material is permeable and the tunnel -must pass at a small depth below the river bed, a method will have to be -adopted which provides for handling the difficulty of inflowing water. -If, on the other hand, the tunnel passes through impermeable material at -a great depth, there will be no unusual trouble from water, and almost -any of the ordinary methods of tunneling such materials may be employed. -Shallow submarine tunnels through permeable material are usually driven -by the shield method or by the compressed air method, or by a method -which is a combination of the first and second. - -It is not an uncommon experience for a submarine tunnel to start out in -firm soil and unexpectedly to find that this material becomes soft and -treacherous as the work proceeds, or that it is intersected by strata of -soft material. The method of dealing with this condition will vary with -the circumstances, but generally if any considerable amount of soft -material has to be penetrated, or if the inflow of water is very large, -the firm-ground system of work is changed to one of the methods employed -for excavating soft-ground submarine tunnels. The Milwaukee water supply -tunnel, described elsewhere, is a notable example of submarine tunnels, -began in firm material which unexpectedly developed a treacherous -character after the work had proceeded some distance. Occasionally the -task of building a submarine tunnel in the river bed arises. In such -cases the tunnel is usually built by means of cofferdams in shallow -water, and by means of caissons in deep water. - -Submarine tunnels under rivers are usually built with a descending grade -from each end which terminates in a level middle position, the -longitudinal profile of the tunnel corresponding to the transverse -profile of the river bottom. Where, however, such tunnels pass under the -water with one end submerged, and the other end rising to land like the -water supply tunnels of Chicago, Milwaukee, and Cleveland, the -longitudinal profile is commonly level, or else descends from the shore -to a level position reaching out under the water. - -The drainage of submarine tunnels during construction is one of the most -serious problems with which the engineer has to deal in such works. This -arises from the fact that, since the entrances of the tunnel are higher -than the other parts, all of the seepage water remains in the tunnel -unless pumped out, and from the possibility of encountering faults or -permeable strata, which reach to the stream bed and give access to water -in greater or less quantities. Generally, therefore, the excavation is -conducted in such a manner that the inflowing water is led directly to -sumps. To drain these sumps pumping stations are necessary at the shore -shafts, and they should have ample capacity to handle the ordinary -amount of seepage, and enough surplus capacity to meet probable -increases in the inflow. For extraordinary emergencies this plant may -have to be greatly enlarged, but it is not usual to provide for these at -the outset unless their likelihood is obvious from the start. The -character and size of the pumping plants used in constructing a number -of well-known tunnels are described in Chapter XII. - -In this book submarine tunnels will be classified as follows: (1) -Tunnels in rock or very compact soils, which are driven by any of the -ordinary methods of tunneling similar materials on land; (2) tunnels in -loose soils, which may be driven, (_a_) by compressed air, (_b_) by -shields, or (_c_) by shields and compressed air combined; (3) tunnels on -the river bed, which are constructed, (_a_) by cofferdams, or (_b_) by -caissons. To illustrate tunnels of the first class, the River Severn -tunnel in England has been selected; to illustrate those of the second -class, the several tunnels discussed in the chapter on the shield system -of tunneling and the Milwaukee tunnel is sufficient; to illustrate those -of the third class, the Van Buren Street tunnel in Chicago, the Harlem, -the Seine and the Detroit River tunnels are selected. - - -THE SEVERN TUNNEL. - -The Severn tunnel, which carries the Great Western Railway of England, -beneath the estuary of a large river, is 4 miles 642 yards long. It -penetrates strata of conglomerate, limestone, carboniferous beds, marl, -gravel, and sand at a minimum depth of 44³⁄₄ ft. below the deepest -portion of the estuary bed. The following description of the work is -abstracted from a paper by Mr. L. F. Vernon-Harcourt.[12] - - [12] Proceedings Inst. C. E., vol. cxxi. - -The first work was the sinking of a shaft, 15 ft. in diameter, lined -with brickwork, on the Monmouthshire bank of the Severn, 200 ft. deep. -After the Monmouthshire shaft had been sunk, a heading 7 ft. square was -driven under the river, rising with a gradient of 1 in 500 from the -shaft on the Monmouthshire shore, so as to drain the lowest part of the -tunnel. Near to the first, a second shaft was sunk and tubbed with iron, -in which the pumps were placed for removing the water from the tunnel -works, connection being made by a cross-heading with the heading from -the first shaft. There was also a shaft on the Gloucestershire shore; -and two shafts inland from the first on the Monmouthshire side, to -expedite the construction of the tunnel. Headings were then driven in -both directions along the line of the tunnel, from the four shafts; and -the drainage heading from the old shaft was continued, in the line of -the tunnel, under the deep channel of the estuary, and up the ascending -gradient towards the Gloucestershire shore, till, in October, 1879, it -had reached to within about 130 yards of the end of the descending -heading from the Gloucestershire shaft. During this period, though the -work had progressed slowly, no large quantity of water had been met with -in any of the headings, in spite of their already extending under almost -the whole width of the estuary. On October 18, 1889, however, a great -spring was tapped by the heading which was being driven landwards from -the old shaft, about 40 ft. above the level of the drainage heading; -and the water poured out from this land spring in such quantity that, -flowing along the heading, falling down the old shaft, and thus finding -its way into the drainage heading and the long heading of the tunnel -under the estuary in connection with it, it flooded the whole of the -workings in communication with the old shaft, which it also filled -within twenty-four hours from the piercing of the spring. - -To obtain increased security against any influx of water from the deep -channel of the estuary into the tunnel, the proposed level portion of -the tunnel, rather more than a furlong long under this part, was lowered -15 ft. by increasing the descending gradient on the Monmouthshire side -from 1 in 100 to 1 in 90, and lowering the proposed rail level on the -Gloucestershire side 15 ft. throughout the ascent, so as not to increase -the gradient of 1 in 100 against the load. A new shaft, 18 ft. in -diameter, was sunk slightly nearer the estuary on the Monmouthshire -shore than the old one; two shafts also were sunk on the land side of -the great spring for pumping purposes; and additional pumping machinery -was erected. The flow from the spring into the old shaft was arrested by -a shield of oak fixed across the heading; and at last, after numerous -failures and breakdowns of the pumps, the headings were cleared of -water, after a diver, supplied with a knapsack of compressed oxygen, had -closed a door in the long heading under the estuary; and the works were -resumed nearly fourteen months after the flooding occurred. The great -spring was then shut off from the workings by a wall across the heading -leading to the old shaft; and, owing to the lowering of the level of the -tunnel, a new drainage heading had to be driven from the bottom of the -new shaft at a lower level, which was made 5 ft. in diameter, and lined -with brickwork, whilst the old drainage heading was enlarged to 9 ft. in -diameter, and lined with brickwork, so as to aid in the permanent -ventilation of the tunnel. The lowering of the level, moreover, -converted the bottom tunnel headings into top headings, so that along -more than a mile of the tunnel the semicircular arch, 26 ft. in -diameter, was built first, and then, after lowering the headings, the -invert was laid and the side walls were built up. Bottom headings were -driven along the remainder of the tunnel, and the work was expedited by -means of break-ups. Ventilation was effected in the works by a fan 18 -inches in diameter and 7 ft. wide, fixed at the top of the new deep -shaft; the rock was bored by drills worked by compressed air; the -blasting was eventually effected exclusively by tonite, owing to its -being freer from deleterious fumes than any other explosive; and the -workings were lighted by Swan and Brush electric lamps. The tunnel is -lined throughout with vitrified brickwork, between 2¹⁄₄ ft. to 3 ft. -thick, set in cement, and has an invert 1¹⁄₂ ft. to 3 ft. in thickness; -the lining was commenced towards the end of 1880, the headings under the -river were joined in September, 1881, and the last length of the tunnel, -across the line of the great spring, was completed in April, 1885. - -Water came in from the river through a hole in a pool of the estuary, -close to the Gloucestershire shore, in April, 1881, during the lining of -a portion of the tunnel, but fortunately before the headings were -joined. This influx was stopped by allowing the water to rise in the -tunnel to tide-level, to prevent the enlargement of the hole, which was -then filled up at low water with clay, weighted on the top with clay in -bags. The great spring broke out again in October, 1883, and flooded the -works a second time; but within four weeks the water had been pumped out -and the spring again imprisoned. During this period an exceptionally -high tide, raised still higher by a southwesterly gale, inundated the -low-lying land on the Monmouthshire side of the estuary, and, flowing -down one of the inland shafts, flooded a section of the tunnel, but the -pumps removed this water within a week. - -In order to construct the portion of tunnel traversing the line of the -great spring, the water was diverted into a side heading below the level -of the tunnel, leading to the old shaft, whence it was pumped, and the -fissure below the tunnel was filled with concrete, over which the -invert was built. An attempt to imprison the spring, on the completion -of this length of tunnel, having resulted in imposing an excessive -pressure on the brickwork, leading to fractures and leakage, a shaft, 29 -ft. in diameter, was sunk at the side of the tunnel at this point in -1886, and pumps were erected powerful enough to deal with the entire -flow of the spring. - -The tunnel was opened for traffic in December, 1886, and gives access to -a double line of railway, connecting the lines converging to Bristol -with the South Wales railway and the western lines. The pumping power -provided at the shaft connected with the great spring, and at four other -shafts, is capable of raising 66,000,000 gallons of water per day, the -maximum amount pumped from the tunnel being 30,000,000 gallons a day. -The ventilation of the tunnel is effected by fans placed in the two main -shafts on each bank of the estuary, and the fan in the Monmouthshire -shaft is 40 ft. in diameter, and 12 ft. wide. The tunnel gives passage -to a large traffic, numerous through-trains between the north and -southwest of England making use of it. - - - - -CHAPTER XVIII. - -SUBMARINE TUNNELING (Continued); THE COMPRESSED AIR METHOD.--THE -MILWAUKEE WATER-WORKS TUNNEL. - - -Tunnels excavated at shallow depth from the bed of the river are liable -to cave in under the great weight of the water and material above the -roof. Besides, the progress of the work will be greatly interfered with -by the water which may reach the tunnel passing through the loose soil -in large quantities. To contend with these two sources of trouble, -different methods of constructing subaqueous tunnels have been devised; -they are: by compressed air, by shield, and finally by a combination of -these two methods, viz., by shield and compressed air. - -The compressed air method was suggested by Mr. Haskin, the promoter and -the first builder of the Hudson River tunnel. In 1874, when he began to -sink the shaft for the construction of his tunnel, several subaqueous -tunnels had already been driven by means of shields. Mr. Haskin had -ideas of his own, and thought he could dispense with the shield and -could trust to compressed air, since he was firmly convinced that -compressed air alone could expel the water and temporarily support the -roof of the excavation prior to the building of the lining masonry. In -other words, he expected to substitute a core of compressed air for the -core of earth removed. In the patent granted him for this method of -tunneling, he expresses himself as follows: “The distinguishing feature -of my system is that, instead of using temporary facings of timber or -other rigid material, I rely upon the air pressure to resist the caving -in of the wall and infiltration of water until the masonry wall is -completed. The pressure is, of course, to be regulated by the -exigencies of the occasion. The effect of such a pressure has been found -to drive water in from the surface of the excavation, so that the sand -becomes dry.” - -The compressed air method was soon found to be inefficient, even in the -construction of the Hudson tunnel where the roof of the excavation was -supported by timbering in the manner indicated in the pilot system. Thus -large subaqueous railway tunnels cannot be driven exclusively by the -compressed air method; still it has been successfully employed in the -construction of small tunnels driven for aqueduct purposes. But the use -of compressed air marked a great progress in the art of submarine -tunneling. - - -THE MILWAUKEE WATER-WORKS TUNNEL. - -The following description of the Milwaukee Water-Works Tunnel is an -example of subaqueous tunnels driven through good soil in the usual -manner employed in land tunnels; but afterward when treacherous material -was encountered, the work was continued by means of compressed air. - -The new water supply intake tunnel for the city of Milwaukee, Wis., is -one of the most difficult examples of tunnel construction which American -engineering practice has afforded. The difficulties were in a large -measure unexpected when the work was decided upon and put under way. The -tunnel began and ended in a hard, impervious clay, practically a rock, -and all the preliminary investigations led to the conclusion that the -same favorable material would be encountered for its entire length. With -such material a brick-lined tunnel 7¹⁄₂ ft. in diameter presented no -unusual problems; but after about 1640 ft. had been excavated from the -shore end the tunnel ran out of the hard clay, and for the next 600 ft. -or more a variety of water-bearing material was encountered, which tried -the skill and patience of the engineer to their utmost. Other -difficulties were indeed met with, but these were of minor importance in -comparison with that of safely and successfully penetrating the -water-bearing drift. - -The work of sinking the shore shafts and excavating the first 1600 ft. -of tunnel did not prove especially difficult. A hard, compact, and -rock-like clay, bearing very little moisture, was encountered all along, -and was blasted and removed in the ordinary manner. The only mishap -which occurred with this portion of the work was the destruction of the -contractor’s boiler plant by fire on Jan. 12, 1891, which allowed the -tunnel to fill with water, and delayed work about a month. By Oct. 21, -1891, 1640 ft. had been driven, averaging about 6²⁄₃ ft. per day, all in -the hard clay. No timbering had been necessary, and except for the first -100 ft. of the tunnel there was very little seepage. On the afternoon of -Oct. 21 water was observed coming out from one of the drill holes in the -heading, but no attention was paid to it. Shortly after a blast was -fired, and was immediately followed by a rush of water from the heading. -An unsuccessful attempt was made to check the flow, and the pumps were -started; but they were unable to keep the water down, and after seven -hours’ hard work the tunnel was abandoned. By the next morning the -tunnel and shaft were full of water. - -Several attempts were made to empty the tunnel; but the limited pumping -capacity was not equal to the task, and it was finally decided to -install larger pumps. The pumping had, however, shown that about 1000 -gallons of water a minute was coming through the leak. With the -increased pumping plant the tunnel was finally laid dry Feb. 13, 1892. -Upon examination the head of the drift was found to be in the same -undisturbed condition in which it was left when the water broke in three -months before. - -A brick bulkhead was built into the end of the brickwork of the tunnel, -and provided with a timber door for passage, and two 10-in. pipes for -the outlet of the water. With these openings closed, the flow was -checked sufficiently to allow the placing of pumps at the bottom of the -shore shaft. Meanwhile the pressure of the water against the bulkhead -caused dangerous leakage, and so after the pumps were in position the -10-in. pipes were opened, relieving the pressure and allowing the water -its normal rate of flow. Trouble with the pumps now arose, and after -various stoppages and breaks the discharge pipe finally fell, disabling -the whole plant. It became necessary to close the 10-in. pipes in the -bulkhead and draw up the pumps. This allowed the tunnel to again fill -with water. - -After thoroughly overhauling the pumping machinery, the contractor again -laid the tunnel dry on March 19; and after the pumps had been -permanently placed so as to take care of the water, an examination of -the work was made. It was found that the water was coming from the -north, and with the hope of avoiding the difficulties of the old -heading, it was decided to make a détour of the south. On April 16 work -was begun at a point about 90 ft. back from the face, and deflecting the -line about 38° toward the south. About 38 ft. from the angle of junction -a brick bulkhead with two 8-in. openings was built into the new bore. -The work progressed successfully for about 75 ft., when water was again -encountered; and upon pushing forward the heading, gravel and sand came -in such quantities that it was found impracticable to continue the work -further. On June 1 the bulkhead was permanently closed, and the work in -this direction was abandoned. - -A further and closer examination was now made of the heading first -abandoned. Upon breaking through the rock-like clay it was found that -the water came from an underground stream flowing from the north through -a well defined channel in red clay. This channel was about 13 ft. above -the grade of the tunnel; and above it in every direction visible was a -bed of hard, dry, red clay, while immediately in front of the face of -the work was a bank of coarse gravel. Fig. 122 is a sketch of the -channel and stream where they entered the work. In this last drawing the -photograph has been followed exactly, no particular being exaggerated in -the slightest. The water from this stream was clear and pure; and a -chemical analysis showed that it was not lake water, but must come from -some separate source. - -[Illustration: FIG. 122.--Sketch Showing Underground Stream, Milwaukee -Water-Works Tunnel.] - -While the engineer did not consider the difficulty of proceeding along -the old line insurmountable, it was decided to be less difficult on the -whole to go back from 150 ft. to 175 ft. and deflect the line to the -north and upward, so as to pass over the underground entrance. Instead -of allowing the water to flow at its normal rate and take care of it by -pumping, the contractors desired to reduce the pumping, and to this end -they constructed a bulkhead just west of the deflection toward the south -with a view of shutting off the water. The water, however, accumulated -with a pressure of some 50 lbs. per sq. in. and penetrated the filling -around the brick lining of the tunnel, preventing the cutting through of -the lining for the new line. A second bulkhead was then built about 20 -ft. west of the first, but with not much better results, for upon -closing it the water was found to leak through the brickwork for a long -distance west. Finally on Aug. 2, 1892, the contractors lifted their -pumps and allowed the tunnel to fill again with water. - -No further work was done on the tunnel by the contractors, although they -continued work on the lake shaft for some months. Difficulties had, -however, arisen here, which will be described further on; and finally a -disagreement arose between the contractors and the city over the delay -in prosecuting the tunnel work and over one or two other questions, -which resulted in the City Council suspending their contract and -ordering the Board of Public Works to go ahead with the work. - -The first step to be taken by the engineer was to purchase adequate -pumping machinery and empty the tunnel. This was effected Jan. 17, 1894; -and as soon as practicable thereafter the two bulkheads were removed and -the tunnel cleaned, tram-car tracks laid, and everything prepared for -work. It was now determined to go ahead on the original line of the -tunnel if possible, and the bulkhead here was removed and work begun. -Meanwhile, a safety bulkhead had been built to replace the first one -torn away. This was provided with a door and drainage pipes. Work was -begun on the original heading, but had proceeded only a little way when -the water broke in, driving out the workmen. This was removed three or -four times, when the flow suddenly increased to 3000 gallons per minute. -An examination of the lake bottom above the break showed that it had -settled down, indicating that the new break connected with the lake -bottom, and making further work along the original line out of the -question. - -The question now arose what it was best to do. It was impracticable to -use a shield, as the material ahead of the break required blasting, and -the pressure from above was enormous. On account of its expense and -difficulty of application the freezing process did not seem advisable, -and the plenum process was likewise out of the question on account of -the great pressure which would be required at this depth. The détour to -the south which had been made by the contractor had been unsuccessful, -and had left the ground in a treacherous condition. To depress the -tunnel was not advisable, for it was not by any means certain that the -bed of gravel could be avoided in that way; and, moreover, it would be -necessary to ascend again further on, and thus leave a trap which would -effectually cut off escape to those at work on the face if water again -broke into the tunnel. - -It was finally decided that the old plan of deflecting the line toward -the north and upward so as to pass over the underground stream should be -tried. A hole was therefore cut through the tunnel lining 1433 ft. from -the shore, and work was begun on a détour of 20° toward the north and an -upward grade of 10%. Fair progress was made on this new line, gradually -ascending into solid rock, until May 10, when the test borings, which -were constantly made in every direction from the face, showed that sand -was being approached. A brick bulkhead was therefore built into the -masonry as a safeguard, should it happen that water was encountered in -large quantities. As the borings seemed to indicate that the top surface -of the rock underlying the sand was nearly level, the lower half of the -tunnel was first excavated, leaving about 18 ins. of the rock to serve -as a roof (Sketch _a_, Fig. 123), and the brick invert was built for a -distance of 52 ft. The rock roof was then carefully broken through for -short distances at a time, and short sheeting driven ahead into the -sand, which proved to be a very fine quicksand flowing through the -smallest openings. Extreme care had to be taken in this work, but little -by little the brickwork was pushed ahead until at a distance of 90 ft. -from the point where the sand was first met, and 208 ft. from the old -tunnel, the sand stopped and the heading entered a hard clay. - -All this work had been done on an ascending grade, and the ascent was -continued about 40 ft. farther in the clay. By this time a sufficient -elevation was gained to pass over the underground stream, and the tunnel -line was changed to head toward the lake shaft, and the grade reduced to -a level. The underground stream was passed without trouble and the -tunnel continued for a distance of 54 ft. without difficulty. On July 10 -the clay in the heading suddenly softened, and before the miners could -secure it by bracing, the water rushed in, followed by gravel, filling -up solidly some 34 ft. of the tunnel before it was stopped by a timber -bulkhead hastily built. - -[Illustration: ~Longitudinal Section Showing Method of Construction in -Rock Covered with Quicksand.~ - -~Sketch “a”.~ - -~Section A-B-C-D.~ - -~Sketch “c”.~ - -~Longitudinal Section of Tunnel.~ - -~Sketch “b”.~ - -~Cross Section Showing Manner of Constructing Lining around Boulder.~ - -~Sketch “d”.~ - -FIG. 123.--Sketch Showing Methods of Lining, Milwaukee Water-Works -Tunnel.] - -Upon examining the lake bottom a cavity over 60 ft. deep and 10 ft. in -diameter was found directly over the end of the tunnel, which had been -caused by the gravel breaking into the tunnel. Having now reached an -elevation where it was possible to use compressed air, it was determined -to put in double air-locks and use the plenum process. The locks were -built, and some 670 cu. yds. of clay were dumped into the hole in the -lake bottom. On Aug. 4 the air-locks were tried with 26 lbs. air -pressure; but, upon a temporary release of the pressure, the water -passed around the locks and back of the tunnel lining for some distance, -and even forced through the lining, carrying considerable clay and fine -sand with it. Upon sounding the lake bottom it was found that the cavity -had again increased to a depth of 65 ft., whereupon an additional 600 -cu. yds. of clay were dumped into it. - -On account of the water leaking through the brickwork, the only dry -place to cut through the brickwork and build in an air-lock was just -ahead of the brick bulkhead. This lock was completed Aug. 27, and to -avoid encountering the danger of the direct connection with the lake at -the end of the drift, it was decided to make another détour to the -north. On Aug. 28, therefore, the brick on the north side of the tunnel -12 ft. back from the end of the brickwork was cut through under 25 lbs. -air pressure, and work proceeded in good, hard clay. The original -air-lock was cut out and a new lock built into this clay about 34 ft. -from the last détour, to be used in case of further difficulties. After -building the tunnel for about 80 ft. from the détour, the soundings -again indicated the approach to gravel and water, and on Oct. 14 the -water broke through from the bottom in such volume and with such force -that the men ran out, closing every air-lock and the valves of every -drain in their haste to escape, until the brick bulkhead was reached. It -was with great difficulty that the portion of the tunnel up to the last -air-lock was recovered and cleaned out. - -It was now recognized that a pressure of from 38 to 40 lbs. of air would -be needed to hold this water, and accordingly another compressor was -added to the plant. With a pressure of 36 lbs. the water was driven out -and the work again started. At this time also an additional 350 cu. yds. -of clay were dumped into the hole in the lake bottom. Altogether, 1620 -cu. yds. of clay had been put into this hole. - -Loose gravel and boulders, some of immense size, were now encountered, -and the work became exceedingly difficult on account of the great escape -of air. The interstices between the gravel and boulders were not filled -with silt or sand, but contained water. Moreover, this material extended -upward to the lake bottom, as was shown by the escape of air at the -surface of the lake. For an area of several hundred square feet the -surface of the water resembled a pot of boiling water. At times the air -would escape very rapidly; and again only a few bubbles would show. - -It need hardly be said that the work in this gravel was very slow. It -was impossible to blast or to tear out the large boulders whole, as so -much surface would be exposed that an inrush of water would take place -despite the air pressure. The method of procedure was to excavate a -heading and build the brick roof arch first, and then to take out the -bench and build the invert. Fig. 123 gives a number of sketches showing -how the work was done. A short piece of heading was taken out, the top -and face of the bench being meanwhile plastered with clay (Sketches _b_ -and _c_, Fig. 123) to reduce the escape of air, and then the roof arch -was built and supported on side sills resting on the bench. Bit by bit -the roof arch was pushed forward until some little distance had been -completed, then the heading was plastered with clay and the bench taken -out little by little and the invert built. All the gravel except the -small area upon which work was actually in progress was kept thoroughly -plastered with clay; and as the air escaped through the completed -brickwork very rapidly, water was allowed to cover a portion of the -invert (see Sketch _c_, Fig. 123), so as to reduce the area of escape. - -When a large boulder was reached, which lay partly within and partly -without the tunnel section, the lining was built out and around it, as -shown in Sketch _d_, Fig. 123. The boulder was then broken and taken -out. All through this gravel bed the cross-section of the lining is made -irregular by the construction of these pockets in the lining to get -around boulders. Sometimes they were on one side and sometimes on the -other, or on both, or at the top or bottom. In fact, there was no -regularity. Despite the hazard and danger of this work, continual -progress was made, though sometimes it was only 4 ft. of completed -tunnel per week, working night and day; and, if some cases of caisson -disease be excepted, the only mishap occurring was a fire which got into -the timber packing behind the lining and caused some trouble. From the -gravel the tunnel ran into clay and quicksand, and then into hard, dry -clay similar to that encountered near the shore. Some difficulty was had -with the quicksand, but it was successfully overcome; and when the hard -clay was struck, the trouble, as far as the work from the shore shaft -was concerned, was virtually over. - -Meanwhile, a different set of afflictions had come upon the engineer and -contractors in sinking the lake shaft and driving the heading toward -shore. This shaft was intended to be built by sinking a cast-iron -cylinder 10 ft. in diameter, made up of sections bolted together. Work -was begun July 5, 1892, and the sinking was accomplished first by -weighting the cylinder, and afterwards by pumping out the sand and water -within it until the pressure from the outside broke through under the -cutting edge and forced the sand into the cylinder, allowing it to sink -a little. From 10 to 30 cu. yds. of sand were carried into the cylinder -each time, and finally it was feared that if the process continued, the -crib, which had been previously erected, would be undermined. On Sept. -6, therefore, the contractors were ordered to discontinue this method of -work. No change was made, however, until Oct. 1, when the cylinder had -reached a depth of 68 ft., and by this time there was quite a large -cavity underneath the crib. This was refilled, and the cylinder pumped -out, and excavation begun inside of it. On Oct. 11 a 2¹⁄₂-ft. deep ring -of brickwork was laid underneath the cutting edge; but in trying to put -in another ring beneath the first, two days later, the sand and water -broke through the bottom, driving the men out, and filling the cylinder -to a depth of 16 ft. with sand. The pumps were started, but the water -could not be lowered to a greater depth than 60 ft. - -At the request of the contractors, the city engineer had a boring made -at the center of the shaft to determine the character of the material to -be further penetrated. This boring showed that sand mixed with loam and -gravel would be found for a depth of 26 ft., then would come 15 ft. of -red clay, and finally a layer of hard clay like that penetrated by the -shore end of the tunnel. About the middle of December the contractors -made another attempt to pump the shaft, but finding that the water came -in at the rate of 25 gallons a minute, abandoned the attempt. In the -latter part of February preparations were made to put an air-lock in the -shaft and use compressed air. Hardly had the work been begun by this -system when, on April 20, 1893, a terrific easterly storm swept the top -of the crib bare of the buildings and machinery, and drowned all but one -of the 15 men at work there. - -This disaster delayed the work for some time, but in June the -contractors erected a new building and new machinery, and resumed work. -Very little progress was made; and the air escaped so rapidly that it -loosened the sand surrounding the shaft and reduced the friction to such -an extent that on July 28 the entire cylinder lifted bodily about 6 ft., -and sand rushed in, filling the lower part of the cylinder to within 45 -ft. of the lake surface. No further work was done by the contractors -although they submitted a proposition to sink a steel cylinder inside -the cast-iron cylinder and extending from 5 ft. above datum to 100 ft. -below datum for $300 per ft. This proposition was refused by the city; -and since work on the tunnel proper had been abandoned by the -contractors some time before, as had already been described, the city -suspended their contract on Oct. 19. - -On Oct. 30 a contract was made with Mr. Thos. Murphy of Milwaukee, Wis., -to sink a steel cylinder inside the old iron cylinder. The water was -first pumped out of the old cylinder, and a timber bulkhead built at -the bottom. On this the steel cylinder was built, and then the bulkhead -was removed. Air pressure was put on, and the excavation proceeded -successfully until the bottom layer of clay was met with, when all -chances for trouble ceased. - -The cylinder, as it was completed, penetrated 9 ft. into the hard clay, -and was underpinned with brickwork for a depth of 29 ft. or more, to a -point 4 ft. below the grade line of the tunnel. At the lower end, the -section of the shaft was changed from a circle to a square. Later the -steel cylinder was lined with brick. - -On March 28, 1894, an agreement was made with Mr. Thos. Murphy to -construct the tunnel from the lake shaft toward the shore. Except that -considerable water was encountered, which, owing to inadequate pumping -machinery, filled the tunnel and shaft at two different times, and had -to be removed, no very great difficulty was had with this part of the -work. - -On July 28, 1895, the headings from the lake and shore shafts met. -Meanwhile the cast-iron pipe intake, the intake crib, etc., had been -completed, and practically all that remained to be done was to clean the -tunnel and lift the pumping machinery at the shore shaft. During the -cleaning, the air pressure had been kept up on account of the leakage -through the brick lining, and, indeed, the pressure was kept up until -the last possible moment, and everything made ready for removing the -air-locks, bulkheads, pumps, etc., in the least possible time. The pumps -were the last to come out. - - - - -CHAPTER XIX. - -SUBMARINE TUNNELING (Continued). - - -THE SHIELD SYSTEM. - - -=Historical Introduction.=--The invention of the shield system of -tunneling through soft ground is generally accredited to Sir Isambard -Brunel, a Frenchman born in 1769, who emigrated to the United States in -1793, where he remained six years, and then went to England, in which -country his epoch-making invention in tunneling was developed and -successfully employed in building the first Thames tunnel, and where he -died in 1849, a few years after the completion of this great work. Sir -Isambard is said to have obtained the idea of employing a shield to -tunnel soft ground from observing the work of ship-worms. He noticed -that this little animal had a head provided with a boring apparatus with -which it dug its way into the wood, and that its body threw off a -secretion which lined the hole behind it and rendered it impervious to -water. To duplicate this operation by mechanical means on a large enough -scale to make it applicable to the construction of tunnels was the plan -which occurred to the engineer; and how closely he followed his animate -model may be seen by examining the drawings of his first shield, for -which he secured a patent in 1818. Briefly described, this device -consisted of an iron cylinder having at its front end an auger-like -cutter, whose revolution was intended to shove away the material ahead -and thus advance the cylinder. As the cylinder advanced the perimeter of -the hole behind was to be lined with a spiral sheet-iron plating, which -was to be strengthened with an interior lining of masonry. It will be -seen that the mechanical resemblance of this device to the ship-worm, on -which it is alleged to have been modeled, was remarkably close. - -In the same patent in which Sir Isambard secured protection for his -mechanical ship-worm he claimed equal rights of invention for another -shield, which is of far greater importance in being the prototype of the -shield actually employed by him in constructing the first Thames tunnel. -This alternative invention, if it may be so termed, consisted of a group -of separate cells which could be advanced one or more at a time or all -together. The sides of these cells were to be provided with friction -rollers to enable them to slide easily upon each other; and it was also -specified that the preferable motive power for advancing the cells was -hydraulic jacks. To summarize briefly, therefore, the two inventions of -Brunel comprehended the protecting cylinder or shield, the closure of -the face of the excavation, the cellular division, the hydraulic-jack -propelling power, and cylindrical iron lining, which are the essential -characteristics of the modern shield system of tunneling. The next step -required was the actual proof of the practicability of Brunel’s -inventions, and this soon came. - -Those who have read the history of the first Thames tunnel will recall -the early unsuccessful attempts at construction which had discouraged -English engineers. Five years after Brunel’s patent was secured a -company was formed to undertake the task again, the plan being to use -the shield system, under the personal direction of its inventor as chief -engineer. For this work Brunel selected the cellular shield mentioned as -an alternative construction in his original patent. He also chose to -make this shield rectangular in form. This choice is commonly accounted -for by the fact that the strata to be penetrated by the tunnel were -practically horizontal, and that it was assumed by the engineer that a -rectangular shield would for some reason best resist the pressures which -would be developed. Whatever the reason may have been for the choice, -the fact remains that a rectangular shield was adopted. The tunnel as -designed consisted of two parallel horseshoe tunnels, 13 ft. 9 ins. wide -and 16 ft. 4 ins. high and 1200 ft. long, separated from each other by a -wall 4 ft. thick, pierced by 64 arched openings of 4 ft. span, the whole -being surrounded with massive brickwork built to a rectangular section -measuring over all 38 ft. wide and 22 ft. high. - -The first shield designed by Brunel for the work proved inadequate to -resist the pressures, and it was replaced by another somewhat larger -shield of substantially the same design, but of improved construction. -This last shield was 22 ft. 3 ins. high and 37 ft. 6 ins. wide. It was -divided vertically into twelve separate cast-iron frames placed close -side by side, and each frame was divided horizontally into three cells -capable of separate movement, but connected by a peculiar articulated -construction, which is indicated in a general way by Fig. 124. To close -or cover the face of the excavation, poling-boards held in place by -numerous small screw-jacks were employed. Each cell or each frame could -be advanced independently of the others, the power for this operation -being obtained by means of screw-jacks abutting against the completed -masonry lining. Briefly described, the mode of procedure was to remove -the poling-boards in front of the top cell of one frame, and excavate -the material ahead for about 6 ins. This being done, the top cell was -advanced 6 ins. by means of the screw-jacks, and the poling-boards were -replaced. The middle cell of the frame was then advanced 6 ins. by -repeating the same process, and finally the operation was duplicated for -the bottom cell. With the advance of the bottom cell one frame had been -pushed ahead 6 ins., and by a succession of such operations the other -eleven frames were advanced a distance of 6 ins., one after the other, -until the whole shield occupied a position 6 ins. in advance of that at -which work was begun. The next step was to fill the 6-in. space behind -the shield with a ring of brickwork. - -[Illustration: FIG. 124.--Longitudinal Section of Brunel’s Shield, First -Thames Tunnel.] - -The illustration, Fig. 124, is the section parallel to the vertical -plane of the tunnel through the center of one of the frames, and it -shows quite clearly the complicated details of the shield construction. -Two features which are to be particularly noted are the suspended -staging and centering for constructing the roof arch, and the top plate -of the shield extending back and overlapping the roof masonry so as to -close completely the roof of the excavation and prevent its falling. -Notwithstanding its complicated construction and unwieldy weight of 120 -tons, this shield worked successfully, and during several months the -construction proceeded at the rate of 2 ft. every 24 hours. There were -two irruptions of water and mud from the river during the work, but the -apertures were effectually stopped by heaving bags of clay into the -holes in the river bed, and covering them over with tarpaulin, with a -layer of gravel over all. The tunnel was completed in 1843, at a cost of -about $5600 per lineal yard, and 20 years from the time work was first -commenced, including all delays. - -[Illustration: FIG. 125.--First Shield Invented by Barlow.] - -The next tunnel to be built by the shield system was the tunnel under -London Tower constructed by Barlow and Greathead and begun in 1869. In -1863 Mr. Peter W. Barlow secured a patent in England for a system of -tunnel construction comprising the use of a circular shield and a -cylindrical cast-iron lining. The shield, as shown by Fig. 125, was -simply an iron or steel plate cylinder. The cylinder plates were thinned -down in front to form a cutting edge, and they extended far enough back -at the rear to enable the advance ring of the cast-iron lining to be set -up within the cylinder. In simplicity of form this shield was much -superior to Brunel’s; but it seems very doubtful, since it had no -diametrical bracing of any sort, whether it would ever have withstood -the combined pressure of the screw-jacks and of the surrounding earth in -actual operation without serious distortion, and, probably, total -collapse. It should also be noted that Barlow’s shield made no provision -for protecting the face of the excavation, although the inventor did -state that if the soil made it necessary such a protection could be -used. The patent provided for the injection of liquid cement behind the -cast-iron lining to fill the annular space left by the advancing -tail-plates of the shield. Although Barlow made vigorous efforts to get -his shield used, it was not until 1868 that an opportunity presented -itself. In the meantime the inventor had been studying how to improve -his original device, and in 1868 he secured additional patents covering -these improvements. Briefly described, they consisted in partly closing -the shield with a diaphragm as shown by Fig. 126. The uninclosed portion -of the shield is here shown at the center, but the patent specified that -it might also be located below the center in the bottom part of the -shield. The idea of the construction was that in case of an irruption of -water the upper portion of the shield could be kept open by air -pressure, and work prosecuted in this open space until the shield had -been driven ahead sufficiently to close the aperture, when the normal -condition of affairs would be resumed. This was obviously an improvement -of real merit. The partial diaphragm also served to stiffen the shield -somewhat against collapse, but the thin plate cutting-edges and most of -the other structural weaknesses were left unaltered. To summarize -briefly the improvements due to Barlow’s work, we have: the construction -of the shield in a single piece; the use of compressed air and a partial -diaphragm for keeping the upper part of the shield open in case of -irruptions of water; and the injection of liquid cement to fill the -voids behind the lining. - -[Illustration: ~Longitudinal Section.~ - -~Cross Section.~ - -FIG. 126.--Second Shield Invented by Barlow.] - -Turning now to the London Tower tunnel work, it may first be noted that -Barlow found some difficulty in finding a contractor who was willing to -undertake the job, so little confidence had engineers generally in his -shield system. One man, however, Mr. J. H. Greathead, perceived that -Barlow’s device presented merit, although its design and construction -were defective, and he finally undertook the work and carried it to a -brilliant success. The tunnel was 1350 ft. long and 7 ft. in diameter, -and penetrated compact clay. Work was begun on the first shore shaft on -Feb. 12, 1869, and the tunnel was completed the following Christmas, or -in something short of eleven months, at a cost of £14,500. - -The shield used was Barlow’s idea put into practical shape by Greathead. -It consisted of an iron cylinder, or, more properly, a frustum of a cone -whose circumferential sides were very slightly inclined to the axis, the -idea being that the friction would be less if the front end of the -shield were slightly larger than the rear end. The shell of the cone was -made of ¹⁄₂-in. plates. The thinned plate cutting-edge of Barlow’s -shield was replaced by Greathead with a circular ring of cast iron. -Greathead also altered the construction of the diaphragm by arranging -the angle stiffeners so that they ran horizontally and vertically, and -by fastening the diaphragm plates to an interior cast-iron ring -connected to the shell plates. This was a decided structural -improvement, but it was accompanied with another modification which was -quite as decided a retrogression from Barlow’s design. Greathead made -the diaphragm opening rectangular and to extend very nearly from the top -to the bottom of the shield, thus abandoning the element of safety -provided by Barlow in case of an irruption of water. Fortunately the -material penetrated by the shield for the Tower tunnel was so compact -that no trouble was had from water; but the dangerous character of the -construction was some years afterwards disastrously proven in driving -the Yarra River tunnel at Melbourne, Australia. To drive his shield -Greathead employed six 2¹⁄₂-in. screw-jacks capable of developing a -total force of 60 tons. The tails of the jack bore against the completed -lining, which consisted of cast-iron rings 18 ins. wide and ⁷⁄₈ in. -thick, each ring being made up of a crown piece and three segments. The -different segments and rings were provided with double (exterior and -interior) flanges, by means of which they were bolted together. The -soil behind the lining was filled with liquid cement injected through -small holes by means of a hand pump. - -[Illustration: FIG. 127.--Shield Suggested by Greathead for the Proposed -North and South Woolwich Subway.] - -[Illustration: FIG. 128.--Beach’s Shield Used on Broadway Pneumatic -Railway Tunnel.] - -The remarkable success of the London Tower tunnel encouraged Barlow to -form in 1871 a company to tunnel the Thames between Southwark and the -City, and Greathead, in 1876, to project a tunnel under the same -waterway known as the North and South Woolwich Subway. Barlow’s -concession was abrogated by Parliament in 1873, without any work having -been done. Greathead progressed far enough with his enterprise to -construct a shield and a large amount of the iron lining when the -contractors abandoned the work. From the brief description of his shield -given by Greathead to the London Society of Civil Engineers, it -contained several important differences from the shield built by him for -the London Tower tunnel, as is shown by Fig. 127. The changes which -deserve particular notice are the great extension of the shield behind -the diaphragm, the curved form of the diaphragm, and the use of -hydraulic jacks. Greathead had also designed for this work a special -crane to be used in erecting the cast-iron segments of the lining. - -[Illustration: FIG. 129.--Shield for City and South London Railway.] - -While these works had been progressing in England, Mr. Beach, an -American, received a patent in the United States for a tunnel shield of -the construction shown by Fig. 128, which was first tried practically in -constructing a short length of tunnel under Broadway for the nearly -forgotten Broadway Pneumatic Underground Railway. This shield, as is -indicated by the illustration, consisted of a cylinder of wood with an -iron-cutting-edge and an iron tail-ring. Extending transversely across -the shield at the front end were a number of horizontal iron plates or -shelves with cutting-edges, as shown clearly by the drawing. The shield -was moved ahead by means of a number of hydraulic jacks supplied with -power by a hand pump attached to the shield. By means of suitable valves -all or any lesser number of these jacks could be operated, and by thus -regulating the action of the motive power the direction of the shield -could be altered at will. Work was abandoned on the Broadway tunnel in -1870. In 1871-2 Beach’s shield was used in building a short circular -tunnel 8 ft. in diameter in Cincinnati, and a little later it was -introduced into the Cleveland water-works tunnel 8 ft. in diameter. In -this latter work, which was through a very treacherous soil, the shield -gave a great deal of trouble, and was finally so flattened by the -pressures that it was abandoned. The obviously defective features of -this shield were its want of vertical bracing and the lack of any means -of closing the front in soft soil. - -[Illustration: FIG. 130.--Shield for St. Clair River Tunnel.] - -[Illustration: ~Longitudinal Section.~ - -~Cross Section.~ - -FIG. 131.--Shield for Blackwall Tunnel.] - -With the foregoing brief review of the early development of the shield -system of tunneling, we have arrived at a point where the methods of -modern practice can be studied intelligently. In the pages which follow -we shall first illustrate fully the construction of a number of shields -of typical and special construction, and follow these illustrations with -a general discussion of present practice in the various details of -shield construction. - -[Illustration: ~Transverse Section.~ - -~Longitudinal Section.~ - -FIG. 132.--Elliptical Shield for Clichy Sewer Tunnel, Paris.] - -[Illustration: ~Longitudinal Section.~ - -~Cross Section.~ - -FIG. 133.--Semi-elliptical Shield for Clichy Sewer Tunnel.] - -Mr. Raynald Légouez, in his excellent book upon the shield system of -tunneling, considers that tunnel shields may be divided into three -classes structurally, according to the character of the material which -they are designed to penetrate. In the first class he places shields -designed to work in a stiff and comparatively stable soil, like the -well-known London clay; in the second class are placed those constructed -to work in soft clays and silts; and in the third class those intended -for soils of an unstable granular nature. This classification will, in a -general way, be kept by the writer. As a representative shield of the -first class, the one designed for the City and South London Railway is -illustrated in Fig. 129. The shields for the London Tower tunnel, the -Waterloo and City Railway, the Glasgow District Subway, the Siphons of -Clichy and Concorde in Paris, and the Glasgow Port tunnel, are of the -same general design and construction. To represent shields of the second -class, the St. Clair River and Blackwall shields are shown in Figs. 130 -and 131. The shields for the Mersey River, the Hudson River, and the -East River tunnels also belong to this class. To represent shields of -the third class, the elliptical and semi-elliptical shields of the -Clichy tunnel work in Paris are shown by Figs. 132 and 133. The -semi-circular shield of the Boston Subway is illustrated by Fig. 134. - -[Illustration: ~Half Transverse Section A-B.~ - -~Half Rear-End Elevation.~ - -~Details of Casting Supporting Ends of Jacks.~ - -~Details of Castings under Ends of Girders.~ - -~Longitudinal Section C-D.~ - -FIG. 134.--Roof Shield for Boston Subway.] - - -=Prelini’s Shield.=--In closing this short review mention will be made -of a new shield designed and patented by the Author and shown in Fig. -135. It is an articulated shield composed of two separated shields whose -outer shells overlap each other. The shields are connected together by -means of hydraulic jacks attached all around the two diaphragms. Between -these diaphragms is a large inclosed space called a safety chamber, -where the men can withdraw in case of accidents and where the air can be -immediately raised to the required pressure. This is an advantage in -case of blow-outs, because the flooding of the tunnel is prevented, -while the accident is limited to only a few feet from the front. On -account of the shield being advanced half at a time it is always under -control and is thus better directed through grade and alignment. -Besides, this shield will not rotate around its axis and consequently it -can be built of any shape, thus permitting the construction of -subaqueous tunnels of any cross-section and even with a wider -foundation, which is impossible to-day with the ordinary rotating -shields of circular cross-section. - -[Illustration: FIG. 135.--Transversal and Longitudinal Section of -Prelini’s Shield.] - - -SHIELD CONSTRUCTION. - - -=General Form.=--Tunnel shields are usually cylindrical or -semi-cylindrical in cross-section. The cylinder may be circular, -elliptical, or oval in section. Far the greater number of shields used -in the past have been circular cylinders; but in one part of the sewer -tunnel of Clichy, in Paris, an elliptical shield with its major axis -horizontal, was used, and the German engineer, Herr Mackensen, has -designed an oval shield, with its major axis vertical. A semi-elliptical -shield was employed on the Clichy tunnel, and semi-circular shields were -used on the Baltimore Belt Line tunnel and the Boston Subway in America. -Generally, also, tunnel shields are right cylinders; that is, the front -and rear edges are in vertical planes perpendicular to the axis of the -cylinder. Occasionally, however, they are oblique cylinders; that is, -the front or rear edges, or both, are in planes oblique to the axis of -the cylinder. One of these visor-shaped shields was employed on the -Clichy tunnel. - - -=The Shell.=--It is absolutely necessary that the exterior surface of -the shell should be smooth, and for this reason the exterior rivet heads -must be countersunk. It is generally admitted, also, that the shell -should be perfectly cylindrical, and not conical. The conical form has -some advantage in reducing the frictional resistance to the advance of -the shield; but this is generally considered to be more than -counterbalanced by the danger of subsidence of the earth, caused by the -excessive void which it leaves behind the iron tunnel lining. For the -same reason the shell plate, which overlaps the forward ring of the -lining, should be as thin as practicable, but its thickness should not -be reduced so that it will deflect under the earth pressure from above. -Generally the shell is made of at least two thicknesses of plating, the -plates being arranged so as to break joints, and, thus, to avoid the use -of cover joints, to interrupt the smooth surface which is so essential, -particularly on the exterior. The thickness of the shell required will -vary with the diameter of the shield, and the character and strength of -the diametrical bracing. Mr. Raynald Légouez suggests as a rule for -determining the thickness of the shell, that, to a minimum thickness of -2 mm., should be added 1 mm. for every meter of diameter over 4 meters. -Referring to the illustrations, Figs. 128 to 132 inclusive, it will be -noted that the St. Clair tunnel shield, 21¹⁄₂ ft. in diameter, had a -shell of 1-in. steel plates with cover-plate joints and interior angle -stiffeners; the shell of the East River tunnel shield, 11 ft. in -diameter, was made up of one ¹⁄₂-in. and one ³⁄₈-in. plate; the -Blackwall tunnel shield, 27 ft. 9 ins. in diameter, had a shell -consisting of four thicknesses of ⁵⁄₈-in. plates; and the Clichy tunnel -shield, with a diameter of 2.06 meters, had a shell 2 millimeters -thick. - - -=Front-End Construction.=--By the front end is meant that portion of the -shield between the cutting-edge and the vertical diaphragm. The length -of this portion of the shield was formerly made quite small, and where -the material penetrated is very soft, a short front-end construction yet -has many advocates; but the general tendency now is to extend the -cutting-edge far enough ahead of the diaphragm to form a fair-sized -working chamber. Excavation is far more easy and rapid when the face can -be attacked directly from in front of the diaphragm than where the work -has to be done from behind through the apertures in the diaphragm. So -long as the roof of the excavation is supported from falling, experience -has shown that it is easily possible to extend the excavation safely -some distance ahead of the diaphragm. In reasonably stable material, -like compact-clay, the front face will usually stand alone for the short -time necessary to excavate the section and advance the shield one stage. -In softer material the face can usually be sustained for the same short -period by means of compressed air; or the face of the excavation, -instead of being made vertical, can be allowed to assume its natural -slope. In the latter case a visor-shaped front-end construction, such as -was used on some portions of the Clichy tunnel, is particularly -advantageous. The following figures show the lengths of the front ends -of a number of representative tunnel shields. - - City and South London 1 ft. - St. Clair River 11.25 „ - Hudson River 5²⁄₃ „ - Mersey River 3 „ - East River 3²⁄₃ „ - Blackwall 6.5 „ - -Two general types of construction are employed for the cutting-edge. The -first type consists of a cast-iron or cast-steel ring, beveled to form a -chisel-like cutting-edge and bolted to the ends of the forward shell -plates. This construction was first employed in the shield for the -London Tower tunnel, and has since been used on the City and South -London, Waterloo and City, and the Clichy tunnels. The second -construction consists in bracing the forward shell plates by means of -right triangular brackets, whose perpendicular sides are riveted -respectively to the shell plates and the diaphragm, and whose inclined -sides slant backward and downward from the front edge, and carry a -conical ring of plating. The shields for the St. Clair River, East -River, and Blackwall tunnels show forms of this type of cutting-edge -construction. A modification of the second type of construction, which -consists in omitting the conical plating, was employed on some of the -shields for the Clichy tunnel. This modification is generally considered -to be allowable only in materials which have little stability, and which -crumble down before the advance of the cutting-edge. Where the material -is of a sticky or compact nature, into which the shield in advancing -must actually cut, the beveled plating is necessary to insure a clean -cutting action without wedging or jamming of the material. - - -=Cellular Division.=--It is necessary in shields of large diameter to -brace the shell horizontally and vertically against distortion. This -bracing also serves to form stagings for the workmen, and to divide the -shield into cells. The following table shows the arrangement of the -vertical and transverse bracing in several representative tunnel -shields. - - +------------------+----------+-------+-------+-------+ - | NAME OF TUNNEL. | DIAMETER.| HORI- |PLATES,| VERT. | - | | |ZONTAL.| DIST. |BRACES.| - | | | | APART.| | - +------------------+----+-----+-------+-------+-------+ - | |Ft. | In. | No. | Ft. | No. | - |Hudson River |19 |11 | 2 | 6.54 | 2 | - |Clichy |19.4| 0 | 2 | 6.54 | None | - |St. Clair River |21 | 6 | 2 | 6.98 | 3 | - |Waterloo (Station)|24 |10¹⁄₂| 2 | 7.12 | None | - |Blackwall |27 | 8 | 2 | 6.0 | 3 | - |East River |11 | ³⁄₄| None | ... | 1 | - +------------------+----+-----+-------+-------+-------+ - -Referring first to the horizontal divisions, it may be noted that they -serve different purposes in different instances. In the Clichy tunnel -shield the horizontal divisions formed simply working platforms; in the -Waterloo tunnel shield they were designed to abut closely against the -working face by means of special jacks, and so to divide it into three -separate divisions; in the St. Clair tunnel they served as working -platforms, and also had cutting-edges for penetrating the material -ahead; and in the Blackwall tunnel shield they served as working -platforms, and had cutting-edges as in the St. Clair tunnel shield, and -in addition the middle division was so devised that the two lower -chambers of the shield could be kept under a higher pressure of air than -the two upper chambers. Passing now to the vertical divisions, they -serve to brace the shell of the shield against vertical pressures, and -also to divide the horizontal chambers into cells; but unlike the -horizontal plates they are not provided with cutting-edges. The St. -Clair, Hudson River, and Blackwall tunnel shields illustrate the use of -the vertical bracing for the double purpose of vertical bracing and of -dividing the horizontal chambers into cells. The Waterloo tunnel shield -is an example, of vertical bracing employed solely as bracing. The -vertical division of the East River tunnel shield was employed in order -to allow the shield to be dissembled in quadrants. - - -=The Diaphragm.=--The purpose of the shield diaphragm is to close the -rear end of the shield and the tunnel behind from an inrush of water and -earth from the face of the excavation. It also serves the secondary -purpose of stiffening the shell diametrically. Structurally the -diaphragm separates the front-end construction previously described from -the rear-end construction, which will be described farther on; and it is -usually composed of iron or steel plating reinforced by beams or -girders, and pierced with one or several openings by which access is had -to the working face. In stable material, where caving or an inrush of -water and earth is not likely, the diaphragm is omitted. The shield of -the Waterloo tunnel is an example of this construction. In more -treacherous materials, however, not only is a diaphragm necessary, but -it is also necessary to diminish the size of the openings through it, -and to provide means for closing them entirely. Sometimes only one or -two openings are left near the bottom of the diaphragm, as in the St. -Clair and Mersey tunnel shields; and sometimes a number of smaller -openings are provided, as in the East River and Hudson River tunnel -shields. - -In highly treacherous materials subject to sudden and violent irruptions -of earth from the excavation face, it sometimes is the case that -openings, however small, closed in the ordinary manner, are -impracticable, and special construction has to be adopted to deal with -the difficulty. The shields for the Mersey and for the Blackwall tunnels -are examples of such special devices. In the Mersey tunnel a second -diaphragm was built behind the first, extending from the bottom of the -shield upward to about half its total height. The aperture in the first -diaphragm being near the bottom, the space between the second and first -diaphragms formed a trap to hold the inflowing material. The Blackwall -tunnel shield, as previously indicated, had its front end divided into -cells. Ordinarily the face of the excavation in front of each cell was -left open, but where material was encountered which irrupted into these -cells a special means of closing the face was necessary. This consisted -of three poling-boards or shutters of iron held one above the other -against the face of the excavation. These shutters were supported by -means of strong threaded rods passing through nuts fastened to the -vertical frames, which permitted each shutter to be advanced against or -withdrawn from the face of the excavation independently of the others. -Various other constructions have been devised to retain the face of the -excavation in highly treacherous soils, but few of them have been -subjected to conclusive tests, and they do not therefore justify -consideration. - - -=Rear-end Construction.=--By the rear end of the shield is meant that -portion at the rear of the diaphragm. It may be divided into two parts, -called respectively the body and the tail of the shield. The chief -purpose of the body of the shield is to furnish a place for the location -of the jacks, pumps, motors, etc., employed in manipulating the shield. -It also serves a purpose in distributing the weight of the shield over a -large area. To facilitate the passage of the shield around curves, or -in changing from one grade to another, it is desirable to make the body -of the shield as short as possible. In the Mersey, Clichy, and Waterloo -tunnel shields, and, in fact, in most others which have been employed, -the shell plates of the body have been reinforced by a heavy cast-iron -ring, within and to which are attached the jacks and other apparatus. -The latest opinion, however, seems to point to the use of brackets and -beams for strengthening the shell for the purpose named, rather than to -this heavy cast-iron construction. In the Hudson River, St. Clair River, -and East River tunnel shields, with their long and strongly braced -front-end construction to carry the jacks, the body of the shield, so to -speak, is omitted and the rear-end construction consists simply of the -tail plating. In the Blackwall shield, the body of the shield shell -provides the space necessary for the double diaphragms and the cells -which they inclose. In a general way, it may be said that the present -tendency of engineers is to favor as short and as light a body -construction as can be secured. - -The tail of the shield serves to support the earth while the lining is -being erected; and for this reason it overlaps the forward ring of the -lining, as shown clearly by most of the shields illustrated. To fulfill -this purpose, the tail-plates should be perfectly smooth inside and -outside, so as to slide easily between the outside of the lining plates -and the earth, and should also be as thin as practicable, in order not -to leave a large void behind the lining to be filled in. In soils which -are fairly stable, the tail construction is often visor-shaped; that is, -the tail-plates overlap the lining only for, say, the roof from the -springing lines up, as in one of the shields for the Clichy tunnel. In -unstable materials the tail-plating extends entirely around the shield -and excavation. The length of the tail-plating is usually sufficient to -overlap two rings of the lining, but in one of the Clichy tunnel shields -it will be noticed that it extended over three rings of lining. This -seemingly considerable space for thin steel plates is made possible by -the fact that the extreme rear end of the tail always rests upon the -last completed ring of lining. - -In closing these remarks concerning the rear-end construction, the -accompanying table, prepared by Mr. Raynald Légouez, will be of -interest, as a general summary of principal dimensions of most of the -important tunnel shields which have been built. The figures in this -table have been converted from metric to English measure, and some -slight variation from the exact dimensions necessarily exists. The -different columns of the table show the diameter, total length, and the -length of each of the three principal parts into which tunnel shields -are ordinarily divided in construction as previously described: - - +---------------------+-----------------------------------+ - | | LENGTH IN FEET. | - | NAME OF SHIELD. +---------+-----+-----+------+------+ - | |DIAMETER.|TAIL.|BODY.|FRONT.|TOTAL.| - +---------------------+---------+-----+-----+------+------+ - |Concorde Siphon | 6.75 | 2.51| 2.55| 1.16| 6.67| - |Clichy Siphon | 8.39 | 2.51| 2.55| 1.16| 6.16| - |Mersey | 9.97 | 5.61| 2.98| 2.98| 11.58| - |East River | 10.99 | 3.51| 0.32| 3.67| 7.51| - |City and South London| 10.99 | 2.65| 2.82| 1.01| 6.49| - |Glasgow District | 12.07 | 2.65| 2.82| 1.01| 6.49| - |Waterloo and City | 12.99 | 2.75| 2.98| 1.24| 6.98| - |Glasgow Harbor | 17.25 | 2.75| 2.98| 1.08| 8.49| - |Hudson River | 19.91 | 4.82| 2.98| 5.67| 10.49| - |St. Clair River | 21.52 | 4.00| 2.98| 11.25| 15.25| - |Clichy Tunnel |23.7-19.8| 4.00| 2.98| 6.88| 17.22| - |Clichy Tunnel |23.8-19.4| 7.44|11.90| 4.46| 23.65| - |Blackwall | 27.00 | 6.98| 5.90| 6.59| 19.48| - |Waterloo Station | 24.86 | 3.34| 5.51| 1.14| 10.00| - +---------------------+---------+-----+-----+------+------+ - -A shield of 60 or 100 tons weight can hardly be directed along the line -of the proposed tunnel and also through curves and grades, especially -when driven through loose or muddy soils. The tunnels of the New York -and Hudson River Railroad under the Hudson, and the tunnel of the New -York Rapid Transit Railway under the East River, show marked evidence of -how troublesome this work is. To avoid these and other inconveniences -encountered in every shield, the Author has designed a new shield which -was briefly described at page 251. - -[Illustration: FIG. 136.--Elevation and Section of Hydraulic Jack, East -River Gas Tunnel.] - - -=Jacks.=--The motive power usually employed in driving modern tunnel -shields is hydraulic jacks. In some of the earlier shields screw-jacks -were used, but these soon gave way to the more powerful hydraulic -device. The manner of attaching the hydraulic jacks to the shield is -always to fasten the cylinder castings at regular intervals around the -inside of the shell, with the piston rods extending backward to a -bearing against the forward edge of the lining. In the older forms of -shield, having an interior cast-iron reinforcing ring construction, the -jack cylinder castings were always attached to this cast-iron ring; but -in many of the later shields constructed without this cast-iron -reinforcing ring, the cylinder castings are attached to the shell by -means of bracket and gusset connections. The number and size of the -jacks employed, and the distance apart at which they are spaced, depend -upon the size of the shield and the character of the material in which -it is designed to work. In stiff and comparatively stable clays, the -skin friction of the shield is comparatively small, and an aggregate -jack-power of from 4 to 5 tons per square yard of the exterior friction -surface of the shield has usually been found ample. The cylinders are -spaced about 5³⁄₄ ft. apart, and have a working diameter of from 5 to 6 -ins., with a water pressure of about 1000 lbs. per sq. in. In soft, -sticky material, giving a high skin friction, the aggregate jack-power -required per square yard of exterior shell surface rises to from 18 to -24 tons; the jacks are spaced about 3 ft. apart; and the working -cylinder diameter and water pressure are, respectively, about 6 or 7 -ins., and from 4000 lbs. to 6000 lbs. per sq. in. With these high -pressures, power pumps are necessary to give the required water -pressure; but where the pressure required does not exceed 1000 lbs. per -sq. in., hand pumps may be, and usually are, employed. Fig. 136 shows -the hydraulic jacks used in the East River Gas Tunnel at New York. The -number of jacks required depends upon the diameter of the shield, and, -of course, upon the distance apart which they are placed. In the City -and South London tunnel shield six jacks were used, and in the Blackwall -shield 24 were used. The mechanical construction of the jacks for tunnel -shields presents no features out of the usual lines of such devices used -elsewhere. The jacks used on the East River tunnel shield are shown by -Fig. 136. - -Two general methods are employed for transmitting the thrust of the -piston rods against the tunnel lining. The object sought in each is to -distribute the thrust in such a manner that there is no danger of -bending the thin front flange of the forward lining ring. In English -practice the plan usually adopted is to attach a shoe or bearing casting -to the end of the piston rod, which will distribute the pressure over a -considerable area. An example of this construction is the shield for the -City and South London tunnel. In the East River and St. Clair River -tunnels built in America, the tail of the piston rod is so constructed -that the thrust is carried directly to the shell of the lining. - - -LINING. - -Either iron or masonry may be used for lining shield-driven tunnels but -present practice is almost universally in favor of iron lining. As -usually built, iron lining consists of a series of successive cast-iron -rings, the abutting edges of which are provided with flanges. These -flanges are connected by means of butts, the joints being packed with -thin strips of wood, oakum, cement, or some other material to make them -water-tight. Each lining ring is made up of four or more segments, which -are provided with flanges for bolted connections similar to those -fastening the successive rings. Generally the crown segment is made -considerably shorter than those forming the sides and bottom of the -ring. The erection of the iron segments forming the successive rings of -the lining may be done by hand in tunnels of small diameter where the -weights to be handled are comparatively light, but in tunnels of large -size special cranes attached to the shield or carried by the finished -lining are employed. The construction of the iron lining for the Hudson -River tunnel is illustrated in Chapter XX., and that for the St. Clair -River tunnel is shown by Fig. 137. - -[Illustration: ~Part Transverse Section.~ - -~Longitudinal Section.~ - -FIG. 137.--Cast-Iron Lining, St. Clair River Tunnel.] - - - - -CHAPTER XX. - -SUBMARINE TUNNELING (Continued). - -THE SHIELD AND COMPRESSED AIR METHOD. THE HUDSON RIVER TUNNEL OF THE -PENNSYLVANIA RAILROAD. - - -The shield and compressed air method of excavating subaqueous tunnels is -used when the distance is small between the roof of the tunnel and the -bed of the river. These tunnels are usually driven from the shafts sunk -from each shore. It is very seldom they can be driven also by an -intermediate shaft. This, however, was done in the case of the Belmont -tunnel under the East River. Here the tunnels passed under the -man-of-war reef where a working shaft was sunk. - -The plant is located at some convenient point near the head shaft. It -consists of a set of boilers to provide the power for the different -machines. They are low and high pressure compressors, the former supply -the air through the tunnel; the latter, the air for working the drills, -in case rock is encountered, and power for hauling and hoisting -purposes. The various pumps force the water for the hydraulic rams that -drive the shield and work the erector. They also remove the water from -the tunnel which always collects in variable quantities at the bottom of -the excavation. Besides the machines for light and ventilation purposes, -the head shaft is provided with an overhead construction where are -housed the hoisting machines, the telephone and other means of -communication with the work at the front. Usually a long trestle is -built in connection with the head shaft, leading to the dumping place -and yard. On this inclined elevated structure are located, also, the -tracks upon which will run the small cars used inside the tunnel for -hauling purposes. - -The shafts are excavated on a square, rectangular or circular plan and -are usually lined with masonry. It is only recently that shafts -excavated through loose soils have been lined with the same cast-iron -lining used in the tunnels, the only difference being that the rings -were laid flat on the ground and attached to those already sunk. - -After the shaft has been sunk to the required level, the tunnel is -driven toward the river by any one of the methods used for land work. At -some convenient distance from the shaft, the dimensions of the tunnel -are enlarged for a length of 20 or 30 ft. In this larger space, called -the shield chamber, the shield is assembled, mounted, and, when -completed, it is slowly pushed toward the river. The tunnel is excavated -from the shield chamber on, with dimensions equal to the exterior shell -of the shield. - -The construction of the shield and the hydraulic jacks used for its -advance are explained in a preceding chapter. - -In very loose soils, a solid bulkhead of masonry is built across the -tunnel, after the shield has advanced to a certain distance and some -rings of the cast-iron lining have been erected. The bulkhead is -provided with three air locks--two near the floor of the tunnel, for -working purposes, and one near the roof, called the emergency lock, -which, as the name suggests, is used only in case of danger. The air -locks are steel cylinders from 10 to 15 ft. long and 6 ft. in diameter, -made up of boiler plates. They are provided with doors at each end, -besides the pipes for the admission and exit of compressed air. The -working locks also have narrow-gauge tracks for hauling purposes. In -rock or more consistent soil the bulkhead is constructed after the -shield is far ahead, since there is no immediate necessity, under these -conditions, to use the compressed air. In both the loose and good soils, -when the shield has been advanced over 500 ft. from the bulkhead, a -second bulkhead, with air locks, is erected in the tunnel. The first is -left in place but used only in case of emergency. - -To direct the shield along the center line and through curves and -grades, accurate measurements are taken, and the distance between the -shield and the last ring inserted in the iron lining is regulated -accordingly. The alignment inside the tunnel is maintained in a very -simple way. For this purpose, points corresponding to the center line -are marked on the roof at distances of 100 ft. Nearly 100 ft. from the -shield, a transit is set up on a strong scaffold spanning the tunnel, -and it is supported by the flanges of the iron lining. A plumb-line is -hung from one of the points of the roof already determined, as -indicating the center line; and the transit man aligns his instrument -with this plumb-line; after this he “plunges” his telescope. A rodman -next places a horizontal rod of special construction between the flanges -of the last ring of the lining. This rod has in the center an open slot -which carries a glass with a black vertical line. The slot is graduated, -the zero of graduation remains in the center while the vertical line is -moved right and left. The rodman places a lamp behind the slot and the -transit-man tells him how to move the dark line until it coincides with -the axis of the tunnel. If the ring, just erected, be a little out of -alignment, it is readjusted by pushing the shield a little more on the -side that has swerved from the axis of the tunnel. As the shield is -pushed forward, it is kept in place by four men with graduated rods, one -man on each side of the shield, one on top and the other on the floor. -As the shield progresses, they repeat aloud in succession, the distance -indicated on the rods, which is the distance from the shield to the -outer circumferential flange of the last ring of the lining. When an -advance of one foot has been made, readings are taken at every inch; and -when very near the required distance, they are taken at every quarter of -an inch. In this way it is not difficult to bring the shield back into -line, in case it may have shifted a little to the right or left. When -curves are met, the rings are no longer cylindrical segments but tores, -so that the segments at one side are longer than those on the other. In -this case, the shield is advanced more on one side by a quantity equal -to the difference of the two sides of the ring to be erected. At each -advance the shield is moved 2 ft. or 2¹⁄₂ ft. ahead, the distance -corresponding to the length of the cast-iron rings of the lining. Within -the space now open between the shield and the lining another ring is -inserted. The ring is composed of different segments provided with -flanges and holes bored so they can be bolted together. The segments of -the lining are very heavy and difficult to handle but they are easily -set by means of the erector. - -When the erector is not mounted on the shield, it is located in the -middle of a girder placed across the iron rings of the lining and just -at the rear end of the shield. The girder, at both extremities, has -flanged wheels resting on rails which are placed on brackets. These -brackets are attached temporarily to the flanges of the iron lining. The -erector is provided with an arm capable to swing in a full circle. Its -movements are regulated by two hydraulic jacks, located horizontally on -the spanning girder. On the extreme end of the revolving arm are -projections with holes for the bolts. Each segmental plate of the lining -has a kind of plug in the center which is cast together with the plate -and is provided with holes for the bolt. In placing the segmental plates -of the lining, the arm of the erector is swung over the plate to be -lifted, then two bolts are passed through the holes in the projection of -the erector, and through those in the plug. The arm of the erector is -then moved upwards until the plate, free from all obstacles, is swung -very near its intended position. There it is adjusted and held until -bolts are inserted to fix it to the plates of the preceding ring. - -In connection with the method of excavating submarine tunnels by means -of shield and compressed air, the excavation varies with the quality of -soil encountered. In compact rock the usual heading and bench method, so -common in land tunnels, is also employed in this case. The shield is -left behind in presence of good rock. - -The men at the front attack the rock with air drilling machines and -charges of dynamite. The holes are driven at a smaller depth than in -land work; very light charges of dynamite are used and only a few holes -fired at each round. Every precaution is taken in order not to disturb -the shield and the bed of the river any more than is possible, because -at a shallow depth the blast would tend to widen the existing crevices -in the rock and thus permit an inflow of water. When the rock is -fissured or disintegrated and the roof of the excavation at the front -requires timbering, the shield should be kept closer to the front. In -this way the quantity of timber for strutting is greatly reduced, so -lessening the probabilities of fires. It is very difficult, in -compressed air, to extinguish fires and in almost every instance the -only way is to flood the tunnel. This was done at the Manhattan end of -the tunnel under the East River for the extension to Brooklyn of the New -York Subway. - -The excavation is made by hand in loose but compact soils such as clay. -The men work on platforms located at the front of the shield and they -are protected from the caving-in of the roof by a hood added for working -through loose soils. The men excavate the material which is shoveled -inside the tunnel and is carried away in small cars. The shield is very -close to the front of the excavation in loose soil. The East Boston -tunnel, under Boston Harbor, connecting with the Boston Subway, was -excavated through blue clay. The minimum distance between the bottom of -the water and the roof of the excavation was 18 ft. The tunnel was -excavated by means of compressed air and the shield which was only used -for the roof. It slid on top of concrete side walls built in two drifts -which were excavated nearly 100 ft. ahead of the shield. The tunnel was -lined with concrete, the arch being reinforced by longitudinal steel -rods which received the thrust of jacks used for advancing the shield. -The material in the drifts under the shield and the bench was removed by -hand and carried away in small cars. - -Subaqueous tunnels driven through very loose soils can be excavated by -simply leaving the doors open while the shield is pushed ahead. The -material, dislodged by the cutting edge of the shield, is forced through -the doors and falls on the floor whence it is removed in small cars. In -very loose soils the excavation has been made in a still more economic -way; the shield with closed doors is simply squeezed through the soil. -This method is financially convenient, because all the excavating and -hauling operations are eliminated and the tunnel progresses from 40 to -50 ft. per day, but clearly indicates a lack of stability. In this -manner, the Hudson River tunnel of the New York and New Jersey Railroad -was constructed. - -The pressure of the air in the tunnel depends upon the depth and as a -rule it varies between 20 and 40 or even more pounds per square inch -above atmospheric pressure. Working in compressed air causes a peculiar -disease commonly known as “bends” or “caisson disease” often proving -fatal. To prevent and remedy the disease, the engineers should order a -set of rules to be strictly observed. The preventative measures should -be, first, to employ only sober, strong and healthy men, never one who -has not successfully passed the examination of the attending physician; -second, to order the lock tenders never to allow any man in or out of -the tunnel unless he has spent at least ten minutes within the locks. -Both compression and decompression should be thorough and it cannot be -in less than this time. A stop of only a few minutes in the locks is not -sufficient and this incomplete compression or decompression is the real -cause of the bends. The men become careless after they have been in the -compressed air for some time, and they try to reduce this tiresome -operation to a minimum, hence the duty of the engineer to strictly -enforce this rule. The remedial measures should consist of constant -medical attendance near the shafts and the erection of a compressed air -hospital where the men affected by bends for lack of decompression may -be attended and cured. - - -THE HUDSON RIVER TUNNELS OF THE PENNSYLVANIA RAILROAD.[13] - -The tunnels constructed under the Hudson River for the Pennsylvania -Railroad, consist of two parallel tubes driven side by side 14 ft. -apart. The tubes are of circular cross-section, 23 ft. exterior -diameter, and are lined with cast-iron rings. The tunnels were driven -from two shafts, one on the eastern shore of the Hudson River near 32nd -St. and 11th Ave., New York; the other at Weehawken, New Jersey, near -the piers of the Erie Railroad. The horizontal distance between the -shafts was 6550 ft. The permanent one at Weehawken was built on a square -plan, 130 ft. to a side. It was lined with concrete masonry and the -walls were battered in such a way as to become the shape of an inverted -frustum of a pyramid. It was provided with five openings at the bottom, -four of these are used by trains that run in the open, the fifth one -leads to a power house near by. During the construction of the tunnels -one-third of this shaft was used for the land portion of the tunnel -under Bergen Hill, while the remaining two-thirds were devoted to the -construction of the tunnel under the river. The working shaft on -Manhattan Island was a side shaft of rectangular plan 30 ft. by 22 ft., -the tunnel proper being connected by two drifts 10 ft. by 10 ft. each. -The shield rooms 23 ft. long, were situated on both sides of the river -just in front of the shafts. On the New York side, the shields, one for -each tube, were built inside the iron lining of the shield chamber, and -the hoisting tackle was slung from the iron lining. The erection on the -Weehawken side was done in the bare rock excavation where timber -falsework was used. After the shields were finished and in position, the -first two rings of the lining were erected in the tail of the shield. -These rings were firmly braced to the rock and the chamber lining; then -the shields were shoved ahead by their own jacks, another ring was built -and so on. - - [13] Condensed from paper by James Forgie, “Eng. News,” Vol. LVI, and - by H. B. Hewett and W. L. Brown, “Proc. Am. Soc. C. E.”, Vol. XXXVI. - -[Illustration: ~Rear Elevation of Shield.~ - -~Vertical Section.~ - -~Half Section A-B.~ - -~Half Section C-D.~ - -~Horizontal Section.~ - -FIG. 138.--General Elevations and Sections of Shield.] - - -=Shield.=--The shields used in these tunnels were designed by Mr. James -Forgie, M. Inst. C. E. and M. Am. Soc. C. E., and were provided with -three innovations: the segmental doors, the sliding platforms and the -removable hood. The shields, Fig. 138, were circular, 23 ft. 6¹⁄₄ ins. -in external diameter, and were 16 ft. long, exclusive of the hood. The -tail of the shield overlapped the lining, the maximum being 6 ft. 4¹⁄₂ -ins. during ordinary working; the minimum, 2 ft. during the operation of -taking any ram out for repairing. The shields had only one transverse -bulkhead made up of two continuous horizontal platforms and three -vertical partitions stiffening angular web plates fore and aft the ram -chambers. They were connected by angles and skin plates which formed a -ring-shaped frame 25 ins. thick radially and nearly 5 ft. long. Between -the vertical and horizontal partitions were left openings which either -were partially or entirely closed by segmental doors pivoted on an axis -parallel to the face of the shield bulkhead. There were nine of such -openings on each shield, the clear width being 2 ft. 7 ins., the height -varying from 2 ft. 2 ins. to 3 ft. 4 ins., according to the location. -The hood at the front of the shield was designed so as to be detached -underground and was made of complete segments to permit easy erection or -detachment. The hood was extended as far as the upper platform, thus -protecting only the roof of the excavation. It was attached to the -shield by means of bolts, and, when removed, it was replaced by the -cast-steel cutting-edge, built in 24 sections and placed all around the -shield. The eight sliding platforms, another characteristic of this -shield, could be extended 2 ft. 9 ins. in front of the shield by means -of hydraulic rams, and, when so extended, were able to stand a pressure -of 7900 lbs. per sq. ft. These sliding platforms were used as hoods for -the protection of the men working through loose soils, while in rock -they enabled the drilling and blasting to be carried on at three levels. -A water trap or bird fountain was constructed, at the rear of the -bulkhead of the shield, by means of angle irons to which steel plates -were bolted. The opening to the face was so spacious that in an -emergency the men could readily escape by getting over this trap into -safety. Besides, with the assistance of compressed air, it was -sufficient to perfectly trap the water-bearing ground, in case the face -collapsed. Including rams and erector, the total weight of the shield -was 193 tons. - - -=Hydraulic Rams.=--The shield was operated by hydraulic pressure. The -machines were designed for a maximum pressure of 5000 lbs., to a -minimum of 2000 lbs., while the average working pressure was 3500 lbs. -per sq. in. The forward movement of the shield was obtained by means of -24 single-acting rams 8¹⁄₂ in. in diameter and with 38 in. stroke. Each -ram exerted a pressure of nearly 100 tons, so that the combined action -of the 24 rams was equal to 2400 tons. Each sliding platform was -operated by two single-acting rams 3¹⁄₂ ins. in diameter and with 2 ft. -9 in. stroke. The rams were attached to the rear face of the shield and -the front ends of the cylinders to the front ends of the sliding -platforms, and since the cylinders were movable and free-sliding so also -were the platforms. - - -=Erector.=--The erector, a box-shaped frame mounted on a central shaft, -revolved in bearings attached to the shield. Inside this frame there was -a differential hydraulic plunger of 4 in. and 3 in. diameters and 48 in. -stroke. To the plunger head were attached two channels which slide -inside the box frame and to the projecting ends of which the grip was -attached. At the opposite end of the box frame was attached a -counter-weight which balances about 700 lbs. of the tunnel segment at 11 -ft. radius. The erector was revolved by two single-acting rams fixed -horizontally to the back of the shield, above the erector pivot, through -double chains and chain wheels which were keyed to the erector shaft. - - -=Air Locks.=--Two bulkhead walls, forming the rear closure of the -pneumatic sections, were built in each end of each tunnel, one just -ahead of the shield chamber, the other about 1200 ft. ahead of the -first. The walls were built of Portland concrete 10 ft. thick, and they -were grouted with Portland cement, under a pressure of nearly 100 lbs. -per sq. in., to make them thoroughly air-tight. Each wall had in it -three locks; for man, material and emergency. Each was equipped with -hand valves arranged to be operated from either outer end or from -within. The floors of the man and material locks were on a level with -the working platform of the tunnel, about 3 ft. 6 ins. above the invert; -the floor of the emergency lock was about 5 ft. above the horizontal -axis of the tunnel. The locks were made of steel plates and shapes, with -iron fittings riveted and bolted together. The man lock was 11 ft. long -of elliptical cross-section, 6 ft. vertical diameter and 5 ft. -horizontal; the material lock was 25 ft. long, with circular -cross-section, 7 ft. diameter, and the emergency lock was 20 ft. long, -of elliptical cross-section, 4 ft. vertical and 3 ft. horizontal -diameters. Fig. 139 shows the elevation of the air lock used in the -Pennsylvania tunnel. - -[Illustration: ~Sectional Elevation~ - -~Horizontal Section~ - -FIG. 139.--Plan and Elevation of First Bulkhead Wall in South Tube -Manhattan.] - - -=Excavation.=--In driving these tunnels almost any kind of material was -encountered, viz., rock, partly rock, and partly loose soil, sand and -gravel, and finally silt. - - -=Rock.=--Much of the rock excavation was made before the shields were -erected in order to avoid the handling of rock through the narrow -openings of the shield doors. Throughout the cross-section the shield -traveled on a cradle of concrete in which 2 or 3 steel rails were -imbedded. At the points where the excavation had been made for the full -section of the tunnel, it was only necessary to trim off the projecting -corners of rock. Where only the bottom heading had been driven the -excavation was completed just in front of the shield; the drilling below -the axis level being done from the heading itself, and above that from -the front sliding platforms of the shield. The holes were placed near -together and were drilled short; very light charges of powder were used -in order to lessen the chance of knocking the shield about too much. - - -=Mixed Face.=--When the rock dipped to such an extent that the front of -the tunnel was excavated partly in rock and partly in loose soil, the -compressed air was turned on, starting with a pressure varying from 12 -to 18 lbs. When the surface of the rock was penetrated, the soft face -was held up at first by horizontal boards braced from the shield until -the shield was shoved. The braces were then taken out and, after the -shield had been shoved, were replaced by others. As the amount of soft -ground in the surface increased, the system of timbering was gradually -changed to one of 2-in. poling-boards. These rested on top of the shield -and were supported by vertical breast-boards which in turn were held by -6-in. by 6-in. walings, braced through the upper doors to the iron -lining and from the sliding platforms of the shield. - - -=Sand and Gravel.=--Sand and gravel were only met at Weehawken, where -two different methods were used. The first method was employed when the -roof of the excavation was through sand. It consisted of excavating the -ground 2 ft. 6 ins. ahead of the cutting-edge, the roof being held in -place by longitudinal poling-boards. These boards rested on the outside -of the skin at their back end, and at the forward end on vertical -breast-boards, braced from the sliding platforms and through the shield -doors to cross timbers in the tunnel. - -The second method of timbering was used in the presence of gravel at the -upper part of the excavation. In such a case, the excavation was only -carried 1 ft. 3 ins. (half a shove) ahead of the cutting-edge, the roof -being supported by transverse boards held by pipes which rested in holes -left in the shield. After a small section of the ground had been -excavated a board supported by a pipe that was inserted underneath and -wedged to it was placed against the ground. These polings were kept -below the level of the hood, so that when the shield was shoved, they -would come inside of it; in addition they were braced with vertical -posts from the sliding platform. The upper part of the face was held by -longitudinal breast-boards braced from the sliding platform by vertical -pieces. The lower part of the face was supported by vertical sheeted -poling, braced to the tunnel through the lower doors. Straw and clay -were used in front of the boards to prevent the escape of air which was -very large, when the tunnel was excavated through sand and gravel. The -average rate of progress in these materials was 5.1 ft. per day. - - -=Silt.=--When silt was encountered, the shield was shoved into the -ground without any excavation being done by hand ahead of the diaphragm. -As the shield advanced the silt was forced through the doors into the -tunnel. Forcing the shield through the silt resulted in raising the bed -of the river, the amount that the bed was raised depending on the -quantity of material brought into the shield. When the whole volume of -the excavation was brought in, the surface of the bed was not affected; -when about 50% was taken in, the surface was raised about 3 ft.; if the -shield was driven blind, the bed was raised about 7 ft. When the shield -was driven blind, the tunnel began to rise for about 2 ins., and the -iron lining was distorted, the vertical diameter increasing and the -horizontal one decreasing by about 1¹⁄₄ ins. It was found, however, that -the tunnel was not affected when part of the excavation was taken, but -if all of it was taken in or the shield was shoved with open doors, the -tunnel was lowered. A powerful aid was thus found for the guidance of -the shield; for, if high, the shield could be brought down by increasing -the quantity of muck taken in, if low, by decreasing it. - -The junction of the shields under the river was made as follows: When -the two shields of one tunnel, which had been driven from opposite sides -of the river, approached within 10 ft. of each other, they were stopped; -a 10-in. pipe was driven between them, and a final check of lines and -levels was made through the pipe. One shield was then started up with -all doors closed, while the doors of the stationary shield were opened -for the muck driven ahead by the moving shield. This was continued until -the cutting-edges came together. All doors in both shields were then -opened and the shield mucked out. The cutting-edges were taken off and -the shields moved together again, edge of skin to edge of skin. As the -sections of the cutting-edges were taken off, the space between the skin -edges was poled with 3-in. stuff. When everything except the skin had -been removed, iron lining was built up inside the skins; the gap at the -junction was filled with concrete and long bolts were used from ring to -ring on the circumferential joint. - - -=Lining.=--The tunnels were lined with cast-iron circular rings of the -segmental bolted type. In some special cases, cast steel was used -instead of cast iron. The rings were made 30 ins. long, with an internal -diameter of 21 ft. 2 ins. and an external one of 23 ft. The rings were -composed of nine equal segments of 77¹⁄₂ ins. external circumferential -length each, except the two segments adjoining the key which were equal -to the other segments with the difference, that one end joint was not -radial but formed so as to make an opening 12.25 ins. wide at the -outside and 12.60 ins. at the inside, which was closed by the key -segment. Each segment had six bolts in the circumferential joint, the -key had one, so that there were 67 bolts in one circumferential joint. -Each of the twelve longitudinal or radial joints had five bolts, in all -127 bolts per ring. The circumferential flanges of each plate were -strengthened by two transverse webs or feathers on each flange. Each -segment was provided with a 1¹⁄₂ in. grout hole closed with a screw -plug. In order to pass around curves, whether horizontal or vertical, or -to correct deviation from the line or grade, tapering was used; by this -is meant the placing of rings in the tunnels which were wider than the -standard rings, either at one side (horizontal tapers or liners), or at -the top (depressors), or at the bottom (elevators). Tapers ¹⁄₂, ³⁄₄ or -even 1 in. were used. The taper rings were made by casting a ring with -one circumferential flange much thicker than usual and then machining it -off to the taper. - - -=Grouting.=--From the exterior of the tunnel already lined with -cast-iron rings, grout was forced through the holes closed by -screw-plugs, at a pressure of 90 lbs. per sq. in. The grout was composed -of 1 Portland cement and 1 sand by volume and was forced in by a -specially constructed machine, so it formed a shell of cement nearly 3 -ins. thick around the exterior of the iron lining. The grouting began at -the lower segment; the cement was forced in until it reached the hole -above, then the hole was plugged, and the grouting was carried on from -the consecutive hole and so on until all the tunnel was finally encased -in grout, as it filled every crevice between the outside of the lining -and the ground as excavated. The cast-iron rings of the tunnel were -covered with a concrete lining which was placed in the following order: -First, on the invert; second, on the duct benches; third, on the arch; -fourth, on the ducts; fifth, on the face of the bench. Before any -concrete was placed, the surface of the iron was cleaned by scrapers and -wire brushes and by washing it with water. The invert was built in -sections 30 ft. long and the duct benches were constructed soon after. -These duct benches were built with several steps for the ducts to be -laid later. They were built by means of a traveling stage on wheels -which ran on tracks on the working platform of the tunnel. The arch was -constructed soon after. First the portion from the duct benches to the -haunches, then the arch proper, was built on traveling centers on tracks -laid on the steps of the duct benches. The concrete was received in -³⁄₄-cu.-yd. dumping buckets, from the flat cars on which they were run; -the buckets were hoisted to the level of the lower platform of the arch -by a small Lidgerwood compressed air hoister. At this level the concrete -was dumped on a traveling car or stage and moved in that to the point -on the form where it was to be placed. For the lower part of the arch -the concrete was thrown directly into the form from this traveling part -of the stage. Fig. 140 shows the cross-section of the tunnel with the -iron lining and concrete. - -[Illustration: Section in Sand and Gravel or Rock - -Section in Hudson River Silt, with foundations - -FIG. 140.--Typical Cross-Sections of One Tube of Pennsylvania Railroad -Tunnel Under the Hudson River.] - - -=Hauling.=--A working platform, made up of 5-ft. sections, was built -inside the tunnel and kept close to the shield. On this platform two -lines of industrial railway tracks with switches and sidings at the -locks, and a heading, were laid for hauling materials and spoils. These -lines converged into a single track in passing through the air locks. At -the shaft elevators, they terminated in a steel plate floor to avoid -switches. Between the locks of the bulkheads was installed an -electrically driven cable system, to haul the loaded muck up grade and -to empty the flat cars. From the first bulkhead to the shaft, the cars -were hauled up grade by a steam hauling engine. At the Manhattan end -there was one 10-H.P. engine for each tunnel, while at Weehawken one -25-H.P. engine served for both tunnels. Each shaft contained two -elevators driven by a double-cable, reversible single-drum -steam-hoisting engine. A grouty frame was built over the shafts, and on -the platforms over this frame were narrow-gauge tracks, extending from -the elevators to the muck-chutes and to points where the lining segments -were loaded on the cars. The elevators were arranged to stop at both the -ground and the grouty platform levels. The rolling-stock at each of the -tunnels consisted of 75 flat cars for moving the tunnel segments, and of -about 50 muck cars, each of 1¹⁄₄ cu. yd. capacity. - - -=Plant.=--The plants located at each end of the tunnel near the shafts -were almost identical. Each consisted of three 500-H.P. Stirtling -boilers, which supplied steam at 150 lbs. pressure. Feed water was -supplied by three 13¹⁄₂ metropolitan injectors, and two Blake duplex -pumps. Two Worthington surface condensers, each of 2250 sq. ft. -condensing surface, took care of the exhaust from the engines and -compressors. Condensing water was pumped from the river through a 16-in. -pipe. The high-pressure air was supplied by a duplex Ingersoll-Sergeant -compressor, with cross-compound steam end 14 × 26 × 30 ins. and simple -water-jacket air cylinders 13¹⁄₄ × 36 ins. Its capacity at 100 r.p.m. -was 1085 cu. ft. free air per minute. The maximum pressure was 130 lbs. -per sq. in. The air for the pneumatic working was supplied by three 14 × -26 × 30 in. duplex Ingersoll-Sergeant compressors. The maximum capacity -of the three was 12,000 cu. ft. free air per minute at 125 r.p.m. and a -discharge pressure of 50 lbs. per sq. in. The suction air was taken from -the outside about 10 ft. above the roof of the engine house. Three -aftercoolers, 32¹⁄₂ ins. × 11 ft. 4 ins., each having 809 sq. ft. -cooling surface of tinned brass tubes, cooled the low-pressure discharge -to within 10° F. of the temperature of the cooling-water. From the -aftercoolers, the air passed into three steel receivers each 54 × 12 -ft., placed outside the engine room and fitted with weighing safety -valves. The receivers were connected to two 10-in. mains; one serving -the north, the other the south tunnel. A fourth receiver of the same -size was built to receive the discharge of the high-pressure compressor, -through a 4-in. pipe. The high-pressure water required for the shield -was furnished by three Blake direct-acting, duplex pumps with outside -packed plungers. The steam end was 16 × 18 ins., the water end 2¹⁄₁₆ × -18 ins. At 55 r.p.m. pumping against 5000 lbs. per sq. in., the capacity -of each pump was 57 gals. per minute. Two of them, one on each tunnel, -were sufficient to run the shields and the third was held in reserve. -The high-pressure water was conveyed to the front by means of a 2-in. -double, extra strong pipe which was buried between the engine room and -the shaft, in a trench, to prevent freezing in cold weather. The -electric current for light and power was supplied by two 100-K.W. -250-volt G.E. direct-current generators directly connected to Ball & -Wood high-speed engines running at 250 r.p.m. The switchboard had two -machine panels, two distributing panels and one panel carrying a circuit -breaker for the traction circuit. - - -=Illumination.=--The tunnel was lighted by electricity, there being two -rows of lamps, one in the crown and one in the south axial fine. The -lamps were 16-c.p., 240-volt, two-wire system, and were spaced 35 ft. -apart in the crown and 12¹⁄₂ ft. apart on the axial line. In addition, -five nests of 5 lamps each were used at the front. Candles were supplied -for miscellaneous and emergency uses. The sockets for electric globes -were fitted to a wooden reflector, coated with white enamel paint on the -inside. - - - - -CHAPTER XXI. - -SUBMARINE TUNNELING (Continued); TUNNELS AT VERY SHALLOW DEPTH. THE -COFFERDAM METHOD. THE PNEUMATIC CAISSON METHOD. THE JOINING TOGETHER -SECTIONS OF TUNNELS BUILT ON LAND. - - -The tunnels on the river bed or at such a shallow depth that only a few -feet of material will remain between the bottom of the river and the -roof of the tunnel can be built in three different ways, viz., (1) by a -cofferdam; (2) by pneumatic caissons; (3) by sinking and joining -together whole sections of tunnels that were built on land. - - -=The Cofferdam Method.=--=The Van Buren Street Tunnel, Chicago -River.=--According to the cofferdam method, the work is attacked at one -of the shores, and the tunnel built in sections of such length as not to -interfere with the flow of water or the navigation of the river. Round -the entire exterior line of the first section a double-walled cofferdam -is built, and strongly braced transversely, so as to withstand the -pressure of the water. When the water is pumped out, a single-walled -cofferdam is built within the first, leaving sufficient distance between -the two to allow of the construction of the masonry. The soil is then -removed within the inner cofferdam, and the tunnel constructed from the -foundation. When the end of the tunnel reaches the channel end of the -cofferdam, a crib-wall is erected over the end of the completed tunnel. -This crib, in turn, forms the end wall of another cofferdam, built in -continuation of the first, so as to allow the second section to be -proceeded with, and at the same time to facilitate the removal of the -cofferdams of the first section. The work goes on continuously in this -way until the distant shore is reached. - - -VAN BUREN STREET TUNNEL, CHICAGO. - -The Van Buren Street tunnel, built to carry a double-track street -railway under the Chicago River, was completed in 1894 by the cofferdam -method. The special features of the tunnel[14] are: (1) the unusually -large dimensions of the cross-section of 30 ft. × 15 ft. 9 ins.; (2) its -construction inside of cofferdams of great length and width; (3) the -construction under some very high buildings calling for great care and -very strong temporary and permanent supports. - - [14] “Eng. News,” April 12, 1892. - -The special feature of the work for our present purpose was the -construction of the tunnel across the river. To accomplish this a -cofferdam was built out from the west shore of the river to its middle, -and the tunnel constructed within it like the building of any other -structure within a cofferdam. Transverse and longitudinal sections of -this cofferdam are shown by Fig. 141. As will be seen, it was a simple -double-wall cofferdam, with a clear width between the walls of 58 ft., -and braced transversely as shown. Inside of this a single-wall cofferdam -of piles was constructed, with a clear width just sufficient to allow -the construction of the masonry within it. When the tunnel end reached -the channel end of the cofferdam, a crib-wall was built over the end of -the completed tunnel, as shown by the drawings. This crib-wall was -intended to form the end wall of another cofferdam, which was built out -from the east shore, and within which the remaining half of the tunnel -was built as the first half had been. The drawings show the character of -the tunnel masonry and of the centering upon which it was built. - -[Illustration: ~TRANSVERSE SECTION OF COFFERDAM AND TUNNEL~ - -~SECTION SHOWING METHOD OF CONSTRUCTING CRIB DAM.~ - -FIG. 141.--Sections of Cofferdam, Van Buren St. Tunnel, Chicago.] - -The Van Buren Street tunnel was the last of the three tunnels under the -Chicago River, constructed according to the cofferdam method. At the -time the tunnels were constructed the bed of the river was 17 ft. -deep. In connection with the harbor and river improvements, the Federal -Government ordered the Chicago River to be lowered so as to give a depth -of 26 ft. of water. This necessitated the lowering of the tunnel roof -and the excavation for a deeper floor which was a very difficult -operation. This work was described in “Eng. News,” Sept., 1906. - - -THE PNEUMATIC CAISSON METHOD.--THE TUNNEL UNDER THE HARLEM RIVER. - -In the early seventies Prof. Winkler proposed to construct a tunnel -under the River Danube to connect the various portions of the Vienna, -Austria, underground railway, and to use caissons in the construction. -Prof. Winkler proposed to build caissons from 30 ft. to 45 ft. long, -with a width depending upon the lateral dimensions adopted for the -tunnel masonry. The caisson was to be made of metal plates and angle -iron with riveted connections on all sides except those running -vertically transverse to the tunnel axis, whose connections were to be -bolted. In the middle of the roof an opening was to be left; this was -for the shaft having the air-locks to allow the passage of men, -materials, and compressed air. - -Across the river two parallel rows of piles were to be driven into the -river bed, to fix the place where the caisson was to be sunk. Then the -first caisson near the shore was to be lowered in the ordinary way, and -a second caisson was to be immediately sunk very close to the first one. -When both caissons had reached the plane of the tunnel floor, the sides -which were in contact were to be unbolted and removed, and the small -space between made water-tight. The chambers of the two caissons were to -be opened into a single large one communicating above by means of two -shafts. At the same time that the masonry was being built in the first -two caissons, from the inverted arch up, a third caisson was to be sunk; -and when by excavation it had reached the plane of the projected tunnel -floor, the partitions were to be removed so that the three caissons were -in communication, forming a large single caisson. Then the outer -partition of the first caisson was to be removed, and the masonry of the -submarine tunnel connected with the portion of the tunnel built on land. -In a similar manner all the caissons were to be sunk; and when the last -one was placed, and the masonry lining constructed, and connected with -the portion of the tunnel built on the other shore of the river, the -partition walls were to be battered down, and the submarine tunnel -completely constructed and open to traffic. - - -=The Harlem River Tunnel.=--The pneumatic caissons method was employed -in the construction of the tunnel under the Harlem River for the New -York Rapid Transit Railway. The tunnel proper consisted of two parallel -tubes riveted to each other and surrounded by a cradle of concrete as -shown in Fig. 121, page 216. The tunnel was built in three -sections:--The first, from the Manhattan shore well towards the middle -of the river; the second, from the shore of the Bronx towards the middle -of the river; and the last, the section uniting the other two and -completing the tunnel. - -Each section was built within a specially constructed working-chamber, -consisting of timber side walls forming a wooden caisson, so constructed -that compressed air could be used. This working-chamber of Mr. McBean -presented some novel features, inasmuch as the caisson was not built on -land, but under water. - -In building the tunnel, the Harlem River was dredged to a certain depth, -so as to leave only 6 ft. or 8 ft. of excavation to be done before -reaching the line of sub-grade of the proposed structure. Two service -platforms were built on piles 10 ft. apart longitudinally, and cut off -at a point above mean high-water mark, braced in the usual manner, and -covered with heavy planks, to serve as the floor of the platform. On -this platform were placed rails for the trains used in the -transportation of materials. These platforms were also used in -maintaining the perfect alignment of the caissons. - -Within the platforms and along the dredged channel four longitudinal -rows of piles were sunk. These piles were accurately brought to line by -beams bolted together, and placed across and above the water-level. A -few beams were also added for the purpose of bracing the piles -transversely, after which they were cut off under water and capped. - -[Illustration: FIG. 142.--Showing Working Platforms and Piles Sunk in -the Dredged Channel.] - -Fig. 142 shows the manner in which the working platforms were -constructed, and also the rows of piles sunk in the dredged channel. -Between the piles a very strong frame was placed, made up of waling -pieces and two transverse beams 14 ins. by 14 ins. each, placed one -below the other at a distance of 5 ft. 8 ins., and strongly braced -together. Guiding-beams were fixed on each side of the frame for the -sheeting piles. The frames were built in sections of different lengths, -and placed directly above the cap-pieces of the pile-bents sunk in the -dredged channel. - -The longitudinal sides of the caisson were constructed by sinking two -rows of sheeting piles, each row being close to a service platform. The -sheeting piles were made up of yellow-pine timbers 12 ins. by 12 ins.; -three piles bolted together formed a section 3 ft. wide. Each section -was grooved and tongued, so as to be firmly connected with the adjacent -sections to be sunk. The lower ends of the piles were cut wedge-shaped, -with a sharp edge to offer a small resistance while penetrating the -soil. The sheeting-piles were then cut off under water, which operation -was successfully carried out by means of a circular saw operated by a -pile-driving machine. The sheeting was also extended between two -platforms to make a bulkhead, and in this way the four sides of the -caisson were built up. Particular attention was always given to the -alignment of the sheeting piles, which was obtained by guiding the piles -with the timbers placed longitudinally, one below the water-line in -connection with the frames located between the pile-bents, and the -second along the inner edge of the service platform, as shown in Fig. -143. - -[Illustration: FIG. 143.--Showing Sheeting-Piles for the Sides of the -Caisson and Trussed Beam for the Roof.] - -The caisson was completed by placing a roof covering the sides. This -roof was 40 ins. thick, made up of three layers of 12-in. beams placed -transversely to the axis of the caisson, while between the beams planks -2 ins. thick were placed lengthwise and bolted together, so as to make a -firm, solid structure. The roof was built ashore, in sections each -varying from 39 ft. to 130 ft. long. The edges of the roof fitted the -sides of the caisson perfectly; and when each section was towed to the -proper spot, it was sunk and made secure. Under the roof were placed six -longitudinal beams, 12 ins. by 14 ins., called “rangers,” resting on the -cap-pieces of the pile-bents that were laid across the space of the -proposed tunnel; while the extreme rangers were used for the purpose of -fitting above the sheeting-piles of the caisson, in order to make the -latter water-tight. The two extreme rangers were provided with -=T=-irons, the flat side being laid on the sheeting-piles, while the web -penetrated the ranger by reason of the weight of the load resting on the -roof, for the purpose of sinking it to the required point. Earth was -next heaped on the roof, and in this way a large working-chamber was -prepared, as shown in Fig. 144. - -[Illustration: FIG. 144.--Showing the Caisson with the Working-Chamber.] - -The working-chamber built on the Manhattan side of the Harlem River was -216 ft. long, provided with two rectangular shafts 7 ft. by 17 ft., -rendered water-tight, and rising above the high-water mark of the river. -Within these shafts the air-locks of the tunnel tubes were placed, so -that the work could be carried on by means of compressed air. The -pressure of the air was used to expel the water, being sufficiently -intense to equilibrate a column of water equal to the depth of the -lowest point of the roof of the caisson. - -When the working-chamber was constructed, the tunnel proper was begun by -excavating the soil down to the required level; the concrete was then -laid on. It was just at this point, when a large part of the roof was -constructed and supported only by the sheeting-piles of the sides of the -caisson, that the writer of this article feared that this novel method -of tunneling would prove a failure. The tendency of the timber to float, -aided as it was by the air pressure within the caisson, was counteracted -only by the weight of the earth heaped on the roof, and by the friction -of the soil against the feet of the sheeting-piles. This friction was -only a small quantity, as the soil was loose, so that it was considered -rather risky and dangerous to place reliance on such a feeble quantity. -This fear was, unfortunately, justified on two occasions, when on -cutting off a portion of the pile-bents some of the sheeting-piles got -loose and water flooded the whole chamber, but, happily, without loss of -life. As the chamber was one of large dimensions, the workmen had time -enough to effect their escape. It may be remarked that during these -troubles only a few of the sheeting-piles were displaced, while the -caisson itself offered a stout and successful resistance, due to its -being strongly braced transversely. The accidents were, therefore, -limited to a few piles, instead of affecting the entire caisson. On the -occasion of the first, the repairs were effected by sinking the piles to -a greater depth, continuing down until rock was encountered. After that, -the water was pumped out and the work resumed. In repairing the second -accident, the sheeting-piles were driven down to bed-rock, and the -surrounding soil strengthened by cement forced through the loose soil -around the piles. This remedy proved effective, and no further trouble -occurred to delay the work on the Manhattan side of the Harlem River. - -[Illustration: FIG. 145.--Showing the Tunnel Constructed within the -Caisson.] - -On the concrete bed of the tunnel the segments of the metal lining were -placed and surrounded by concrete, as required by the plans and -specifications (Fig. 145). The contractors had planned to unbolt the -roof from its holdings, to remove by means of dredgers the earth which -had been heaped on it, and thus set the roof afloat, after which it was -to be towed within the two working platforms already erected on the -Bronx shore. But Mr. McBean devised a simpler and more economic, but at -the same time more dangerous, way of constructing this second section of -the tunnel. He thought that the upper half of the tunnel proper could be -used instead of the timber roof, thereby reducing the capacity of the -working chamber, and limiting the use of compressed air. In this way he -dispensed with the removal of timber, and also of the earth heaped on -the roof. - -In building this Bronx section, a channel was dredged along the line of -the tunnel to a depth of 5 ft. from the foundation-bed of the proposed -tunnel. The working platforms were constructed on both sides of this -channel, quite similar to those erected on the other half of the tunnel; -and between them pile-bents were sunk, capped with 12-in. by 12-in. -beams. Over the cap-pieces rangers were placed longitudinally, which -also rested on the sides of the wooden working caisson, Fig. 146. The -sheeting-piles were cut off at level, but much lower down than in the -first half of the tunnel. - -The roof was built on floats made of 12-in. by 12-in. timber laid -transversely 4 ft. apart and supporting a floor of 3-in. by 12-in. -planks rendered water-tight. The sides of the floats were made by -verticals, 4 ins. by 6 ins., and planks, 3 ins. by 12 ins., carefully -caulked. A temporary floor was built on the base of the float, -consisting of transverse beams, 16 ins. by 16 ins., placed 8 ft. apart. -A center piece, 10 ins. by 16 ins., was laid so as to correspond with -the axis of the tunnel; and on each side of it, other parallel beams, 16 -ins. by 16 ins., corresponding to each center of the circular metal -lining of the tunnel; the beams, longitudinal and transversal, were -strongly bolted together. The temporary floor was completed by boarding -the spaces left between the various beams. - -[Illustration: FIG. 146.--Showing Sides of the Caisson and Supports for -the Roof.] - -On this float, the upper half of the tunnel was constructed by erecting -the segments of the metal lining, which were strongly supported, so as -to prevent any settling or distortion; the concrete was then built up in -a large flange with vertical suspension rods, four to each bar. The -rings of the tunnel were 6 ft. each, the weight of each lining being -41,000 lbs., the concrete covering 618 cubic feet. The second part of -the tunnel was 300 ft. long, with roof constructed in three -sections--two of 90 ft. in length each and the third of 84 ft. Each of -these sections alternated with a smaller section, 12 ft. long, provided -with air-locks. The shortest of the three sections was the first one set -up, and was constructed close to the Bronx side of the Harlem River. For -this purpose the two extreme ends of the section were closed by means of -steel plates forming diaphragms, built 6 ft. inward, thus leaving one -ring projecting out at each end. Openings were left on the top of these -projecting rings for access by divers. The exterior of the upper half -section of the permanent tunnel was filled with water until it was -lowered into position. It was directed by means of tackles attached to -vertical eye-bars, which were strongly fixed to the flanges of the -springing line of the arch, and bolted to the beams of the temporary -floor. In this way the roof was towed into place, and lowered by means -of stone ballast, until it rested on the cap-pieces and frames of the -pile abutments, the sides of the roof remaining just on top of the -sheeting-piles that formed the sides of the caisson, as shown in Fig. -147. Perfect alignment was obtained by wires strung at each end and -along the side of the roof, corresponding to points fixed on the working -platforms and sighted with transits. Such accuracy was obtained that the -circumferential flanges of the outer 6-ft. ring were brought into -contact with those of the 12-ft. section already constructed. A diver -then entered by the opening left in the projecting ring, and bolted this -section of the roof to the preceding one. By removing the iron -diaphragm, the consecutive sections were united into one. When the diver -completed his work, the opening was closed up, and compressed air used -to keep the water out of the box included between the roof and the -temporary flooring. - -[Illustration: FIG. 147.--Showing the Roof of the Caisson Formed by the -Upper Half of the Tunnel.] - -The remaining sections of the tunnel roof were built in the same way, -until the last abutted against the part of the work constructed within -the caisson under the high wooden roof on the Manhattan side of the -river. The following method was adopted for the purpose of connecting -the few parts of the tunnel which had been differently constructed. The -diaphragm at the end of the last section of the tunnel roof was -constructed so as to abut against the last circumferential flanges of -the iron lining without leaving a projecting ring. It was continued -above the metal and concrete lining of the roof in a rectangular form, -and of the same height and width as the wooden bulkhead of the -working-chamber on the Manhattan side of the river. The diaphragm was -made of riveted plates and angles, with an opening 20 ins. by 30 ins., -bolted so as to be removable at will. The diaphragm was of the same -height as the roof and was connected with a roof-plate to the rangers -supporting the thick wooden roof. Other steel plates, placed vertically, -were riveted to the diaphragm and bolted to the caisson. All this work -was carried on by divers. The wooden bulkhead was cut to the -springing-line of the arch; and between the two parts of the tunnel, -built by different methods, a bulkhead was placed, made of steel plates -14 ins. long, which prevented the entrance of water into the -working-chamber. - -[Illustration: FIG. 148.--Showing the Tunnel Completed by Building the -Lower Half within the Caisson.] - -When the different sections were joined together, and all the openings -closed and made water-tight, cement-grout was poured on the roof, and -earth was heaped up to a height of 5 ft. The 300 ft. of the roof, -resting on sheeting-piles and provided with diaphragms at the extreme -ends, formed a water-tight working-chamber, or caisson, communicating -with the exterior by means of the shafts and air-locks. The lower -portion of the tunnel was built under air-pressure. The pile-bents were -first cut off at the plane of the tunnel sub-grade, after which the -foundation-bed of concrete was laid. The lower segments of the iron -lining were then placed in position, and the structure made continuous -by building up the lateral walls, consisting of concrete (Fig. 148). No -accidents occurred while building the second part of the tunnel. - -The Harlem River tunnel was completed in contract time, although the -opening of the subway was delayed by difficulties encountered in -tunneling through rock in the borough of the Bronx. The writer -endeavored to obtain information regarding the expense per linear foot, -but all his efforts were rewarded with a general assurance that it -proved to be the cheapest method. - - -SINKING AND JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND. THE -SEINE. THE DETROIT RIVER TUNNELS. - -In the year 1896, Mr. Erastus Wyman secured a patent for building -subaqueous tunnels close to the river, by sinking and joining together -small sections of tunnels previously built on land. Each section would -have been provided with a long vertical tube for the air-lock when -compressed air was to be admitted to expel the water and permit the -construction of the lining within the sunken shell. Thus each section of -the tunnel would have acted as a pneumatic caisson; being, however, an -improvement on Professor Winkler’s suggestion inasmuch as the caisson -was a portion of the tunnel itself, instead of a simple inclosure for -facilitating the construction of the shield. Mr. Wyman proposed to use -this method in the construction of a tunnel between South Brooklyn and -Stapleton, Staten Island; a charter was granted him but the tunnel was -never built. - - -=The Tunnel under the Seine River.=--The caisson method of building -tunnels under water was used at Paris, France, in the construction of -the Metropolitan Railroad under the Seine River. - -The caissons designed by Mr. L. Chagnaud were for a double track line. -They were sunk, ends to ends, and formed a portion of the tunnel lining -which was enveloped by a framework of metal embedded in concrete. -Built-up frames carried a shell of steel plating on the sides, from toes -to springing lines, and on the sides and roof of the working-chamber. A -temporary plate diaphragm closed the open ends. This construction formed -a vessel capable of floating with a very light draft. - -The method of sinking the caissons was as follows: The caisson was -erected on the river bank and when completed it was launched and towed -into position between pile stagings which served the double purpose of -guiding the descent at the beginning of the sinking and of forming a -working platform. The caisson when launched and, consequently, before -the cast-iron lining had been put in place within it, weighed 280 metric -tons; but, beyond some difficulty in taking it under the bridges in the -way, the towing was accomplished without serious trouble. - -[Illustration: FIG. 149.--Transversal Section of the Caissons for the -Tunnel under the Seine River.] - -Previous to placing the caisson in position between the stagings, the -portion of the river bed it was to rest upon had been leveled by -dredging. Once in position, the first work was the erecting of the -cast-iron lining segments within the framework. Work was then begun by -filling the annular space between the lining and the shell with -concrete; this additional weight gradually sunk the caisson to the river -bottom. The working shafts, made up of steel cylinders, were placed as -the sinking progressed to this point. - -[Illustration: ~Section A-B.~ - -~Section C-D.~ - -~Plan at Joint.~ - -FIG. 150.--Showing the Joining of the Caissons at the Pont Mirabeau -Tunnel under the Seine River.] - -After the caissons had been sunk to the required place and in -continuation of one another, a space of nearly 5 ft. was left between -them. The construction of the tunnel within the bank of earth -separating the two caissons was as follows: A cofferdam was built around -this space. It was formed by two diaphragms closing the ends of the -tunnel, and by two longitudinal walls sunk as temporary caissons, one on -each side of the tunnel and inclosing their ends. This cofferdam was -covered with a metal working-chamber whose lower edges rested on top of -the four walls of the cofferdam. The joints were made tight by means of -rubber or packed clay. The water in the cofferdam was then pumped out, -the earth excavated, and the masonry built in continuation of the two -ends of the tunnel sections. The submerged sections of the tunnel which -were allowed to remain full of water to render them more stable and to -save effort in pumping them, were now made dry; the diaphragms were -removed from the ends of the caisson tunnels and the work made -continuous. Fig. 149 shows the cross-section of the caissons. - -At the Pont Mirabeau crossing of the Seine, a slightly different method -was used, described in “Eng. News,” May 18, 1911. The caissons were sunk -to the required line and grade with an intervening longitudinal space of -15³⁄₄ ins. between two adjoining caissons. At each end of this space, -which was filled with the river marl, was sunk against the edges of the -caissons a hollow cylinder 20 ins. outside diameter. The interior of -these cylinders was excavated and filled with concrete, thus forming a -continuous wall on both sides of the two adjoining caissons. The earth -from the intervening space was then removed and concrete deposited from -bottom opening tremies up to the level of the top of the caisson. After -nearly one month the tunnel was entered from the shaft and an opening -the shape and size of the tunnel section cut through the diaphragms of -the 15³⁄₄-in. wall and the concrete tunnel lining made continuous -between the two sections. Fig. 150 shows the method of joining the -caissons. - - -=The Detroit River Tunnel.=[15]--With some modifications which permitted -dispensing with compressed air, the tunnel under the Detroit River was -built for the Michigan Central Railroad, connecting Detroit with -Windsor, Canada. The tunnel is 6625 ft. long; of this, however, only -2625 ft. are under the river, while the approach on the American side is -2000 ft. long and that on the Canadian side, 4000 ft. The tunnel -consists of two parallel circular tubes 23 ft. in diameter, built up of -³⁄₈-in. steel plate. They are placed 26 ft. apart, center to center, and -are connected by diaphragms at 12-foot intervals. - - [15] Condensed from a paper by B. H. Ryder. - -Each section of the subaqueous tunnel is approximately 262 ft. long. -There are ten of these sections and an eleventh a little over 60 ft. -long. These tubes were built at the shipyards of the Great Lakes -Engineering Works at St. Clair, about 30 miles from Detroit. After the -assembling was completed, the ends of each tube were closed by temporary -wooden bulkheads to make them float, and the outside sheathed -horizontally with heavy timbers bolted to the diaphragms. This sheathing -running lengthwise of the tube made a form or pocket, into which the -inclosing jacket of concrete was placed. The sections were then launched -and towed down to the tunnel site and sunk separately in a trench on the -river bottom that had been previously dredged to receive them. This -trench was dug to a width of 50 ft. and depth varying from 25 to 50 ft. -by clamshell buckets, swung from a scow, working to a depth below the -water level of 60 to 90 ft. - -As a foundation for the sections, a grillage was constructed on the -surface and sunk in place in the trench by derricks swung from a scow. -The grillage was placed underneath each joint between the sections and -built up of I-beams imbedded in concrete. This grillage is the width of -the trench and about 30 ft. long, with posts projecting downward from -the four corners, and these were seated into the river bottom, by means -of pile drivers, to the desired grade. - -Then the eleven sections of the tunnel were lowered and connected, one -at a time. By the aid of air tanks placed on each section the movement -was controlled until the final sinking upon the grillage in the trench. -This operation called into play the greatest engineering skill and -ingenuity. When it is considered that the current velocity at the river -bed is about 2 ft. per second and much higher along the surface, some -idea can be gained of the problems to be overcome. The movement of the -enormous sections must be absolutely under control. Thirty-five-ton -blocks of concrete were sunk in the river bottom up and down stream to -act as anchors, and through them cables were rigged and connected back -to the hoisting engines on the derrick scows. These were prevented from -moving by spuds at each corner, securely driven into the river bottom at -depths sometimes as great as 90 ft. Controlling cables were also run -from the sections to the tremie scow to pull one structure close to the -adjoining section previously sunk, and the divers made the necessary -connection. Fig. 151 shows cross-sections and plans of the tunnel as -given in “Eng. Record,” March 2, 1907. - -[Illustration: ~HALF CROSS SECTION Y-Y~ - -~HALF CROSS SECTION Z-Z~ - -~HALF HORIZONTAL SECTION X-X~ - -~HALF TOP VIEW~ - -FIG. 151.--Cross-Sections and Plans of the Detroit River Tunnel.] - -Steel masts had been previously attached to each end of the sections to -enable the engineers on shore to determine the alignment and locate the -exact position during the sinking. - -Concrete was then deposited in the pockets, completely surrounding the -tubes, forming a solid monolithic structure from end to end. - -This was done by means of the tremie process. - -A 32-ft. by 160-ft. scow was equipped with a concrete mixing plant and -the tremie pipes, three in number, through which the concrete was -deposited. Each pipe is 12 ins. in diameter, of spiral riveted steel, 80 -ft. long. These pipes could be raised or lowered, reaching from the -receiving hoppers on the scow to the bottom of the trench. When the -pipes were filled with concrete and lowered into position, a continuous -flow was maintained. As fast as the concrete escaped at the bottom end -of the pipe it was replenished at the top; this process continuing until -the entire space surrounding the section was filled to the desired -level, and under the pressure produced not only by the depth of water -under which it was submerged, but also by the weight of the long column -of concrete contained in the tubes. It is interesting to note that this -is the first time a large amount of concrete has been deposited at a -depth of 70 ft. by this method, and upon the accomplishment of this task -in a measure depended the successful building of the tunnel. - -Inside the tubes was placed a lining of reinforced concrete 20 ins. -thick. Side walls were built up from this ring to provide ducts, which -carry the electrical cables for the distribution of power, lighting, -signal and telegraph wires. They also serve to provide a footwalk along -the side of the tunnel. - -There are cross passages in the tunnel every 200 ft., and also various -niches for the different equipment needed in connection with the -signaling, telephone and fire alarm system. The tunnel is lighted with -800 16-candle-power incandescent lights. - -The track construction is new. There is no ballast used, the ties being -laid in concrete. A ditch in the center of each track carries the -rainfall that will flow down from the summits to sumps which are drained -by centrifugal pumps. - -One remarkable feature of its construction is that compressed air was -not used in the building of the subaqueous tunnel, but it was necessary -in building the approach tunnels. This is contrary to the usual program -where compressed air is required in subaqueous work, and not ordinarily -used in approach or land tunnel construction. - -The trains are operated by very heavy electric locomotives, operated by -the third-rail system. - -The tunnel was constructed under the supervision of W. S. Kinnear, Chief -Engineer of the Detroit River Tunnel Co.; Butler Bros. of New York were -the general contractors. - - - - -CHAPTER XXII. - -ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER CONSTRUCTION. - - -In the excavation of tunnels it often happens that the disturbance of -the equilibrium of the surrounding material by the excavation develops -forces of such intensity that the timbering or lining is crushed and the -tunnel destroyed. To provide against accidents of this kind in a -theoretically perfect manner would require the engineer to have an -accurate knowledge of the character, direction and intensity of the -forces developed, and this is practically impossible, since all of these -factors differ with the nature and structure of the material penetrated. -The best that can be done, therefore, is to determine the general -character and structure of the material penetrated, as fully as -practicable, by means of borings and geological surveys, and then to -employ timbering and masonry of such dimensions and character as have -withstood successfully the pressures developed in previous tunnels -excavated through similar material. If, despite these precautions, -accidents occur, the engineer is compelled to devise methods of checking -and repairing them, and it is the purpose of this chapter to point out -briefly the most common kinds of accidents, their causes, and the usual -methods of repairing them. - - -=Accidents During Construction.=--Accidents may happen both during or -after construction, but it is during construction, when the equilibrium -of the surrounding material is first disturbed, and when the only -support of the pressures developed is the timber strutting that they -most commonly occur. - - -=Causes of Collapse.=--Collapse in tunnels may be caused: (1) by the -weight of the earth overhead, which is left unsupported by the -excavation; (2) by defective or insufficient strutting; and (3) by -defective or weak masonry. - -(1) The danger of collapse of the roof of the excavation is influenced -by several conditions. One of these is the method of excavation adopted. -It is obvious that the larger the volume of the supporting earth is, -which is removed, the greater will be the tendency of the roof to fall, -and the more intense will be the pressures which the strutting will be -called upon to support. Thus the English and Austrian methods of -tunneling, where the full section is excavated before any of the lining -is placed, and where, as the consequence, the strutting has to sustain -all of the pressures, present more likelihood of the roof caving in than -any of the other common methods. - -The character and structure of the material penetrated also influence -the danger of a collapse. A loose soil with little cohesion is of course -more likely to cave than one which is more stable. Rock where strata are -horizontal, or which is seamy and fissured, is more likely to break down -under the roof pressures than one with vertical strata and of -homogeneous structure. Soft sod containing boulders whose weight -develops local stresses in the roof timbering is likely to be more -dangerous than one which is more homogeneous. A factor which greatly -increases the danger of collapse, especially in soft soils, is the -presence of water. This element often changes a soil which is -comparatively stable, when dry, into one which is highly unstable and -treacherous. The liability of the material to disintegration by -atmospheric influences and various other conditions, which will occur to -the reader, may influence its stability to a dangerous extent, and -result in collapse. - -(2) Collapse is often the result of using defective or insufficient -strutting. Of course, in one sense, any strutting which fails under the -pressures developed, however enormous they may be, can be said to be -insufficient, but as used here the term means a strutting with an -insufficient factor of safety to meet probable increases or variations -in pressure. Insufficient strutting may be due to the use of too light -timbers, to the spacing of the roof timbers too far apart, to the -yielding of the foundations, to insufficient bearing surface at the -joints, etc. Collapse is often caused by the premature removal of the -strutting during the construction of the masonry. The masons, to secure -more free space in which to work, are very likely, unless watched, to -remove too many of the timbers and seriously weaken the strutting. - -(3) The third cause of collapse is badly built masonry. Poor masonry may -be due to the use of defective stone or brick, to the thinness of the -lining, to poor mortar, to weak centers which allow the arch to become -distorted during construction, to poor bonding of the stone or bricks, -to the premature removal of the centers, to driving some of the roof -timbers inside it, etc. - - -=Prevention of Collapse.=--Tunnels very seldom collapse without giving -some previous warning of the possible failure, and also of the manner in -which the failure is likely to occur. From these indications the -engineer is often able to foresee the nature of the danger and take -steps to check it. The danger may occur either during excavation or -after the lining is built. During excavation the danger of collapse is -indicated beforehand by the partial crushing or deflection of the -strutting timbers. If the timbers are too light or the bearing surfaces -are too small, crushing takes place where the pressures are the -greatest, and the timbers bend, burst, or crack in places, and the -joints open in other places. The remedy in such cases is to insert -additional timbers to strengthen the weak points, or it may be necessary -to construct a double strutting throughout. When the distance spanned by -the roof timbers is too great, failure is generally indicated by the -excessive deflection of these timbers, and this may often be remedied by -inserting intermediate struts or props. In some respects the best -remedy under any of these conditions is to construct the masonry as -soon as possible. - -When collapse is likely to occur after the masonry is completed, its -probability is generally indicated by the cracking and distortion of the -lining. A study of the cause is quite likely to show that it is the -percolation of water through the material surrounding the lining which -causes cavities behind the lining in some places, and an increase of the -pressures in other places. When it is certain that this water comes from -the surface streams above, these streams may often be diverted or have -their beds lined with concrete to prevent further percolation. When -percolating water is not the cause of the trouble, a usually efficient -remedy is to sink a shaft over the weak point, and refill it with -material of more stable character. These, and the remedies previously -suggested, are designed to prevent failure without resorting to -reconstruction. When they or similar means prove insufficient, -reconstruction or repairs have to be resorted to. - - -=Repairing Failures.=--Tunnels may collapse in several ways: (1) The -front and sides of the excavation may cave in; (2) the floor or bottom -may bulge or sink; (3) the roof may fall in; (4) the material above the -entrances may slide and fill them up. - -(1) One of the most common accidents is the caving of the front and -sides of the excavation. This may often be prevented by taking care that -the face of the excavation follows the natural slope of the material -instead of being more or less nearly vertical. When, however, caving -does occur it may usually be repaired by removing the fallen material, -strongly shoring the cavity, and filling in behind with stone, timber, -or fascines. - -(2) The bulging or rising of the bottom of the tunnel may usually be -considered as a consequence of the squeezing together of the side walls. -It usually occurs in very loose soils, and is chiefly important from the -fact that the reconstruction of the side walls is made necessary. The -sinking of the tunnel bottom is a more serious occurrence. It seldom -happens unless there is a cavity beneath the floor, due either to -natural causes or to the fact that mining operations have gone on in the -hill or mountain penetrated by the tunnel. When the bottom of the tunnel -sinks, three cases may be considered: (_a_) when the sinking is limited -to the middle of the tunnel floor; (_b_) when only a portion of the -foundation masonry is affected; and, (_c_) when the entire lining is -disturbed. In the first case repairs are easily made by filling in the -cavity with new material. In the second case the unimpaired portion of -the masonry is temporarily supported by shoring while the injured -portion is removed and rebuilt on a firm foundation. The remaining -cavity is then filled. In the case of the complete failure of the -lining, the method of repairing employed when the roof falls, and -described below, is usually adopted. - -(3) The most dangerous of all failures is the falling of the tunnel -roof. In such casualties two cases may be considered: (_a_) When the -falling mass completely fills the tunnel section, and (_b_) when it -fills only a portion of the section. - -[Illustration: FIG. 152.--Tunneling through Caved Material by Heading.] - -When the whole section is filled by the fallen material, the problem may -be considered as the excavation of a new tunnel of short length inside -the old tunnel, and under rather more difficult conditions. The first -task, particularly if men have been imprisoned behind the fallen -material, is to open communication through it between the two uninjured -portions of the tunnel. It is advisable to do this even when there is no -danger to life because of imprisoned workmen, since it enables the work -of repairing to be conducted from both directions. The excavation of a -passageway through the fallen material is rendered difficult, both -because the fallen material is of an unstable character, and also -because it is usually filled with the lining masonry, timbering, etc. -When, therefore, the accident has happened before the full section of -the original material has been removed, the first heading or drift is -driven through this original material rather than through the fallen -débris. Any of the regular soft-ground methods of tunneling may be -employed, but it is usually better to select one which allows the -masonry to be built with as little excavation as possible at first. For -this reason the German method of tunneling is particularly suited to -repair work of this nature. The Belgian method may also be used to -advantage, particularly when the caving extends to the surface of the -ground above, and the upper portion of the débris is, therefore, -practically the same material as that through which the original tunnel -was driven. The greatest defect of the Belgian method for making repairs -is that the roof arch is supported by a rather unstable mass of mingled -earth, stone, and timber, which constitutes the bottom layer of the -fallen material. The method of strutting the work when the German or -Belgian method is used is shown by Fig. 152. It sometimes happens that -the fallen débris is so unstable that it will not carry safely the arch -masonry in the Belgian method or the strutting in the German method, and -in these cases one of the full-section methods of excavation is usually -adopted. The nature of the strutting employed is shown by Fig. 153. When -the section has been opened and the new masonry built, great care should -be taken to fill the cavity behind the masonry with timber or stone; and -should the disturbance reach to the ground surface it is often a good -plan to sink a shaft through the disturbed material, and fill it with -more stable material. - -[Illustration: FIG. 153.--Tunneling through Caved Material by Drifts.] - -When the fallen débris fills only a part of the section, the first thing -to provide against is the occurrence of any further caving; and this is -usually done by building a protecting roof above the line of the future -roof masonry. Figs. 154 and 155 show two methods of constructing this -temporary roof, which it will be noticed is filled above with cordwood -packing. As soon as the temporary roof is completed, the lining masonry -is constructed. - -[Illustration: FIGS. 154 and 155.--Filling in Roof Cavity Formed by -Falling Material.] - -[Illustration: FIG. 156.--Timbering to Prevent Landslides at Portal.] - -(4) Landslides which close the tunnel entrance are repaired in a variety -of ways. Fig. 156 shows a common method of preventing the extension of a -landslide which has been started by the excavation for the entrance -masonry. Fig. 157 shows a method often adopted when the slope is quite -flat and the amount of sliding material is small. It consists -essentially of removing the fallen material and building a new portal -farther back; that is, the open cut is extended and the tunnel is -shortened. When the amount of the sliding material is very large, the -contrary practice of lengthening the tunnel and shortening the open cut, -as shown by Fig. 158, may be adopted. - -[Illustration: FIG. 157.--Shortening Tunnel Crushed by Landslide at -Portal.] - - -=Accidents After Construction.=--Accidents after the completion of the -tunnel may be divided into two classes: first, those which entirely -obstruct the passage of trains, of which the collapse of the roof is the -most common; and second, those which allow traffic to be continued while -the repairs are being made, such as the bulging inward of a portion of -the lining without total collapse. In the first case the first duty of -the engineer is to open communication through the fallen débris, so that -passengers at least may be transferred from one part of the tunnel to -the other and proceed on their way. This is done by driving a heading, -and strongly timbering it to serve as a passageway. If the tunnel is -single tracked this heading is afterwards enlarged until the whole -section is opened. In double-track tunnels the method generally adopted -is to open first one side of the section and timber it strongly, so as -to clear one track for traffic. While the trains are running through -this temporary passageway the other half of the section is opened and -repaired; the traffic is then shifted to the new permanent track, and -the temporary structure first employed is replaced with a permanent -lining. When the accident is such that the repairs can be made without -obstructing traffic entirely, various modes of procedure are followed. -In all cases great care has to be exercised to prevent accident to the -trains and to the tunnel workmen. The work should be done in small -sections so as to disturb as little as possible the already troubled -equilibrium of the soil; the strutting should be placed so as to give -ample clearing space to passing trains, and the trains themselves should -be run at slow speeds past the site of the repairs. To illustrate the -two kinds of accidents and the methods of repairing them, which have -been mentioned, the accidents at the Giovi tunnel in Italy and at the -Chattanooga tunnel in America have been selected. - -[Illustration: FIG. 158.--Extending Tunnel through Landslide at Portal.] - - -=Giovi Tunnel Accident.=--In September, 1869, at a point about 220 ft. -from the south portal of the Giovi tunnel, a disturbance of the masonry -lining for a length of about 52 ft. was observed. Accurate measurements -showed that the lining was not symmetrical with respect to the vertical -axis of the sectional profile. It was concluded that owing to some -disturbance of the surrounding soil unsymmetrical vertical and lateral -pressures were acting on the masonry. Close watch was kept of the -distorted masonry, which for some time remained unchanged in position. -In 1872, however, new crevices were observed to have developed, and -shortly afterwards, in January, 1873, the injured portion of the masonry -caved in, obstructing the whole tunnel section. The fallen material -consisted chiefly of clay in a nearly plastic state. The surface of the -ground above was observed to have settled. Investigation showed also -that the cause of the caving was the percolation of water from a nearby -creek. The water had soaked the ground, and decreased its stability to -such an extent that the masonry lining was unable to withstand the -increased vertical and lateral pressures. - -The mode of procedure decided upon for repairing the damage was: (1) To -open at least one track for the temporary accommodation of traffic; (2) -To remove permanently the causes which had produced the collapse; (3) To -build a new and much stronger lining. Close to the western side wall, -which was still standing, the débris was removed, and the opening -strongly strutted in order to allow the laying of a single track to -reëstablish communication. At the same time a shaft was sunk from the -surface above the caved portion of the tunnel, for the double purpose of -facilitating the removal of the fallen material and of affording -ventilation. The depth of the surface above the tunnel was 41.6 ft., -which made the construction of the shaft a comparatively easy matter. -The shaft itself was 6¹⁄₂ ft. wide and 18 ft. long, with its longer -dimensions parallel to the tunnel, and it was lined with a rectangular -horizontal frame and vertical-poling board construction. After -temporary communication had been opened on the western track of the -tunnel, the remainder of the fallen earth was removed and the excavation -strutted. The new masonry lining was then built. - -To remove permanently the cause of the cave-in, which was the -percolation of water from a close-by stream, this stream was diverted to -a new channel constructed with a concrete bed and side walls. - -The failure of the original lining occurred by cracks developing at the -crown, haunches, and springing lines. The new lining was made -considerably thicker than the original lining, and at the points where -failure had first occurred in the original arch cut-stone _voussoirs_ -were inserted in the brickwork of the new arch as described in Chapter -XIII. - - -=Chattanooga Tunnel.=--The Western & Atlantic Ry. passes through the -Chattanooga mountains by means of a single-track tunnel 1,477 ft. long, -constructed in 1848-49. The lining consisted of a brickwork roof arch -and stone masonry side walls. After the tunnel had been opened to -traffic, this lining bulged inward at places, contracting the tunnel -section to such an extent that it was decided to reconstruct the -distorted portions. After careful surveys and calculations had been -made, it was decided to take down and reconstruct about 170 ft. of the -lining. - -Owing to contracted space in the tunnel, it was necessary to remove all -men, tools, and material, whenever trains were to pass through; and in -order to do this a work-train of three cars was fitted up with necessary -scaffolds, and supplied with gasoline torches for lighting purposes. -Mortar was mixed on the cars, and all material remained on them until -used. Débris torn out of the old wall was loaded on the cars, and hauled -to the waste dump. A siding was built near the West end of the tunnel -for the use of this train, and a telephone system was installed between -the entrances and the working-train. On account of the contracted -working-space and the greater ease with which brick could be handled, -it was decided to rebuild the walls out of brick instead of stone. - -In tearing out the old wall a hole was first cut through the three -bottom courses of the arch and gradually widened. When the opening -became four or five feet long, a small jack was placed near the center -of it and brought to a bearing against the arch to sustain it. After -cutting the opening to a length of from 7 to 10 ft. depending on the -stability of the earth backing, the jack was removed and a piece of 8×16 -in. timber placed under the arch and brought up to a bearing with jacks. -One end of the timber rested on the old wall, the other on a seat built -into the adjoining section of new wall. Wedges were then driven under -the ends of timber and the jacks removed. With this timber in place, the -old wall could be taken down with ease, the only trouble being that -small stones and earth fell in from above and behind the arch. This was -obviated by placing a 2 in. plank across the opening and just back of -the 8×16 in. timber. At several points, however, the earth backing was -saturated with water, and it became necessary to put in lagging as the -old wall was removed. This timbering would be taken out as the new work -was built up. - -A suitable foundation for the new wall was secured at a depth from 2 to -4 ft., and a concrete footing was used. The section of the new wall was -then built up as near as possible to the 8×16 in. timber; the timber was -then removed and the new wall built up and keyed under the arch. - -The new wall had a minimum width of 2¹⁄₂ ft. at the top, and 4 ft. at -the base of rail, and was provided with weep holes at intervals. To -facilitate matters, work was carried on simultaneously at two or three -different places, the intention being to get one place torn out and -ready for the bricklayers by the time they completed a section of the -new wall at another place. - -In rebuilding the arch, sections extending from the springing line up as -far as was necessary to obtain the desired clearance, and from 2¹⁄₂ to -4 ft. in length, were removed. Near the sides, the earth above the arch -was a stiff clay, which was self-sustaining; but near the center there -occurred a stratum of gravel and clay saturated with water. This gave -considerable trouble, falling through almost continuously until -timbering could be placed. One end of this timber rested on the old -arch, the other on the adjoining section of the new work. As the new -work was to be set 6 to 13 ins. back from the old, it was necessary to -block up this distance on top of the old arch, to carry the end of the -lagging timber, in order that the timber should be clear of the new -arch. - -Owing to the small clearance between the car roof and the arch, a -special form of centering was required, one that would occupy as small -space as possible. Bar iron 1 in. thick, 4 ins. wide, and 20 ft. long -was curved to a radius of 6¹⁄₂ ft., and on the underside of this was -riveted a 6-in. plate ¹⁄₄ in. thick. This plate projected 1 in. on the -sides of the centering, and carried the ends of the 1 in. boards used -for lagging. The rivets were counter-sunk on the outside of the -centering to present a smooth surface next the arch. - -In keying up a section of the new work, a space about 18 ins. square had -to be left open for the use of the workmen. As soon as the next section -had been torn out, this space was built up. In building up the last -section, this space had to be filled from below, which proved to be a -tedious undertaking. The opening was gradually reduced to a size of 10 × -18 in., and the top ring then completed and keyed up, the adhesion of -mortar holding the bricks in place until the key could be driven home. -The next ring was treated in a similar manner, and so on to the face -ring. Altogether 412 lin. ft. of the walls and 178 lin. ft. of the arch -were taken down and rebuilt, amounting in all to 607 cu. yds. of masonry -at the total cost of $7,440, or about $12.25 per cu. yds. - -The regular trains arrived so frequently at the tunnel that slightly -over two hours was the longest working-time between any two trains, and -usually less than one hour at a time was all that it could be worked. In -addition to the regular trains, a large number of extra trains, moving -troops, had to be accommodated. Work was in progress eight months, and -during that time there was no delay to a passenger train. The repairs -were completed in August, 1899. The work was under the direction of Mr. -W. H. Whorley, engineer of the Western & Atlantic R. R., and foreman of -construction, A. H. Richards. A recent examination failed to reveal any -sign of settlement cracks at the junction points of the new and old -work. - - - - -CHAPTER XXIII. - -RELINING TIMBER-LINED TUNNELS WITH MASONRY. - - -The original construction of many American railway tunnels with a timber -lining to reduce the cost and hasten the work has made it necessary to -reline them, as time has passed, with some more permanent material. In -most cases the work of removing the old lining and replacing it with the -new masonry has had to be done without interfering with the running of -trains, and a number of ingenious methods have been developed by -engineers for accomplishing this task. Three of these methods which have -been employed, respectively, in relining the Boulder tunnel on the -Montana Central Ry., in Montana, the Mullan tunnel on the Northern -Pacific Ry., in Montana, and the Little Tom tunnel on the Norfolk & -Western R. R., in Virginia, have been selected as fairly representative -of this class of tunnel work. - - -=Boulder Tunnel.=--This tunnel penetrates a spur of the main range of -the Rocky Mountains, at an elevation at the summit of grade of 5,454 -ft., and is 6,112 ft. in length. Its alignment is a tangent, with the -exception of 150 ft. of 30′ curve at the north end. The material -penetrated is blue trap-rock with seams for 4,950 ft. from the north -end, and syenitic boulders with the intervening spaces filled with -disintegrated material for the remaining 1,160 ft. The dimensions and -character of the old timber lining and of the new masonry lining -replacing it are shown in Figs. 159 and 160. - -The form of masonry adopted consisted of coarse rubble side walls of -granite, 13 ft. 8 ins. high, and generally 20 ins. thick, with a full -center circular arch of four rings of brick laid in rowlock form. When -greater strength was needed the thickness of the side walls was -increased to 30 ins. and that of the arch to six rings of brick. - -[Illustration: ~Cross Section.~ - -~Longitudinal Section.~ - -~Cross Section.~ - -~Cross Section.~ - -FIGS. 159 and 160.--Relining Timber-Lined Tunnel.] - -The first plan adopted in putting in the masonry was to remove all the -timbering; but owing to the large number of falls and slides this was -abandoned, and the plan followed was to leave in the three roof segments -of the timbering with the overlying cord-wood packing and débris. In -carrying on the work the first step was to remove the side timbers. This -was done by supporting the roof timbers, as shown in Fig. 159; that is, -the first and fourth arch rib of an 8-ft. section containing four arch -ribs were supported by temporary posts. The intermediate arch ribs were -supported against the downward pressure by 6 × 6 in. timbers, extending -from the side ribs near the tops of the temporary posts to the opposite -sides of the intermediate roof segments, as shown in the longitudinal -section, Fig. 160. To resist the pressure from the sides, 4 × 6 in. -braces were placed across the tunnel from near the center of the -intermediate segments to the upper ends of the hip segments, as shown in -the cross-section, Fig. 159. The hip segments were then sawed off below -the notch, and the side timbering removed and the masonry built. - -The stone was conveyed into the tunnel on flat cars, and laid by means -of small derricks located on the cars. Two derricks were used, one for -each side wall, and the work on both walls was carried on -simultaneously. - -The arch was built upon a centering, the ribs of which were 5¹⁄₂ ins. -less in diameter than the distance between the side walls, so as to -permit the use of 2³⁄₄ ins. lagging. Each center had three ribs, made in -1-in. or 2-in. board segments, 10 ins. thick and 14 ins. deep. These -ribs were mounted on frames, which followed the opposite walls, and were -4 ft. apart, making the total length of the center out to out about 9 -ft. The frames, upon which the ribs were supported, are shown in Fig. -161. As will be seen, they were mounted on dollys to enable the center -to be moved from one section to another. Jacks were used to raise and -lower the center into its proper position. - -[Illustration: ~Cross Section.~ - -~Longitudinal Section.~ - -FIG. 161.--Relining Timber-Lined Tunnel, Great Northern Ry.] - -The arch was built up from the springing lines on both sides at the same -time, four masons being employed. The rings were built beginning with -the intrados, which was brought up, say, a distance of about 2 ft. from -the springing line. Then the back of the ring was well plastered with -from ³⁄₈ in. to ¹⁄₂ in. of mortar, and the second ring brought up to the -same height and plastered on the back, and so on until the last ring was -laid. After bringing the full width of the arch up some distance, new -laggings were placed on the ribs for an additional height of 2 ft. and -the same process was repeated. All the space between the extrados of the -masonry arch and the old lining was compactly filled with dry rubble. -When high enough so that the hip segments had a foot or more bearing on -the masonry the segments were securely wedged and blocked up against the -brickwork, and the longitudinal 4 × 6 in. timbers removed. The -remaining space was now clear for completion of the arch, and both sides -were brought up until there was not sufficient space for four masons to -work, when the keying was completed by two masons beginning at the -completed and working back toward the toothed end. The brickwork was -built from the top of a staging-car. - -[Illustration: ~Cross Section.~ - -~Longitudinal Section.~ - -FIG. 162.--Relining Timber-Lined Tunnel, Great Northern Ry.] - -In a few instances where slides occurred after the removal of the slide -timbering, the method of re timbering the tunnel shown in Fig. 162 was -adopted. Two side drifts were first run 2¹⁄₂ ft. wide by 4 ft. high, and -the plate timbers placed in position and blocked. Cross drifts were then -run, and the roof segments placed, and the core down to the level of the -bottoms of the side drifts taken out. The lower wall plates were then -placed and the hip segments inserted. The bench was then taken down by -degrees, the side plates being held by jacks, and the posts placed one -at a time. As the masonry at the points where slides occur consists of -30-in. walls and six-ring arch, the timbering was 22 ft. wide in the -clear, with other dimensions as shown in Fig. 162. - -Only a single crew of brick and stone masons was employed. In order to -prepare the sections for these masons it was necessary to have timber -and trimming crews at work throughout the whole day of 24 hours, so that -an engine and two train crews were in constant attendance. The single -mason crews were able to complete 8 ft. of side wall and arch in 24 -hours. The number of men actually employed at the tunnel was 35. This -included electric-light maintenance, and all other labor pertaining to -the work. The tunnel was lighted by an Edison dynamo of 20 arc light -capacity, one arc light being placed on each side of the tunnel at all -working-places. Each lamp carried a coil of wire 20 or 30 ft. long to -allow it to be shifted from place to place without delay. - - -=Mullan Tunnel.=--This tunnel is 3,850 ft. long, and crosses the main -range of the Rocky Mountains, about 20 miles west of Helena, Mont. The -tunnel is on a tangent throughout, and has a grade of 20% falling toward -the east. The summit of the grade, west of the tunnel, is 5,548 ft. -above sea level, and the mountain above the line of the tunnel rises to -an elevation of 5,855 ft. Owing to the treacherous nature of the -material through which the tunnel passed, it had been a constant menace -to traffic ever since its construction in 1883, and numerous delays to -trains had been caused by the falls of rock and fires in the timber -lining. For these reasons it was finally decided to build a permanent -masonry lining, and work on this was begun in July, 1892. - -[Illustration: ~_With Wall Plates._~ - -~_Without Wall Plates._~ - -~Old Timber Sections.~ - -~_Minimum Section._~ - -~_Average Section._~ - -~Permanent Work.~ - -FIG. 163.--Relining Timber Lined Tunnel, Great Northern Ry.] - -The original timbering consisted of sets spaced 4 ft. apart _c._ to -_c._, with 12 × 12 in. posts supporting wall plates, and a five-segment -arch of 12 × 12 in. timbers joined by 1¹⁄₂-in. dowels. The arch was -covered with 4-in. lagging, and the space between this and the roof was -filled with cordwood. Except where the width had been reduced by -timbering placed inside the original timbering to increase the strength, -the clear width was 16 ft., and the clear height 20 ft. above the top of -the rail. Fig. 163 shows the timbering and also the form of masonry -lining adopted. The side walls are of concrete and the arch of brick. -This new masonry, of course, required the removal of all the original -timbering. The manner of doing this work is as follows: A 7-ft. section, -_A B_, Fig. 164, was first prepared by removing one post and supporting -the arch by struts, _S S_. After clearing away any backing, and -excavating for the foundation of the side wall, two temporary posts, _F -F_, were set up, and fastened by hook bolts. Fig. 146, _L_, and a -lagging was built to form a mold for the concrete. Several of these -7-ft. sections were prepared at a time, each two being separated by a -5-ft. section of timbering. - -[Illustration: ~Section, with Concrete Car.~ - -~With Wall Plate.~ - -~Without Wall Plate.~ - -~Longitudinal Section.~ - -FIG. 164.--Construction of Centering Mullan Tunnel.] - -The mortar car was then run along, and enough mortar (1 cement to 3 -sand) was run by the chute into each section to make an 8-in. layer of -concrete. As the car passed along to each section, broken stone was -shoveled into the last preceding section until all the mortar was taken -up. The walls were thus built up in 8-in. layers, and became hard enough -to support the arches in about 10 to 14 days. The arches were then -allowed to rest on the wall, and the posts of the remaining 5-ft. -sections were removed, and the concrete wall built up in the same way as -before. - -The average progress per working-day was 30 ft. of side wall, or about -45 cu. yds.; and the average cost, including all work required in -removing the timber work, train service, lights and tools, engineering -and superintendence, and interest on plant, was $8 per cubic yard. - -[Illustration: FIG. 165.--Centering Mullan Tunnel.] - -The centering used for putting in the brick arches is shown in Fig. 165. -From 3 ft. to 9 ft. of arch was put in at a time, the length depending -upon the nature of the ground. To remove the old timber arch, one of the -segments was partly sawed through; and then a small charge of giant -powder was exploded in it, the resulting débris, cordwood, rock, etc., -being caught by a platform car extending underneath. From this car the -débris was removed to another car, which conveyed it out of the tunnel. -The center was then placed and the brickwork begun, the cement car shown -in Fig. 164 being used for mixing the mortar. The size of the bricks -used was 2¹⁄₂ + 2¹⁄₂ + 9 ins., four rings making a 20-in. arch and -giving 1.62 cu. yds. of masonry in the arch per lin. ft. of tunnel. The -bricks were laid in rowlock bond, two gangs, of three bricklayers and -six helpers each, laying about 12 lin. ft. per day. The brickwork cost -about $17 per cu. yd. The total cost of the new lining averaged about -$50 per lin. ft. - -[Illustration: ~Cross Section.~ - -~Longitudinal Section.~ - -FIG. 166.--Relining Timber-Lined Tunnel, Norfolk and Western Ry.] - - -=Little Tom Tunnel.=--The tunnel has a total length of 1,902 ft., but -only 1,410 ft. of it were originally lined with timber. This old timber -lining consists of bents spaced 3 ft. apart, and located as shown by the -dotted lines in the cross-section, Fig. 166. Instead of renewing this -timber, it was decided to replace it with a brick lining. Although the -tunnel was constructed through rock, this rock is of a seamy -character, and in some portions of the tunnel it disintegrates on -exposure to the air. In removing the timber to make place for the new -lining some of the roof was found close to the lagging, but often also -considerable sections showed breakages in the roof extending to a height -varying from 1 ft. to 12 ft. above the upper side of the timbering. This -dangerous condition of the roof made it necessary that only a small -section of the timber lining should be removed at one time. It made it -necessary, also, that the brick arch should be built quickly to close -this opening, and finally that all details of centers, etc., should be -arranged so as to furnish ample clearance to trains. The accompanying -illustrations show the solution of the problem which was arrived at. - -[Illustration: FIG. 167.--Relining Timber-Lined Tunnel, Norfolk and -Western Ry.] - -Referring to the transverse and longitudinal sections shown by Fig. -166, it will be seen that two side trestles were built to carry an -adjustable centering for the roof arch. Two sections of these trestles -and centerings were used alternately, one being carried ahead and set up -to remove the timbering while the masons were at work on the other. The -manner of setting up and adjusting the trestles and centerings is shown -by Fig. 166 and also by Fig. 167, which is an enlarged detail drawing of -the set screw and rollers for the centering ribs. The following is the -bill of material required for one set of trestles and one center: - - Trestles: - Caps and sills 8 pieces 8 × 8 ins. × 20 ft. - Posts 18 „ 8 × 8 „ × 11 „ - Braces 16 „ 6 × 4 „ × 7 „ - Centerings: - Ribs 27 „ 2 × 18 „ × 7 „ - Bracing 12 „ 2 × 8 „ × 7 „ - Support to crown lagging 2 „ 6 × 6 „ × 10 „ - Crown lagging 20 „ 3 × 6 „ × 2 „ - Side lagging 30 „ 3 × 6 „ × 10 „ - Side strips 2 „ 2 × 12 „ × 9 „ - Blocking for rollers 1 „ 5 × 8 „ × 12 „ - - 6 screw and roller castings complete with bolts and lever; 114 bolts - ³⁄₄-ins. in diameter; 7¹⁄₂ U. H. hexagonal nut and 2 cast washers - each. - -With this arrangement the progress made per day varied from 2 lin. ft. -to 3 lin. ft. of lining complete. By work complete is meant the entire -lining, including stone packing between the brickwork and the rock. On -Feb. 23, 1900, 363 ft. of lining had been completed, at a cost of $33.50 -per lin. ft. This cost includes the cost of removing the old timber, the -loose rock above it, and all other work whatsoever. - - - - -CHAPTER XXIV. - -THE VENTILATION AND LIGHTING OF TUNNELS DURING CONSTRUCTION. - - -VENTILATION. - -In long tunnels, especially when excavated in hard rock, proper -ventilation is of great importance, because the air cannot be easily -renewed, and the amount of oxygen consumed by miners horses and lamps -during construction is very large. The gases produced by blasting also -tend to fill the head of excavation with foul air. Pure atmospheric air -contains about 21% of oxygen and only 0.04% of carbonic acid; when the -latter gas reaches 0.1% the fact is indicated by the bad odor; at 0.3% -the air is considered foul, and when it reaches 0.5% it is dangerous. It -is generally admitted that the standard of purity of the air is when it -contains 0.08% of carbonic acid. - -A large quantity of carbonic acid in the air is easily detected by -observing the lamps, which then give out a dim red light and smoke -perceptibly; the workmen also suffer from headache and pains in the -eyes, and breathe with difficulty. Naturally, miners cannot easily work -in foul air and, therefore, make very slow progress. It is, therefore, -to the interest of the engineer to afford good ventilation, not only -because of his duty to care for the safety and health of his men, but -also for reasons of economy, so that the men may work with the greatest -possible ease, thus assuring the rapid progress of the work. - -It would be impossible to change completely the atmosphere inside a -tunnel, as the gases developed from blasting will penetrate into all the -cavities and gather there, but the fresh air carried inside by -ventilation has a very small percentage of carbonic acid, mixes with -that which contains a greater quantity, and dilutes it until the air -reaches the standard of purity. We have not here considered the gases -developed from the decomposition of carboniferous and sulphuric rocks, -which may be met with in some tunnels, and which render ventilation -still more necessary. Tunnels may be ventilated either by natural or -artificial means. - - -=Natural Ventilation.=--It is well known that if two rooms of different -temperatures are put in communication with each other, e.g., by opening -a door, a draft from the colder room will enter the other from the -bottom, and a similar draft at the top, but with a contrary direction, -will carry the hot air into the colder room, thus producing perfect -ventilation, until the two rooms have the same temperature. Now, during -the construction of tunnels the temperature inside may be considered as -constant, or independent of the outside atmospheric variations; hence -during summer and winter, there will always be a draft affording -ventilation, owing to the difference of temperature inside and outside -the tunnel. In winter time the cold air outside will enter at the bottom -of the entrances and headings, or along the sides of the shafts, and the -hot air will pass out near the top of the headings or entrances or the -center of the shafts; in summer the air currents will take the contrary -direction. - -Natural ventilation in tunnels is improved when the excavation of the -heading reaches a shaft, because the interior air can then communicate -with the exterior at two points, at different levels. In such cases a -force equal to the difference in weight between a column of air in the -shaft and a similar one of different density at the entrance of the -tunnel, will act upon the mass of air in the tunnel and keep it in -movement, thus producing ventilation. Consequently, during winter, when -the outside air has greater weight than that inside, the air will come -in by the headings and go out by the shaft, and in the summer it will -enter at the shaft and pass out at the entrance. Sometimes to afford -better ventilation shafts 8 or 12 in. in diameter are sunk exclusively -for the purpose of changing the air. When the inside temperature is -equal to that outside, as often happens during the spring and autumn, -there are no drafts, and consequently the air in the excavation is not -renewed and becomes foul; then fires are lighted under the shaft and a -draft is artificially produced. The hot air going out through the shaft, -as through a chimney, allows the fresh air to come in as in ordinary -ventilation. - -When the head of the excavation is very far from the entrances, or when -the mountain is too high to allow excavation by shafts, it is quite -impossible to secure good natural ventilation, especially during the -spring and autumn months, and the engineer has to resort to some -artificial means by which to supply fresh air to the workmen. - - -=Artificial Ventilation.=--Artificial ventilation in tunnels may be -obtained in two different ways, known as the vacuum and plenum methods. -Their characteristic difference consists in this, that in the vacuum -method the air is drawn from the inside and the vacuum thus produced -causes the fresh air from the outside to rush in, while the plenum -method consists in forcing in the fresh air which dilutes the carbonic -air produced inside the tunnel by workingmen and explosives. In the -vacuum method the pressure of the atmosphere inside the tunnel is always -less than the pressure outside, while in the plenum method the pressure -within is always greater than that outside. Ventilation is the result of -this difference of pressure, as the tendency of the air toward -equilibrium produces continuous drafts. Both these methods have their -advantages and disadvantages; but in the presence of hard rock, when -explosives are continually required, the vacuum method is considered the -best, because the gases attracted to the exhaust pipes are expelled -without passing through the whole length of the tunnel, thus avoiding -the trouble that a draft of foul air will give to the workmen who are -within the tunnel. In both these methods it is necessary to separate -the fresh air from the foul one; and this is done by means of pipes -which will exhaust and expel the foul air in the vacuum method, or force -to the front a current of fresh air when the plenum method is used. -Artificial ventilation may also be obtained by compressed air which is -set free after it has driven the machines, especially in tunnels -excavated through rock, when rock drilling machines moved by compressed -air are employed. - - -=Vacuum Method Contrivances.=--The most common of the vacuum appliances -consists in the simple arrangement of a pipe leading from the head of -the tunnel out through the fire of a furnace. The air in the pipe is -rarefied by the heat of the furnace and then set free from the other end -of the pipe, thus creating a partial vacuum in the pipe, into which the -foul air of the head rushes, the fresh air from the entrance taking its -place, and thus ventilating the tunnel. A similar arrangement may be -used with shafts, and the foul air may be driven out by a furnace which -is placed either at the top or bottom of the shaft. Such furnaces act -the same as those commonly used for heating purposes in the houses, with -this difference, that, instead of fresh air being forced in, foul air is -expelled. Another simple arrangement for producing a vacuum is by means -of a steam jet which is thrown into the pipe, and which helps the -expulsion of the air by heating it, thus producing a different density -which originates a draft besides that mechanically originated by the -force of the steam jet, which tends to carry out the foul air of the -pipes. - -Foul air may also be expelled by means of exhaust fans which are -connected with pipes near the entrance of the tunnel. The fan consists -of a box containing a kind of a paddle wheel turned by steam or water -power and arranged so as to revolve at a high speed. The air inside the -pipe is forced out by blades attached to the wheel, and thus the foul -air of the front is driven away and fresh air from the entrance rushes -in to take its place, and perfect ventilation is obtained. - -The best manner of expelling foul air from tunnels, according to the -vacuum method, is by means of bell exhausters. This consists of two sets -of bells connected by an oscillating beam and balancing each other. Each -set consists of a movable bell, which covers and surrounds a fixed bell -with a water joint. In the central part of the fixed bell there are -valves which open upwards, and on the bottom of each movable bell there -are valves which open from the outside. When one bell ascends, the -valves at the bottom are closed, the air beneath is then rarefied, and a -vacuum is produced; the valves in the central part of the fixed bell -filled with water are opened, and there is an aspiratory action from the -pipe leading to the headings, and the foul air is thus carried away. The -apparatus makes about ten oscillations per minute, and the dimensions of -the bells depend upon the quantity of air to be exhausted in a minute. -In the St. Gothard tunnel, where these bell exhausters were used, they -exhausted 16,500 cu. ft. of air per minute. - - -=Plenum Method Contrivances.=--Fresh air may be driven into tunnels to -dilute the carbonic acid by two different ways, viz., by water blast and -by fans. Water when running at a great velocity produces a movement in -the air which may be sometimes usefully and economically employed for -ventilating tunnels. Water falling vertically is let run into a large -horizontal zinc pipe having a funnel at the outer end; into this the air -attracted by the velocity of the water is forced. By an opening at the -bottom the water is afterward withdrawn from the pipe, and there remains -only the air which is pushed forward by the air which is being -continually sucked in by the velocity of the water. - -The best and most common means of ventilation by the plenum method is by -fans. There are numerous varieties of these fans in the market, but they -all consist of a kind of fan wheel which by rapid revolution forces the -fresh air into the pipe leading to the headings of the tunnel or to the -working places. Instead of a large single fan, such as is used for -mining purposes, it is better to have a number of small fans acting -independently of each other, conveying the fresh air where it is needed -through independent pipes. - - -=Saccardo’s System.=--A new method of ventilating tunnels was devised by -Mr. Saccardo for the ventilation of the Pracchia tunnel along the -Bologna and Lucca Railway in Italy. At the highest end of the tunnel the -mouth was contracted inward in a funnel shaped form so as to just admit -a train. Immediately at this contraction, a lateral tunnel, 50 feet -long, branched off from one side of the main tunnel. At the mouth of -this lateral tunnel was installed a fan which forced air into the tunnel -and with 70 revolutions per minute delivered 3.532 cu. ft. of air per -second at a water pressure of 1 in. This air current was directed inward -through a second contraction or funnel, parallel to the one at the -entrance and 23 ft. beyond it. In operation the action of the artificial -air current was to suck in a considerable volume of outside air, while -the air pressure was sufficient to counterbalance the movement of air -produced by a train moving at a velocity of 16.1 ft. per second. Mr. -Saccardo’s method was employed in ventilating a tunnel on the Norfolk -and Western Railway with satisfactory results. - - -=Compressed Air.=--In the excavation of tunnels in hard rock a number of -rock drilling machines are employed which are moved by compressed air at -a pressure of not less than five atmospheres. At each stroke about 100 -cu. ins. of compressed air are set free, and at an average of 10 strokes -per minute there would be 5000 cu. ins. of air at five atmospheres or -25,000 cu. ins., or a little more than 175 cu. ft. of fresh air at -normal pressure set free every minute by each of the machines employed. -But the air exhausted from the drilling machine is foul. - -Regarding ventilation by compressed air, Mr. Adolph Sutro, in a lecture -delivered to the mining students of the University of California, said: - - “I will note a curious fact which I have never seen explained, and - which is worthy of close investigation by means of experiments. In the - Sutro tunnel we found that the compressed air used for driving the - machine drills, after having been compressed and expanded and - discharged from the drills, was not wholesome to breathe, and the men - and mules would all crowd around the end of the blower pipe to get - fresh air. Whether the air in being compressed has parted with some of - its oxygen or because vitiated from some other cause, I do not know, - and I hope that this subject will at some future day be carefully - examined into.” - -In the December, 1901, number of “_Compressed Air_,” a magazine -especially devoted to the useful application of compressed air, is read: - - Compressed air wasted from power drills is so contaminated with oil - from the cylinders that it cannot be taken into consideration as - ventilation. It is as important to displace it with pure air as it is - to drive out or draw off other vitiated air. The ventilation should be - an independent supply provided by fan or blower, delivering by pipe at - the point where miners are working. - - -=Quantity of Air.=--The quantity of air to be introduced into tunnels -must be in proportion to the oxygen consumed by the men, the animals, -and the explosions. It is allowed that the quantity of air required for -breathing purpose and explosions is as follows: - - 1 workman with lamp needs 240 cu. yds. of fresh air in 24 hours. - 1 horse „ 850 „ „ „ „ - 1 lb. gunpowder 100 „ „ „ - 1 lb. dynamite 150 „ „ „ - -In a long tunnel excavated through hard rock the number of workmen all -together may be assumed at 400 at each end, and each workman is supposed -to be furnished with a lamp. No less than ten horses are employed, and -the average quantity of dynamite consumed is 600 lbs. per day. From the -data given the consumption of air by workmen and lamps would be: 240 × -400 = 96,000 cu. yds.; the consumption of air by horses would be 850 × -10 = 8500 cu. yds.; the consumption of air by dynamite would be 150 × -600 = 90,000 cu. yds.; making a total consumption of air per day of -194,500 cu. yds., or about 8000 cu. yds. per hour. - -To obtain good ventilation, then, it will be necessary to furnish every -hour a quantity of fresh air amounting to not less than 8000 cu. yds. -Since, however, a large quantity of pure air is expelled with the foul -air, it is necessary greatly to increase this quantity. - -It may be observed, in closing, that the water having its particles -divided, as in a fog or mist, rapidly precipitates the gases produced by -explosions. Now, when hydraulic machines are used, there is a hollow -ball pierced by holes that are almost imperceptible, from which the -compressed water spreads in very subtile particles, and this causes the -fall of the gases from explosions. Such a method of precipitating gases -is very good, but does not have the advantage of supplying new oxygen to -replace that consumed by the men, animals, lamps, and explosions; -besides, it has the defect of increasing the quantity of water to be -removed. In tunnels the pipes used either for conveying the fresh air or -for carrying away the foul air, are of iron, having a diameter of about -8 in.; they are fixed along the side walls about 3 ft. above the -inverted arch. - - -LIGHTING. - -The object and necessity of a perfect lighting of the tunnel-workings -during construction are so obvious that they need not be enlarged upon. -Comparatively few tunnels require lighting after completion; and these -are generally tunnels for passenger traffic under city streets, of which -the Boston Subway is a representative American example. Considering the -methods of lighting tunnels during construction, we may, for sake of -convenience, chiefly, divide the means of supplying light into (1) lamps -and lanterns usually burning oil; (2) coal-gas lighting; (3) acetylene -gas lighting; and (4) electric lighting. - - -=Lamps and Lanterns.=--Lamps and lanterns are commonly employed by -engineers for making surveys inside the tunnel, and to light the -instrument. For ranging in the center line, a convenient form of lamp -consists of an oil light inclosed in glass chimney covered with sheet -metal, except for a slit at the front and back through which the light -shines, and on which the observer sights his instrument. To direct the -operations of his rodmen the engineer usually employs a lantern, either -with white or colored glass, much like the ordinary railway trainman’s -lantern, which he swings according to some prearranged code of signals. - -Lamps and lanterns are used by the workmen both for signaling and for -lighting the workings. For signaling purposes red lanterns are usually -placed to denote the presence of unexploded blasts or other points of -possible danger; and colored or white lights are usually placed on the -front and rear of spoil and material trains. For lighting purposes, two -forms of lamps are employed, which may be somewhat crudely designated as -lamps for individual use and lamps for general lighting. Individual -lamps are usually of small size, and burn oil; they may be carried in -front of the miner’s helmet, or be fixed to standards, which can be set -up close to the work being done by each man. Miners’ safety lamps should -be employed where there is danger from gas. A great variety of lamps for -mining and tunneling purposes are on the market, for descriptions of -which the reader is referred to the catalogues of their manufacturers. - -Lamps for general lighting are always of larger size than lamps for -individual use. A common form consists of a cylinder ten or twelve -inches in diameter, provided with a hook or bail for suspension, and -filled with benzine, gasolene, or other similar oil. Connected with this -cylinder is a pipe of considerable length and small diameter through -which the benzine or gasolene vapor runs, and burns when lighted with a -brilliant flame. Lamps of this type burning gasolene were extensively -employed in building the Croton Aqueduct tunnel. Various patented forms -of lamps for burning coal-oil products are on the market, for -descriptions of which the manufacturers’ catalogues may be consulted. - - -=Coal-gas Lighting.=--A common method of lighting tunnel workings is by -piping coal-gas into the headings and drifts from some nearby permanent -gas plant, or from a special gas works constructed especially for the -work. Gas lighting has the great advantage over lamps and lanterns of -giving a light which is more brilliant and steady. Its great objection -is the danger of explosion caused by leaks in the pipes, by breaks -caused by flying fragments of rock, and by the carelessness of workmen -who neglect to turn off completely the burners when they extinguish the -lights. In nearly every tunnel where gas has been used for lighting, the -records of the work show the occurrence of accidents which have -sometimes been very serious, particularly when fire has been -communicated to the tunnel timbering. - - -=Acetylene Gas Lighting.=--The comparatively recent development of -acetylene gas manufactured from carbide of calcium has given little -opportunity for its use in tunnel lighting, and the only instance of its -use in the United States, so far as the author knows, is the water-works -tunnel conduit for the city of Washington, D. C. Col. A. M. Miller, U. -S. Engineer Corps, who is in charge of this work, describes the method -adopted in his annual report for 1899 as follows:-- - - “It had been the practice to do all work underground by the light of - miners’ lamps and torches. This means of illumination is very poor for - mechanical work. The fumes and smoke from blasting, added to the smoke - from torches and lamps, render the atmosphere underground, especially - when the barometer conditions were unfavorable to ventilation, very - offensive and discomforting to the workmen. An investigation of the - subject of lighting the tunnel by other means, more especially at the - locality where the mechanics were at work,--brick and stone masons, - and the workmen on the iron lining,--resulted in the selection of - acetylene gas as the most available and economical in this special - emergency. Accordingly, an acetylene gas plant for 300 burners was - erected at Champlain-Avenue shaft, and one for 60 lights at Foundry - Branch. The engine-houses at the shafts, the head-houses, and - localities in the tunnel, when required, are lighted by these plants. - - “Gas pipes were carried down the Champlain-Avenue shaft and along the - tunnel both in an easterly and westerly direction, with cocks for - burners at proper intervals every 30 feet; and this system sufficed - for illumination from Hock Creek to Harvard University, a distance of - over two miles. The plant erected at Foundry Branch was in like manner - utilized for the illumination from that point in both directions. - - “By connecting with the stopcocks by means of a rubber hose, a - movable light, chandelier, or ‘Christmas-tree’ of any required number - of burners is used, thus concentrating the light in the immediate - vicinity of the work, and also enabling the illumination to be carried - into the cavities or ‘crow-nests,’ so called, behind the defective old - lining. - - “This method of illuminating has proved very satisfactory and quite - economical. It is especially valuable as enabling good work to be - done, and facilitating a thorough inspection of the same.” - - -=Electric Lighting.=--By far the most perfect, and at present the most -commonly employed means for lighting tunnel workings, is electricity. -The light furnished by electric lamps is steady and brilliant, and does -not consume oxygen or give off offensive gases. The wires are easily -removed and extended, and the lamps are easily put in place and removed. -About the only objection to the method is the fragility of the lamps, -which are easily broken by the flying stones and the concussion produced -by blasting. - - - - -CHAPTER XXV. - -THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR THE WORK. - - -=Cost.=--The cost of a tunnel will depend upon the cost of the two -principal operations required in its construction, viz., the excavation -of the cross section and the lining of the excavation with masonry, -metal, or timber. These two operations may in turn be subdivided, in -respect to expense, into cost of labor and cost of materials. It is a -comparatively simple matter to calculate the cost of the building -materials required to construct a tunnel; but it is very difficult to -estimate with accuracy what the cost of labor will be. The reason for -this is that it is impossible to foresee exactly what the conditions -will be; the character of the material may change greatly as the work -proceeds, increasing or decreasing the cost of excavation; water may be -encountered in quantities which will materially increase the -difficulties of the work, etc. Nevertheless, while accurate preliminary -estimates of cost are not practicable, it is always desirable to attempt -to obtain some idea of the probable expense of the work before beginning -it, and the more usual means of getting at this point will be discussed -here. - -Two methods of estimating the cost of tunnel work are employed. The -first is to calculate the probable expense of the various items of work, -based upon the available data, per unit of length, and then add to this -a margin of at least 10% to allow for contingencies; the second is to -apply to the new work the unit cost of some previous tunnel built under -substantially the same conditions. In the first method it is usual to -consider the strutting and hauling as constituting a part of the work -of excavation. To estimate the cost of excavation involves the -consideration of three general items, viz., the excavation proper, the -strutting of the walls of the excavation, and the hauling of the -excavated materials and the materials of construction. - -The cost of excavating the preliminary headings or drifts is greater per -unit of material removed than that of excavating the enlargement of the -section. The cost of bottom drifts is also always greater than that of -top headings, the material penetrated remaining the same. Mr. Rziha -gives the comparative unit costs of excavating drifts, headings, and -enlargement of the profile as follows:-- - - Bottom drifts $9.20 per cu. yd. - Top headings 4.80 „ „ „ - Enlargement of profile 2.84 „ „ „ - -The cost of hauling increases with the length of the tunnel. This fact -and amount of this increase are indicated by the following actual prices -for the Arlberg tunnel:-- - - Top heading $6.76 per cu. yd., increasing 37 cts. per mile - Bottom drift 7.40 „ „ „ „ 26 „ „ „ - Enlargement of profile 2.70 „ „ „ „ 10 „ „ „ - -In all the prices given above, the cost of strutting and hauling is -included in the cost of excavation. - -The cost of excavation is not always the same for the same character of -materials in different tunnels. The following figures show the prices -paid for the excavation of calcareous rock in four different German -tunnels:-- - - Berliner Nordhausen Wetzler R.R. $1.24 per cu. yd. - Ofen 1.30 „ „ „ - Stafflach 2.76 „ „ „ - Gries 1.92 „ „ „ - -The method of tunneling has little influence upon the cost of the work, -as shown by the following figures from tunnels excavated through -calcareous rock by different methods:-- - - Ofen tunnel Austrian method $93.19 per lin. ft. - Dorremberg tunnel Belgian method 86.08 „ „ „ - Stafflach tunnel English method 91.69 „ „ „ - -The Martha and Merten tunnels, excavated through soft ground by the -Austrian and German methods respectively, cost $87.95 and $87.55 per -lin. ft. respectively. In the excavation of the various sections of the -tunnel for the new Croton Aqueduct in America, the following prices were -paid:-- - - Excavation of heading $8 to $10.00 per cu. yd. - Tunnel in soft ground 8 to 9.00 „ „ „ - Tunnel in rock 7 to 8.50 „ „ „ - Brick masonry 10.00 „ „ „ - Timber in place $40 per M. ft. B. M. - -It is the practice in America to include the work of hauling under -excavation, but not to include the strutting, which is paid for -separately. In some cases only the market price of the timber is paid -for separately, the cost of setting up being included in the price of -excavation. The writer prefers the European practice of including the -total cost of timbering under excavation, since the two operations are -so closely connected, and since the contractor employs the same timber -over and over again. Knowing the dimensions of the several members of -the strutting, it is a simple, although somewhat tedious, - -process to calculate the total quantity required. An idea of the -quantity of timber required for strutting in soft ground may be had from -the data given on page 55. The quantity will decrease as the cohesion of -the material penetrated increases, until it becomes so small in hard -rock-tunnels as to cut very little figure in the total cost. - -The cost of hoisting excavated materials through shafts depends upon the -depth from which it is hoisted, and upon the character of hoisting -apparatus employed. The following table, showing the cost of hoisting -for different lifts and by different methods, is given by Rziha, the -cost being in francs per cubic meter:-- - - +-------+----------+----------------------+-------------+ - | HEIGHT| WINDLASS.| HORSE GINS. |STEAM HOISTS.| - | IN +----------+----------+-----------+-------------+ - |METRES.| |ONE HORSE.|TWO HORSES.| | - | | Francs | Francs | Francs | Francs | - | |per Cu. M.|per Cu. M.| per Cu. M.| per Cu. M. | - +-------+----------+----------+-----------+-------------+ - | 15 | 0.172 | 0.077 | 0.062 | 0.035 | - | 30 | 0.212 | 0.087 | 0.070 | 0.045 | - | 45 | 0.257 | 0.100 | 0.080 | 0.050 | - | 60 | 0.305 | 0.112 | 0.092 | 0.082 | - | 90 | 0.410 | 0.152 | 0.110 | 0.087 | - | 120 | 0.535 | 0.195 | 0.135 | 0.092 | - | 150 | 0.722 | 0.240 | 0.157 | 0.112 | - +-------+----------+----------+-----------+-------------+ - -Mr. Séjourné, a French engineer, who has been connected with the -construction of numerous tunnels by the Belgian method where he was in -position to secure comparative figures, has given the following rules -for calculating the cost of tunnels. Assuming _A_ to represent the cost -of excavating a cu. yd. in the open air, the cost of excavating the same -quantity underground in driving headings will be from 9 _A_ to 11 _A_, -and in enlarging the profile it will be about 5 _A_. The cost of -constructing single-track tunnels varies with the thickness of the -lining, and may be calculated by the following formulas: - - Without lining, _C_ = 5.5 _A_. - With roof arch only, _C_ = 6.4 + 6.4 _A_. - With lining 18 in. thick, _C_ = 9.4 + 7 _A_. - With lining 2 ft. thick, _C_ = 11 + 8 _A_. - -In these formulas _C_ is the cost per cu. yd. of excavation, including -the masonry. For double-track tunnels the amounts given by the above -formulas may be used by reducing them about 7¹⁄₂% or 8%. - -The second method of estimating the cost of tunnel work consists in -assuming as a unit the unit cost of tunnels previously excavated under -similar conditions. Mr. La Dame gives the following unit prices for a -number of tunnels driven through different materials: - - +-------------------+--------+-------------+--------+----------------+ - | NATURE OF SOIL. |TUNNELS,| EXCAV. PER |COST PER| MAX. AND MIN. | - | | NO. OF | CU. YD. |LIN. FT.| PER LIN. FT. | - +-------------------+--------+-------------+--------+----------------+ - |Granite-gneiss | 56 |$3.07 @ $3.85| $100. |$61.46 @ $190.40| - |Schist | 39 | 1.38 @ 1.53| 75.42| 43.11 @ 70.68| - |Triassic | 3 | ... | 90.85| 84.75 @ 93.33| - |Jurassic | 69 | 1.23 @ 1.38| 77.86| 35.24 @ 157.2 | - |Cretaceous | 34 | 0.61 @ 0.77| 59.60| 27.37 @ 92.25| - |Tertiary and modern| 39 | 0.33 @ 0.61| 105.80| 51.52 @ 188.36| - +-------------------+--------+-------------+--------+----------------+ - -In the following table is given a list of tunnels excavated through -different soils, from the most compact to very loose materials, and -driven according to the various methods which have been illustrated. - -DOUBLE-TRACK TUNNELS. - - +--------------+-----------------+--------+-------------+ - |NAME OF |QUALITY OF SOIL. |COST PER| METHOD OF | - |TUNNELS. | |LIN. FT.| TUNNELING. | - +--------------+-----------------+--------+-------------+ - |Mt. Cenis |Granitic, |$273.73 |Drift. | - |St. Gothard |... | 193.63 |Heading. | - |Stammerich |Granitic, | 157.90 |English. | - |Stalle |Broken schist, | 290.58 |Austrian. | - |Bothenfels |Dolomite, | 115.64 |English. | - |Dorremberg |Calcareous, | 86.08 |Belgian. | - |Stafflach |Calcareous, | 91.69 |English. | - |Ofen |Calcareous, | 93.19 |Austrian. | - |Wartha |Grewack, | 87.95 |Austrian. | - |Mertin |Grewack, | 87.55 |German. | - |Schloss Matrei|Clay schist, | 94.25 |English. | - |Trietbitte |Clay and sand, | 229.0 |German. | - |Canaan |Clay-slate, | 69.50 |Wide heading.| - |Church-Hill |Clay with shells,| 178.0 |... | - |Bergen No. 1 |Trap rock, | 182.31 |... | - +--------------+-----------------+--------+-------------+ - -SINGLE-TRACK TUNNELS. - - +--------------+--------------------+----------+-------------+ - | NAME OF | QUALITY OF SOIL. | COST PER | METHOD OF | - | TUNNELS. | | LIN. FT. | TUNNELING. | - +--------------+--------------------+----------+-------------+ - |Mt. Cenis |Gneiss, |$82.27 |Heading. | - |Stalletti |Granite and quartz, | 62.75 |Austrian. | - |Marein |Clay schist, | 64.36 |English. | - |Welsberg |Gravel, |165.07 |Austrian. | - |Sancina |Clay of 1st variety,|129.40 |Belgian. | - |Starre |Clay of 2d variety, |191.61 |Belgian. | - |Cristina |Clay of 3d variety, |307.42 |Italian. | - |Burk |... | 83.90 |Wide heading.| - |Brafford Ridge|... | 85.33 |Wide heading.| - |Dunbeithe |Limestone, | 70.47 |Wide heading.| - |Fergusson |Sandstone, | 37.46[16]|Wide heading.| - |Port Henry |Limestone, | 80.00[17]|Wide heading.| - |Points |Granite, | 72.00[16]|Wide heading.| - +--------------+--------------------+----------+-------------+ - - [16] Are unlined. - - [17] Lined with timber. - -The Habas tunnel through quicksand, between Dax and Ramoux, France, -cost $118.50 per lin. ft. The cost of the Boston subway was $342.40 per -lin. ft. The Severn and Mersey tunnels, constructed through rock under -water, cost respectively $208.38 and $263 per lin. ft. The First Thames -Tunnel, driven by Brunel’s shield, cost $1661.66 per lin. ft. The Hudson -River and St. Clair River tunnels, excavated through soft ground by -means of shields and compressed air, cost respectively $305 and $315 per -lin. ft. The Blackwall double-track tunnel under the River Thames, which -is the largest tunnel ever built by the shield system, cost $600 per -lin. ft. - -In making estimates of the cost of projected tunnel work based on the -cost of tunnels previously constructed through similar materials, it is -important to keep in mind the date and location of the work used as the -basis for calculations. For example, a tunnel excavated in Italy, where -labor is very cheap, will cost less than one excavated in America, where -labor is dear, all other conditions being the same. Other reasons for -variation in cost due to difference of date and location of construction -will suggest themselves, and should be taken into full consideration in -estimating the cost of the new work. - - -=Time.=--The time required to excavate a tunnel depends upon the -character of the material penetrated and upon the method of work -adopted. Tunnels driven through soft ground by hand require about the -same time to construct as tunnels driven through hard rock by the aid of -machinery. Tunnels can be driven through hard rock at about as great a -speed as through soft or fissured rock, chiefly because the work of -blasting is more efficient in hard rock, and because no time is required -in timbering. The following table shows the average rate of progress in -different parts of the tunnel excavation through both hard and soft -materials in feet per month:-- - - +---------------+--------------------+--------------------+-----------+ - | QUALITY | HEADING. | EXCAVATION |ENLARGEMENT| - | OF SOIL. | | OF SHAFTS. |OF PROFILE.| - | +----------+---------+---------+----------+-----------+ - | | By hand. | By | By hand.| By | By hand. | - | | | machine.| | machine. | | - +---------------+----------+---------+---------+----------+-----------+ - |Very loose soil|16.7- 26.8| | 6.6-16.7| | 6.6- 16.7| - |Loose soil |33.4-100 | |16.7-33.4| | 16.7- 33.4| - |Soft rock |66.8 |233.8-334|33.4-66.8|66.8-132.6| 33.4- 50 | - |Hard rock |50 - 66.8|233.8-334|33.4-50 |66.8-132.6| 66.8-100 | - |Very hard rock |33.4 |233.8-334|16.7-33.4|66.8-132.6| 66.8-100 | - +---------------+----------+---------+---------+----------+-----------+ - -The following tables showing the average rate of progress have been -compiled from the actual records made in the tunnels named: - - +-------------+-------------+--------+--------------+-------------+ - | NAME OF | DIMENSIONS |MONTHLY | CHARACTER OF |OBSERVATIONS.| - | TUNNEL. | IN FEET. |PROGRESS| MATERIAL. | | - | | |IN FEET.| | | - +-------------+-------------+--------+--------------+-------------+ - |Excavation of| | | | | - |headings by | | | | | - |hand: | | | | | | - | Mount Cenis |10 × 10 | 65.8 |Schist, |Bottom drift.| - | Sutro | 6.7 × 5.7 | 70.14 |Quartzose, |... | - | St. Gothard | 8.4 × 8.7 | 70.14 |Granite, |Top heading. | - | | | | | | - |Excavation of| | | | | - |headings by | | | | | - |machine: | | | | | | - | Mount Cenis |10 × 10 | 188.7 |Calcareous | | - | | | |schist, |Bottom drift.| - | Sutro | 8.15 × 10 | 227.45 |Quartzose, |... | - | St. Gothard | 8.4 × 8.7 | 339.45 |Granite, |Top heading. | - | Trari | 8 × 9.35| 167 |Gneiss, |Top heading. | - | Arlberg | 8.35 × 9.35| 474.2 |Mica schist, |Bottom drift.| - | Palisades |16 × 7 | 160 |Trap rock, |Top heading. | - | Busk |15 × 7 | 126 |Granite, |Top heading. | - | Cascade |16 × 8 | 180 |Basaltic rock,|Top heading. | - | Franklin |15 × 7 | 240 |... |Top heading. | - +-------------+-------------+--------+--------------+-------------+ - -The following table shows the monthly progress of completed tunnel in -feet excavated through rock: - - +---------------+--------+----------+------------+ - |NAME OF TUNNEL.|PROGRESS|MATERIAL. | METHOD. | - | |IN FEET.| | | - +---------------+--------+----------+------------+ - |Cascade | 207 |Basalt, |Top heading.| - |Palisades | 186 |Trap rock,|Top heading.| - |Busk | 190 |Granite, |Top heading.| - |Tennessee Pass | 169.5 |Granite, |Top heading.| - +---------------+--------+----------+------------+ - -The average monthly progress in feet of excavating tunnels through -treacherous ground may be quite generally assumed to be for: (1) clay of -the first variety from 43.4 ft. to 60 ft.; for clay of the second -variety from 33.4 ft. to 43.4 ft.; for clay of the third variety from -23.3 ft. to 33.4 ft., and for quicksand from 30 ft. to 50 ft. The -monthly progress in feet made in sinking the shafts of the Hoosac and -Musconetcong tunnels in America was as follows:-- - - +---------------+------------+---------+--------+------------+ - |NAME OF TUNNEL.| DIMENSIONS | DEPTH |PROGRESS|CHARACTER OF| - | | IN FEET. |IN FEET. |IN FEET.| MATERIAL. | - +---------------+------------+---------+--------+------------+ - |Hoosac: | | | | | - | East shaft |15.4 × 27.7| 1035 | 21.7 |Mica schist.| - | West shaft | 8 × 16 | 267 | 16.7 |Gneiss. | - |Musconetcong: | | | | | - | Vertical shaft| 8.35 × 16.7| 113.5 | 100 |Loose rock. | - | Inclined shaft| 8.35 × 26 | 304. | 32 |Loose rock. | - +---------------+------------+---------+--------+------------+ - -The average monthly progress of sinking shafts in treacherous soils may -be assumed to be as follows: clay of first variety, 50 ft. to 75 ft; -clay of second variety, 36.75 to 50 ft; clay of third variety, 23.4 ft. -to 36.75 ft; quicksand, 16.7 ft. to 33.4 ft. - -For the reason that the details change with the various conditions -encountered in every work, all the tunnel operations have been treated -in a general way, purposely avoiding to give any detail. Also the rate -of progress and items of cost of tunnels have been given in a broad -manner because they greatly vary in the different works. This -information, however, can be easily obtained by consulting the -Engineering Magazines, where are reported all the tunnel works of -America and Europe, and where are given so many details which are very -valuable to expert engineers in charge of similar works, but not to -students and people who are looking only for general knowledge. - - - - -INDEX - - - Accidents and Repairs in the Belgian Method, 152 - Accidents in Tunnels: - After Construction, 308 - Baltimore Belt Line, 165 - Chattanooga Tunnel, 311 - During Construction, 301 - General Discussion, 301 - Giovi Tunnel, 309 - Repairing of, 304 - Acetylene Gas Lighting, 334 - Air Compressors, Description of, 87 - Air Locks, 264-272 - Air Pressure, 268 - American Method: - General Description, 172 - Excavation, 172 - Strutting, 174 - Hauling, 175 - Arrangement of Drill Holes, 90 - Artificial Ventilation, 327 - Austrian Method of Tunneling: - Advantages and Disadvantages, 180 - Excavation, 176 - General Description, 176 - Lining, 180 - Strutting, 177 - Average Progress in Tunnels, 342 - - Baltimore Belt Line Tunnel, General Description, 160 - Barlow’s Shield, 242 - Beach’s Shield, 246 - Belgian Method: - Accidents and Repairs, 152 - Advantages and Disadvantages, 152 - Excavation, 145 - General Description, 144 - Lining, 148 - Hauling, 150 - Strutting, 146 - Bench, 131 - Bends, 268 - Blackwall’s Tunnel Shield, 248 - Blasting-cone, 33 - Blickford Match, 31 - Boston Subway: - General Descriptions, 203 - Roof Shield, 251 - Boulder Tunnel Relined, 315 - Box-cars, 61 - Box Strutting, 51 - Brandt Drilling Machine, 28, 112 - Brown, W. L., 269 - Brunel’s Shield, 240 - - Caissons, 293 - Canals and Pipe Lines, 86 - Cascade Tunnel, 98 - Center-cut, 91 - Center Line: - Curvilinear Tunnels, 14 - Determination of, 9 - Rectilinear Tunnels, 9 - Simplon Tunnel, 106 - Submarine Tunnels, 265 - Triangulation, 12 - Transferred through Center Shafts, 13 - Transferred through Side Shafts, 14 - Value’s Device, 10 - Centers: - For Arches, 68 - English Method, 169 - Ground Molds, 66 - Italian Method, 184 - Lagging, 71 - Leading Frames, 67 - Setting Up, 70 - Striking, 71 - Chattanooga Tunnel, Accident, 311 - City and South London Railway Shield, 250 - Classification of Tunnels, 42 - Coal-gas Lighting, 333 - Cofferdam Method of Tunneling, 281 - Van Buren Street Tunnel, Chicago, 282 - Collapse of Tunnels, 302 - Compressed Air: - For Power, 87 - For Ventilation, 330 - Concrete Lining, 75 - Fort George Tunnel, 139 - Murray Hill Tunnel, 126 - Cost of: - Double-track Tunnels, 340 - Hauling, 338 - Headings, 337 - Hoisting, 338 - Single-track Tunnel, 340 - Submarine Tunnels, 341 - Subways, 209-217 - Tunnels, 336 - Craven, Alfred, 39 - Craven’s Sunflower, 39 - Cross-section: - Dimensions of, 20 - Form of, 18 - Hudson River Tunnel Pennsylvania Railroad, 277 - Crown-bar (see American Method). - Subways, 204-211 - Croton Aqueduct Tunnel, 95 - Culverts, 80 - - Detroit River Tunnel, 296 - Diamond Drilling Machine, 27 - Directing the Shield, 265 - Drift, 37 - Drift Method: - General Discussion, 102 - Murray Hill Tunnel, 123 - Simplon Tunnel, 103 - Drilling Machines: - Brandt, 112 - Ingersoll, 26 - Drills: - Diamond, 27 - Hand, 23 - Mountings for, 25 - Percussion, 24 - Power, 24 - Rotary, 27 - Dumping Cars, 60 - - Electric Firing, 32 - Electric Lighting, 335 - English Method: - Advantages and Disadvantages, 171 - Centers, 169 - Excavation, 166 - General Discussion, 166 - Lining, 170 - Strutting, 167 - Enlargement of the Profile, 38 - Entrances, 81 - Erector, 272 - Excavation: - American Method, 172 - Arrangement of Drill Holes, 90 - Austrian Method, 176 - Belgian Method, 145 - Center-cut, 91 - Enlargement of Profile, 38 - English Method, 166 - Fort George Tunnel, 136 - German Method, 155 - Headings, 37, 91 - Hudson River Tunnel of Pennsylvania Railroad, 273 - Italian Method, 182 - Murray Hill Tunnel, 124 - Quicksand Method, 189 - Pilot Method, 193 - Shield and Compressed Air Method, 267 - Simplon Tunnel, 110 - Excavating Machines: - For Earth, 22 - For Rock, 23 - Explosions, 33 - Dynamite, 30 - Gunpowder, 28 - Nitroglycerine, 29 - Quantity of, 34 - Storage of, 30 - - Failure of Tunnel Roof, 305 - Forgie, James, 269 - Fort George Tunnel, 135 - Foundations for Lining, 76 - Fox, Charles B., 103 - Frame Strutting, 49 - Fuses, 31 - - Geological Survey, 3 - German Method: - Advantages and Disadvantages, 159 - Excavation, 155 - General Description, 155 - Hauling, 158 - Strutting, 156 - Giovi Tunnel Accident, 309 - Graveholz Tunnel, 98 - Greathead’s Shield, 245 - - Hand Drills, 23 - Harlem River Tunnel, 285 - Hauling: - American Method, 175 - Belgian Method, 150 - Italian Method, 185 - German Method, 158 - Hudson River Tunnel of Pennsylvania Railroad, 278 - Motive Power, 61 - By Way of Entrances, 59 - Simplon Tunnel, 111 - By Way of Shafts, 62 - Heading and Bench Method: - Fort George Tunnel, 135 - General Discussion, 130 - St. Gothard Tunnel, 1 - Headings, 37, 91 - Hewett, H. B., 269 - History of Tunnels, xiii - Hoisting Machines: - General Discussion, 62 - Elevators, 64 - Horse Gins, 63 - Windlass, 63 - Hoosac Tunnel, 93 - Hopkins, Stephen W., 135 - Hudson River Tunnel of Pennsylvania Railroad, 269 - Hydraulic Jacks, 260, 271 - Hydraulic Rams, 271 - - Illumination: - Acetylene Gas, 334 - Coal-gas, 333 - Electric, 335 - Hudson River Tunnel of Pennsylvania Railroad, 280 - Lamps and Lanterns, 330 - Inclination of Strata, 6 - Ingersoll Drilling Machine, 26 - Inverted Arch Lining, 77 - Iron and Masonry Lining, 74 - Iron Lining, 73, 261, 276 - Iron Strutting, 55 - Full Section, 56 - Headings, 56 - Shafts, 57 - Italian Method: - Advantages and Disadvantages, 188 - Excavation, 182 - General Description, 182 - Modifications, 186 - Strutting, 183 - - Jacks, 260, 271 - Joining the Caissons, 295 - - Lagging, 71 - Lamps and Lanterns, 330 - Lighting (see Illumination). - Lining: - Austrian Method, 180 - Belgian Method, 148 - Concrete, 126, 139 - English Method, 170 - Foundations, 76 - General Observations, 78 - German Method, 158 - Hudson River Tunnel Pennsylvania Railroad, 276 - Invert, 77 - Iron, 73, 261, 276 - Iron and Masonry, 74 - Italian Method, 185 - Masonry, 74 - Quicksand Method, 191 - Roof Arch, 77 - Side Tunnels, 79, 83 - Side Walls, 77 - Subways, 207-213 - Timber, 72 - Thickness of Masonry, 78, 83 - Little Tom Tunnel Relined, 321 - Loose Soil (see Soft Ground). - - Masonry (see Centers). - Masonry Culverts, 80 - Masonry (see Lining). - Masonry Lining, 74 - Masonry Niches, 81 - McBean, Daniel, 285 - Mechanical Installations for Tunnel Work, 84 - Milwaukee Tunnel, 226 - Mont Cenis Tunnel, 92 - Monthly Progress of Tunnels, 342 - Mullan Tunnel Relined, 319 - Murray Hill Tunnel, 123 - - Natural Ventilation, 326 - New York Rapid Transit Subway, 209 - Niagara Falls Power Tunnel, 97 - Niches, 81 - - Open Cut or Tunnel, 1 - Open-cut Tunneling: - General Discussion, 195 - Parallel Longitudinal Trenches, 197 - Single Trench, 196 - Single Narrow Trench, 197 - Transverse Trenches, 200 - Tunnels on the Surface, 200 - - Palisade Tunnel, 94 - Pennsylvania Railroad Shield, 270 - Percussion Drills, 24 - Pilot Method of Tunneling, 192 - Plank Centers, 69 - Platform Cars, 59 - Plenum Method of Ventilation, 329 - Pneumatic Caissons, 287 - Polar Protractor, 39 - Portals, 81 - Power Drills, 24 - Power Plants: - Air Compressors, 87 - Canals and Pipe Lines, 86 - Cascade Tunnel, 98 - Croton Aqueduct Tunnel, 95 - General Description, 84 - Graveholz Tunnel, 98 - Hoosac Tunnel, 93 - Hudson River Tunnel Pennsylvania Railroad, 279 - Mont Cenis Tunnel, 92 - Murray Hill Tunnel, 128 - Niagara Falls Power Tunnel, 97 - Palisades Tunnel, 94 - Receivers, 89 - Reservoirs, 86 - Simplon Tunnel, 117 - Sonnstein Tunnel, 99 - St. Clair River Tunnel, 99 - St. Gothard Tunnel, 133 - Steam, 85 - Strickler Tunnel, 96 - Turbines, 86 - Prelini’s Shield, 251 - Presence of Water, 7 - Prevention of Collapse, 303 - Progress in Sinking Shafts, 343 - Progress of Excavation, 342 - Progress of the Work, 342 - Progress in Simplon Tunnel, 122 - - Quantity of Air for Ventilation, 331 - Quicksand Tunneling: - General Discussion, 188 - Removing the Seepage Water, 191 - Quantity of Timber in Strutting, 54 - - Receivers, 89 - Relining Tunnels, 315 - Boulder Tunnel, 315 - Little Tom Tunnel, 321 - Mullan Tunnel, 319 - Repairing of Accidents in Tunnels, 308 - Reservoirs, 86 - Roof Arch Lining, 77 - Roof Shield for Boston Subway, 251 - Roof of Caissons, 287-291 - Rotary Drills, 27 - Ryder, B. H., 296 - - Saccardo System of Ventilation, 330 - Saunders, W. L., 88 - Seepage Water, 191 - Seine River Tunnel, 293 - Setting up Centers, 70 - Severn Tunnel, 221 - Shafts, Description of, 40 - Shaler, Ira A., 142 - Shield and Compressed Air Method, 263 - Shield Construction: - Diaphragm, 256 - Cellular Division, 255 - Dimensions of Shields, 259 - Front End, 254 - General Form, 252 - Rear End, 257 - Shell, 253 - Shield Method: - Barlow Shield, 242 - Beach’s Shield, 245 - Blackwall Tunnel Shield, 248 - Brunel Shield, 240 - City and South London Railway Shield, 250 - Greathead’s Shield, 245 - History, 238 - Prelini’s Shield, 251 - St. Clair River Tunnel Shield, 247 - Side Shafts, 41 - Side Tunnels Lining, 79 - Side Walls Lining, 77 - Simplon Tunnel, 103 - Soils Encountered in Tunnels, 3 - Sonnstein Tunnel, 99 - Stations of Subways, 207-216 - St. Clair River Tunnel Shield, 247 - St. Gothard Tunnel, 132 - Steam Power Plant, 85 - Stratification of the Soils, 6 - Strickler Tunnel, 96 - Striking the Centers, 71 - Strutting: - American Method, 174 - Austrian Method, 177 - Belgian Method, 146 - Dimensions of Timber, 54 - English Method, 167 - Fort George Tunnel, 137 - Full Section, 51 - German Method, 156 - Headings, 48 - Italian Method, 183 - Murray Hill Tunnel, 125 - Pilot Method, 193 - Quantity of Timber, 54 - Shafts, 52 - Iron: Full Section, 56 - Headings, 56 - Shafts, 57 - Submarine Tunneling: - Cofferdam Method, 281 - Compressed Air Method, 225 - Detroit River Tunnel, 296 - General Discussion, 218 - Harlem River Tunnel, 285 - Hudson River Tunnel Pennsylvania Railroad, 269 - Lining, 261 - Milwaukee Water-Works Tunnel, 226 - Pneumatic Caisson Method, 284 - Seine River Tunnel, 293 - Severn Tunnel, 221 - Shield and Compressed Air Method, 263 - Shield System, 238 - Sinking and Joining Sections Built on Land, 293 - Van Buren Street Tunnel, 282 - Subways: - Boston, 203 - Cost of, 209-217 - Cross-sections, 204-211 - General Discussion, 195-202 - Lining, 207-213 - New York Rapid Transit Railway, 209 - Stations, 207-216 - Sutro, Adolph, 330 - - Tamping, 32 - Thickness of Lining Masonry, 78, 83 - Thomson Excavating Machine, 22 - Timber Lining, 72 - Timbering (see Strutting). - Tremies, 299 - Trussed Centers, 70 - Tunnel or Open Cut, 1 - Tunnels: - Baltimore Belt Line, 160 - Classification of, 42 - Fort George, 135 - Murray Hill, 123 - Simplon, 103 - St. Gothard, 132 - Hard Rock, 84 - Drift Method, 102 - Comparison of Methods, 141 - Heading and Bench Method, 152 - Heading Method, 130 - Soft Ground: - American Method, 172 - Austrian Method, 176 - Belgian Method, 144 - English Method, 166 - German Method, 155 - Italian Method, 182 - Pilot Method, 192 - Quicksand Method, 188 - Submarine: - Detroit River Tunnel, 296 - Harlem River Tunnel, 285 - Hudson River Tunnel of Pennsylvania Railroad, 269 - Milwaukee Tunnel, 226 - Seine River Tunnel, 293 - Severn Tunnel, 221 - Van Buren Street Tunnel, Chicago, 282 - Under City Streets: - General Description, 201 - Boston Subway, 203 - Turbines, 86 - - Vacuum Method of Ventilation, 328 - Value, Beverley R., 10 - Van Buren Street Tunnel, 282 - Ventilation, 325 - Artificial, 327 - Compressed Air, 330 - Natural, 326 - Plenum Method, 329 - Quantity of Air, 331 - Saccardo’s System, 330 - Simplon Tunnel, 120 - Vacuum Method, 328 - Vernon-Harcourt, L. F., 221 - - Working Platforms, 286 - Wyman, Erastus, 293 - - - - - Transcriber’s Notes - - - Inconsistencies in spelling and hyphenation have been retained except - as mentioned below; non-English words and phrases have not been - corrected except as listed below. The (minor) differences between the - Table of Contents and the chapter headings have not been rectified. - - Page 36/132: Figs. 14 and 61 are identical. - - Page 92/93, Sommeilier: possibly an error for Sommeiller. - - Page 134, Soummelier: possibly an error for Sommeiller. - - Page 174, Footnote 11: presumably Fig. 92, indicating the planes of - the sections, is from the same publication. - - Page 176, Austrian method: Dresden and Leipsic, and the Oberau Tunnel, - are (and were in 1837) in Saxony, Germany (or Prussia). - - Page 179, The short transverse beam _c_, Fig. 90: there is no short - transverse beam visible in Fig. 90, nor is it clear which other figure - might be intended; there is therefore no hyperlink to the - illustration. - - Page 279, Stirtling boiler: possibly an error for Stirling boiler. - - Pages 337 and 342, Arlberg: possibly an error for Aarlberg. - - Page 340, Wartha: possibly an error for Martha; Mertin: possibly an - error for Merten. - - - Changes made - - Footnotes, tables and illustrations have been moved out of text - paragraphs; some table data have been re-arranged for better - legibility. In some of the formulas brackets have been added for - clarity. - - Several obvious minor typographical and punctuation errors have been - corrected silently. - - Page 12, footnote 3: Chapter IX. changed to Chapter X. - - Page 35: on page 155 changed to on page 135 - - Page 36: on page 34 changed to on page 35 - - Page 53: The lagging plank may be ... changed to The lagging planks - may be ... - - Page 113: (1) changed to (_I_) (2×) - - Page 117: ... and it in this clearing ... changed to ... and it is in - this clearing ... - - Page 130: as indicated by Fig. 58 changed to as indicated by Fig. 61 - - Page 136: as indicated in the Fig. 63 changed to as indicated in the - Fig. 65 - - Page 146: as shown by Fig. 63 changed to as shown by Fig. 69 - - Page 149: underpining changed to underpinning - - Page 150: Since the roof arch rests for some time ... changed to Since - the roof arch rests are for some time ...; as shown by Fig. 66 changed - to as shown by Fig. 72 - - Page 172: illustrated in Fig. 12 changed to illustrated in Fig. 11 - - Page 175: page 127 changed to page 123 - - Page 179: as at _b_, Fig. 90 changed to as at _b_, Fig. 97 - - Page 204: The third type of section is shown by Fig. 116 changed to - The third type of section is shown by Fig. 117 - - Page 218: Malinö changed to Malmö - - Page 261: Fig. 118 shows the hydraulic jacks changed to Fig. 136 shows - the hydraulic jacks - - Page 282: shown by Fig. 119 changed to shown by Fig. 141 - - Page 297: towed down to the tunnel side changed to towed down to the - tunnel site - - Page 315: shown in Figs. 141 and 142 changed to shown in Figs. 159 and - 160 - - Page 324: shown by Fig. 148 changed to shown by Fig. 166 - - Page 338: given on page 50 changed to given on page 55 - - Page 340: Scloss Matrei changed to Schloss Matrei - - Page 341: _Time._ changed to =Time.= - - Page 348, entry Ryder: page number 296 added; Sounstein changed to - Sonnstein (2×). - - - - - -End of the Project Gutenberg EBook of Tunneling: A Practical Treatise., by -Charles Prelini - -*** END OF THIS PROJECT GUTENBERG EBOOK TUNNELING: A PRACTICAL TREATISE. *** - -***** This file should be named 60043-0.txt or 60043-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/0/0/4/60043/ - -Produced by deaurider, Harry Lamé and the Online Distributed -Proofreading Team at http://www.pgdp.net (This file was -produced from images generously made available by The -Internet Archive) - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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