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diff --git a/29420.txt b/29420.txt new file mode 100644 index 0000000..d96cbb5 --- /dev/null +++ b/29420.txt @@ -0,0 +1,5402 @@ +The Project Gutenberg EBook of American Rural Highways, by T. R. Agg + +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 + + +Title: American Rural Highways + +Author: T. R. Agg + +Release Date: July 16, 2009 [EBook #29420] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK AMERICAN RURAL HIGHWAYS *** + + + + +Produced by Tom Roch, Richard J. Shiffer and the Online +Distributed Proofreading Team at https://www.pgdp.net (This +file was produced from images produced by Core Historical +Literature in Agriculture (CHLA), Cornell University) + + + + + +[Transcriber's Note: Every effort has been made to replicate +this text as faithfully as possible, including obsolete and +variant spellings and other inconsistencies. Text that has been +changed to correct an obvious error is noted at the end of this +ebook. + +Also, on pages 47-48, the Greek letter theta is represented by +THETA. In chemical and mathematical notations, a subscript is +enclosed in braces and preceded by an underscore (e.g., H_{2}O.)] + + + + + AGRICULTURAL ENGINEERING SERIES + + E. B. MCCORMICK, CONSULTING EDITOR + + + FORMERLY DEAN OF ENGINEERING DIVISION + KANSAS STATE AGRICULTURAL COLLEGE + + + + + AMERICAN + + RURAL HIGHWAYS + + + + _McGraw-Hill Book Co. Inc._ + + PUBLISHERS OF BOOKS FOR + + + Coal Age -- Electric Railway Journal + Electrical World -- Engineering News-Record + American Machinist -- Ingenieria Internacional + Engineering & Mining Journal -- Power + Chemical & Metallurgical Engineering + Electrical Merchandising + + + [Illustration: _Frontispiece_] + + + + + AMERICAN + RURAL HIGHWAYS + + + BY + + T. R. AGG, C.E. + + + PROFESSOR OF HIGHWAY ENGINEERING + IOWA STATE COLLEGE + + + + FIRST EDITION + + + + MCGRAW-HILL BOOK COMPANY, INC. + NEW YORK: 239 WEST 39TH STREET + LONDON: 6 & 8 BOUVERIE ST., E. C. 4 + 1920 + + + COPYRIGHT, 1920, BY THE + McGRAW-HILL BOOK COMPANY, INC. + + + + +PREFACE + + +AMERICAN RURAL HIGHWAYS was written for use as a text or reference in +courses dealing with rural highways and intended for agricultural +engineers, students in agriculture and for short courses and extension +courses. The reader is assumed to have familiarity with drawing and +surveying, but the text is adapted primarily for students who do not +receive training along the lines of the usual course in Highway or +Civil Engineering. + +The text is intended to familiarize the student with the relation of +highway improvement to national progress, to indicate the various +problems of highway administration and to set forth the usual methods +of design and construction for rural highways in sufficient detail to +establish a clear understanding of the distinguishing characteristics +and relative serviceability of each of the common types of roadway +surface. + +Experience with classes made up of students in agriculture or +agricultural engineering and with trade school students in road making +served as a guide in the selection and arrangement of the material. +Detailed discussion of tests of materials and of the theory of design +has to a considerable extent been eliminated as being outside of the +scope of the course for which the text is intended. + +In the preparation of American Rural Highways reference was had to +many books on highway subjects and to current periodical literature. +Wherever direct extracts were made from such source, appropriate +acknowledgment appears in the text. + + T. R. AGG + + AMES, IOWA, + AUGUST 18, 1920. + + + + +CONTENTS + + +PREFACE vii + + +CHAPTER I + +THE PURPOSE AND UTILITY OF HIGHWAYS + +Transportation Problem--National in Scope--Development in +Traffic--Location or Farm to Market Traffic--Farm to Farm +Traffic--Inter-City Traffic--Inter-County and Inter-State +Traffic--Rural Education--Rural Social Life--Good Roads and +Commerce 1-12 + + +CHAPTER II + +HIGHWAY ADMINISTRATION + +Township Administration--County Administration--State +Administration--Federal Administration--Special Assessments--Zone +Method of Assessing--General Taxation--Vehicle Taxes--Sinking +Fund Bonds--Annuity Bonds--Serial Bonds--Comparison of Methods +of Issuing Bonds--Desirability of Road Bonds 13-28 + + +CHAPTER III + +DRAINAGE OF ROADS + +The Necessity for Drainage--Importance of Design--Surface +Drainage--Run-off--Ordinary Design of Ditches--Underground +Water--Tile Drains--Lying Tile--Culverts--Length of Culvert-- +Farm Entrance Culverts--Metal Pipe--Clay and Cement Concrete +Pipe--Concrete Pipe--Endwalls for Culverts--Reinforced Concrete +Box Culverts--Drop Inlet Culverts 29-41 + + +CHAPTER IV + +ROAD DESIGN + +Necessity for Planning--Road Plans--Problems of Design-- +Preliminary Investigations--Road Surveys--Alignment-- +Intersections--Superelevation--Tractive Resistance--Rolling +Resistance--Internal Resistance--Air Resistance--Effect of +Trades--Energy Loss on Account of Grades--Undulating Roads-- +Guard Railing--Width of Roadway--Cross Section--Control of +Erosion--Private Entrances--AEsthetics 42-62 + + +CHAPTER V + +EARTH ROADS + +Variations in Soils--Variation in Rainfall--Cross Sections +Elevating Grader--Maney Grader--Slip Scraper--Fresno +Scraper--Elevating Grader Work--Use of Blade Grader-- +Costs--Maintenance--Value of Earth Roads 63-73 + + +CHAPTER VI + +SAND-CLAY AND GRAVEL ROADS + +The Binder--Top-soil or Natural Mixtures--Sand-clay on Sandy +Roads--Sand-clay on Clay or Loam--Characteristics--Natural +Gravel--The Ideal Road Gravel--Permissible Size of Pebbles-- +Wearing Properties--Utilizing Natural Gravels--Thickness +of Layer--Preparation of the Road--Trench Method--Surface +Method--Maintenance 74-88 + + +CHAPTER VII + +BROKEN STONE ROAD SURFACES + +Design--Properties of the Stone--Kinds of Rocks used for +Macadam--Sizes of Stone--Earth Work--Foundation for the +Macadam--Telford Foundation--Placing the Broken Stone-- +Rolling--Spreading Screenings--Bituminous Surfaces--Maintenance +Characteristics 89-97 + + +CHAPTER VIII + +CEMENT CONCRETE ROADS + +Destructive Agencies--Design--Concrete Materials--Fine +Aggregate--Proportions--Measuring Materials--Preparation of the +Earth Foundation--Placing Concrete for Two-course Road--Curing +the Concrete--Expansion Joints--Reinforcing--Bituminous Coatings +on Concrete Surfaces--Characteristics--Maintenance 98-105 + + +CHAPTER IX + +VITRIFIED BRICK ROADS + +Vitrified Brick--Paving Brick--Repressed Brick--Vitrified Fiber +Brick--Wire-cut-lug Brick--Tests for Quality--Other Tests-- +Foundation--Sand Bedding Course--Sand Mortar Bedding Course-- +Green Concrete Bedding Course--Bituminous Fillers--Mastic +Fillers--Marginal Curb 106-115 + + +CHAPTER X + +BITUMINOUS ROAD MATERIALS AND THEIR USE + +Classes of Bituminous Materials--Coal Tar--Water Gas Tar--Natural +Asphalt--Petroleum Asphalt--Mixtures--Classification According to +Consistency--Road Oils--Liquid Asphalts--Asphalt Cements-- +Fillers--Bitumen--Specifications--Surface Treatments--Applying +the Bituminous Binder--Finishing the Surface--Patching-- +Penetration Macadam--Foundation--Upper or Wearing Course-- +Patching Characteristics--Hot Mixed Macadam--Foundation--Sizes +of Stone--Mixing the Wearing Stone--Placing and Wearing +Surface--Seal Coat--Characteristics--Asphaltic Concrete-- +Bitulithic or Warrenite--Topeka Asphaltic Concrete--Foundation +--Placing the Surface--Characteristics 116-129 + + +CHAPTER XI + +MAINTENANCE OF HIGHWAYS + +Petrol Maintenance--Gang Maintenance--Maintenance of Earth, +Sand-clay, Gravel and Macadam Roads 130-134 + + +Index 135 + + + + +AMERICAN RURAL HIGHWAYS + + + + +CHAPTER I + +THE PURPOSE AND UTILITY OF HIGHWAYS + + +THE DEVELOPMENT OF HIGHWAY SYSTEMS + +=Transportation Problem.=--Public highways, like many other familiar +things, are utilized constantly with little thought of how +indispensable they are to the conduct of the business of a nation or +of the intimate relation they bear to the everyday life of any +community. The degree to which a nation or a community perfects its +transportation facilities is an index of its industrial progress and +public highways constitute an important element in the national +transportation system. It is to be expected that the average citizen +will think of the public highway only when it affects his own +activities and that he will concern himself but little with the broad +problem of highway improvement unless it be brought forcibly to his +attention through taxation or by publicity connected with the +advancement of specific projects. + +=National in Scope.=--The improvement and extension of the highway +system is of national importance just as is development and extension +of railways, and concerted action throughout a nation is a +prerequisite to an adequate policy in regard to either. It is +inconceivable that any community in a nation can prosper greatly +without some benefit accruing to many other parts of the country. +Increased consumption, which always accompanies material prosperity, +means increased production somewhere, and people purchase from many +varied sources to supply the things that they want. Good +transportation facilities contribute greatly to community prosperity +and indirectly to national prosperity, and the benefits of highly +improved public highways are therefore national in scope. This fact +has been recognized in Europe, notably in England, France and Belgium, +where the public highways are administered largely as national +utilities. + +Until recent years, highway improvement in the United States has been +subordinated to other more pressing public improvements, but during +the World War the inadequacy of the transportation system of the +United States became apparent. While such an unprecedented load upon +transportation facilities may not recur for many years, it has become +apparent that more rapid progress in highway improvement is necessary +and in the United States the subject is now likely to receive +attention commensurate with its importance. + +=Development of Traffic.=--The character and extent of the highway +improvement needed in any locality is dependent entirely on the +demands of traffic. In sparsely settled areas, particularly those that +are semi-arid or arid, the amount of traffic on local roads is likely +to be small and the unimproved trails or natural roads adequate. But +as an area develops either on account of agricultural progress or the +establishment of industrial enterprises, the use of the public +highways both for business and for pleasure increases and the old +trails are gradually improved to meet, at least to some degree, the +new demands of traffic. In sparsely settled areas, it is possible for +the public to accommodate its use of the highways to the physical +condition thereof, and business is more or less regulated according to +the condition of the roads. This is not always pleasant or economical +but is the only possible arrangement. In populous districts, with +diversified activities, it becomes imperative to have year-round +usable roads in order to transact with reasonable dispatch the regular +business of the industries. Anything less will handicap normal +community progress. + +The advent of the motor driven vehicle in the United States has +resulted in a greatly increased use of the public highways of +agricultural areas, even of those that are sparsely populated, because +of the convenience of the motor vehicle both for passenger and for +freight service. Probably in excess of 90 per cent of the tonnage +passing over the rural highways in the United States is carried by +motor vehicles. This class of traffic has really just developed and no +one can predict what it will be in ten years, yet it has already +introduced into the highway problem an element that has revolutionized +methods of construction and maintenance. + +A different set of traffic conditions exists in those parts of the +United States where large areas are devoted primarily to industrial +pursuits, the agricultural development being of secondary importance. +Public highways connecting the industrial centers are indispensable +adjuncts to the business facilities in such communities and are +ordinarily subjected to a very large volume and tonnage of traffic +consisting principally of motor vehicles. The roads first selected for +improvement will not be those serving the agricultural interests of +the district, but rather those serving the industrial centers. +Inter-city roads of great durability and relatively high cost are +necessary for such traffic conditions. + +Not infrequently the transportation needs will require a system of +both inter-city and rural highways in the same community. There are +few areas in the United States where there is no agricultural +development. It is apparent therefore that the nature of the highway +systems and the administrative organization under which they are built +and maintained will differ in various states or areas according to +the nature of development of that area agriculturally and +industrially. In planning improvements of highway systems, it is +recognized that one or more of several groups of traffic may be +encountered and that the extent and nature of the improvement must be +such as will meet the requirements of all classes of traffic, the most +important being first provided for, and that of lesser importance as +rapidly as finances permit. + + +KINDS OF TRAFFIC ON PUBLIC HIGHWAYS + +=Local or Farm to Market Traffic.=--In strictly agricultural +communities the principal use of the highways will pertain to +agricultural activities and most of it will be between the farm and +the most convenient market center. In the ordinary state, the number +of rural families will not average more than six to eight per square +mile, but in some districts it may reach twenty families per square +mile. The travel from the district around a market center will +originate in this rather sparsely populated area and converge onto a +few main roads leading to market. The outlying or feeder roads will be +used by only a few families, but the density of traffic will increase +nearer the market centers and consequently the roads nearer town will +be much more heavily traveled than the outlying ones. It is apparent +therefore that considerable difference may exist in the kind of +construction adequate for the various sections of road where farm +traffic is the principal consideration. This traffic is made up of +horse drawn wagons, transporting farm products and of horse drawn and +motor passenger vehicles, the motor traffic comprising 80 per cent or +more of the volume of traffic and a greater per cent of the tonnage. +Motor trucks are now employed to some extent for marketing farm +products and, where surfaced highways have been provided, this class +of traffic is superseding horse drawn traffic. + +=Farm to Farm Traffic.=--In the ordinary prosecution of farming +operations, a considerable amount of neighborhood travel is +inevitable. Farmers help each other with certain kinds of work, +exchange commodities such as seed, machinery and farm animals and +visit back and forth both for business and pleasure. To accommodate +this traffic, it is desirable to provide good neighborhood roads. +Traffic of this sort follows no particular route and can to some +extent accommodate itself to the condition of the highways without +entailing financial loss, although some discomfort and some +inconvenience may result from inadequate highway facilities. This +traffic will be partly motor and partly horse drawn, but the +proportion of motor driven is large. + +=Inter-city Traffic.=--In strictly agricultural districts there is a +large amount of travel between towns, both for business and for +pleasure. The pleasure travel is mostly in motor vehicles and a +considerable part of the business traffic is the same, although horse +drawn vehicles are employed to some extent. + +In industrial districts there is a large volume of this class of +traffic consisting of motor passenger vehicles used for business and +for pleasure and of motor freight vehicles used for general business +purposes. In addition, there is certain to be a large amount of motor +truck freight traffic incident to the particular industrial pursuits +of the cities. Where adequate public highways connect industrial +centers, there is invariably a very large amount of inter-city +traffic, due in part to the needs of industry and in part to +concentration of population in industrial centers. + +=Inter-County and Inter-State Traffic.=--Automobile touring is a +popular means of relaxation, especially on the part of those who live +in the cities, although it is by no means confined to them. Traffic of +this kind follows the routes where roads are best and passes entirely +across a county, attracted by some public gathering. Often it is +inter-state in character, made up of tourists who are traveling to +distant pleasure resorts. Such traffic at present constitutes a +relatively small part of the travel on public highways, except on +certain favorable routes, but as the wealth of the country increases +and good touring roads are numerous, long distance travel will +increase and will eventually necessitate the construction of a number +of well maintained national highways, located with reference to the +convenience of the automobile tourist. + + +PUBLIC HIGHWAYS AND COMMUNITY LIFE + +It is well to recognize the intimate relation public highways bear to +the economic progress of a nation. Normal development of all of the +diverse activities of a people depends very largely upon the highway +policy that is adopted and whether the actual construction of +serviceable roads keeps pace with transportation needs. + +=Rural Education.=--It has become increasingly apparent during the +World War that the demand upon North America for food stuffs is to +become more and more insistent as the years pass. Already the +consumption in the United States has approached quite closely to the +average production and yet the population is constantly increasing. +The time is not far distant when greater production will be required +of the agricultural area in North America in order to meet the home +demand for foodstuffs, and many thousands of tons will be needed for +export. This need can only be met by agricultural methods that will +increase greatly the present yield of the soil. The adoption of better +agricultural methods must of necessity be preceded by the technical +training of the school children who will be the farmers of the next +generation, which can best be accomplished in graded schools with well +equipped laboratories and with suitably trained teachers. The problem +of providing such schools in rural communities has, in some instances, +been solved by consolidating a number of rural school districts and +constructing a well equipped building to accommodate the students from +an area several miles square. An educational system of this sort can +reach its highest usefulness only when adequate public highways +facilitate attendance of pupils. The whole trend of rural educational +progress is toward a system which is predicated upon a comprehensive +highway policy in the district. + +=Rural Social Life.=--Closely allied to the rural educational problem +is the rural social problem. Motor cars and good roads do a great deal +to eliminate the isolation and lack of social opportunity that has +characterized rural life in the United States. A high order of +citizenship in rural communities is essential to the solution of many +problems of rural economics, and such citizens will not live away from +the social opportunities of modern life. The rural school house and +the rural church may become social centers and local plays, moving +picture shows and lectures and entertainments of other kinds made +available to those who live in the country. Their enjoyment of these +social opportunities will be much more general if the public highways +are at all times in a condition to be traveled in comfort. Good homes +and good schools on good roads are prerequisites to the solution of +many rural problems. + +If there is opportunity for those who live in the cities to get some +adequate idea of rural life and the conditions under which farming +operations are carried on it will correct many misunderstandings of +the broad problems of food production and distribution. Reference has +frequently been made to the seeming desire on the part of city people +to get into the country, and, by facilitating the realization of this +desire, a great social service is rendered. + +=Good Roads and Commerce.=--That good highways are almost as necessary +as are railroads to the commercial development of a nation is +recognized but, unlike the railroads, the highways are not operated +for direct profit and the responsibility of securing consideration of +the demand for improvements is not centralized. Therefore, sentiment +for road improvement has been of slow growth, and important projects +are often delayed until long after the need for them was manifest. +Movements to secure financial support for highway improvement must go +through the slow process of legislative enactment, encountering all of +the uncertainties of political action, and the resulting financial +plan is likely to be inadequate and often inequitable. + +The whole commercial structure of a nation rests upon transportation, +and the highways are a part of the transportation system. The highway +problem can never receive adequate consideration until public highways +are recognized as an indispensable element in the business equipment +of a nation. + +During the World War all transportation facilities were taxed to the +limit, and motor trucks were utilized for long distance freight +haulage to an extent not previously considered practicable. As a +result, the interest in the motor truck as an addition to the +transportation equipment of the nation, has been greatly stimulated. +Many haulage companies have entered the freight transportation field, +delivering commodities by truck to distances of a hundred miles or +more. + +The part the motor truck will play in the future can only be +estimated, but it seems clear that the most promising field is for +shipments destined to or originating in a city of some size and a +warehouse or store not on a railroad spur, and especially when the +shipments are less than car load lots. The delays and expense incident +to handling small shipments of freight through the terminals of a +large city and carting from the unloading station to the warehouse or +other destination constitute a considerable item in the cost of +transportation. + +Mr. Charles Whiting Baker, Consulting Editor of _Engineering +News-Record_, states:[1] + + [1] Engineering News Record, July 10, 1919. + + "It costs today as much to haul a ton of farm produce ten miles + to a railway station as it does to haul it a thousand miles over + a heavy-traffic trunk-line railway. It often costs more today to + transport a ton of merchandise from its arrival in a long train + in the freight yard on the outskirts of a great city to its + deposit in the warehouse of a merchant four or five miles away + than it has cost to haul it over a thousand miles of railway + line." + +Nevertheless it seems probable that new methods of operating the motor +truck transport, and possibly new types of trucks or trucks and +trailers will be developed so that freight traffic over many roads +will be of considerable tonnage and an established part of the +transportation system of the nation. In the article above referred to +are given the following data relative to the cost of hauling on +improved roads by motor truck and these cost estimates are based on +the best information available at this time. They should be considered +as approximate only, but serve to indicate the limitations of the +truck as a competitor of the steam railway. + + TABLE 1 + + TRUCK OPERATION COSTS, FROM REPORTS BY SIX MOTOR TRUCK + OPERATORS, DIRECT CHARGES PER DAY + + +---------+-------+-------+-------+-------+-------+-------+----------- + | A | B | C | D | E | F | Average + | | | | | | | Total + +---------+-------+-------+-------+-------+-------+-------+----------- + Driver | $5.00 | $5.20 | $5.00 | $5.00 | $5.17 | $5.50 | $5.13 + Tires | 3.00 | 3.75 | 2.00 | 2.00 | 2.00 | 3.00 | 2.68 + Oil, etc. | .30 | ... | .30 | .50 | .25 | .25 | .35 + Gasoline | 3.00 | 4.00 | 3.50 | 4.65 | 2.08 | 3.75 | 3.50 + | | | | | | | ------ + | | | | | | | $11.66 + +---------+-------+-------+-------+-------+-------+-------+----------- + + + INDIRECT CHARGES PER DAY + + -------------+------+------+------+------+------+------+------------ + | | | | | | | Average + | A | B | C | D E | F | Total + -------------+------+------+------+------+------+------+------------ + Depreciation | $3.50| $4.19| $3.60| $3.40| $3.67| $4.00| $3.77 + Interest | 1.20| 1.26| 1.08| 1.22| 1.10| 1.00| 1.15 + Insurance | 1.50| 2.54| 1.26| 2.10| .86| .50| 1.47 + Garage | 1.00| 1.20| 1.00| 1.00| .89| 1.00| 1.01 + Maintenance | .50| ...| .50| ...| 1.00| ...| .75 + Overhaul | 1.33| 2.75| 1.80| 1.60| 2.00| 3.00| 2.07 + License | .17| .27| .20| .20| .20| .20| .20 + Body upkeep | .25| ...| .30| .10| .40| ...| .27 + | | | | | | | ---- + | | | | | | | $10.69 + Supervision | .50| 2.93| 2.05| 1.90| ... | ... | 1.90 1.90 + Lost time | 2.20| ... | 1.67| 3.40| 2.50| 1.97| 2.57 2.57 + | -----| -----| -----| -----| -----| -----| ----------- + | 23.45| 28.09| 24.26| 28.07| 22.12| 24.17| 26.82 + -------------+------+------+------+------+------+------+------------ + + TABLE 2 + + OVERHEAD CHARGES PER YEAR FOR A 5-TON CAPACITY GASOLINE + MOTOR TRUCK RUNNING AN AVERAGE OF 50 MILES PER DAY + FOR 240 DAYS PER YEAR + + Driver's wages[1] $1500 + Depreciation (20% on $6000 investment) 1200 + Interest (6% on $6000 investment) 360 + Insurance 450 + Garage (rental, upkeep, etc.) 300 + Maintenance, minor repairs and supplies, tire chains, tools, + lamps, springs, equipment, etc. (estimated) 300 + Complete overhaul once a year 600 + License fee 60 + Body upkeep, repairs, painting, etc. 90 + Supervision 696 + ----- + Total per annum $5556 + + Overhead charges per day for 240 days in the year, + actual operation $23.15 + Overhead charges per mile for 50 miles per day .463 + + [1] In the above table the driver's wages have been placed under + overhead charges because the driver is paid by the month and his + wages continue even though the truck is idle because of repairs, + bad weather or lack of business, unless, of course, the idleness + should be of long duration, when the driver might be laid off. + + DIRECT CHARGES PER DAY AND PER MILE FOR 5-TON TRUCK + OPERATED AS ABOVE + + ------------------------------------------------+---------+--------- + | Cost | Cost + | per day | per mile + ------------------------------------------------+---------+--------- + Tires (based on present tire guarantee) | $3.00 | $0.06 + Lubricants | .50 | .01 + Gasoline (3-1/2 miles per gal., 14 gal. at 25c) | 3.50 | .07 + | ----- | ----- + | 7.00 | 0.14 + ------------------------------------------------+---------+--------- + + Total of overhead and direct charges for 240 days per year operation, + per day $30.15 + Per mile .603 + Cost per ton-mile for full loads one way and empty returning .2412 + Cost per ton-mile for full loads one way and half load returning .16 + +The significance of these figures becomes apparent when they are +compared with the cost of hauling freight over trunk-line railways +with heavy traffic where the cost per ton-mile, including terminal +charges, ranges from 1.7 _mills_ per ton-mile to 4.4 _mills_ per +ton-mile. + +In view of these facts it seems reasonable to suppose that motor +vehicles for use on the public highways are more likely to be employed +to supplement the rail transport than to compete with it. To the +actual cost of operation of motor trucks given in Table 2, there +should be added the proportionate cost of maintaining the highway for +the use of the truck, which is partly covered by the item "License +Fee" in the table. The license fee would necessarily be considerably +larger if it were to compensate adequately for the wear on the +highways over which the trucks operate. This will still further +increase the cost of hauling by motor truck. + +Motor trucks are employed for many kinds of hauling where their speed +and consequently their daily capacity is an advantage over team +hauling that is decidedly worth while. It probably could be shown +that for many kinds of hauling, teams are more economical than motor +trucks, but when promptness and speed and the consequent effect on +dependent activities are considered, the motor truck often has a +distinct advantage, and the use of the truck to replace horse drawn +vans is progressing rapidly. This is true not only in the cities, but +also in the smaller towns and in the country. Motor trucks have been +adopted in a great many communities for delivery of farm products to +market, and this use of the truck is certain to increase rapidly. But +trucks in this service will use the secondary roads as well as the +main or primary roads. + +These observations emphasize the extent to which the highway policy of +the nation must be predicated on the use of the highways by motor +vehicles. + + + + +CHAPTER II + +HIGHWAY ADMINISTRATION + + +The systems of highway administration extant in the various political +units in the United States present a patchwork of overlapping +authority and undetermined responsibility. Highway laws are being +constantly revised by state legislatures and with each revision there +is some change in administrative methods and often the changes are +revolutionary in character. In most states, the trend is away from +county and township administration and toward state administration, +with provision for considerable participation by the federal +government. + +It will be pertinent to consider briefly the present functions of each +of the administrative authorities having duties in connection with +highway work in the United States, although these duties vary greatly +in the several states and change periodically with the action of +legislatures. + +=Township Administration.=--Township or "Town" authority is a survival +of the old New England town government and the town board consists of +three or more trustees who hold office for fixed terms. The usual term +is three years, but is less in some states. The incumbent is generally +a man who has other responsibilities of a public or private nature and +who gives but little of his time to highway matters. In some states +the pay is a fixed annual salary and in others a per diem with some +limitation on the amount that may be drawn in any one year, which +limitation may be statutory or may be by common consent. + +The township highway commissioners or trustees have jurisdiction over +certain of the roads in the township, usually best described as all +roads not by law placed under the jurisdiction of some other +authority. In certain instances, the township authorities have charge +of all of the roads in the township, which would mean that no "county" +or "state" roads happened to be laid out in that township. It is a +matter of general observation that the trend of legislation is toward +removing from the jurisdiction of the township officials all roads +except those upon which the traffic is principally local in character. +The actual mileage of roads in the United States that is at present +administered by township officials is large, probably constituting not +less than seventy per cent of the total mileage. + +In most states the township officials are responsible for the +maintenance of the roads under their jurisdiction and also supervise +such new construction as is undertaken. This includes the construction +of culverts and bridges as a rule, but in some states the county board +of supervisors is responsible for all of the bridge and culvert work +on the township roads. In other states, the township board is +responsible only for bridges or culverts that cost less than a certain +amount specified by law (usually about $1000) and the county board +provides for the construction and upkeep of the more expensive bridges +and culverts. + +Funds for the work carried out by the township road officials are +obtained by general taxation, the amount that may be levied being +limited by statute and the actual levy being any amount up to the +maximum that the township board deems necessary for its purposes. It +is the general observation that the tax levy is usually the maximum +permitted by law. + +In many states, township officials are permitted to issue bonds for +road construction, almost invariably, however, with the restriction +that each issue must be approved by the voters of the township. There +is always a provision that the total amount of bonds outstanding must +not exceed the constitutional limit in force in the state. In several +states, the townships have large amounts of road bonds outstanding. + +=County Administration.=--In some states the county is the smallest +administrative unit in the road system. A county board, called the +board of county supervisors or board of county commissioners +consisting of from three to fifteen members, is the administrative +authority. Its members are elected for fixed terms which vary in +length from one to five years. The county board usually has many +public responsibilities other than highway administration, and is +generally made up of men with considerably more business ability than +the average township board. + +The county board has jurisdiction over all of the highways in the +county in some states, and in others it has charge of only the more +important highways. In most states, the laws set forth specifically +what highways shall be under the jurisdiction of the county +authorities. + +In addition to having direct supervision of the improvement and +maintenance of the roads assigned to county administration, the county +boards in some states arrange for the construction of all culverts and +bridges on the roads that are under township supervision, or at least +the more expensive bridges and culverts on such roads. Sometimes this +is accomplished by granting county aid for township bridges, under +which system the county pays a part of the cost of the construction of +bridges on the township roads. The amount of aid varies, but is +generally about one-half of the cost, and the township and county +officials jointly assume the responsibility of arranging for the +construction by contract or otherwise. + +The county board obtains funds for road work through a direct tax on +all property in the county, the maximum rate being limited by statute. +County boards are also authorized to issue bonds for road construction +under statutory restrictions and limitations similar to those +effective in the township as to total amount issued, and many +millions of dollars' worth of highway bonds have been issued by +county authorities in the United States. + +=State Administration.=--In a state, the administrative authority in +highway matters is vested in a board of commissioners usually +consisting of three or more members. In a few states, the +administrative authority is delegated to a single commissioner. Where +the authority is vested in a board, that board is usually appointed by +the governor. In several states one or more members of the commission +hold that position _ex officio_; for example, in several states the +governor is by law a member of the commission, in others the secretary +of state or the dean of engineering at the State University or the +state geologist is a member of the commission. Where the +administrative authority is a single commissioner he may be elected +along with other state officers, but this is the case in only a few +states. + +The authority of the state highway department varies in the several +states, but in general the departments serve in the dual capacity of +general advisers to the county and township authorities on road +matters and as the executive authority responsible for the +construction of those highways that are built entirely or in part from +state or federal funds. + +State highway departments consist of the commission or commissioner, +and the technical and clerical staff required to perform the duties +imposed on the state organization. To some extent the state highway +departments are able to encourage economical and correct construction +of highways by the township and county authorities by furnishing them +standard plans and specifications and by formulating regulations to +govern the character of construction, but such efforts are likely to +be more or less ineffective unless the state authority has supervision +of the allotment of state or federal funds to the various counties and +townships. Nevertheless, most state highway departments do a great +deal of advisory work in connection with the highway construction +carried out by county and township authorities. + +State highway departments are supported by funds obtained in various +ways, laws differing greatly in this respect. The necessary support is +in some states appropriated from funds obtained by general taxation, +and is in others obtained from automobile license fees. In still +others, the funds are secured by a combination of the two methods +mentioned above. In addition to these support funds, a certain part of +the money obtained as federal aid may be employed for the engineering +and inspection costs on federal aid roads. The above mentioned funds +are required to maintain the state highway department. In addition, +the departments have supervision of the expenditures of construction +funds which can be used for road construction and maintenance, and may +not be expended for salaries or other overhead expense. + +In a number of states, automobile license fees are set aside for +financing road construction and maintenance, and the work paid for +from the fees is carried out under the supervision of the state +highway department. + +In a number of instances, state bonds have been issued for road +construction, and the expenditure of the proceeds of the sale of road +bonds has usually been supervised by the state highway department. + +All federal aid funds allotted to a state must be expended under the +direction of the state highway department. + +=Federal Administration.=--Federal authority in highway work is vested +in the Bureau of Public Roads of the United States Department of +Agriculture. The official head is the Secretary of Agriculture, but +the administrative head is the Director of the Bureau. In this Bureau +are the various instrumentalities needed for carrying on +investigations and furnishing information to the various states on +highway subjects. The Bureau also supervises the construction of +federal aid roads in a general way through district engineers, each +of whom looks after the work in several states. + +Funds for the support of the Bureau of Public Roads are obtained from +congressional appropriations to the Department of Agriculture and from +a percentage of the funds appropriated for federal aid. + +Federal aid is money appropriated by Congress to be distributed to the +various states to stimulate road construction. It is granted to the +states on the condition that the states will expend at least an equal +amount on the projects involved. The states in turn usually give a +suitable part of the state allotment to each county. There are various +limitations as to the amount of federal aid per mile of road and the +type of construction that may be employed, but these are matters of +regulation that change from time to time. + +It will be seen that each of the administrative authorities, except +the Bureau of Public Roads, is to some extent subservient to a higher +authority, and the Bureau of Public Roads is supervised by the United +States Congress. Considerable diplomacy is required on the part of any +administrative authority if his contact with other officials is to be +without friction. This is especially true in connection with the +formulation of a policy regarding the types of construction to be +adopted for an improvement. The responsibility for the selection is +variously placed on the township, county or state authority, the laws +not being uniform in this respect. If state or federal funds are +allotted to an improvement, the state authority either makes the +selection of the type of construction or the selection is made by some +subordinate authority subject to the approval of the state highway +department. Where the improvement is paid for exclusively with +township or county funds, the selection is often made by the township +or county authority without review by higher authority. Many abuses +have crept into highway administration through the unscrupulous +methods of promoters of the sale of road materials or road machinery. +A great deal of the selling activity of the agents for these +commodities is entirely irreproachable, but it is well known that such +is not always the case. As a result, the tendency of legislation is to +require the state highway department to approve contracts for +materials or construction entered into by the township or county +authorities. The state highway departments can secure the requisite +technical experts to determine the merits of materials and equipment +and, in spite of some glaring examples of inefficiency or worse, have +made a good record for impartiality and integrity as custodians of the +funds for which they are responsible. + + +HIGHWAY FINANCE + +The paramount problem in highway administration is the development of +an adequate financial plan for carrying on road improvement. The +necessary expenditures are enormous, although the money so expended is +probably much less than the actual benefit resulting from the +improvements. + +=Special Assessments.=--There is presumed to be a direct and +recognized benefit conferred on farm lands by the construction of +improved highways adjacent thereto. Therefore, it is equitable to +charge a part of the cost against the lands so benefited. + +The principle of paying for public improvements by a special +assessment upon private property has been long established and a large +proportion of the public improvements in the cities and towns have +been made financially possible through the medium of special +assessments on abutting and adjacent property. The same principle has +been applied to the financing of drainage projects for reclaiming farm +lands. Recently the special assessment method has come into limited +use in financing rural highway improvements. The policy in such cases +is to assess the abutting and adjacent property in a zone along the +improved road for a percentage of the cost of the improvement. The +amount so assessed does not ordinarily exceed one-fourth of the total +cost of the improvement and may be considerably less. The assessment +is spread over an area extending back from one to six miles from the +improved road. The assessment area is generally divided into about +four zones parallel to the road. The zone next the road is assessed at +a rate arbitrarily determined as a fair measure of the benefit, and +each succeeding zone is assessed at a somewhat lower rate. Generally +about three-fourths of the total assessment is placed on the half of +the assessment area lying next to the road. + +Many systems of making assessments have been proposed which are +mechanical in application after the area and rate of distribution of +benefit have been established, but in practice it is always found +necessary to make adjustments on individual parcels of land because of +variation in benefits received and it is impossible to eliminate the +exercise of human judgment in equalizing the assessments. + +=Zone Method of Assessing.=--The area to be assessed on each side of +the improved road is divided into zones usually four in number, but a +larger or smaller number of zones may be adopted. The rate for each +zone is then arbitrarily determined. For a typical case, the first of +four zones would receive an assessment of 50 per cent of the amount to +be borne by the area; the second zone 25 per cent, the third 15 per +cent and the fourth 10 per cent. Other percentages sometimes adopted +are 45, 25, 20 and 10 and 60, 20, 15 and 5. The set of percentages +first mentioned seems to insure the most equitable distribution for an +area all of which is substantially equally productive. + +When a road, for the improvement of which an assessment is being made, +lies on two or more sides of a parcel of land all of which is within +the assessment area, the rate is arbitrarily reduced to relieve that +parcel of land somewhat, or the assessment is first spread as above +outlined and afterward equalized as judgment dictates. + +In applying the zone method some difficulty is encountered in +determining an equitable distribution on those parcels of land lying +partly in one zone and partly in another, but the rate may be arrived +at with reasonable accuracy by pro-rating in accordance with the exact +conditions. + +In. Fig. 1, let it be assumed that the assessment area is to be two +miles wide, one mile on each side of the road and the various +ownerships to be indicated by the parcels of land numbered 1 to 8, as +shown. Each zone for the assessment of the 3-1/4 mile section is 1/4 +mile wide and the rates for the several zones are 50, 25, 15 and 10 +per cent respectively. Let it be assumed that the portion of the cost +of the 3-1/4 miles of road to be assessed on the area shown is +$20,000. The assessment would then be as follows: + + ------+-------+----------------------+------------+------------- + | | Rate x frontage on | Amount of | + Parcel| Rate | improved road = | Assessment | Assessment + | | assessment units | per unit[1]| + 1 | 2 | 3 | 4 | 5 + ------+-------+----------------------+------------+------------- + 1 | a 50 | 50 x 2640 = 132,000 | $0.016655 | $1558.46 + | b 75 | 75 x 1320 = 99,000 | | 1153.90 + 2 | 40 | 40 x 2640 = 105,600 | | 1230.77 + 3 | 10 | 10 x 2640 = 26,400 | | 307.69 + 4 | 25 | 25 x 1320 = 33,000 | | 384.66 + 5 | [2]85 | 85 x 5280 = 448,800 | | 5230.88 + 6 | 15 | 15 x 5280 = 79,200 | | 923.08 + 7 | [2]65 | 65 x 7920 = 514,800 | | 6000.00 + 8 | 35 | 35 x 7920 = 277,200 | | 3230.77 + | | ------------------- | | ----------- + | | 1,716,000 | | $20000.00 + ------+-------+----------------------+------------+------------- + + [1] The assessment per unit is obtained by dividing the total + assessment by the total of column three. + + [2] On these two parcels, it is decided that more than half of + the zone rate should apply to the half of the zone toward the + improved road, but some modification of the rates adopted might be + justified. + +[Illustration: Fig. 1] + +The assessment of the cost of the east and west one-mile section of +road is made up in like manner, and let it be assumed that the portion +of the cost of this road that is to be assessed on the area shown is +$5500. The assessment area will be one mile wide and each zone +one-fourth mile in width and the rates for each zone the same as +before. + + ------+-------+----------------------+------------+------------- + | | Rate x frontage on | Amount of | + Parcel| Rate | improved road = | Assessment | Assessment + | | assessment units | per unit | + ------|-------+----------------------+------------+------------- + 1 | a 75 | 75 x 1320 = 99,000 | $0.010417 | $1031.25 + | b 15 | 15 x 2640 = 39,600 | | 412.49 + 2 | 75 | 75 x 2640 = 198,000 | | 2062.53 + 3 | 50 | 50 x 1320 = 66,000 | | 687.51 + 4 | a 25 | 25 x 1320 = 33,000 | | 756.25 + | b 15 | 15 x 2640 = 39,600 | | + 5 | 10 | 10 x 3300 = 33,000 | | 343.73 + 6 | 10 | 10 x 1980 = 19,800 | | 206.24 + ------|-------| | |------------- + | | 528,000 | | 5500.00 + ------+-------+----------------------+------------+------------- + +It will be noted that the combined assessment for the two sections of +road is especially heavy on parcels 1, 2 and 3. In order to prevent +unjust charges against such properties, laws usually limit the total +assessment against any parcel of land to a fixed percentage of a fair +market value or of the assessed value. The assessment on these parcels +would be reduced as seemed expedient and the deficit would be +distributed over the remainder of the area in the same manner as the +original assessment was spread. In practice such re-distribution is +ordinarily made by the arbitrary adjustment in accordance with what +the authorized officials consider to be fair and equitable. The method +outlined is merely a mechanical means of securing distribution and +must not be considered as an infallible method of making the +assessment. It is always necessary to review the results in the light +of the actual benefits to be presumed for each parcel of land. +Nevertheless, the method outlined will prove equitable in a majority +of cases. + +=General Taxation.=--There is a general community benefit derived from +the construction of good roads in that the actual cost of marketing +farm products is lessened with a resulting lowering of the price to +the consumer. The benefit also accrues from the greater facility with +which all community business may be conducted. The introduction of +better opportunities for social, religious and educational activities +in the rural districts which results from improved highways is also a +community benefit of no mean importance. A part of the cost of road +improvement may therefore be equitably paid from funds obtained by +general taxation. + +A considerable portion of the current expense of maintaining the +township and county highway work and at least a part of the cost of +maintaining state highway activities is met from funds obtained by +general taxation. Likewise, the funds required for the amortization of +bond issues are often obtained from general taxation although vehicle +license fees are sometimes used for that purpose. + +General taxes are levied on all taxable property in a political unit +under statutory provisions regulating the amount of the levy and the +purpose for which the revenue is to be used. In the aggregate, the +road taxes are large but in the township or county the rate is +generally small compared to some other taxes, such as the school tax. + +=Vehicle Taxes.=--The great direct benefit derived by those who +actually operate vehicles over the roads justifies the policy of +requiring a vehicle to pay a license fee in lieu of other taxes, the +funds so obtained to be used for the construction and maintenance of +public highways. In practice, this method has already been applied to +motor vehicles in most states and has proven to be an important source +of revenue. Its application to horse-drawn vehicles has not been +attempted, due probably to the fact that such horse-drawn vehicles as +use the public highways are also employed about the farm or in the +towns and the determination of an equitable basis for taxation +involves many difficulties. + +The rate of the fee for motor vehicles should be based on their +destructive effect on the road so far as that is possible. The scale +of fees should therefore take account of weight and speed of vehicle +and if the license is in lieu of all other taxes, it should also be +graduated with the cost of the vehicle. + +When funds are thus derived, every precaution should be taken to +insure that the money is used judiciously for construction and +especially for maintenance on those roads most useful to motor +traffic. + +=Highway Bonds.=--Bond issues for road improvement afford a means of +constructing roads and paying for them while they are being used. A +very large volume of such bonds are outstanding in the United States. +Road bonds should be issued only for durable types of improvement and +the life of the bond should be well within the probable useful life of +the road surface. It is customary and highly desirable that the +general nature and extent of the improvement be established before the +bonds are issued. It is desirable that bond issues be subject to +approval by referendum before issue and that is provided in every +instance. + +Highway bonds are of three classes known as Sinking Fund, Annuity and +Serial Bonds, respectively. The earlier bonds issued were almost all +of the sinking fund class, but in recent years the serial bond has +been widely employed and is probably the most satisfactory to +administer. + +=Sinking Fund Bonds.=[1]--When this type of bond is employed, the +amount of the expenditure for road improvement is determined upon and +the length of the period during which tax payments shall be made is +settled. To employ a concrete example, it may be assumed that $100,000 +is to be expended for road work and is to be paid at the end of ten +years. The interest rate on the bonds will vary with the condition of +the bond market and the stability of the political unit issuing the +bonds, but is usually about 5 per cent. Knowing these factors, the +amount to be added to the sinking fund each year is computed. In order +to pay the interest on the bonds, a tax of suitable rate is levied, +and in order to retire the bonds at the end of the period, a sum is +set aside each year which is supposed to be invested and draw interest +which will be added to the principle, and the principle and interest +comprise the sinking fund. The principle of the sinking fund is +obtained by tax levies, a sum being added to the principle of the +sinking fund each year. + + [1] For a more detailed discussion of highway bonds see Bulletin + 136, U. S. Dept. of Agriculture, which is the basis of this + discussion. + +The success of this method of financing depends upon the proper +administration of the sinking fund. It must be invested with fidelity +and the fund be kept intact. Usually the sinking fund cannot be +invested at as high a rate of interest as the bonds bear and there is +some loss as a result. Road bonds bearing 5 per cent interest can +usually be sold at par while the sinking fund will usually net about 3 +or 3-1/2 per cent interest. The total cost of a bond issue will be +greater by the sinking fund method than by either of the other methods +described. + +=Annuity Bonds.=--Annuity bonds are drawn in such a manner that the +amount of the payment for principle and interest is the same each year +during the life of the bond. When the amount of the issue and the rate +of interest has been determined and the amount of the desired annual +payment has been determined, the number of years the bonds must run is +computed. + +This method is convenient in that the amount of the tax to be levied +each year remains constant. + +=Serial Bonds.=--Serial bonds are drawn so that a uniform amount of +the principle is retired each year after retirement starts and the +total interest payments decrease each year after the first bonds are +retired. The first bond may not be retired for a number of years after +the issue of the bonds, but when it once starts retirement proceeds at +a constant rate annually. + +=Comparison of Methods of Issuing Bonds.=--The relative costs of +financing by either of the three methods depends upon the rate of +interest in each case and the net rate secured on the sinking fund +provided for retiring sinking fund bonds. + +For comparative purposes, some typical examples are given in Table 3. +These illustrate the differences in total cost of securing $100,000 by +each of the three methods at various interest rates. + + TABLE 3 + + TOTAL COST OF A LOAN OF $100,000 FOR 20 YEARS, INTEREST + COMPOUNDED ANNUALLY + + ---------+---------------------------------------+---------+--------- + Annual | Sinking Fund Compounded | | + Interest | Annually at | | + on Bonds +----------+----------------+-----------+ Annuity | Serial + |3 per cent| 3-1/2 per cent | 4 per cent| | + ---------+----------+----------------+-----------+---------+--------- + 4 | $154,431 | $150,722 | $147,163 |$147,163 | $142,000 + 4-1/2 | 164,431 | 160,722 | 157,163 | 153,752 | 147,250 + 5 | 174,431 | 170,722 | 167,163 | 160,485 | 152,500 + 5-1/2 | 184,431 | 180,722 | 177,163 | 167,359 | 157,750 + 6 | 194,431 | 190,722 | 187,163 | 174,369 | 163,000 + ---------+----------+----------------+-----------+---------+--------- + +=Desirability of Road Bonds.=--In theory the bond method of financing +enables the highway authorities to construct a large mileage of roads +in a few years and spreads the cost over the period during which the +public is being benefited. Better prices are obtained on contracts for +a large mileage than for smaller jobs, and the community can receive +the benefit more quickly than where construction proceeds piecemeal +with current funds. The vital consideration is to insure that the term +of the bonds is well within the useful life of the road, and that +ample provision is made to maintain the roads during that period. +Under proper restrictions the bond method of financing is to be +commended. The bonds are an attractive investment and readily +marketable on satisfactory terms. + + + + +CHAPTER III + +DRAINAGE OF ROADS + + +=The Necessity for Drainage.=--The importance of drainage for all +roads subject to the effects of storm or underground water has always +been recognized by road builders, but during recent years constantly +increasing attention has been given to this phase of road +construction. It is unfortunate that there has in the past been some +tendency to consider elaborate drainage provisions less necessary +where rigid types of surfaces were employed. It has become apparent, +from the nature of the defects observed in all sorts of road surfaces, +that to neglect or minimize the importance of drainage in connection +with either earth roads or any class of surfaced roads is to invite +rapid deterioration of some sections of the roadway surface and to add +to maintenance costs. + +The degree to which lack of drainage provisions affect the +serviceability of the road surface varies with the amount of +precipitation in the locality and the manner in which it is +distributed throughout the year. In the humid areas of the United +States, which are, roughly, those portions east of a north and south +line passing through Omaha and Kansas City, together with the northern +part of the Pacific slope, precipitation is generally in excess of 30 +inches per year and fairly well distributed throughout the year, but +with seasonal variations in rate. In these areas, the effect of the +precipitation, both as regards its tendency to lower the stability of +soils and as an eroding agent, must be carefully provided against in +highway design. + +Outside of the areas mentioned above, the precipitation is much less +than 30 inches per year and its effect as an agent of erosion is of +greatest significance, although in restricted areas there may be short +periods when the soil is made unstable by ground water. + +=Importance of Design.=--The drainage system for a proposed road +improvement ought to be designed with as much care as any other +element, and, to do so, a study must be made of all factors that have +any bearing on the drainage requirements and the probable +effectiveness of the proposed drainage system. The well established +principles of land drainage should be followed so far as applicable. + +The basic principle of road drainage is to minimize the effect of +water to such an extent that there will always be a layer of +comparatively dry soil of appreciable thickness under the traveled +part of the road. This layer should probably never be less than two +feet thick and for soils of a structure favorable to capillary action +it should be at least three feet thick. The means employed to +accomplish the requisite drainage will be as various as the conditions +encountered. + +=Surface Drainage.=--The drainage method which is by far the most +nearly general in application is that which utilizes open ditches, and +the system which employs these ditches is usually referred to as +surface drainage. The full possibilities of this method of minimizing +the effects of storm water are rarely fully utilized in road +construction. Very frequently, deterioration of a road surface is +directly attributable to failure to provide adequately for the removal +of the storm water or water from the melting of snow that has fallen +on the road, or water that flows to the road from land adjacent +thereto. Surface water can usually most cheaply and expeditiously be +carried away in open ditches, although special conditions are +occasionally encountered which require supplementary tile drains. The +cross section commonly adopted for roads lends itself naturally to the +construction of drainage ditches at the sides of the traveled way, and +these are usually the principal dependence for the disposal of storm +water. + +=Run-off.=--The capacity required of side ditches to insure +satisfactory surface drainage will be affected by the amount and +nature of the precipitation in the region where the road is built. The +annual rainfall in a region may amount to several feet, but may be +well distributed throughout the year with an absence of excessive +rainfall for short periods, that is, flood conditions may rarely +occur. In other areas, the annual rainfall may be comparatively small +but the precipitation occurs at a very high rate, that is, flood +conditions may be common, or it may be at a low rate extending over a +considerable period. These peculiarities must be known before an +adequate drainage system can be planned. + +It is almost universally true in the United States that precipitation +at a very high rate will be for a relatively short duration, and +during these short periods, which usually do not exceed thirty +minutes, a portion of the water that falls on the areas adjacent to +the road and that drains to the road ditches will soak into the soil +and therefore not reach the ditches along the road. The extent to +which the water is taken up by the soil will vary with the porosity +and slope of the land and the character of the growth thereon. +Cultivated land will absorb nearly all of the water from showers up to +fifteen or twenty minutes duration; grass land a somewhat smaller +percentage; and hard baked or other impervious soil will absorb a +comparatively small amount. Rocky ground and steep slopes will absorb +very little storm water. + +The surface of the road is designed to turn water rapidly to the +ditches, but when the material is the natural soil, there is always +considerable absorption of storm water. Surfaces such as sandclay, +gravel and macadam do not absorb to exceed 10 per cent of the +precipitation during short showers. Bituminous surfaces, brick and +concrete pavements, do not absorb an appreciable amount of storm +water. + +Generally it is best to assume that if a rain lasts for forty-five +minutes or more, all of the water will run off, as the soil will reach +a state of saturation in that time. This is not true of deep sand, but +is for nearly all other soils. + +The ditch capacity needed will therefore depend upon the area drained, +the character of the soil, the slopes and the rainfall characteristics +of the region, and upon the nature of the road surface. + +For a required capacity, the cross section area of the ditch will vary +inversely as the grade, because the velocity of flow increases with an +increase in the grade of the ditch. If the surface water must be +carried along the road for distances exceeding five or six hundred +feet, the ditch must be constructed of increasing capacity toward the +outlet in order to accommodate the accumulated volume of water. + +The velocity of flow varies not only with the grade, but with the +shape of the cross section, cleanness of the channel, the depth of the +water in the channel, alignment of the channel and the kind of +material in which the channel is formed. It is not necessary to go to +great refinement in the design of the side ditches for the ordinary +case where the water is carried along the road for only a few hundred +feet. The ditches are made of ample capacity by using the commonly +accepted cross section for a road, which will be discussed in a later +paragraph. But where large areas must be drained by the road ditches, +it is desirable carefully to design the side ditches. The basis for +that design is too lengthy to be included herein, and reference should +be made to a standard treatise on the subject. + +=Ordinary Design of Ditches.=--For grades of one per cent or less on +roads in the humid area, the bottom of the ditch should be at least +three and one-half feet lower than the traveled surface of the road, +except for very sandy soil. For grades greater than one per cent, this +depth may be decreased one foot, and for grades of four per cent and +upward, the depth may be still less. These general rules for depth are +susceptible of variation but are believed to be the minimum except in +arid or semi-arid climates. It is far better to be too liberal in +ditch allowance than to be too conservative. In arid or semi-arid +regions, the ditch design will be based on the necessity of providing +for flood flow and preventing damage through erosion. Ordinary +drainage requirements will be satisfactory with the ditch about one +foot deep. + +If the topography is such that it is evident considerable storm water +will flow from the adjacent land to the road ditches, the design must +be modified to take this into account. Sometimes such water can be +diverted by ditches well back from the road, and thus prevented from +flowing into the side ditches along the roadway. It is especially +desirable to divert water, which would otherwise flow down the slope +of a cut, by means of a ditch on the hill-side above the upper edge of +the slope of the cut. + +Ditches are not effective unless they afford a free flow throughout +their length and have an outlet to a drainage channel of ample +capacity. Therefore, ditch grades should be established by survey, +especially if the gradient is less than one per cent, and the +construction work should be checked to insure that the ditch is +actually constructed as planned. A few high places in the ditch will +greatly reduce the effectiveness, although these may appear at the +time of construction to be slight. Constricted places, such as might +be due to a small amount of loose earth left in the ditch, are always +to be avoided. + +Where the side ditch passes from a cut to the berm alongside a fill, +the ditch should be excavated throughout in the undisturbed natural +soil, five feet or more from the toe of the slope of the fill, and +along the filled portion of the road there should be a berm of three +or four feet between the toe of the slope of the fill and the near +edge of the ditch. + +=Underground Water.=--In a preceding paragraph, mention was made of +the fact that only a part of the storm water runs off over the surface +of the ground, the larger part being absorbed by the soil. The water +thus absorbed flows downward through the pores in the soil until it is +deflected laterally by some physical characteristic of the soil +structure. The movement of underground water is affected by many +circumstances, but only two conditions need be discussed herein. + +Underground water, like surface water, tends to attain a level +surface, but in so doing it may need to flow long distances through +the pores of the soil, and to overcome the resistance incident to so +doing some head will be required. That is to say, the water will be +higher at some places than at others. If a cut is made in grading the +road, the road surface may actually be lower than the ground water +level in the land adjoining the road. As a result, the water will seep +out of the side slopes in the cut and keep the ditches wet, or even +furnish enough water to occasion a flow in the ditch. Similarly, the +higher head of the underground water near the top of a hill may result +in ground water coming quite close to the surface some distance down +the hill. The remedy in both cases is tile underdrains alongside the +road to lower the ground water level so that it cannot affect the road +surface. + +Sometimes the ground water encounters an impervious stratum as it +flows downward through the soil, or one that is less pervious than the +surface soil. When such is the case, the water will follow along this +stratum, and should there be an outcrop of the dense stratum, a spring +will be found at that place. This may be on a highway. The impervious +stratum may not actually outcrop but may lie only a few feet under the +surface of the road, in which case, the road surface will be so water +soaked as to be unstable. The so-called "seepy places" so often noted +along a road are generally the result of this condition. This +condition can be corrected by tile laid so as to intercept the flow at +a depth that precludes damage to the road. Commonly, the tile will be +laid diagonally across the road some distance above the section where +the effect of the water is noted, and will be turned parallel to the +road at the ditch line and carried under one of the side ditches to an +outlet. + +=Tile Drains.=--Where the soil and climatic conditions are such that +the roadway at times becomes unstable because of underground water +rising to a level not far below the road surface, the ground water +level is lowered by means of tile underdrains. The function of the +tile drains in such cases is precisely the same as when employed in +land drainage; to lower the ground water level. + +=Laying Tile.=--The tile lines are usually laid in trenches parallel +to the center line of the road near the ditch line and at least 4 feet +deep so as to keep the ground water level well down. They must be +carefully laid to line and grade. A good outlet must be provided and +the last few joints of pipe should be bell-and-spigot sewer pipe with +the joints filled with cement mortar. The opening of the tile should +be covered with a coarse screen to prevent animals from nesting in the +tile. + +It is frequently necessary to lay a line of tile at the toe of the +slope in cuts to intercept water that will percolate under the road +from the banks at the sides. In some cases, it is desirable to +back-fill the tile trench with gravel or broken stone to insure rapid +penetration of surface water to the tile. In other instances, it is +advantageous to place catch basins about every three or four hundred +feet. These may be of concrete or of tile placed on end or may be +blind catch basins formed by filling a section of the trench with +broken stone. When a blind catch basin is used, the top should be +built up into a mound, and for a tile or concrete catch basin, a +grating of the beehive type should be used, so that flow to the tile +will not be obstructed by weeds and other trash that is carried to the +catch basin. + +=Culverts.=--Culverts and bridges are a part of the drainage system +and the distinction between the two is merely a matter of size. +Generally, structures of spans less than about eight feet are classed +as culverts, but the practice is not uniform. In this discussion +culverts will be defined as of spans of 8 feet or less. + +Numerous culverts are required to afford passage for storm water and +small streams crosswise of the road, and their aggregate cost is a +large item in the cost of road improvement. The size of the waterway +of a culvert required in any location will be estimated by an +inspection of the stream and existing structure, and by determining +the extent and physical characteristics of the drainage area. +Sometimes there is sufficient evidence at the site to indicate quite +closely the size required, but this should always be checked by +run-off computations. The drainage area contributing water to the +stream passing through the culvert under consideration is computed +from contour maps or from a survey of the ground, and the size of +culvert determined by one of the empirical formulas applicable to that +purpose. In these formulas, the solution depends upon the proper +selection of a factor "C" which varies in accordance with the nature +of the drainage area. Two of these that are quite widely used are as +follows: + + _Myers' Formula: a = CA_ + +Where _a_ = area of cross section of culvert in square feet. _A_ = +area in acres of the drainage area above culvert. _C_ a factor varying +from 1 for flat country to 4 for mountainous country or rocky soil, +the exact value to be selected after an inspection of the drainage +area. + +_Talbot's Formula_: Area of waterway in square feet = + + _C_ [Square root of] ((Drainage area in acres)^3) + + Transcriber's Note: The above formula used the mathematical + square root symbol in the original. One should read it as "C + times the square root of the Drainage area in acres cubed." + +_C_ being variable according to circumstances thus: + +"For steep and rocky ground _C_ varies from 2/3 to 1. For rolling +agricultural country, subject to floods at times of melting snow, and +with length of valley three or four times its width, _C_ is about 1/3, +and if stream is longer in proportion to the area, decrease _C_. In +districts not affected by accumulated snow, and where the length of +valley is several times its width, 1/5 or 1/6 or even less may be +used. _C_ should be increased for steep side slopes, especially if the +upper part of the valley has a much greater fall than the channel at +the culvert. The value of _C_ to be used in any case is determined +after an inspection of the drainage area." + +[Illustration: Fig. 2. Design of Pipe Culvert and Bulkhead] + +=Length of Culvert.=--The clear length between end walls on a culvert +should be at least equal to the width of the roadway between ditches. +This is a minimum of 20 feet for secondary roads and ranges from 24 to +30 feet for main roads. The headwall to the culvert should not be a +monument, but should be no higher than needed to prevent vehicles from +leaving the roadway at the culvert. + +=Farm Entrance Culverts.=--At farm entrances, culverts are required to +carry the farm driveway across the side ditch of the road. These +culverts are usually about 16 feet along, and should be of a size +adequate to take the flow of the side ditch. The farm entrance culvert +should be of such design that it can be easily removed to permit +cleaning out the ditches with a road grader. + + +TYPES OF CULVERTS + +Culverts constructed of concrete and poured in place are called box +culverts because of the rectangular form of the cross section. +Culverts of pre-cast pipe are known as pipe culverts. Several forms of +pipe culvert are in general use. + +[Illustration: Fig. 3.--Typical Concrete Box Culvert] + +=Metal Pipe.=--These may be of cast iron, steel or wrought iron. The +cast iron pipe is very durable but expensive and heavy to handle and +is not widely used in highway construction. Steel pipe has been +employed to a limited extent but its durability is questioned. At +least it is known that the pipe made from uncoated, light sheet steel +is not very durable. Sheet iron and sheets made from alloy iron +coated with spelter have been extensively used and seem to be durable, +especially when laid deep enough to eliminate possibility of damage +from heavy loads. To insure reasonable resistance to corrosion, the +metal sheets should be coated with at least one and one-half ounces of +spelter per square foot of sheet and the sheets should not be lighter +than 16 gauge for small sizes and should be heavier for the larger +sizes. + +=Clay and Cement Concrete Pipe.=--The ordinary burned clay bell and +spigot pipe that is employed for sewer construction is sometimes used +for culverts. It must be very carefully bedded, preferably on a +concrete cradle and the joints filled with cement mortar. Culverts of +this type have a tendency to break under unusual loads, such as +traction engines or trucks. They may be damaged by the pressure from +freezing water, particularly when successive freezing and thawing +results in the culvert filling with mushy snow, which subsequently +freezes. + +=Concrete Pipe.=--Reinforced concrete pipe is a satisfactory material +for culverts, if the pipe is properly designed. The pipe should be +carefully laid on a firm earth bed with earth carefully back-filled +and tamped around the pipe. The joints in the pipe should be filled +with cement mortar, or should be of a design that will be tight. + +=Endwalls for Culverts.=--A substantial retaining wall is placed at +each end of the culvert barrel, whatever the type. This is to prevent +the end of the culvert from becoming choked with earth and to retain +the roadway at the culvert. It also indicates to the drivers the +location of the end of the culvert. The endwall extends a foot or more +below the floor of the culvert to prevent water from cutting under the +barrel. Plain concrete or stone masonry are most commonly used for +culvert endwalls. + +[Illustration: Fig. 4.--Two Types of Drop Inlet Culvert] + +=Reinforced Concrete Box Culverts.=--The pipe culvert is limited in +application to the smaller waterways. Reinforced concrete is +extensively used for culverts of all sizes, but especially for the +larger ones. These are usually constructed with endwalls integral with +the barrel of the culvert. Culverts of this type must be designed for +the loads anticipated to insure suitable strength and stability, and +must be constructed of a good quality of concrete. Figs. 2 and 3 show +designs for pipe and box culverts. + +[Illustration: Fig. 5.--Drop Inlet Culvert] + +=Drop Inlet Culverts.=--In some locations erosion has begun in the +fields adjacent to a culvert and it will probably continue until the +stream above the culvert has eroded to about the level of the floor of +the culvert. This is a reason for placing the culvert as high as the +roadway will permit, so long as the area above the culvert will be +properly drained. Considerable reclamation of land is possible if the +culvert is constructed with a box at the inlet and as shown in Fig. 4. +The area up-stream from the culvert will not erode below the level of +the top of the box at the inlet end. + +Where the stream crossing the road has eroded to considerable depth or +has considerable fall, as would sometimes be the case on side hill +roads, the culvert barrel would follow the general slope of the ditch +but should have a drop inlet. This type of culvert is shown in Fig. +5. + + + + +CHAPTER IV + +ROAD DESIGN + + +=Necessity for Planning.=--Sometimes highway improvement is the result +of spasmodic and carelessly directed work carried out at odd times on +various sections of a road, finally resulting in the worst places +being at least temporarily bettered. The grade on the steepest hills +is probably reduced somewhat and some of the worst of the low lying +sections are filled in and thereby raised. Short sections of surfacing +such as gravel or broken stone may be placed here and there. From the +standpoint of the responsible official, the road has been "improved," +but too often such work does not produce an improvement that lasts, +and sometimes it is not even of any great immediate benefit to those +who use the roads. In nearly every instance such work costs more in +money and labor that it is worth. + +Lasting improvement of public highways can be brought about only +through systematic and correlated construction carried on for a series +of years. In other words, there must be a road improvement policy +which will be made effective through some agency that is so organized +that its policies will be perpetuated and is clothed with enough +authority to be capable of enforcing the essential features of good +design and of securing the proper construction of improvements. + +Details of highway construction and design must vary with many local +conditions and types of surface. The limits of grades and the many +other details of design may properly be adopted for a specific piece +of work only after an adequate investigation of the local requirements +and in the light of wide experience in supervising road improvement. + +New ideas are constantly being injected into the art of road building, +but these are disseminated somewhat slowly, so that valuable devices +and improvements in methods remain long unknown except to the +comparatively few who have the means for informing themselves of all +such developments. + +It follows then that the logical system of conducting road improvement +is through an agency of continuing personnel which will supervise the +preparation of suitable plans and direct the construction in +accordance with the most recent experience. + +=Road Plans.=--The information shown on the plans prepared for road +improvement varies somewhat with the design and with the ideas of the +engineer as to what constitutes necessary information, but in general +the plans show the existing road and the new construction contemplated +in an amount of detail depending principally upon the character of the +construction. Simple plans suffice for grade reduction or reshaping an +earth road surface, while for the construction of paved roads, the +plans must be worked out in considerable detail. The essential +requirement is that there be given on the plans all information +necessary to enable the construction to be carried out according to +the intentions of the engineer, that all parts of the work fit +together, that the culverts are of the proper size and located at the +proper places, ditches drain properly, grades are reduced to the +predetermined rate, that excavated material is utilized and that an +exact record of the work done is retained. Plans are indispensable to +economical road construction and the preparation of the plans is the +work of the expert in road design, that is, the highway engineer. + +=Problem of Design.=--The problem of road design is to prepare plans +for a road improvement with the various details so correlated as to +insure in the road constructed in accordance therewith the maximum of +safety, convenience and economy to the users thereof. The degree to +which the design will be effective will depend to a considerable +extent upon the financial limitations imposed upon the engineer, but +skill and effort on the plans will do a great deal to offset financial +handicap and no pains should be spared in the preparation of the +plans. Moreover, the plans must afford all of the information needed +by the contractor in preparing a bid for the work. + +=Preliminary Investigation.=--The first step in road improvement is to +secure an adequate idea of the existing conditions on the road or +roads involved. The detail to which this information need go will +depend entirely upon the purpose of the preliminary investigation, for +before a definite plan is prepared, it may be necessary to choose the +best from among several available routes. For this purpose, it is not +always necessary to make an actual instrument survey of the several +routes. A hasty reconnaissance will usually be sufficient. This is +made by walking or riding over the road and noting, in a suitable book +or upon prepared blanks, the information needed. The items of +information recorded will usually be as follows: distances, grades, +type of soil on the road and nature of existing surface, character of +drainage, location of bridges and culverts and the type of each with +notes as to its condition, location of railway crossings and notes as +to type, location of intersecting roads, farm entrances, and all +similar features that have a bearing on the choice of routes. These +data can be obtained in a comparatively short time by a skilled +observer who may drive over the road in a motor car. Sometimes it may +be desirable to make a more careful study of some certain sections of +road and this may be done by waking over the section in question in +order to make a more deliberate survey of the features to be +considered than is possible when riding in a motor car. + +Factors other than relative lengths of routes will obviously determine +the cost of improvement and the comparative merits of the improved +roads. Some special characteristic of a road, such as bad railroad +crossings or a few bad hills, may eliminate a route, or availability +of materials along a route may offset disadvantages of alignment or +grade. + +In special cases, complete surveys of routes may be required finally +to select the best route, but these instances are few in number. + +=Road Surveys.=--When a road has been definitely selected for +improvement, a careful survey is made to furnish information for the +preparation of the plans. This will consist of a transit survey and a +level survey. + +The transit survey is made by running a line between established +corners following the recorded route of the road, or if no records are +available or the road is irregular in alignment, by establishing +arbitrary reference points and running a line along the center line of +the existing road or parallel thereto. The topography is referenced to +this line in such completeness that it can be reproduced on the plans. +The level survey consists in taking levels on cross sections of the +road at one hundred foot intervals, and oftener if there are abrupt +changes in grade. Special level determinations are made at streams, +railroad crossings, intersecting roads or lanes and wherever it +appears some special features of the terrain should be recorded. + +From the surveys and such other information as has been assembled +relative to the project, a plan is prepared which embodies a design +presumed to provide for an improvement in accordance with the best +highway practice. + + +THE PROBLEM OF DESIGN + +It will be convenient to consider separately the components of a road +design, although in the actual design the consideration of these +cannot be separated because all parts of the plan must fit together. + +=Alignment.=--The alignment of the road is determined to a +considerable extent by the existing right-of-way, which may follow +section lines, regardless of topography, as is the case with many +roads in the prairie states, or it may follow the valleys, ridges, or +other favorable location in hilly country. In many places the roads of +necessity wind around among the hills in order to avoid excessive +grades. In designing an improvement, it is generally desirable to +follow the existing right-of-way so far as possible. But the element +of safety must not be lost sight of, and curves should not preclude a +view ahead for sufficient distance to insure safety to vehicles. The +necessary length of clear view ahead is usually assumed to be 250 +feet, but probably 200 feet is a satisfactory compromise distance when +a greater distance cannot be obtained at reasonable cost. To secure +suitable sight distance, the curves must be of long radii, and where +possible the right-of-way on the inside of the curve should be cleared +of trees or brush that will obstruct the view. Where the topography +will not permit a long radius curve and the view is obstructed by an +embankment or by growing crops or other growth, it is desirable to +separate the tracks around the curve to eliminate the possibility of +accidents on the curve. This is readily accomplished if the road is +surfaced, but if it is not surfaced, the same end is accomplished by +making the earth road of ample width at the curve. + +Relocations should be resorted to whenever they shorten distances or +reduce grades sufficiently to compensate for the cost. + +=Intersections.=--At road intersections, it is always difficult to +design a curve that entirely meets the requirements of safety because +there is not enough room in the right-of-way, and enough additional +right-of-way must be secured to permit the proper design. It is not +necessary to provide an intersection that is adapted to high speed +traffic, where main roads cross, but, on the contrary, a design that +automatically causes traffic to slow up has distinct advantages. + +Where a main route, improved with a hard surface, crosses secondary +roads, it is satisfactory to continue the paved surface across the +intersecting road at normal width and make no provision for the +intersecting road traffic other than a properly graded approach at the +intersection. + +=Superelevation.=--On all curved sections of road, other than +intersections, account is taken of the tendency of motor cars to skid +toward the outside of the curve. This tendency is counteracted by +designing the cross section with superelevation. + +[Illustration: Fig. 6] + +In Fig. 6, _F_ represents the tangential force that tends to cause +skidding. _W_ represents the weight of the vehicle in pounds, THETA += the angle of superelevated surface _c-d_, with the horizontal _c-a_. +_R_ represents the radius of the curve upon which the vehicle is +moving. _w_ is the component of the weight parallel to the surface +_c-d_, _v_ = velocity of the vehicle in feet per second. _m_ = mass +of vehicle = _W/g THETA_ + + + _w_ = _W_ tan _THETA_ + + + _mv^2_ _wv^2_ + _F_ = ------- = ------ + _R_ _gR_ + +If _F_ = _w_ there will be no tendency to skid; hence the rate of +superelevation necessary in any case is as follows: + + + _Wv^2_ + _W_ tan _THETA_ = ------- + _gR_ + + + _v^2_ + tan _THETA_ = ------- + _gR_ + + +The amount of superelevation required, therefore, varies as the square +of the velocity and inversely as the radius of the curve. + +Theoretically, the amount of the superelevation should increase with a +decrease in the radius of the curve and should also increase as the +square of the speed of the vehicle. On account of the variation in +speeds of the vehicles, the superelevation for curves on a highway can +only be designed to suit the average speed. At turns approaching +ninety degrees, the curve is likely to be of such short radius that it +is impossible to maintain the ordinary road speed around the curve, +even with the maximum superelevation permissible. It is good practice +to provide the theoretical superelevation on all curves having radii +greater than 300 feet for vehicle speeds of the maximum allowed by +law, which is generally about 25 miles per hour. Where the radii are +less than 300 feet, the theoretical superelevation for the maximum +vehicle speeds gives a superelevation too great for motor trucks and +horse drawn vehicles and generally no charge is made in superelevation +for radii less than 300 feet, but all such curves are constructed with +the same superelevation as the curve with 300 foot radius. + +The diagram in Fig. 7 shows the theoretical superelevation for various +curve radii. + +[Illustration: Fig. 7. Curves showing Theoretical Superelevation for +Various Degrees of Curve for Various Speeds of Vehicle] + +At the intersection of important highways, the problem is complicated +by the necessity for providing for through traffic in both directions +and for traffic which may turn in either direction and the engineer +must provide safe roadways for each class of traffic. + +=Tractive Resistance.=--The adoption of a policy regarding the grades +on a road involves an understanding of the effect of variation in the +character of the surface and in rate of grade upon the energy required +to transport a load over the highway. The forces that oppose the +movement of a horse drawn vehicle are fairly well understood and their +magnitude has been measured by several observers, but comparatively +little is known about the forces opposing translation of rubber tired +self-propelled vehicles. + +The resistance to translation of a vehicle is made up of three +elements: resistance of the road surface to the rolling wheel, +resistance of the air to the movement of the vehicle and internal +friction in the vehicle itself. + +=Rolling Resistance.=--When the wheel of a vehicle rolls over a road +surface, both the wheel and the surface are distorted. If the wheel +has steel tires and the road surface is plastic, there will be +considerable distortion of the road surface and very little of the +wheel. A soft rubber tire will be distorted considerably by a brick +road surface. Between these extremes there are innumerable +combinations of tire and road surface encountered, but there is always +a certain amount of distortion of either road surface or wheel, or of +both, which has the same effect upon the force necessary for +translation as a slight upward grade. When both the tire and the road +surface strongly resist distortion (as steel tires on vitrified brick +paving), the resistance to translation is low but the factor of impact +is likely to be introduced. Where impact is present, energy is used up +in the pounding and grinding of the wheels on the surface, and this +factor increases as the speed of translation, and may be a +considerable item. Impact is especially significant on rough roads +with motor vehicles, particularly trucks, traveling at high speed. +These two factors (impact and rolling resistance) combined constitute +the major part of the resistance to translation for horse drawn +vehicles. + +=Internal Resistance.=--For horse drawn vehicles, the internal +resistance consists of axle friction, which is small in amount. For +self-propelled vehicles, the internal resistance consists of axle +friction and friction in the driving mechanism, of which gear +friction and the churning of oil in the gear boxes is a large item. +Internal friction is of significance in all self-propelled vehicles +and especially so at high speeds. + +=Air Resistance.=--At slow speeds, the resistance of still air to +translation is small, but as the speed increases, the air resistance +increases rapidly and at the usual speed of the passenger automobile +on the road becomes a very considerable part of the total resistance +to translation. This factor has no significance in connection with +horse drawn vehicles, but is to be taken into account when dealing +with self-propelled vehicles at speeds in excess of five miles per +hour. + +Many determinations of tractive resistance with horse drawn vehicles +have been made from time to time and these show values that are fairly +consistent when the inevitable variations in surfaces of the same type +are taken into account. Table 4 is a composite made up of values +selected from various reliable sources and Table 5 is from experiments +by Professor J. B. Davidson on California highways. + + TABLE 4 + + AVERAGE TRACTIVE RESISTANCE OF ROAD SURFACES TO STEEL TIRED + VEHICLES + + Surface Tractive force per ton + + Earth packed and dry 100 + Earth dusty 106 + Earth muddy 190 + Sand loose 320 + Gravel good 51 + Gravel loose 147 + Cinders well-packed 92 + Oiled road--dry 61 + Oiled road--wet 108 + Macadam--very good 38 + Macadam--average 46 + Sheet asphalt 38 + Asphaltic concrete 40 + Vitrified brick--new 56 + Wood block--good 33 + Wood block--poor 42 + Cobblestone 54 + Granite tramway 27 + Asphalt block 52 + Granite block 47 + + TABLE 5 + + TRACTIVE RESISTANCES TO STEEL TIRED VEHICLES[1] + + ----------+-----------------+-----------------+-----------+----------- + | | Condition | Tractive | Resistance + Test No. | Kind of Road | of Road | Total lb. | per ton lb. + ----------+-----------------+-----------------+-----------+----------- + 29-30-31 | Concrete |Good, excellent | 83.0 | 27.6 + | (unsurfaced) | | | + [2]11-12 | Concrete |Good, excellent | 90.0 | 30.0 + | (unsurfaced) | | | + 26-27-28 | Concrete 3/8-in.|Good, excellent | 147.6 | 49.2 + | surface | | | + | asphaltic oil | | | + | and screenings| | | + 13-14 | Concrete 3/8-in.|Good, excellent | 155.0 | 51.6 + | surface | | | + | asphaltic oil | | | + | and screenings| | | + 9-10 | Macadam, |Good, excellent | 193.0 | 64.3 + | water-bound | | | + 22-23 | Topeka on |Good, excellent | 205.5 | 68.5 + | concrete | | | + 8 | Gravel |Compact, good | 225.0 | 75.0 + | | condition | | + [3]45-48 | Oil macadam |Good, new | 234.5 | 78.2 + [4]46-47 | Oil macadam |Good, new | 244.0 | 81.3 + 38 | Gravel |Packed, in | 247.0 | 82.3 + | | good condition | | + 18-19-20 | Topeka on plank |Good condition, | 265.0 | 88.3 + | | soft, wagon | | + | | left marks | | + 34 | Earth road |Firm, 1-1/2-in. | 276.0 | 92.0 + | | fine loose dust| | + 24-25 | Topeka on plank |Good condition, | 278.0 | 92.6 + | | but soft | | + 1-2-5 | Earth road |Dust 3/4 to 2 in.| 298.0 | 99.3 + 3-3 | Earth |Mud, stiff, firm | 654.0 | 218.0 + | | underneath | | + 6-7 | Gravel |Loose, not packed| 789.0 | 263.0 + ----------+-----------------+-----------------+-----------+----------- + + [1] Prof. J. B. Davidson in _Engineering News-Record_, August 17, + 1918. + + [2] Graphic record indicates that the load was being accelerated + when test was started. + + [3] Drawn with motor truck at 2-1/2 miles per hour. + + [4] Drawn with motor truck at 5 miles per hour. + +Comparatively few data are available showing the tractive resistance +of motor vehicles, but the following tables are based on sufficient +data to serve to illustrate the general trend. + +These data on the tractive resistances of an electric truck with solid +rubber tires on asphalt and bitulithic, wood, brick and granite block, +water-bonded and tar macadam, cinder and gravel road surfaces were +obtained by A. E. Kennelly and O. R. Schurig in the research division +of the electrical engineering department of the Massachusetts +Institute of Technology, and are published in Bulletin No. 10 of the +division. + +An electric truck was run over measured sections, ranging from 400 to +2600 feet in length, surfaced with these various materials, at certain +speeds per hour, ranging from about 8 to about 15.5 miles per hour. +The result of the observations of speeds, tractive resistances, +conditions of surfaces, etc., were collected and studied in various +combinations. + + TABLE 6 + + ----------------------+-----------------------+-----------+---------- + | | Tractive | Tractive + | |Resistance |Resistance + Type of Surface | Condition of Surface | in lbs. | in lbs. + | | per ton | per ton + | | 10 miles |12.4 miles + | | per hr. | per hr. + ----------------------+-----------------------+-----------+---------- + Asphalt | Good | 20.4 | + Asphalt | Poor | 22.6 | 25.5 + Wood block | Good | 24.2 | 25.3 + Brick block | Good | 24.6 | 26.6 + Granite block | Good | 40.3 | 45.75 + Brick block | Slightly worn | 25.1 | 28.0 + Granite block with | | | + cement joints | Good | 25.5 | 30.2 + Macadam, water bonded | Dry and hard | 23.3 | 25.8 + Macadam, water bonded | Fair, heavily oiled | 35.9 | 38.7 + Macadam, water bonded | Poor, damp, some holes| 36.3 | 41.6 + Tar macadam | Good | 25.7 | 28.0 + Tar macadam | Very soft | 36.8 | 38.7 + Tar macadam | Many holes, soft, | | + | extremely poor | 52.4 | 60.6 + Cinder | Fair, hard | 27.5 | 30.6 + Gravel | Fair, dusty | 30.4 | 33.0 + ----------------------+-----------------------+-----------+---------- + +[Illustration: Fig. 8] + +=Effect of Grades.=--Grades increase or decrease the resistance to +translation due to the fact that there is a component of the weight of +the vehicles parallel to the road surface and opposite in direction to +the motion when the load is ascending the hill and in the same +direction when the vehicle is descending. In Fig. 8 _W_ represents the +weight of the vehicle, acting vertically downward, _w_ is the +component of the weight perpendicular to the road surface and _W_{2}_ +is the component parallel to the road surface. + + _W_{2}_ = _W_ tan _THETA_. + + tan _THETA_ = 0.01 x per cent of grade. + + _W_{2}_ = 0.01 _W_ x per cent grade. + + _W_{2}_ = 0.01 x 2000 x per cent of grade, for each ton + of weight of vehicle. + + Hence _W_{2}_ = 20 lbs. per ton of load for each one per cent of + grade. + +The gravity force acting upon a vehicle parallel to the surface on a +grade is therefore 20 lbs. per ton for each one per cent of grade and +this force tends either to retard or to accelerate the movement of the +vehicle. + +Let _F_ = the sum of all forces opposing the translation of a vehicle. + + _F = f_{r} + f_{i} + f_{p} + f_{a} + f_{g}_ (1) + +where + + _f_{r}_ = rolling resistance of road surface. + _f_{i}_ = resistance due to internal friction in the vehicle. + _f_{p}_ = resistance due to impact of the road surface. + _f_{a}_ = resistance due to air. + _f_{g}_ = resistance due to grade, which is positive when + ascending and negative when descending. + +All of the above in pounds per ton of 2000 lbs. + +Let _T_ = the tractive effort applied to the vehicle by any means. + +_T_ >= must be greater than _F_ in order to move the vehicle. + +By an inspection of (1), it will be seen that for a given vehicle and +any type of road surface, all terms are constant except _f_{a}_ and +_f_{g}_. _f_{a}_ varies as the speed of the vehicle and the driver can +materially decrease _f_{a}_ by reducing speed. _f_{g}_ varies with the +rate of grade. For any vehicle loaded for satisfactory operation on a +level road with the power available, the limiting condition is the +factor _f_{g}_. If the load is such as barely to permit motion on a +level road, any hill will stall the vehicle. Therefore, in practice +the load is always so adjusted that there is an excess of power on a +level road. If draft animals are employed the load is usually about +one fourth of that which the animals could actually move by their +maximum effort for a short period. With motor vehicles, the excess +power is provided for by gearing. + +If it be assured a load of convenient size is being moved on a level +road by draft animals, there is a limit to the rate of grade up which +the load can be drawn by the maximum effort of the animals. + +Tests indicate that the horse can pull at a speed of 2-1/2 miles per +hour, an amount equal to 1/8 to 1/10 of its weight, and for short +intervals can pull 3/4 of its weight. The maximum effort possible is +therefore six times the average pull, but this is possible for only +short intervals. A very short steep hill would afford a condition +where such effort would be utilized. But for hills of any length, that +is, one hundred feet or more but not to exceed five hundred feet, it +is safe to count on the draft animal pulling three times his normal +pulling power for sustained effort. + +The limiting grade for the horse drawn vehicle is therefore one +requiring, to overcome the effect of grade, or _f_{g}_, a pull in +excess of three times that exerted on the level. + +A team of draft animals weighing 1800 lbs. each could exert a +continuous pull of about 1/10 of their weight or 360 lbs. If it be +assumed that the character of the vehicle and the road surface is such +that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 100 lbs. per gross ton on +a level section of road, then the gross load for the team would be 3.6 +tons. The same team could for a short time exert an additional pull of +three times 360 lbs. or 1080 lbs. For each 1 per cent of grade a pull +of 20 lbs. per ton would be required or _f_{g}_ for the 3.6 tons load +would be 72 lbs. for each per cent of grade. At that rate, the +limiting grade for the team would be fifteen per cent. + +If, however, the character of the vehicle and the road surface were +such that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 60 lbs. per gross +ton on a level section of road, the gross load for the team on the +level would be 6 tons, and the limiting grade 9 per cent. + +The above discussion serves to illustrate the desirability of adopting +a low ruling or limiting grade for roads to be surfaced with a +material having low tractive resistance and the poor economy of +adopting a low ruling grade for earth roads or roads to be surfaced +with material of high tractive resistance. + +It may be questioned whether horse drawn traffic should be the +limiting consideration for main trunk line highways, but it is +certain that for a number of years horse drawn traffic will be a +factor on secondary roads. + +In the case of motor vehicles, excess power is provided by means of +gears and no difficulty is encountered in moving vehicles over grades +up to 12 or 15 per cent, so that any grade that would ordinarily be +tolerated on a main highway will present no obstacle to motor +vehicles, but the economy of such design is yet to be investigated. + +=Energy Loss on Account of Grades.=--Whether a vehicle is horse drawn +or motor driven, energy has been expended in moving it up a hill. A +part of this energy has been required to overcome the various +resistances other than grade, and that has been dissipated, but the +energy required to translate the vehicle against the resistance due to +grade has been transformed into potential energy and can be partially +or wholly recovered when the vehicle descends a grade, provided the +physical conditions permit its utilization. If the grade is so steep +as to cause the vehicle to accelerate rapidly, the brakes must be +applied and loss of energy results. The coasting grade is dependent +upon the character of the surface and the nature of the vehicle. In +the cases discussed in the preceding paragraph, the coasting grades +would be five per cent and three per cent respectively. For horse +drawn vehicles then the economical grades would be three and five per +cent, which again emphasizes the necessity of lower grades on roads +that are surfaced than on roads with no wearing surface other than the +natural soil. + +The theory of grades is somewhat different when motor vehicles are +considered, since it is allowable to permit considerably higher speed +than with horse drawn vehicles before applying the brakes and the +effect of grade can be utilized not only in translating the vehicle +down the grade, but also in overcoming resistances due to mechanical +friction and the air. On long grades, a speed might be attained that +would require the use of the brake or the same condition might apply +on very steep short grades. There is at present insufficient data on +the tractive resistance and air resistance with motor vehicles to +permit the establishing of rules relative to grade, but experience +indicates a few general principles that may be accepted. + +If a hill is of such rate of grade and of such length that it is not +necessary to use the brake it may be assumed that no energy loss +results so far as motor vehicles are concerned. Where there is no turn +at the bottom of the hill and the physical condition of the road +permits speeds up to thirty-five or forty miles per hour grades of +five per cent are permissible if the length does not exceed five +hundred feet and grades of three per cent one thousand feet long are +allowable. It is a rather settled conviction among highway engineers +that on trunk line highways the maximum grade should be six per cent, +unless a very large amount of grading is necessary to reach that +grade. + +=Undulating Roads.=--Many hills exist upon highways, the grade of +which is much below the maximum permissible. If there are grades +ranging from 0 to 4 per cent, with a few hills upon which it is +impracticable to reach a grade of less than six per cent, it is +questionable economy to reduce the grades that are already lower than +the allowable maximum. It is especially unjustifiable to incur expense +in reducing a grade from two per cent to one and one-half per cent on +a road upon which there are also grades in excess of that amount. The +undulating road is not uneconomical unless the grades are above the +allowable maximum or are exceptionally long or the alignment follows +short radius curves. + +=Safety Considerations.=--On hills it is especially desirable to +provide for safety and curves on hills are always more dangerous than +on level sections of road. Therefore, it is desirable to provide as +flat grades as possible at the curves and to cut away the berm at the +side of the road so as to give a view ahead for about three hundred +feet. Whether a road be level or on a hill, safety should always be +considered and the most important safety precaution is to provide a +clear view ahead for a sufficient distance to enable motor vehicle +drivers to avoid accidents. + +[Illustration: Fig. 9.--Types of Guard Rails] + +=Guard Railing.=--When a section of road is on an embankment, guard +rails are provided at the top of the side slope to serve as warnings +of danger, and to prevent vehicles from actually going over the +embankment in case of skidding, or if for any reason the driver loses +control. These are usually strongly built, but would hardly restrain a +vehicle which struck at high speed. But they are adequate for the +protection of a driver who uses reasonable care. A typical guard rail +is shown in Fig. 9, but many other designs of similar nature are +employed. At very dangerous turns a solid plank wall six or eight feet +high is sometimes built of such substantial construction as to +withstand the severest shock without being displaced. + +Trees, shrubs and the berms at the side of the road in cuts are +particularly likely to obstruct the view and should be cleared or cut +back so far as is necessary to provide the proper sight distance. + +=Width of Roadway.=--For roads carrying mixed traffic, 9 feet of width +is needed for a single line of vehicles and 18 feet for 2 lines of +vehicles. In accordance with the above, secondary roads, carrying +perhaps 25 to 50 vehicles per day, may have an available traveled way +18 feet wide. Those more heavily traveled may require room for three +vehicles to pass at any place and therefore have an available traveled +way 30 feet wide. Greater width is seldom required on rural highways, +and 20 feet is the prevailing width for main highways. + +=Cross Section.=--The cross section of the road is designed to give +the required width of traveled way, and, in addition, provide the +drainage channels that may be needed. In regions of small rainfall the +side ditches will be of small capacity or may be entirely omitted, but +usually some ditch is provided. The transition from the traveled way +to ditch should be a gradual slope so as to avoid the danger incident +to abrupt change in the shape of the cross section. The depth of ditch +may be varied without changing to width or slope of the traveled part +of the road as shown in Fig. 10. + +[Illustration: Fig. 10] + +=Control of Erosion.=--The construction of a highway may be utilized +to control general erosion to some extent, particularly when public +highways exist every mile or two and are laid out on a gridiron +system, as is the case in many of the prairie states. The streams +cross the highways at frequent intervals and the culverts can be +placed so as effectually to prevent an increase in depth of the +stream. This will to some extent limit the erosion above the culvert +and if such culverts are built every mile or two along the stream, +considerable effect is produced. + +Where small streams have their origin a short distance from a culvert +under which they pass, it is sometimes advisable to provide tile for +carrying the water under the road, instead of the culvert, and, by +continuing the tile into the drainage area of the culvert, eliminate +the flow of surface water and reclaim considerable areas of land. + +Erosion in the ditches along a highway can be prevented by +constructing weirs across the ditch at frequent intervals, thus +effectually preventing an increase in the depth of the ditch. + +Wherever water flows at a velocity sufficient to produce erosion or +where the drainage channel changes abruptly from a higher to a lower +level, paved gutters, tile or pipe channels should be employed to +prevent erosion. + +=Private Entrances.=--Entrance to private property along the highway +is by means of driveways leading off the main road. These should +always be provided for in the design so as to insure easy and +convenient access to the property. The driveways will usually cross +the side ditch along the road and culverts will be required to carry +the water under the driveway. Driveways that cross a gutter by means +of a pavement in the gutter are usually unsatisfactory, and to cross +the gutter without providing a pavement is to insure stoppage of the +flow at the crossing. The culvert at a driveway entrance must be large +enough to take the ditch water readily or it will divert the water to +the roadway itself. Generally end walls on such culverts are not +required as in the case of culverts across a highway. + +=Aesthetics.=--Much of the traffic on the public highways is for +pleasure and relaxation and anything that tends to increase the +attractiveness of the highways is to be encouraged. Usually the +roadside is a mass of bloom in the fall, goldenrod, asters and other +hardy annuals being especially beautiful. In some states wild roses +and other low bushes are planted to serve the two-fold purpose of +assisting to prevent erosion and to beautify the roadside. In humid +areas trees of any considerable size shade the road surface and are a +distinct disadvantage to roads surfaced with the less durable +materials such as sand-clay or gravel. It is doubtful if the same is +true of paved surfaces, but the trees should be far enough back from +the traveled way to afford a clear view ahead. Shrubs are not +objectionable from any view-point and are to be encouraged for their +beauty, so long as they do not obstruct the view at turns. + + + + +CHAPTER V + +EARTH ROADS + + +Highways constructed without the addition of surfacing material to the +natural soil of the right-of-way are usually called earth roads. But +if the natural soil exhibits peculiar characteristics or is of a +distinct type, the road may be referred to by some distinctive name +indicating that fact. Hence, roads are referred to as clay, gumbo, +sandy or caliche roads as local custom may elect. In each case, +however, the wearing surface consists of the natural soil, which may +have been shaped and smoothed for traffic or may be in its natural +state except for a trackway formed by the vehicles that have used it. + +=Variations in Soils.=--The nature of the existing soil will obviously +determine the serviceability and physical characteristics of the road +surface it affords. That is to say that even under the most favorable +conditions some earth roads will be much more serviceable than others, +due to the better stability of the natural soil. Some soils are dense +and somewhat tough when dry and therefore resist to a degree the +tendency of vehicles to grind away the particles and dissipate them in +the form of dust. Such soils retain a reasonably smooth trackway in +dry weather even when subjected to considerable traffic. Other soils +do not possess the inherent tenacity and stability to enable them to +resist the action of wheels and consequently grind away rapidly. Roads +on such soils become very dusty. These are the extremes and between +them are many types of soils or mixtures of soils possessing varying +degrees of stability, and, in consequence, differing rates of wear. +Similarly the various soils exhibit different degrees of stability +when wet. + +It is to be expected that soils will differ with the geographical +location, for it is well known that there is a great variation in +soils in the various parts of the world. But wide differences are also +encountered in the soil on roads very near each other and even on +successive stretches of the same road. It is for this reason that +earth roads often exhibit great differences in serviceability even in +a restricted area. + +=Variation in Rainfall.=--The stability of a soil and its ability to +support the weight of vehicles varies greatly with the amount of water +in the soil. A certain small amount of moisture in the soil is +beneficial in that practically every soil compacts more readily when +moist than when dry because the moisture aids in binding together the +particles. But most soils also become unstable when the amount of +water present is in excess of that small amount referred to above and +the stability decreases very rapidly as the amount of water in the +soil increases. + +The serviceability of an earth road will change continually as the +moisture content of the soil changes and consequently the general +utility of the earth road system in any locality is dependent to a +considerable extent upon the amount and seasonal distribution of +precipitation. The methods of maintaining earth roads appropriate to +any locality must of necessity be adapted to the climatic conditions, +and the amount of work required to give the highest possible degree of +serviceability will be exceedingly variable from season to season and +from place to place. In regions of great humidity, earth roads may be +expected to have a low average of serviceability, while in arid +regions they may possess sufficient durability for a considerable +volume of traffic. The design adopted for earth roads and the methods +of maintenance followed should therefore be carefully evolved to meet +the soil and climate conditions where the roads are located. These +will differ greatly throughout a state or even a county. + +=Cross Sections.=--The general principles of road design were set +forth in Chapter IV. In Fig. 11 are shown typical cross sections for +earth roads adapted to various conditions as indicated. It is not +apparent that one form of ditch is particularly preferable to the +other and since some engineers prefer the V section and others the +trapezoidal section both are shown. It would appear that the V shaped +ditch is somewhat the easier to construct with the blade grader while +the trapezoidal is readily excavated with the slip or fresno scraper. +The ditch capacity required and consequently the dimensions will +depend upon the drainage requirements, as was pointed out in Chapter +III. + +[Illustration: Fig. 11. Cross Section for Earth Roads] + + +EARTH ROADS IN REGIONS OF CONSIDERABLE RAINFALL + +In the zones where the annual precipitation exceeds 30 inches +distributed over several months, earth roads will be unserviceable for +a considerable period each year unless they are constructed so as to +minimize the effect of water. This is done by providing for the best +possible drainage and by adopting a method of maintenance that will +restore the surface to a smooth condition as quickly as possible after +a period of rainy weather or after the "frost comes out" in the +spring. + +Before the construction of the desired cross section is undertaken, +all of the grade reduction should be completed, except for minor cuts +which can be handled with the elevating grader in the manner that will +be described presently. + +Where any considerable change in grade is to be effected, the earth +can be moved in several ways and of these the most economical cannot +be readily determined. Ordinarily a contractor or a county will use +the equipment that happens to be at hand even though some other might +be more advantageous. + +=Elevating Grader.=--Where the topography is such as to permit its +use, the elevating grader is employed in grade reduction to load the +earth into dump wagons in which it is hauled to the fill or waste +bank. The elevating grader consists essentially of a heavy shear plow +or disc plow which loosens the earth and deposits it on a moving +canvas apron. The apron carries the material up an incline and +deposits it into a wagon which is driven along under the end of the +apron. When the wagon is loaded, the grader is stopped while the +loaded wagon is hauled out and an empty one drawn into position. The +motive power for the elevating grader is either a tractor or five or +six teams of mules. For many kinds of work, particularly where +frequent turning is necessary or where the ground is yielding, mules +are preferable to a tractor. The apron is operated by gearing from the +rear wheels of the grader. Generally four mules are hitched to a +pusher in the rear of the grader and six or eight in the lead. This +method of grade reduction is particularly advantageous when the +material must be hauled a distance of 500 yards or more, because wagon +hauling in such cases is the most economical method to employ. A +tractor may be used to draw the elevating grader and one having a +commercial rating of 30 to 45 horsepower is required. + +=Maney Grader.=--If the haul is long and the nature of the cut will +not permit the use of the elevating grader because of excessive grades +or lack of room for turning, a grader of the Maney type may be used. +This consists of a scoop of about one cubic yard capacity, suspended +from a four-wheel wagon gear. When loading, the scoop is let down and +filled in the same manner as a two-wheeled scraper or "wheeler." The +pull required to fill a Maney grader is so great that a tractor is +ordinarily employed in place of a "snap" team. The tractor is hitched +at the end of the tongue, without interfering with the team drawing +the grader. One team readily handles the grader after it is loaded. +For this service a tractor having a commercial rationing of 25 to 30 +horsepower is required. + +=Wheel Scraper.=--For moving earth for distances between 150 and 500 +yards, the wheel scraper of a capacity of about 1-1/2 yards is quite +generally employed. The soil must be loosened with a plow before it +can conveniently be loaded into the wheeler and a heavy plow is +ordinarily employed for that purpose. Two furrows with the plow will +loosen a strip of earth about as wide as the scoop of the scraper and +if more is loosened it will be packed down by the scrapers wheeling in +place to load. A helper or "snap" team is employed to assist in +loading, after which the wheel scraper is handled by one team. + +=Slip Scraper.=--The slip scraper differs from the wheel scraper in +that the scoop is not suspended from wheels but is dragged along the +ground. It is drawn by one team and the capacity is two to five cubic +feet, but the material spills out to some extent as the scraper is +dragged along and the method is not suitable for long hauls, 100 feet +being about the economical limit. + +=Fresno Scraper.=--The Fresno scraper is one form of slip scraper +requiring four horses or mules for efficient work. It differs +somewhat from the ordinary slip scraper in shape and is of larger +capacity, but is a drag type of scraper much favored in the western +states. + + +SHAPING TO PROPER CROSS SECTION + +If a road has been graded so that the profile is satisfactory or if +the existing profile of the location is satisfactory, and the surface +is to be shaped to a prescribed cross section, either the elevating +grader or the blade grader may be employed. + +=Elevating Grader Work.=--If the elevating grader is used in shaping +the earth road, the apron will be lowered and the material will be +excavated at the sides of the road and deposited on the middle +portion. If slight changes in grade are desired, wagons will accompany +the grader and catch under the apron at the high places and haul the +material to the low places. After the earth has been deposited it must +be worked over to secure the correct cross section and be made +passable for vehicles. This requires that clods be broken, weeds and +grass that are mixed with the earth be removed by harrowing and +forking and that the surface be carefully smoothed with a blade +grader. This latter operation will have to be repeated several times +before a satisfactory surface is secured. But this miscellaneous work +is highly important and under no circumstances ought to be neglected. +Nothing so detracts from an otherwise creditable piece of work as +failure to provide a smooth surface for the use of vehicles. It is +especially uncomfortable for the users of a highway if sods and weeds +in quantity are left in the road after it has been graded. The humus +that will be left in the soil as the vegetable matter decays increases +the porosity of the road surface making it more absorbent than soil +without humus. This increases the susceptibility to softening from +storm water or ground water. + +The tractor can advantageously be used to draw the elevating grader on +this class of work, but will be greatly handicapped if there are wet +sections along the road, through which the tractor must be driven. In +many cases its use is prohibited by such conditions and for all-round +service of this character, mules are preferred for motive power. + +[Illustration: Fig. 12.--Tractor-grader Outfit] + +=Use of Blade Grader.=--Heavy blade graders designed to be drawn by a +tractor are suitable for shaping the earth road. Some of these have +blades 12 feet long and excellent control for regulating the depth of +cutting. Often two such graders are operated tandem. These machines +have a device which permits the operator to steer the grader +independently of the tractor. Thus the grader can be steered off to +the side to cut out the ditches, while the tractor continues to travel +on the firm part of the road. Earth moved with the blade grader is +usually fairly free from large lumps and can readily be smoothed to a +satisfactory surface for the use of traffic. The sods and weeds will +be drawn into the road along with the earth just as they are when the +elevating grader is employed. Precaution must therefore be taken to +eliminate them before the vegetable matter decays, and to smooth the +surface for the use of traffic. + +=Costs.=--The cost of shaping an earth road in the manner described +above will vary through rather wide limits because the nature and +amount of work to be done varies so greatly. Some roads can be graded +satisfactorily for $300.00 per mile, while others will cost $700.00. +But $425.00 per mile may be taken as an average for blade or elevating +grader work plus a moderate amount of grade reduction in the way of +removing slight knolls. For the amount of grade reduction necessary in +rolling country, followed by grader shaping, $1000.00 to $1800.00 per +mile will be required. The method is not adapted to rolling country +where the roads are undulating and require some grade reduction on +every hill. For hilly roads one of the methods described for grade +reduction will be required and the cost will obviously depend upon the +amount of earth moved. Averages of cost figures mean nothing in such +cases as the cost may reach $10,000.00 per mile, or may be as low as +$2000.00 per mile. + +=Maintenance.=--Regardless of the care with which an earth road has +been graded, it will be yielding and will readily absorb water for a +long time after the completion of the work. The condition of the +surface will naturally deteriorate rapidly during the first season it +is used unless the road receives the constant maintenance that is a +prerequisite to satisfactory serviceability. The road drag is +generally recommended for this purpose, and if a drag is properly used +it will serve to restore the shape of the surface as fast as it is +destroyed by traffic. + +Good results with the drag depend upon choosing the proper time to +drag and upon doing the work in the right way when using the drag. The +best time to drag is as soon after a rain as the road has dried out +enough to pack under traffic. If the work is done while the road is +too wet, the first vehicles traveling the road after it has been +dragged will make ruts and to a considerable extent offset the good +done by the drag. If the road is too dry, the drag will not smooth the +irregularities. A little observation will be required to determine the +proper time for dragging on any particular soil, but usually after a +rain or thaw there is a period lasting a day or two when conditions +are about right. + +[Illustration: Fig. 13.--Road Drag] + +The drag is used merely to restore the shape of the surface and to do +so a small amount of material is drawn toward the middle of the road. +But there must not be a ridge of loose material left in the middle +after the work is completed. Some patrolmen start at one side of the +road and gradually work across the road on successive trips, finally +finishing up at the side opposite that at which the start was made. +The next dragging should start on the opposite side from the first if +that method is followed. + +By shifting his weight on the drag, the operator can adjust the +cutting edge so that very little loose material is moved crosswise of +the road and that is the proper method to pursue. In that case no +ridge will remain at the middle of the road. If a slight one is left +it should be removed by a final trip with the drag. + +In addition to the dragging, weeds must be cut along the road about +twice a year, the ditches must be kept cleaned out and culverts open. + +All of the maintenance for 10 miles of earth road can be accomplished +by one man giving his entire time to the work, and that is the only +method that has proven adequate to the problem. + + +EARTH ROADS IN ARID REGIONS + +In areas where the rainfall is less than 18 inches per year, and +especially where it is 10 inches or less, an entirely different road +problem exists. The effect of precipitation is of significance +primarily from the standpoint of erosion, and the design of cross +section and ditches and the culvert provisions are entirely different +from those necessary in humid regions. + +Frequently the rainfall in semi-arid regions will be seasonal and +provision must be made to care for a large volume of water during the +rainy season, but, in general, road design is adapted to prevention of +erosion rather than to elimination of ground water effects, or the +softening effects of surface water. Generally the rainy period does +not last long enough to warrant expensive construction to eliminate +its general effects. In fact, the saturation of the soil is more +likely to be a benefit than otherwise. + +Earth roads are likely to be satisfactory except where the traffic is +sufficient to grind the surface into dust to such an extent that an +excessive dust layer is produced. In such locations the problem is one +of providing a durable surface unaffected by long continued dry +weather. + +Grade reduction will have the same importance as in humid areas and +will be carried out in the same way. + +Maintenance will consist in repairing the damage from occasional +floods and in removing or preventing accumulations of drifting sand +or dust. Crude petroleum oils have been satisfactory for maintenance +in such locations when used on stable soils. + +=Value of Earth Roads.=--The serviceability of the earth road depends +to a large extent upon the care exercised in its maintenance. The only +part of earth road construction that is permanent is the grade +reduction. The cross section that is so carefully shaped at +considerable cost may flatten out in one or two years, especially if +the road goes through unusually wet periods. Traffic will continually +seek a new track during the period when the road is muddy and is as +likely to cross the ditch to the sod near the fence as to use any +other part of the road. Continual and persistent maintenance is +therefore essential to even reasonable serviceability. At best the +earth road will be a poor facility for a considerable period each year +in the regions of year-around rainfall. In most localities, roads of +distinctly minor importance are of necessity only earth roads and for +the comparatively small territory they serve and the small amount of +traffic, they probably serve the purpose. For roads of any importance +in the humid areas of the United States, the earth road cannot carry +satisfactorily the traffic of a prosperous and busy community. + + + + +CHAPTER VI + +SAND-CLAY AND GRAVEL ROADS + + +In Chapter IV, mention was made of the variation in serviceability of +road surfaces composed of the natural soil existing on the +right-of-way of the road. It has been found that soils of a clayey +nature in which there is a considerable percentage of sand usually +afford a serviceable road surface for light or moderate traffic, +especially in areas where climatic conditions are favorable. A study +of these soils, together with the construction of experimental roads +of various mixtures of sand and clay, has led to a fairly +comprehensive understanding of the principles of construction and +range of capacity of this type of road surface, which is known as the +sand-clay road. + +The sand-clay road surface consists of a natural or artificial mixture +of sand and clay, in which the amount of clay is somewhat greater than +sufficient to fill the voids in the dry sand. It may be assumed that +the sand contains 40 per cent of voids and that at least 45 per cent +of clay is required to fill the voids and bind the sand grains +together, because the clay spreads the sand grains apart during the +mixing, thus having the effect of increasing the voids. As a matter of +experiment, it is found to be impractical to secure by available +construction methods mixtures of sufficient uniformity to render it +necessary to exercise great exactness in proportioning the components, +but reasonable care in proportioning the materials is desirable. + +Successful utilization of this type of surface requires considerable +study of available materials and investigations of their behavior when +combined. Extensive and exhaustive experiments have been conducted +with sand-clay mixtures in various places where they are widely used +for road surfaces and the following general principles have been +deduced. + +=The Binder.=--In the sand-clay road, stability is obtained by +utilizing the bonding properties possessed to some degree by all +soils. Naturally this characteristic may be expected to vary widely +with the several types of soil. It is generally considered to be a +common property of clay, but the term clay is a general one that is +often applied to soils differing greatly in physical characteristics +and the term therefore loses its significance in this connection. +Those soils that are properly and technically called clay are +decidedly sticky when wet and are the best materials for sand-clay +construction. Of the clays, those that produce a tough sticky mud are +best. This can be tested by mixing a small quantity into a stiff mud +and molding it into a ball and immersing in water. If the ball retains +its shape for some little time, it is likely to prove a very +satisfactory binder, but, if it becomes plastic and loses its shape, +it will be an inferior binder, as a general rule. The ball clay, as +the former is called, may be of any color common to soils, not +necessarily yellow or reddish as is sometimes supposed. Likewise, +balls of mixtures containing varying percentages of sand and the +binder to be used may be made up and immersed in water. The mixture +that holds its shape longest is of course the best combination of the +materials and indicates the mixture to use in the construction. + +An ideal, or even a fairly satisfactory soil for a binder may not +exist in the vicinity of a proposed improvement, and consequently an +inferior binder is frequently the only material available. + +Sometimes deposits of clay or gravel contain a considerable percentage +of gypsum which serves as a binder and is particularly effective when +used in combination with clay and sand or gravel. + +In many places a soil of the type used for adobe and called "caliche" +may be found and this is an excellent binder for sand or gravel. + +=Top-Soil or Natural Mixtures.=--Deposits consisting of a natural +mixture of sand and clay in which the ingredients happen to exist in +about the correct relative proportions for sand-clay road surfaces are +found in many localities. These mixtures are commonly referred to as +top-soil. If the deposits are somewhat deficient either in sand or +clay, they can be utilized if the proper corrections in the +proportions are made during construction. Very satisfactory road +surfaces are sometimes constructed with mixtures that appear to be far +from ideal in composition, but experience and frequent trials are +needed to determine the best way in which to handle these mixtures. + +=Sand-Clay Surfaces on Sandy Roads.=--Sand-clay surfaces may be +constructed on naturally sandy roads either by adding clay and mixing +it with the sand to secure the desired composition, or a layer of a +natural sand-clay mixture, caliche or sand-clay-gypsum may be placed +on top of the sand. + +The most widely used method is to mix clay or other binder with the +sand. Since there is no need to provide for ditches to carry storm +water on a deep sand soil, the sand is graded off nearly flat across +the road and no ditches are provided. The clay is dumped on the road +in a layer about 8 inches thick and is then mixed into the sand. It is +desired to mix enough sand with the clay to produce a mixture composed +of approximately 1/3 clay and 2/3 sand. The mixing is accomplished in +various ways, the most common being to use a heavy plow at first and +to follow this with a heavy disc harrow. The mixing is a tedious and +disagreeable process, but its thorough accomplishment is +indispensable. The mixing is most readily done when the materials are +saturated with water and in practice it is customary to depend upon +rain for the water, although in the final stages water may be hauled +and sprinkled on the road to facilitate final completion of the +mixing. After the mixing has been completed, the surface is smoothed +with the blade grader and is kept smooth until it dries out. Repeated +dragging will be required, during the first year especially, and to +some extent each year in order to keep the surface smooth, but the +dragging can be successfully accomplished only when the road is wet. + +[Illustration: Fig. 14.--Cross Sections for Sand-Clay Roads] + +In regions where several months of continued hot, dry weather is to be +expected each year, the sand-clay mixture is likely to break through +unless it is of considerable thickness and generally the surface layer +is made much thicker than for regions where the annual rainfall is +fairly well distributed. This is especially necessary when the binder +is of inferior quality. It is not uncommon in such cases to make the +sand-clay surface as much as two feet thick. + +As the mixing progresses it may appear that patches here and there are +deficient in either clay or sand and the mixture in these places is +corrected by the addition of a little sand or clay as may be +required. + +If the top-soil is used it is deposited on the sand in the required +quantity and is remixed in place to insure uniformity. If either sand +or clay is needed to give a satisfactory mixture, the proper material +is added and mixed in as the work progresses. The surface is finally +smoothed by means of the grader and drag. + +=Sand-Clay on Clay or Loam.=--If the existing road is of clay or loam, +ample drainage will be required as discussed in Chapter IV. The +surface may be constructed of a natural sand-clay mixture or of a sand +mixed with the natural soil. If the former, the surface of the +existing road is prepared by grading so as to insure good drainage and +the natural mixture is then deposited and the surface completed as +described in the preceding section. + +If the surface is formed by mixing sand with the existing soil, the +sands may be deposited in a layer about six inches thick which will +gradually mix with the soil as the road is used. A second application +of sand may follow in a year or two if it is needed. Such a road +surface will lack uniformity of composition and it seems preferable to +mix the sand with the soil by plowing and discing as previously +described. + +=Characteristics.=--Sand-clay road surfaces do not have sufficient +durability for heavily traveled highways, but will be satisfactory for +a moderate amount of traffic. These surfaces have maximum +serviceability when moist, not wet, and consequently are not as +durable in dry climates as in humid areas. They are likely to become +sticky and unstable in continued wet weather and to become friable and +wear into chuck holes in long continued dry weather. At their best, +they are dustless, somewhat resilient and of low tractive resistance. + +GRAVEL ROAD SURFACES + +[Illustration: Fig. 15.--Cross Sections for Gravel Highways] + +=Natural Gravel.=--Gravel is the name given to a material consisting +of a mixture of more or less rounded stones, sand and earthy material, +which is found in natural deposits. These deposits exist in almost +every part of North America, being especially numerous in the +glaciated areas, but by no means confined to them. Gravel deposits +consist of pieces of rock varying in size from those of a cubic yard +or more in volume to the finest stone dust, but with pieces ranging in +size from that which will pass a 3-inch ring down to fine sand +predominating. The larger pieces are usually more or less rounded and +the finer particles may be rounded or may be angular. Many varieties +of rocks are to be found among the gravel pebbles, but the rocks of +igneous origin and possessing a considerable degree of hardness +generally predominate. Intermixed with the pieces of rock there is +likely to be clay or other soil, the quantity varying greatly in +different deposits and even in various places in the same deposits. + +Often there are found deposits of material which are by the layman +termed gravel, which are really clayey sand or sand containing a few +pebbles, but which are of value to the road builder for the sand clay +type of surfacing. The term gravel is exceedingly general and unless +specifically defined, gives little indication of the exact nature of +to which it is applied. + + TABLE 7 + + SHOWING CEMENTING PROPERTIES OF SEVERAL SAMPLES OF + GRAVEL + + -----------------+---------------------------- + | Cementing Value + Per Cent Clay by +---------------+------------ + Weight | As Received | Washed + -----------------+---------------+------------ + 4.4 | 276 | 43 + 6.4 | 105 | 285 + 5.1 | 241 | 70 + 14.5 | 500 | 279 + 8.5 | 500 | 112 + 10.1 | 300 | 267 + 14.8 | 500 | 107 + 7.5 | 184 | 198 + 16.5 | 500 | 428 + 2.0 | 185 | 239 + 1.5 | 500 | 500 + 4.5 | 212 | 204 + 2.5 | 116 | 363 + -----------------+---------------+------------ + +The value of any gravel for road surfacing depends upon the degree to +which it possesses the properties of an ideal gravel for road +surfacing. Ideal gravel is seldom encountered, but a consideration of +its characteristics serves to establish a measure by which to estimate +the probable value of any deposit. + +=The Ideal Road Gravel.=--The ideal road gravel is a mixture of +pebbles, sand and earthy material, the pieces varying from coarse to +fine in such a manner that when the gravel is compacted into a road +surface the spaces between the larger pebbles are filled with the +finer material. The pebbles are of a variety of rock that is highly +resistant to wear so that the road surface made from the gravel will +have the quality of durability. The gravel possesses good cementing +properties, insuring that the pieces will hold together in the road +surface. The cementing property may be due to the rock powder in the +deposit or to earthy material mixed with the rock particles, or to +both. Table 7 shows the results of a number of tests made upon gravels +and indicates that the cementing property of the gravel does not +always depend upon the clay content. + +=Permissible Size of Pebbles.=--The larger pebbles in the gravel are +less likely to crush under loads than smaller pebbles of the same sort +of rock, but if the rock is of some of the tougher varieties such as +trap, there is very little likelihood of even the smaller pebbles +crushing. If the pebbles are of rock of medium toughness, the smaller +pebbles might be crushed under the heavier loads. It is the usual +practice to permit gravel to be used for the foundation course in +which the pebbles are as large as will pass a 3-1/2-inch circular +screen opening, and for the wearing course, as large as will pass a +2-1/2-inch circular screen opening. If larger pebbles are allowed in +the wearing course, the surface is certain to become rough after a +time. If the gravel is to be placed in a single course as is a very +common practice, then the maximum size should not exceed that which +will pass a 2-1/2-inch circular screen opening. + +The Wisconsin Highway Commission has constructed a very large mileage +of excellent gravel roads and the sizes specified for their roads are +as follows: + + "_Bottom Course Gravel_.--Bottom course shall consist of a + mixture of gravel, sand and clay with the proportions and various + sizes as follows: + + "All to pass a two-inch screen and to have at least sixty and not + more than seventy-five per cent retained on a quarter-inch + screen; at least twenty-five and not more than seventy-five per + cent of the total coarse aggregate to be retained on a one-inch + screen; at least sixty-five and not more than eighty-five per + cent of the total fine aggregate to be retained on a two + hundred-mesh sieve." + + "_Top Course Gravel_.--Top course shall consist of a mixture of + gravel, sand and clay with the proportions of the various sizes + as follows: + + "All to pass a one-inch screen and to have at least fifty and not + more than seventy-five per cent retained on a quarter-inch + screen; at least twenty-five and not more than seventy-five per + cent of the total coarse aggregate (material over one-fourth inch + in size) to be retained on a one-half-inch screen; at least + sixty-five and not more than eighty-five per cent of the total + fine aggregate (material under one-fourth inch in size) to be + retained on a two hundred-mesh sieve." + + "_Screened Gravel and Sand Mixtures_.--Where it is impossible to + obtain run of bank gravel containing the necessary binder in its + natural state, screened gravel shall be used and the necessary + sand and clay binder added as directed by the engineer. Gravel + and sand shall be delivered on the work separately. Clay binder + shall be obtained from approved pits and added as directed by the + engineer." + + "_Run of Bank Gravel_.--When run of bank gravel is permitted + either for one course or two course work, the size shall not + exceed that specified for bottom or top course. If necessary, the + contractor shall pass all the material through a two-inch screen + for the bottom course, and through a one-inch screen for the top + course. When the work consists of only one course, the material + shall be of the sizes as specified for the top course. The + necessary binder shall be contained in the material in its + natural state, excepting that a small percentage of clay binder + may be added as directed by the Engineer." + +=Wearing Properties.=--A certain amount of grinding action takes place +on the road surface under the direct action of wheels, especially +those with steel tires. Where rubber tired traffic predominates, this +action is much less severe than where steel tired vehicles +predominate, but the tendency exists on all roads. In addition, there +is distortion of the layer of gravel under heavy loads which causes +the pieces of stone in the surface to rub against each other and to +wear away slowly. + +The gravel road in the very best condition is slightly uneven but +there is comparatively little jar imparted to vehicles, and, +consequently, little impact on the surface. When somewhat worn, the +impact becomes a factor of some importance and the pounding of +vehicles has a very destructive action on the surface. Soft pebbles +will be reduced to dust in a comparatively short time. + +The degree to which any gravel resists the destructive action of +traffic depends upon the varieties of rock represented by the pebbles +in the gravel. If the pebbles are mostly from rocks of good wearing +properties, that quality will be imparted to the road surface. If +mostly from rocks of little durability, the same characteristic will +be imparted to the road surface. A very good general notion of the +probable durability of gravel can therefore be obtained by a careful +visual examination of the material and classification of the rock +varieties represented by the pebbles. + +=Utilizing Natural Gravels.=--Gravel road construction is advantageous +only when it can be accomplished at low first cost. This usually +presupposes a local supply of gravel that can be utilized, or at any +rate a supply that need not be shipped a long distance. In the nature +of things, such deposits are likely to be deficient in some of the +desirable characteristics, and may be deficient in most of them. By +various means, the defects in the materials can be partially corrected +while constructing the road. + +If the gravel deposit consists of layers of varying composition as +regards size and clay content, the material may be loosened from the +exposed face and allowed to fall to the bottom of the pit thereby +becoming mixed to a sufficient extent to produce a reasonably uniform +product. If deficient in clay, it often proves feasible to add a small +part of the clay over-burden, thereby insuring enough binder. +Sometimes adjoining deposits will consist one of relatively fine +material, the other of relatively coarse. These may be mixed on the +work by first placing the coarse material in a layer about 5 inches +thick and adding the finer material in a similar layer. The two will +mix very rapidly during the operations of spreading and shaping. + +When deposits contain pebbles larger than will pass a 3-1/2-inch ring, +these larger stones will prove to be undesirable if placed on the +road, as they are almost sure to work to the surface of the gravel +layer and become a source of annoyance to the users of the road. +Oversize stone can be removed while loading the gravel or while +spreading it, if care is exercised and not too large a proportion is +oversize. It is preferable however to remove the oversize by means of +screens at the pit. Usually on large jobs the oversize is crushed and +mixed with the supply so as to utilize what is really the best part of +the material. + +Gravels deficient in bonding material are often encountered in +deposits where there is insufficient overburden to give enough +additional binder or where the overburden is of a material unsuitable +for binder. Such materials may be utilized by adding binder in the +form of clay after the gravel has been placed on the road. + +Almost any gravel deposit can be utilized in some way if the material +is of a durable nature, regardless of other characteristics. The +serviceability of a gravel road will depend largely on how nearly the +gravel approaches the ideal, but variations in the manipulations will +do much to overcome deficiencies in materials. + +=Thickness of Layer.=--The thickness of the layer of gravel required +depends both upon the type of soil upon which it is placed and the +nature of the traffic to which the road will be subjected. Gravel +surfaces should not ordinarily be constructed on highways carrying +heavy truck traffic, but if gross loads of three or four tons are the +heaviest anticipated, the gravel will be reasonably stable. On such +roads, a layer of well compacted gravel ten inches thick will support +the loads if a well drained earth foundation is provided. If but +little truck traffic is anticipated and loads up to three tons on +steel tires are the average, a layer 8 inches thick will be +sufficient. In dry climates, a layer six inches thick will be +adequate if it can be kept from raveling. + +On secondary roads, carrying principally farm-to-market traffic, and +not a great volume of that, the above thicknesses may be reduced about +one-fourth. + +The exact thickness needed for any particular road is a matter for +special study on account of the variations in the gravels and in the +supporting power of the soil upon which they are placed. + + +PLACING GRAVEL + +=Preparation of the Road.=--The roadway that is to be surfaced with +gravel is first brought to the desired grade and cross section. It +would be advantageous if this could be done a year before the gravel +is placed so that no settlement of the earth foundation would occur +after the gravel surface is completed. But if that is impractical, the +grading may be done just prior to placing the gravel, providing +appropriate methods are adopted for securing compacted fills. + +=Trench Method.=--Two distinct methods of placing the gravel are in +general use, known as the trench method and the surface or feather +edge method respectively. The method to adopt for any particular road +will depend largely on certain conditions that will be explained +later. + +In the trench method, a trench of the proper width and depth for +receiving the gravel is excavated in the earth road surface and the +gravel is placed therein. + +The trench is formed by plowing a few furrows and scraping out the +loosened earth with a blade grader. The loose material is generally +moved out laterally to build up earth berms or "shoulders" alongside +the gravel. Into this trench the gravel is dumped in the proper +quantity to give the required thickness after being compacted. + +The greatest care must be exercised in spreading the gravel to +eliminate unevenness where the loads were deposited. An ordinary blade +grader is one of the best and most economical implements to use for +spreading the gravel. When the gravel has been deposited in the trench +for a distance of a thousand feet or more, the spreading is +accomplished by dragging the surface repeatedly with the blade grader, +the work being continued until all waviness disappears. The gravel is +then thoroughly and repeatedly harrowed with a heavy stiff tooth +harrow to mix thoroughly the fine and coarse gravel so as to produce +as nearly a uniform mixture as may be. The gravel is then finally +smoothed with the blade grader. + +The gravel may be compacted by rolling or may be allowed to pack from +the action of traffic. The former is greatly to be preferred where +practicable. The rolling is performed with a three-wheeled +self-propelled roller weighing about 8 tons and must be done while the +gravel is wet. Generally a sprinkling wagon is used to wet down the +gravel, but advantage is always taken of rains to facilitate the work. +The gravel must be spread in layers not over 5 or 6 inches thick to +get the desired results, which means that for an ordinary gravel road +about 10 inches thick, the gravel will be placed in two layers of +about equal thickness, each of which will be rolled. + +The gravel will compact slowly even if it is not rolled, but generally +does not become stable until the material is thoroughly soaked by +rains. Then it will begin to pack, but will become badly rutted and +uneven during the process. During this period the surface must be kept +smooth by means of the blade grader. The drag does not suffice for +this purpose, tending to accentuate the unevenness rather than to +correct it. + +If gravel is placed in a trench in dense soil and rainy weather +ensues, sufficient water will be held in the trench to cause +unevenness from foundation settlement and the gravel will become mixed +with the soil to some extent and be thereby wasted. Trenches cut from +the road bed upon which the gravel is placed, to the side ditches, +will relieve this condition by affording an outlet for the surplus +water. Nevertheless some difficulty may be expected if the trench +method is used and wet weather prevails. If it is possible to close +the road against traffic until the road is dry the method is +applicable. Moreover, in long-continued dry weather, the dispersion +and loss of considerable gravel from the action of automobile traffic +is avoided because the gravel is held between substantial earth berms +and the gravel will pack better and hold its shape longer when +constructed by the trench method than otherwise. + +=Surface Method.=--The surface method is one in which the gravel is +placed on the graded earth road surface without earth shoulders to +hold the gravel in place. It is also sometimes called the feather-edge +method. Except for the manner of placing as just mentioned, the +several operations are conducted in the same general manner as for the +trench method. The gravel does not compact as quickly as in the trench +method and a considerable loss of material is likely to result from +the effect of automobile traffic while the gravel is loose. But it has +the advantage of being free from difficulties in wet weather and in +some locations is therefore preferable to the trench method. It is +particularly applicable to those projects on which the placing of +gravel continues throughout the winter, the gravel being dumped and +spread, to be finally smoothed and finished in the early summer. + +=Bonding.=--Where gravels deficient in binder are utilized, clay for +binder is sometimes added as the gravel is placed on the road. This +may be done by spreading the clay on top of the lower course of +gravel, placing the upper layer and sprinkling and rolling until the +clay squeezes up through the surface layer. It may also be +accomplished by spreading dry clay on the upper course before it is +harrowed and then harrowing to mix it with the gravel. Both methods +are practiced, but the former is believed to be preferable. A third +method is to separate the sand and pebbles and to mix the clay binder +with the sand and then spread the sand on top of the pebbles and mix +by harrowing. + +=Maintenance.=--Gravel surfaces require careful maintenance, +especially during the first season the road is used. The gravel will +compact slowly and during the process will be rutted and otherwise +disturbed by traffic. It is important during this period to restore +the shape once a week or at least twice a month. The light blade +grader is usually employed for the purpose so long as the gravel is +somewhat loose. Later a drag of the type known as the planer will +prove to be the most effective. Figure 16 shows a type of drag that is +very satisfactory for use on gravel roads. + +[Illustration: Fig. 16.--Road Planer] + + + + +CHAPTER VII + +BROKEN STONE ROAD SURFACES + + +The broken stone road surface, or macadam road as it is usually +termed, consists of a layer of broken stone, bonded or cemented +together by means of stone dust and water. The surface may or may not +be coated with some bituminous material. + +=Design.=--It has been an accepted assumption that the macadam road +surface is somewhat more stable than the gravel road surface of equal +thickness, and since this is probably the consensus of opinion of +engineers familiar with both types, it may be accepted until +experimental data are available on the subject. + +The thickness of the layer of macadam required for a road will depend +upon the same factors that were considered in connection with the +thickness of the gravel surface, i.e., kind of stone used, character +of earth foundation and nature of the traffic. + +The standard macadam surface where good earth foundation is to be had +and where the loads do not exceed about four tons has for years been +eight inches thick. For heavier loads or inferior foundation, a +somewhat greater thickness would be employed, but the best practice +would probably provide a foundation course of the Telford type for +doubtful foundation conditions, especially for the extremely uncertain +cases. For soils of very good supporting strength such as very sandy +loam or deep sand or for arid regions where stable foundation is +always assured the thickness of the macadam might be reduced to six +inches. It should be borne in mind that the broken stone road is not +adapted to the traffic carried by trunk line highways in populous +districts, but is rather a type permissible on secondary roads and +usually adequate for local roads. It should never be employed for +roads carrying any considerable volume of passenger automobile traffic +or motor truck traffic. If surfaced with a bituminous material it will +carry up to 1200 passenger automobiles per day, but not to exceed +fifty trucks. + +=Properties of the Stone.=--The stone employed for the broken stone +road should possess the qualities of hardness and toughness and should +be capable of resisting abrasion sufficiently well to have reasonable +life under the traffic to which it is subjected. Since the traffic may +vary from very light on some roads to far beyond the limit of the +economical capacity of this type of pavement on others, it follows +that any particular deposit of stone might be durable enough for some +roads, while for others it might be entirely inadequate. As a general +rule it has been found that stone that wears away at a moderate rate +will, when used for water-bound macadam surface, result in a smoother +trackway than one that will wear very slowly. It is not therefore +altogether certain that the most durable stone to be had should be +selected for a particular road. This is especially true now that the +water-bound macadam surface has been largely superseded for trunk line +highways and other heavily traveled roads, and is employed in +locations where service conditions are not severe. + +The stone employed for the water-bound macadam surface must possess +good cementing properties, because the surface depends for stability +primarily upon the bonding action of the dust from the broken stone. +This is in contrast to the gravel road, where little dependence is +placed upon the bonding effect of the rock dust. In preparing the +stone for macadam surfaces, the ledge rock is crushed and screened, +and in that way a supply of the finer particles, which are a part of +the output of the crusher, is obtained for use in bonding the +surface. This finely broken material, usually called screenings, is +essential to the construction of the water-bound type of surface. +Rocks vary considerably in the cementing properties of the dust, but +usually the rocks classed as "trap," such as andesite, gabbro and +rhyolite, and schist and basalt possess good cementing properties. +Limestones usually possess good cementing properties, but some of the +dolomitic limestones are of low cementing value. Quartz, sandstone and +the granites are of low cementing value. + +=Kinds of Rocks Used for Macadam.=--Limestone and chert are the two +sedimentary rocks, employed most extensively for broken stone roads. +These rocks are found in widely distributed areas and vary in physical +characteristics from very soft material of no use to the road builder +to materials possessing considerable durability. It is desirable to +carefully test out the deposits of these materials before using to +ascertain the probable value of the rock, for the construction of the +road surface. + +Of the igneous rocks, those classed as trap are best known to the road +builder and many of the deposits of trap rock afford an excellent +material for broken stone roads where the severest conditions of +traffic are encountered. The trap rocks are tough and durable and +generally possess excellent cementing properties. + +Granite and sandstone are seldom used for water-bound macadam as they +possess poor cementing properties and a binder of some kind must be +added to cement the pieces together. For this purpose clay or the +screenings from some other variety of stone may be utilized. + +Some other materials are occasionally employed for the construction of +macadam surfaces. Of these, oyster or marine shells, burnt shale, and +slag are most common. + +Shells and slag are of rather low durability but possess good +cementing properties. Shale is a makeshift suitable only for very +light traffic roads. + +=Sizes of Stone.=--The stone for the wearing course of a macadam road +should be as large as practicable, because the larger the pieces the +more durable the surface. If the individual stones are too large it is +difficult to secure a smooth surface, and large stones will be readily +loosened by tipping as the wheels roll over them. These considerations +limit the size to a maximum of that which will pass a 2-1/2-inch +screen. Stone of excellent wearing qualities may be somewhat smaller, +but never less than that which will just pass a 1-1/2-inch screen. + +For the lower course, the size is not particularly important except +where the earth foundation is such as to require special construction. +It is not uncommon to use the same size of stone for both upper and +lower course and yet in many instances stone up to that which will +just pass a 3-1/2-inch screen is used for the lower course. Stone much +smaller in size may also be used successfully, but if the stone is +broken to a smaller size than is required, unnecessary expense is +incurred. + +The bonding material is the finer portion of the product of the +crusher, which is called screenings. This material may be so finely +crushed as to pass a one-fourth inch screen, or may be so coarse as to +just pass a one-half inch screen, but in any case must contain all of +the dust and fine material produced by the crusher. + +Where the soil and drainage conditions demand an especially stable +foundation course, the Telford type is used. The Telford foundation +consists of a layer of stones of various dimensions that can be laid +so as to give a thickness of 8 inches. These large stones are placed +by hand and therefore the size requirements are not rigid. Stones +having one dimension about 8 inches and the others not over 10 or 12 +inches are satisfactory. + +=Earth Work.=--A thoroughly drained and stable earth foundation is +essential to success with the macadam type of surface. Before placing +the stone, the road must be shaped to the proper cross section and +all grade reduction work completed. Preferably heavy fills should have +a year to settle before the macadam surface is placed. Side ditches, +necessary culverts and tile drains should be constructed as required +for drainage. The earth work is often carried out in connection with +the construction of the macadam surface, being completed just ahead of +the surfacing. In that case, the fills must be carefully rolled as +they are placed. The road bed may be shaped in connection with the +other earthwork. If the road has been brought to a satisfactory grade +some time prior to placing the macadam, the road bed for the broken +stone will be prepared as needed for placing the stone. + +=Foundation for the Macadam.=--Macadam surfaces are quite generally +placed in a trench as described in the trench method for placing +gravel. It is an almost universal practice to compact the layer of +stone by rolling with an 8- or 10-ton power roller, and if the stone +is not held between substantial earth berms or shoulders, the rolling +merely serves to spread the stone out over the road bed instead of +compacting it. If an attempt is made to roll broken stone which has +been placed on a yielding foundation, no benefit results, but on the +contrary the stone is likely to be forced down into the soil. To +insure that the layer of broken stone can be compacted by rolling, it +is first necessary to roll the earth foundation until it becomes hard +and unyielding. If soft or yielding places appear during the rolling +these should be corrected by tile drains or by removing the earth from +the spongy place and back-filling with material that will compact when +rolled. + +It is not always easy to determine why these soft places exist in what +appears to be a well drained roadway, especially since they are as +likely to be found on fills as anywhere else. Apparently they are due +to local pockets of porous soil held by denser soil so that the water +does not readily drain away. It is usually true that such places are +observed during the season of frequent precipitation more often than +during other seasons of the year. + +In dry climates, the difficulties of securing suitable foundations for +the broken stone road are largely eliminated, but it may be observed +that this type of surface is not suitable for such climates unless +some sort of bituminous binder is employed to hold the stones in +place. The cementing power of the stone dust is inadequate when the +surface is continually dry. + +[Illustration: Fig. 17.--Cross Section for Macadam] + +=Telford Foundation.=--When the Telford type of foundation is +employed, the earth subgrade is prepared and then the Telford stone +placed carefully by hand. The spaces between the large stones are +filled with the spalls broken from the larger stones in fitting them +in place. When completed the base is rolled with a heavy roller to +secure a firm unyielding layer. The thickness is generally about eight +inches. Any fairly sound stone may be used for the Telford base. + +=Placing the Broken Stone.=--It has been found impracticable properly +to roll a greater thickness than about 5 or 6 inches of loose stone, +therefore, the stone for the macadam surface is usually placed in two +layers, the first or lower layer being rolled before the next layer is +placed. The stone is hauled in dump wagons, trucks or dump cars, +dumped on the road bed and spread by hand rakes or by means of a blade +grader and is then rolled. To insure the proper thickness the loads +are accurately spaced to spread to the proper thickness. + +=Rolling.=--A three-wheeled or "macadam" type of roller, of the +self-propelled type, is best for compacting the broken stone road. The +weight varies from eight to fifteen tons, but for most conditions the +ten or twelve ton size seems to be preferable. On Telford base +construction, a heavier machine is desirable and for very hard stone +it may be successfully employed. + +The first trip with the roller is made along the edge of the stone and +each successive trip is made a little nearer the middle until finally +one half of the strip of stone has been rolled. The roller is then +taken to the opposite side of the roadway and the operation repeated +on the other half. The rolling is continued until the stone is +thoroughly compacted, which is evidenced by the fact that the roller +makes but a slight track in the surface. + +The second layer of stone is then placed and rolled in the same manner +as the first. + +=Spreading Screenings.=--After the upper course has been rolled, the +screenings are spread on it from piles alongside the road, enough +being used to fill the voids in the layer of stone and furnish a +slight excess. As the screenings are spread they are rolled to work +them into the voids. When these are filled, the surface is sprinkled +thoroughly by means of an ordinary street sprinkling cart and again +rolled. In this way the dust and water are mixed into a mortar which +fills the crevices between the stones. This mortar hardens in a few +days, giving a bond that is weak, but sufficient for the purpose if +the traffic is not too heavy. A broken stone road finished in this way +is called a water-bound macadam, and is ready for traffic in three or +four days after completion. + +=Bituminous Surfaces.=--On account of the inadequacy of the +water-bound macadam when subjected to motor traffic and to obviate the +tendency of broken stone surfaces to loosen in dry weather, there has +been developed a method of covering the surface with a bituminous +material such as tar or asphalt. This will be described in detail in a +later chapter. + +=Maintenance.=--Even under favorable conditions as regards kind and +amount of traffic the macadam road requires constant maintenance. The +first effect of traffic will be to brush away the fine materials used +for bonding the surface, thus exposing the larger stones in such a way +that they are rather easily loosened and removed from the surface by +wheels and the hoofs of animals. This finer material must be replaced +as fast as it is removed so as to protect the surface. Either stone +dust or clayey sand may be used, but clay if used alone is likely to +be sticky when wet and prove to be worse than the condition it was +expected to correct. In time, ruts and depressions will appear, either +as the gradual effect of wear, which will inevitably effect some +portions of the surface more than others, or on account of subsidence +of the foundation. Uneven places are repaired by first loosening the +stone, then restoring the cross section by adding new material and +tamping or rolling it in place. + +If a bituminous coating has been applied, it will eventually peel off +in places and these places must be recoated as soon as practicable. + +Eventually the surface will be worn to such an extent that an entirely +new wearing surface must be added. This is done by loosening the +entire surface to a depth of 3 or 4 inches and then adding a new layer +of broken stone. The loosening is sometimes accomplished by means of +heavy spikes inserted in the roller wheels, and at others by means of +a special tool known as a scarifier. + +The new surface is placed and rolled in precisely the same manner as +the wearing surface of the original construction, but the layer may +not be as thick as the original wearing course. A new course will not +bond to the old surface unless the old macadam has been thoroughly +broken up first. + +=Characteristics.=--The water-bound macadam is a dusty, somewhat rough +surface of low durability for rubber tired vehicles. It has long been +the standard rural highway for steel tired vehicles, but cannot carry +any considerable amount of motor traffic. It is easily repaired. When +finished with a bituminous surface its durability is greatly increased +and the dust is eliminated. It does not seem to be sufficiently rigid +for truck traffic, unless placed on exceptionally good foundation. + + + + +CHAPTER VIII + +CEMENT CONCRETE ROADS + + +The cement concrete road is one of the later developments in highway +construction, but the type has had sufficient use to show that it is +one of the satisfactory types for heavy mixed traffic, and, where the +proper materials are available, it is one of the economical types of +construction. + +=Destructive Agencies.=--It is well to have clearly in mind at the +outset that the concrete in a road surface is subjected to certain +destructive agencies not usually significant in connection with the +use of concrete, and these are so often disregarded that the average +serviceability of the concrete road surface is sometimes much lower +than it would be if built with due regard for the effect of traffic on +concrete surfaces. In most structural uses of concrete, its strength +in compression only is utilized, and the factor of safety is such as +to eliminate to some extent failures due to inferior materials or +workmanship. + +The concrete road surface is subjected to compression under wheel +loads, to bending, causing tension in the concrete, to abrasion from +wheels, and to tension and compression due to effect of temperature. +The weight of the wheel loads may cause sufficient distortion of the +road slab to produce rupture. The aggregates may be crushed under +wheel loads if the material is too soft. Abrasion from steel tired +vehicles wears away the concrete unless it is hard and durable. +Changes in dimension due to the effect of change in temperature +introduce tension or compression into the road slab and may result in +cracks. Freezing and thawing in the subgrade subjects the slab to +vertical movement and discontinuous support with the result that +longitudinal and transverse cracks occur. + +The foregoing indicates the importance of securing good concrete for +road surfacing, and that is accomplished by using suitable aggregates, +by proper design of the road surface and by following established +construction methods. + +=Design.=--The widths usually adopted for concrete roads are: for +single track roads, 9 or 10 feet, and for double track roads, 18 or 20 +feet. The thickness is 6 to 8 inches at the middle, varying with +climatic conditions and with the kind of soil upon which the concrete +is laid. The thickness at the edge is 1 inch less than at the middle +except that 6-inch surfaces are usually of uniform thickness, the +total crown being 2 inches. The thickness of the two course pavement +is the same as would be used for a single course pavement in the same +location. The surface of either width has a total crown of one or two +inches to insure water running off the surface. The earth foundation +is often flat, the crown being obtained by making the slab thicker at +the middle than at the edge. Fig. 18 shows cross section for concrete +roads. + +[Illustration: Fig. 18.--Cross Section for Concrete Highway] + +In the state of California, concrete roads four or five inches thick +and surfaced with a bituminous carpet mat have been successfully +constructed. Similar designs have been used in a few other places, but +for general practice it is unsafe to depend upon such a thin slab. +Climatic and soil conditions probably account for the success of the +thin roads in California. + +=Concrete Materials.=--The coarse aggregate for the concrete may be +broken stone or pebbles screened from natural gravel. Durability is +necessary, but it is also important to have uniformity in the concrete +so that the road surface will wear uniformly and consequently keep +smooth. Supplies of broken stone are likely to contain a small +percentage of soft pieces and such of these as are at the surface when +the concrete is finished will crush under traffic, leaving a pit in +the surface. Pebbles screened from gravel are also likely to be +variable in durability and should be carefully inspected if they are +to be used as aggregate for concrete roads. The harder limestones, +some sandstones, pebbles from many of the gravel deposits and +practically all of the igneous rocks make satisfactory aggregates for +the concrete road. + +Sometimes none of the coarse aggregates readily available are +sufficiently durable or uniform for the wearing surface of the +concrete road, but a suitable aggregate may be obtained at relatively +high price by shipping considerable distances. In such cases what is +known as the two course type of concrete road is employed. The wearing +course usually is about 2 inches thick and is constructed with +selected aggregates of good quality shipped in for the purpose. The +lower course is constructed of aggregates which do not possess the +desired qualities for a wearing course, but which are satisfactory for +concrete not subjected to abrasion. The aggregates for the wearing +course will be selected with the same regard for uniformity and +durability that would be the case if they were for the one course +pavement. + +Bank run gravel, or run of the crusher stone, is generally not +sufficiently uniform as regards proportion of fine and coarse material +to produce uniformity in the concrete, and the use of aggregates of +that character is not permissible for the wearing course, but under +proper inspection they may be used for the lower course of two course +pavements. + +=Fine Aggregate.=--The fine aggregate is generally natural sand, but a +mixture of natural sand and stone screenings is sometimes employed. +The fine aggregate of whatever character must be clean, free from +organic matter and sand, must contain no appreciable amount of mica, +feldspar, alkali, shale or similar deleterious substances and not +exceed two and one-half per cent of clay and silt. The sand is of such +a range of sizes that all will pass the one-fourth-inch sieve and that +not exceeding about five per cent will pass the 100-mesh sieve. + +=Proportions.=--Various mixtures for the concrete are employed because +these may properly vary to some extent with the exact character and +grading of the aggregates. Experience seems to have shown that the +concrete used for the wearing surface should have a crushing strength +of at least 2500 pounds per square inch, and the mixture adopted is +based on the requirements that will give the desired crushing +strength. The common mixture for the one course pavement is one part +cement, two parts sand and three and one-half parts coarse aggregate. +For the wearing course of the two-course type of pavement, a mixture +of the same kind is very often specified. + +While these are perhaps the most widely adopted proportions, many +others have been used, especially where the aggregates exhibit +peculiarities or the traffic conditions are unusual. It is desired to +emphasize that the purpose is to obtain concrete of the desired +strength and there can be no such thing as "standard" proportions. + +=Measuring Materials.=--In considering the methods employed for +measuring aggregates, emphasis should be placed on the futility of +rigid requirements for the aggregates, both as regards quality and +range of sizes, if the materials are carelessly proportioned at the +mixer. If even reasonably near uniform wearing qualities are to be +secured throughout the entire area of the concrete road surface, +successive batches of concrete must be alike, and to insure that, the +aggregates including the water in each batch of concrete must be mixed +in exactly the same proportions. The aggregates are measured in +various ways, all essentially alike in that the intent is to insure +exactly the same amount of each ingredient for each batch of concrete. + +One method is to place bottomless boxes in wheelbarrows, fill the +boxes level full and then lift off the box. Another is to use a +wheelbarrow with a bed of such shape that the contents will be a +multiple of 1 cubic foot when level full. For the larger jobs, the +aggregates are hauled in industrial cars, each having sufficient +capacity for a batch of concrete. The car body is provided with a +partition so as to separate the fine and coarse material. + +The water is measured in a tank which automatically refills to the +same level each time it is emptied and when adjusted for a mixture +will introduce the proper amount of water for each batch. It is highly +important to use the least amount of water that will produce workable +concrete. + +=Preparation of the Earth Foundation.=--The concrete road is generally +placed directly on the natural soil which has been brought to the +proper cross section. Some engineers advocate that in preparing the +subgrade, the earth be thoroughly rolled; others prefer not to roll +the subgrade. If fills of considerable depth are constructed, they +should either be rolled as built or else should be allowed to settle +for some months before the concrete road is placed, preferably the +latter. + +=Placing the Concrete.=--The concrete is placed between substantial +side forms of a height equal to the thickness of the concrete road +slab at the edge, and is shaped roughly by means of shovels. + +Various methods have been developed for striking the surface to the +exact shape desired and smoothing it. If hand finishing methods are +employed, a plank template is cut to the prescribed cross section and +the concrete is shaped by drawing the template along the side forms. +Sometimes the template is used as a tamper, being moved along very +slowly accompanied by an up and down motion that tends to tamp the +concrete. The template is then drawn along a second time to smooth the +surface finally. + +After the surface has been struck off by hand, it is finally smoothed, +first by rolling crosswise with a slight hand roller about 8 inches in +diameter and 30 inches long. The final finish is effected by dragging +a piece of web belting back and forth across the surface. + +Machines designed to tamp the concrete and strike it off to the +required cross section are also employed for finishing. The machine is +power operated and is carried on wheels that run on the side forms, +and the machine moves slowly along as the tamping progresses. The +concrete is tamped, struck off to shape and smoothed with the belt at +one operation. This method of finishing produces denser and stronger +concrete than can be produced by hand finishing methods. + +=Placing Concrete for Two-course Road.=--The methods employed for the +two-course concrete road are much the same as for the one-course road. +The concrete for the lower course is placed and struck off by means of +hand tools, and after that course has progressed a few feet, the upper +course is placed and finished as has been described for the one-course +road. + +=Curing the Concrete.=--The setting action of cement is a chemical +process, not merely a drying out of the water introduced in mixing the +concrete. The chemical action is progressive for a long time, but is +more rapid during the first few hours than during the later periods, +and the concrete reaches about three-fourths of its maximum strength +at the end of seven days. During the setting period and particularly +during the first few days, plenty of water must be available to the +cement. + +To prevent too rapid loss of water from the concrete during the +setting period, the surface must be protected from the wind and sun. +This is accomplished by first covering with canvas as soon as the +concrete has hardened sufficiently and by later covering with earth, +to a depth of two inches. The earth covering is kept wet for about ten +days and is left in place for about one month. + +In some places the ponding method of curing is adopted. The surface is +divided into sections by earthen dikes and the space inside the dikes +filled with water to a depth of two or three inches. The water +covering is maintained for two weeks or longer. + +No traffic is permitted on the surface for one month, and in cold +weather traffic may be kept off the surface for a longer period. + +=Expansion Joints.=--To permit the concrete slab to accommodate itself +to changes in dimension due to temperature changes, expansion joints +1/2 inch wide are placed about every thirty feet. These consist of a +sheet of some prepared bituminous material placed in position as the +concrete is poured. + +Experience seems to indicate that in spite of the expansion joints, +the concrete will crack more or less and many engineers think it +advisable to omit expansion joints in constructing the pavement and +when cracks develop to pour bituminous material into them, thus +forming expansion joints. + +The prevailing practice in rural highway construction is to omit the +expansion joints, but they are commonly adopted in city pavements. + +=Reinforcing.=--To minimize the cracking, either bar or wire mesh +reinforcing is used in the concrete. If bars are used they are placed +in the concrete as it is poured so as to form a belt around each +section about 15 feet square. If the mesh type is employed, a part of +the layer of concrete is placed and smoothed off and a strip of the +mesh laid in place. Additional concrete is then poured on top of the +mesh to bring the slab to the required thickness. + +=Bituminous Coatings on Concrete Surfaces.=--The concrete road surface +is sometimes coated with a layer of bituminous material and stone +chips or gravel pebbles. This is particularly advisable where no +really satisfactory aggregates are available and the concrete surface +would not possess sufficient durability. The bituminous material is +applied hot to the surface and is then covered with stone chips or +gravel pebbles, ranging in size from 3/4 inch down to 1/4 inch, the +resulting coating being about 3/4 inch thick. Many failures of this +type of surface have been recorded due to the difficulty of securing +adhesion to the concrete. This seems to be due in part to inability to +get the proper bituminous materials and in part to climatic effects. +Considerable progress has been made in developing this type of surface +and it may eventually become a satisfactory maintenance method. + +=Characteristics.=--The concrete road is of a granular texture and is +not slippery. It is of course rigid and noisy for steel tired +vehicles. It is an excellent automobile road and its low tractive +resistance makes it a desirable surface for horse drawn vehicles. It +possesses a high degree of durability if properly constructed. It is +likely to crack indiscriminately but as a general rule the cracks are +not a serious defect. + +=Maintenance.=--The cracks that appear in the concrete surface are +filled once or twice a year, tar or asphalt being employed. The dust +and detritus is cleaned out of the cracks and the hot filler poured +in, with enough excess overflowing to protect the edges. + + + + +CHAPTER IX + +VITRIFIED BRICK ROADS + + +Vitrified brick roads consist of a foundation course of Portland +cement concrete, broken stone or slag macadam, or of brick laid flat, +the first named being by far the most generally used, and a wearing +course of vitrified brick. + +=Vitrified Brick.=--Vitrified brick are made from clay of such a +character that when heated to the required temperature they will fuse +into a glassy texture. Brick roads are constructed on roads carrying +the severest of traffic and the brick must therefore be tough and of +high resistance to wear. + +Not all of the clays from which brick may be manufactured will produce +a product suitable for road construction, and paving brick, even +though truly vitrified, are of different degrees of durability, +depending upon the nature of the clay and the care exercised in the +manufacture. + +Paving brick are manufactured by the stiff mud process, which means +that the clay is molded into form in a relatively dry condition. To +accomplish this, considerable pressure is exerted in forcing the +column of clay through the dies, which form the prism from which the +brick are cut. If the clay is unsuitable in character or is not +properly ground and mixed, the brick will possess planes of weakness +between the various layers of clay which have been pressed together, +and these planes, called laminations, are a source of weakness if too +marked. It is usual to specify that the brick used for road surfaces +shall be free from marked laminations. + +If the brick is not properly burned it will be only partly vitrified +and therefore not of maximum durability. It is customary to specify +that the brick shall show a glassy fracture indicating complete +vitrification. + +Various defects of a minor nature occasionally develop in the brick +during the successive steps in the manufacturing process. Check cracks +resulting from the burning or from too rapid cooling are often +encountered, but unless these are deep, that is 3/16 inch or more, +they do not impair the wearing quality of the brick, nor indicate +structural weakness. Kiln marks are formed on some of the brick due to +the weight of the brick above in the kiln. These depressions are not +objectionable unless the brick are so distorted that they will not lie +evenly in the pavement. + +Spacing lugs or raised letters are formed on one face of the brick to +insure sufficient space between the brick for the filler. These lugs +or letters are not less than 1/8 inch nor more than 1/4 inch high and +of such design that they will not obstruct the free flow of filler +into the joints between the brick. + +Several varieties of paving brick are to be had, the difference being +principally in the design or size. + +=Repressed Brick.=--In this type of brick the spacing lugs are formed +by pressing the green brick, after it has been cut to size, into a +mold on one face of which are recessed letters or other devices into +which the clay is pressed, thus forming the spacing lugs. + +=Vertical Fiber Brick.=--These brick are designed to be laid with one +wire-cut face up and spacing is provided by two or more beads on the +side of the brick. Sometimes the vertical fiber brick has no spacing +lug, it being contended that the irregularities of the brick are such +as to provide all of the space required. In practice this does not +always work out, as the brick are so regular in shape that when laid +there is too little space between the brick to permit the introduction +of a suitable filler. The use of brick without spacing lugs is just +beginning and is not yet a generally accepted practice. + +=Wire-cut-lug Brick.=--This is a type of non-repressed brick which has +spacing lugs provided by cutting one face in a special manner which +provided lugs for spacing. In this type the wire cut face is the one +between the brick as they are laid in the pavement. + +=Tests for Quality.=--The standard test for quality of paving brick is +the rattler test. The brick rattler consists of a barrel of 14 sides +24 inches long, mounted so as to rotate at a speed between 29.5 and +30.5 revolutions per minute. The duration of a test is 1800 +revolutions. Ten brick constitute a charge and these are placed in the +rattler along with 300 lbs. of cast iron spheres. The spheres are of +two sizes, the smaller being 1-7/8 inch in diameter when new, and the +larger 3-3/4 inches in diameter when new. Ten of the larger spheres +are used and the balance of the charge is made up of the small size. + +When tested in the standard manner the loss allowable for the several +classes of service are as follows: + + ------------+---------------+---------------- + | | Maximum Loss + Traffic | Average Loss | for any Brick + ------------+---------------+---------------- + Heavy | 20 per cent | 24 per cent + Medium | 22 per cent | 26 per cent + Light | 25 per cent | 28 per cent + ------------+---------------+---------------- + +=Other Tests.=--Sometimes the absorption test is specified for paving +brick, but it is rarely a vitrified brick that will pass the rattler +tests which fails to pass a reasonable absorption test. Absorption of +water in an amount exceeding 4 per cent indicates incomplete +vitrification and failure of such brick is almost certain during the +rattler tests. + +The cross breaking test is also sometimes employed, but generally +only to check the general quality of the brick. Failure in service +more frequently occurs from excessive wear than from any other cause +and the cross breaking test has little significance, except for brick +less than 3 inches thick, which are to be laid on a sand bedding +course. + +=Foundation.=--The foundation for brick roads is usually of Portland +cement concrete, the thickness varying with the nature of the traffic +and the kind of soil upon which the pavement is built. For well +drained soils and normal highway traffic, 5 inches is the ordinary +thickness of foundation. Under favorable conditions such as locations +with sandy soils or in semi-arid or arid regions where the soil is +always stable, the foundation may be four inches thick, and a +considerable mileage of brick road has been built with concrete +foundations less than four inches thick. + +In other locations the soil and traffic conditions require a base six +inches or more in thickness, and the proper thickness can be +determined only after all of the factors involved are known and have +been analyzed. It is impractical to adopt a standard thickness of +foundation that will be equally economical for all locations and all +kinds of traffic. As the brick pavement is essentially a heavy traffic +type of surface, the design cannot be varied greatly with similar +foundation conditions because the weight of individual loads is the +significant factor and this does not vary so much as the volume of +traffic. A variation in volume of traffic may be compensated for by a +variation in the quality of the brick as already set forth. + +The mixtures for the concrete foundation vary widely because of the +variation in the aggregates employed. If the fine and coarse aggregate +for the concrete are of good quality a mixture of one part cement, two +and one-half parts sand and five parts of coarse aggregate would +insure concrete of adequate strength. A somewhat leaner mixture is +sometimes employed and would be satisfactory if the aggregates were of +exceptional concrete making quality. Mixtures of sand and pebbles +(unscreened gravel) may also be used if care is exercised to secure a +mixture of adequate strength. The proportion will of necessity vary +with each particular material and the discussion of the various +considerations involved may be obtained from various standard works on +concrete and concrete materials. + +Broken stone macadam is sometimes utilized for the foundation course +of the brick pavement and such foundations are constructed as +water-bound, which is described in a previous chapter. The thickness, +like that of the concrete foundation, varies with the soil conditions +and the weight of the loads that are expected to use the road. The +macadam is placed in a single layer and is rolled and bonded with +screenings as described in the chapter dealing with water-bound +macadam. Six inches is a common thickness for the macadam base. This +type of foundation should be employed only where the soil is quite +stable and where material costs are such as to insure that the macadam +base is materially cheaper than one of concrete. This would usually be +in locations where the cost of cement is high because of long hauls +and where suitable macadam materials may be obtained close at hand. + +Old macadam roads are sometimes utilized for the foundation for the +brick surface, but the instances where this is permissible are +comparatively few in number. When an old macadam is to be used it is +reshaped to the proper cross section and re-rolled and bonded so as to +afford a stable foundation of the proper cross slope. + + +BEDDING COURSE FOR BRICK SURFACES + +In order to equalize the variations in size and shape of the brick, +they are laid on a bedding course composed of material into which the +brick may be forced by rolling. In this way the upper surfaces of all +brick can be brought to the proper elevation to insure smoothness and +easy riding qualities. Several kinds of bedding course are now +employed. + +=Sand Bedding Course.=--The sand bedding course has been referred to +as a sand cushion, but as a matter of experience the cushion effect is +slight, although sometimes pavements have become uneven because the +brick have pushed down into the sand after the pavement was used for a +time. The sand for the bedding course should preferably be fine +grained, all particles passing the eight mesh sieve, but ordinary +concrete sand is satisfactory. The sand need not be clean, as a +comparatively large percentage of silt or clay does not impair the +usefulness of the material. + +[Illustration: Fig. 19.--Cross Sections for Brick Highways] + +=Sand Mortar Bedding Course.=--In order to eliminate the tendency for +the straight sand bedding course to shift because of the impact of +traffic on the brick, a lean cement mortar is sometimes employed +rather than the straight sand. Sand and cement in the ratio of one +part cement to four or five parts of sand are mixed dry, and after the +brick have been rolled, is moistened to furnish water to hydrate the +cement. The sand employed is ordinary clean concrete sand. + +=Green Concrete Bedding Course.=--In the monolithic type of brick +road construction, the brick are laid directly on the green concrete +base before the concrete has taken a set and the irregularities of the +brick are taken up by rolling them until bedded in concrete. + + +FILLERS FOR BRICK SURFACES + +The spaces between the brick are filled with some material that will +prevent the brick from being displaced and prevent water getting to +the bedding course. A suitable filler must adhere to the brick and +fill completely the spaces between them. It must withstand traffic so +as to remain intact in the joints and when in place it must be rigid +enough to prevent displacement of the brick. + +=Cement Grout Filler.=--One of the most commonly used fillers for +brick pavements consists of a grout composed of Portland cement and +fine sand. When properly mixed and applied the grout filler meets all +requirements for a filler except that it is non-elastic and some means +must be adopted for caring for pavement expansion. + +=Bituminous Fillers.=--Asphaltic materials and tars are widely used as +fillers for brick pavements. Such fillers are of high melting point +and consequently solid at ordinary temperature. They are poured into +the joints hot and when they cool are firm enough to comply with the +requirements for a filler. In addition, they have enough ductility to +accommodate the expansion of the pavement due to temperature changes. + +=Mastic Fillers.=--Mastic consists of a mixture of about equal volumes +of fine sand and a solid bituminous material. The mixture is prepared +at high temperature and is worked into the joints between the brick +while hot. When cool it resembles the straight bituminous filler +except that the mastic is somewhat more resistant to wear than the +straight bituminous filler. + +EXPANSION JOINTS + +It is recognized that brick will expand and contract with changes in +temperature. When a bituminous or mastic filler is employed there is +sufficient yield to the filler to accommodate the change in dimension +in the brick, but when the grout filler is used either the expansion +joint must be provided or the pavement must be designed to withstand +the compression due to expansion of the brick. Expansion joints may +consist of a sheet of bituminous mastic prepared for the purpose and +set in place in the pavement. The sheet of joint material is simply +inserted between courses of brick at the proper place. + +Another method of forming an expansion joint consists in placing a +strip of wood between courses of brick at the place where a joint is +required. After the pavement has been grouted, the wooden strip is +pulled out and the joint is filled with a suitable bituminous filler. + +=Marginal Curb.=--If the sand bedding course is employed, it is +necessary to provide curbing along the sides of the brick to hold the +bedding course in place. The curb is usually constructed integral with +the base and of concrete of the same mixture as the base. The width of +the curb is usually six inches and the top of the curb is at the same +elevation as the edge of brick surface. + + +CONSTRUCTION OF THE SURFACE + +Before the construction of a brick surface should be undertaken on a +road, the drainage should be provided for even more completely than +for a less costly type of surface since it does not pay to jeopardize +the stability of the pavement by failure to provide adequately for the +stability of the supporting soil. Grades should also be reduced to the +economical limit. + +The earth subgrade is brought to the proper elevation and cross +section and is thoroughly rolled. If there are places where the soil +will not compact properly under rolling, these places are corrected by +taking out the material and back filling with new material that will +properly compact under the roller. + +The aggregates for the concrete may be distributed along on the +prepared subgrade or may be stored in stock piles or bins at +convenient points. If stored on the subgrade, a traction mixer is +employed which is drawn along the road as the work progresses, the +materials being placed directly in the mixer. If stored at a central +point, they may be transported to the mixer on the road and dumped +directly into the mixer, or the mixer may be set up at the storage +piles and the concrete hauled in trucks to the road where it is +deposited and shaped. + +The concrete is spread to the proper thickness and tamped either by +hand or by machinery. If the marginal curb is to be employed, it is +constructed immediately after the concrete for the base has been +finished but before the cement begins to set. + +After the foundation concrete has set, the bedding course is spread +and struck off to the proper thickness. When the bedding course +consists of sand-cement mortar, the sand and cement are mixed dry and +spread to prescribed thickness. It is considered to be desirable to +roll the sand bedding course with a light hand roller before the brick +are placed, but the sand-cement bedding course is not rolled. The +bedding course must be carefully shaped by means of a templet or +strike board before the brick are placed. + +The brick are laid in straight courses across the pavement, with the +spacing lugs all in the same direction if brick with spacing lugs are +employed, and with the lugs in contact with the brick of adjoining +courses. If brick without spacing lugs are used they are laid loosely +so that there will be room for the filler between the brick of +adjoining courses. + +After the brick have been laid they are rolled to bed them in the sand +or sand-mortar bedding course and thus secure a smooth surface. For +this purpose a light, power driven, tandem roller is used and the +rolling is continued until the brick are thoroughly bedded. Any +defective brick that are noted are removed and replaced with good +brick and after this culling has been completed the surface is once +more thoroughly rolled. If a cement-sand bedding course is employed, +the surface is sprinkled just after the final rolling so that water +will flow down between the brick and moisten the bedding course +sufficiently to cause the cement to set. In some cases, the +sand-cement bedding course is sprinkled just before the brick are laid +but in warm weather the setting would take place before the brick +could be rolled if that were done. In cool weather the setting is +sufficiently slow to permit rolling before the bedding course hardens. + +The filler is applied to the surface after the rolling. If the +bituminous type of filler is employed, the hot filler is poured onto +the surface and worked into the joints by means of squeegees, with +comparatively little material left on the surface. In some instances +cone-shaped pouring pots are employed and the material is poured +directly into the joints. + +The cement grout filler is applied in the same general manner as the +bituminous filler. The grout, consisting of equal parts of sand and +cement, is mixed to a thin consistency and poured onto the surface and +is then worked into the joints with squeegees. Two or more +applications are usually required to effect a complete filling of the +joints. The surface should be covered with sand and be kept moist +until the cement grout has set. + + + + +CHAPTER X + +BITUMINOUS ROAD MATERIALS AND THEIR USE + + +Tars and asphaltic materials of various kinds are widely used for road +construction and maintenance, especially for road surfaces subjected +to motor traffic. Materials of this character that are employed in +highway work possess varying degrees of adhesiveness, and while they +may be semi-solid or viscous liquids at air temperature, they melt on +the application of heat and can be made sufficiently fluid to mix with +the mineral aggregates that may be used in the road surface. Upon +cooling, the bituminous materials return to the previous state and +impart a certain amount of plasticity to the mixture, at the same time +serving as a binding or cementing agent, which is sufficiently stable +for many classes of road construction. + +=Classes of Bituminous Materials.=--Bituminous materials may be +classified, according to the source from which they are obtained, as +coal tars, water gas tars, native or natural asphalts and oil or +petroleum asphalts. + +=Coal Tar.=--Coal tar is obtained as a by-product in the manufacture +of illuminating gas from coal. It is also obtained in the manufacture +of coke from coal. The tar thus obtained is manufactured into products +that are used for dust layers on gravel or macadam roads, binders for +macadam and gravel surfaces, fillers for brick, wood block and stone +block pavements and for expansion joints. These various materials +differ mainly in their consistency at air temperature. (They may +differ widely in chemical composition, but that need not be considered +herein.) + +=Water Gas Tar.=--Water gas tar is obtained as a by-product in the +manufacture of illuminating gas from crude petroleum. It is used for +the same kinds of construction as coal tar, and the products utilized +for the several purposes, like the coal tars, differ mainly in +consistency. + +=Natural Asphalt.=--Natural asphalt is found in deposits at many +places in the world, existing in beds or pools where it has exuded +from the earth or as veins in cavities in the rocks. It is of varying +composition and consistency, but those kinds in most general use are +solid or very viscous liquids at air temperature. Of the deposits that +have been developed on a commercial scale, the Trinidad lake in the +British West Indies and Bermudez deposit in Venezuela are best known. +Both of these materials are too hard in the natural state to be used +for road construction, and are softened, or fluxed as it is called, +with fluid petroleum oil before being used. + +=Petroleum Asphalt.=--Petroleum asphalt is a residue remaining after +the fluid products have been distilled from petroleum. Residues of +this sort are not always suitable for road construction, but a number +of brands of road material are obtained from this source. Oil asphalts +are used for dust layers, for binders for macadam roads, for asphalt +cements for sheet pavement surfaces, and for fillers for block +pavements and expansion joints. + +=Mixtures.=--Water gas tars and asphalts are sometimes mixed to +produce road materials, and likewise native asphalts and residues +obtained from petroleum are sometimes mixed to produce asphalt cements +for paving mixtures. + +=Classification according to Consistency.=--The various bituminous +materials may be classified according to consistency in discussing the +various uses to which they may be put. + +=Road Oils.=--Road oils are fluid petroleum oils of such consistency +that they may be applied cold or by heating slightly. They are used +as dust layers on earth, gravel and macadam surfaces. Their efficacy +depends upon the binding properties of the small amount of asphaltic +material that is contained in the oil. + +=Liquid Asphalts.=--These are somewhat less fluid than the road oils, +and must always be heated before application, but are viscous liquids +at ordinary temperature. These materials are obtained from crude +petroleum or semi-solid native bitumens, in which case they are +usually called malthas. Both coal tars and water gas tars of +semi-solid consistency are also employed for the same class of +construction as the liquid asphalts. + +These materials are used for carpeting mediums on macadam roads and as +cementing agents in the construction of hot-mixed macadam. + +=Asphalt Cements.=--The solid asphaltic materials used for hot-mixed +types of construction are called asphalt cements. They may be +petroleum residues or native asphalts fluxed with petroleum oils. They +are solids at ordinary temperature and must be heated to a temperature +in excess of two hundred and fifty degrees before they are +sufficiently fluid to use. Asphalt cements are used for sheet asphalt +and asphaltic concrete construction and for hot-mixed bituminous +macadam. + +=Fillers.=--Fillers are solid asphalts or tars that are used for +filling expansion joints in rigid pavements and for filling the spaces +between the blocks in brick, wood block and stone block pavements. + +=Bitumen.=--Bituminous materials are all soluble to a greater or +lesser extent in carbon disulphide and the soluble portion is called +bitumen. It is the bitumen that gives to the materials the cementing +properties utilized in road construction. Mixtures of mineral +aggregates and bituminous materials for various purposes are +proportioned with bitumen as a basis. Therefore, less of an asphalt +containing one hundred per cent bitumen will be used than of one +containing less than one hundred per cent of bitumen. + + TABLE 8 + + PROPERTIES OF ASPHALTIC ROAD MATERIALS + + (A) Material + (B) Specific Gravity + (C) Consistency + (D) Solubility in CS_2, Per Cent + (E) Solubility of Bitumen in CCl_4, Per Cent + (F) Solubility of Bitumen in 86 deg. Naphtha, Per Cent + (G) Fixed Carbon, Per Cent + (H) Flash Point + (I) Ductility + -------------------------+---------+----------+----------+------------ + (A) | (B) | (C) | (D) | (E) + -------------------------+---------+----------+----------+------------ + Mexican oil asphalts |1.03-1.05|As desired| 99.5-99.9|99.5-99.9 + California oil asphalts |1.02-1.04|As desired| 99.9 | 99.9 + Texas oil asphalts |1.01-1.03|As desired| 99.9 | 99.9 + Bermudez natural asphalt|1.07 | 25 | 95 | 99+ + Trinidad natural asphalt |1.40 | 7 | 56-57 | 100 + Bermudez asphalt cement |1.04-1.06|Up to 135 | 95-97 |99.5 or more + -------------------------+---------+----------+----------+------------ + + -------------------------+---------+----------+----------+------------ + (A) | (F) | (G) | (H) | (I) + -------------------------+---------+----------+----------+------------ + Mexican oil asphalts | 70-80 | 13-16 | 200 deg.C. up| 60-100 + California oil asphalts | 75-80 | 10-12 | 200 deg.C. up| 100+ + Texas oil asphalts | 75-80 | 12-14 | 200 deg.C. up| 50-100 + Bermudez natural asphalt| 68-70 | 13-14 | ... | ... + Trinidad natural asphalt | 64-65 | 10-11 | ... | ... + Bermudez asphalt cement | 77-80 | 11-12 | 175-200 | 25-50 + -------------------------+---------+----------+----------+------------ + +=Specifications.=--Some properties of bituminous materials can be +varied in the process of manufacture, while others are inherent in the +material and cannot be changed in the process of manufacture. +Specifications must therefore be drawn with care to insure that the +requirements can be met by satisfactory materials. But certain +properties, such as specific gravity, may vary greatly among materials +equally satisfactory for construction purposes. One should not be +misled by apparent differences in the characteristics of materials, +because these may simply be natural peculiarities which have no +bearing on the usefulness of the material. There are given in Table 8 +the properties of some of the commonly used bituminous materials and +the properties that can be varied in the process of manufacture are +indicated with an asterisk. A variation in these properties will +usually result in some change of other properties, but generally not a +great change. + + +SURFACES IN WHICH BITUMINOUS MATERIALS ARE UTILIZED + +I. Surface Treatments + +Attention has been directed to the rapid deterioration of water-bound +macadam when subjected to passenger automobile traffic. + +In water-bound macadam the stones are held in place by a weak cement +composed of stone dust and water, and this cement is not sufficiently +strong to hold the stones in place when they are subjected to the +shear of automobile tires. In finishing the water-bound macadam +surface, the spaces between the stones are filled with screening and +in addition a layer about one-fourth inch thick is left on the +surface. + +The automobile traffic first brushes aside all of the screenings and +smaller particles of rock, exposing the larger stones. These gradually +loosen as the road is used and are brushed aside. When this effect +begins, the road is said to be raveling. Various lengths of time may +elapse from the time the road is first finished until raveling begins, +depending upon the character of the stone, the weather and the amount +of motor traffic. During the period before raveling starts, it is +comparatively easy to restore the road surface at any time by the +addition of screenings or clay and sand. Usually there will be a few +small areas of the surface that, on account of faulty construction, +will ravel or become rutted much earlier than the remainder of the +surface. These can be repaired by the methods described in the chapter +on "Water-bound Macadam Construction." When the surface begins to +ravel seriously, maintenance becomes much more difficult and in order +to prevent raveling and the difficulties of maintenance thereafter, +the macadam surface is often coated with a bituminous material. + +[Illustration: Fig. 20.--Oiling a Gravel Road] + +If there is any dust or screenings on the road surface, the bituminous +material will not adhere to the stones and will soon flake off under +traffic. The surface of the macadam must therefore be thoroughly +cleaned before the bituminous material is applied. The usual practice +is to finish the road as water-bound macadam, and permit traffic on it +for a sufficient length of time to show any weak places in the surface +and at the same time thoroughly to season the surface. If any +defective places appear, they are repaired and when the surface +exhibits satisfactory stability, but before it begins to ravel, the +bituminous surface is applied. There will ordinarily be some stone +dust and some screenings remaining on the surface at the time +bituminous treatment is undertaken, and there may also be some caked +mud or other foreign material. All of this must be removed so as to +expose the stones throughout. + +=Applying the Bituminous Binder.=--The bituminous binder may be +delivered in tank cars, which is desirable if the work is near a +railroad siding, or ample tank wagon service is available for long +hauls so that the tank will not be held up too long. Often it is +desirable to purchase the binder in barrels and haul these to the site +of the work in advance of beginning the construction of the surface. + +The bituminous material may be applied by means of hand spreading cans +not unlike an ordinary garden watering pot, except that a slotted +nozzle is substituted for the ordinary perforated one. If hand methods +are employed for spreading, the bituminous material is heated in open +kettles and then spread on the surface, the quantity required usually +being about one-half gallon per square yard of surface. The +temperature of the binder should be great enough to insure fluidity +and the road should be dry at the time of the application. As soon as +the material has been spread, the surface is finished with a dressing +of chips. + +=Finishing the Surface.=--For surface dressing the best material is +stone chips ranging in size from about 1 inch down to one-fourth inch. +But the chips must be of durable material, or they will quickly grind +into dust. They must be free from dust when applied, as the presence +of any considerable amount of dust interferes with the proper +finishing of the surface. The stone chips are rolled into the surface, +a sufficient quantity being used to just cover the surface. + +=Patching.=--It almost always happens that some small areas will not +be properly cleaned or that for some unknown reason the coating peels +off the surface. Such places must be promptly patched to prevent them +enlarging under the action of traffic. This work is usually done by +patrolmen, who inspect the road at frequent intervals and make the +necessary repairs. The patrolman is equipped with a small heating +kettle, a spreading can and the necessary brushes, tampers and +miscellaneous tools needed for the repair work. The place to be +patched is carefully cleaned, coated with bituminous binder and stone +chips and tamped until dense and solid. Repairs made in this way are +exceedingly important in that they arrest deterioration in its early +stages and maintain a high degree of serviceability. + + +II. Penetration Macadam + +A considerable mileage of macadam has been constructed in which an +attempt was made to eliminate the difficulties of maintenance by a +method of construction that involves applying a bituminous binder in +such a manner as to permit it to penetrate two inches or more into the +surface. It is expected that the binder will coat the stones to such +an extent as to increase materially the stability of the bituminous +macadam over the surface treated one. It is also expected that less +difficulty will be encountered in maintaining a surface of bituminous +material and stone chips on this type of road than on the water-bound +macadam. The extent to which these expectations have been realized has +varied to a marked degree and although some excellent surfaces have +been constructed by this method, the results have as a rule been +neither uniform nor entirely satisfactory. It seems to be apparent +that good results cannot be obtained unless the materials are entirely +suitable and the construction is carried out with unusual skill. + +=Foundation.=--The foundation or lower course consists of a layer of +broken stone six inches thick placed on a well drained and thoroughly +rolled earth subgrade. In exceptional cases, the Telford type of +foundation might be employed. + +The lower course of broken stone is finished in the same manner as +water-bound macadam, being bonded with stone screenings or with fine +gravel of high clay content. + +Since this course is in reality the foundation of the surface, it is +necessary to secure stability by appropriate construction methods, +exactly as in constructing water-bound macadam. + +[Illustration: Fig. 21.--Type of Roller used on Gravel and Macadam +Roads] + +=Upper or Wearing Course.=--The wearing course consists of a layer of +stone about two and one-half inches thick. The stone is placed and +rolled and the spaces between the stones partially filled with some +suitable bituminous material. The bituminous material is usually +applied by means of a mechanical spreading device connected to a tank +wagon. The bituminous materials employed for this class of +construction are semi-solid in character and must be heated to give +them sufficient fluidity for application. They may be heated in the +tank wagon which is used for the application or they may be heated in +separate tanks and transferred to the distributing wagon for +spreading. Some kind of a nozzle or group of nozzles is employed for +spreading the material so that it can be delivered in the form of a +spray or at least in a thin fan-shaped stream and can be distributed +in a fairly uniform layer over the stone. The binder will cool rather +rapidly after it is applied, but meanwhile will flow into the openings +between the stones and will form over the surface stones a coating of +slight thickness. + +The surface of the macadam is next covered with a layer of chips of +tough rock, similar to the material used for the final dressing in +surface treatments. These are carefully brushed into the openings +between the larger stones by means of heavy brush brooms. This is an +exceedingly important part of the work and often a much neglected part +of the construction. + +The surface is then covered with a second application of bituminous +material, somewhat less in quantity than required for the first +treatment and the surface again covered with stone chips and brushed. + +The surface is then thoroughly rolled and is ready for traffic. + +=Patching.=--As in the case of surface treatments, there are likely to +be places that, on account of defects in the construction, will fail +soon after the road is placed under traffic. These will quickly +enlarge unless they are repaired promptly. The repairs are made by +loosening the stone in the area affected and adding new stone as +needed and then pouring on the necessary amount of bituminous material +to coat the stones. Allowance must be made for the compression of the +material by tamping so that a depression does not result. The stones +are carefully tamped to place and covered with chips which are also +tamped. + +=Characteristics.=--The penetration macadam is a surface well adapted +to motor traffic if the individual vehicles are not too heavy. It is +likely to squeeze out of shape under motor truck traffic, becoming +seriously uneven and uncomfortable for traffic. Its durability is +materially affected by the construction methods followed. + + +III. Hot Mixed Macadam + +The wearing course of the mixed macadam is composed of graded broken +stone or gravel and a bituminous binder. Usually the bituminous +material only is heated prior to the mixing, but sometimes the stone +is also heated. + +=Foundation.=--The lower course, which serves as the foundation, is +either broken stone macadam, gravel or concrete. + +Where a foundation of broken stone is used, it is constructed of the +materials and in the manner described for the foundation of the +penetration macadam. Quite often a badly worn macadam or gravel road +is used for the foundation and a new wearing course provided by adding +a mixed macadam surface. If such is the case, the old surface is +worked over so as to restore the shape sufficiently and to insure that +it is everywhere of sufficient thickness. + +Portland cement concrete is sometimes used as a foundation for the +mixed macadam, but not often. Usually if the traffic is of a character +requiring a concrete foundation, it is desirable to use a better +wearing course than the mixed macadam, and the asphaltic concrete or +sheet asphalt type of surface is employed. It is necessary to finish +the surface of the concrete base with some device that will leave the +surface rough to prevent the macadam from creeping. A knobbed tamper +which leaves numerous irregular depressions about 2 inches in diameter +and three-fourths inch deep is often employed. + +=Sizes of Stone.=--For the wearing surface, stone ranging in size from +2 inches down to one-fourth inch is usually employed. If the stone is +of good quality the maximum size may be but 1-1/2 inches, but soft or +even medium stone of that size are likely to crush under traffic. The +stone for the base course should preferably be from 3 inches down, but +any available size will be satisfactory if the layer is well rolled +and bonded. The base course is constructed in the same manner as +water-bound macadam and any material satisfactory for the base course +of macadam will serve for the base course of mixed macadam. Screenings +having good bonding properties will also be required for the base +course. + +=Mixing and Wearing Surface.=--Several methods are employed in mixing +the wearing surface. The simplest is to mix by hand with shovels. The +aggregates are heated in improvised heaters which may consist of +nothing more than a metal pipe two or three feet in diameter, around +which the stone is piled. The mixing platform is usually a metal plate +sometimes arranged so that it can be heated by means of a fire +underneath. The bituminous material is heated in kettles. For some +mixtures, the stone is not heated, but the bituminous material is +always heated. The batch of stone is placed on the mixing platform, +the bituminous material added and the materials mixed by hand. + +Machine mixing is practiced much more extensively than hand mixing, +being both more rapid and cheaper. The mixer is similar to a concrete +mixer except that the drum is arranged so that it can be heated. The +hot stone and the bituminous binder are put into the drum and mixed +for the requisite length of time. Sometimes the stone is mixed cold, +the bituminous material only being heated. + +=Placing the Wearing Surface.=--The hot mixture is carted to the road +and spread to such thickness that after rolling the wearing surface +will be not less than two inches thick. The hot mixture is dumped and +then spread by means of shovels to the approximate thickness and the +spreading completed by means of rakes. The surface is then rolled +either with a tandem or a three-wheeled roller until thoroughly +compressed. + +=Seal Coat.=--After the rolling has been completed, the surface is +covered with hot bituminous cement and dressed with pea gravel or +stone chips and again rolled. Traffic may be permitted in twenty-four +hours. + +=Characteristics.=--The mixed macadam is a somewhat resilient surface +of excellent riding qualities and considerable durability for medium +traffic. It is likely to creep and become uneven when subjected to +heavy loads. The seal coat will wear off in two or three years and +will require replacing. + + +IV. Asphaltic Concrete + +Asphaltic concrete is a name given to a road surface mixture which is +composed of graded stone, graded sand and asphalt cement. This type is +designated as asphaltic concrete because of the analogy of the mixture +to Portland cement concrete. + +Asphaltic concrete is of two general types known as bitulithic, or +Warrenite, and Topeka asphaltic concrete, respectively, the +differences being in the nature of the mixture. + +=Bitulithic or Warrenite.=--The stone employed for these types is +graded down from a size about equal to one-half of the thickness of +the wearing course, and stone passing a 1-1/4 or 1-1/2-inch screen is +usually specified. From the maximum size the stone is graded down to +the finest particles produced by the crusher. The range of sizes of +stone will vary with the source of the supply, and in order to secure +the desired density in the mixture, varying amounts of graded sand and +mineral dust, such as ground limestone or Portland cement, are added +to the broken stone. Usually the resulting mixture contains less than +fifteen per cent of voids, and to this carefully graded mineral +aggregate there is added enough asphalt cement to bind together the +particles. + +=Topeka Asphaltic Concrete.=--In this type of asphaltic concrete, the +mineral aggregate consists of a mixture of carefully graded sand and +of broken stone of such size that all will pass a one-half-inch screen +and graded down to the fine dust produced by the crusher. To this +mixture is added about nine per cent of Portland cement or limestone +dust. The voids in the mixture are usually about twenty-five per +cent. + +It will be seen that the essential differences between the Bitulithic +and Topeka types are these: the Topeka type contains a larger +percentage of voids and stone of a smaller maximum size than the +Bitulithic. Both types have been extensively employed for city paving, +but the Bitulithic and Warrenite types have also been used to some +extent for rural highways. The Topeka type has been used but little +for rural highways. + +=Foundation.=--The foundation for the asphaltic concrete may be an old +macadam road, a base course constructed of broken stone or Portland +cement concrete, the latter being used much more extensively than +either of the other types. + +Sometimes asphaltic concrete is used for resurfacing water-bound +macadam or gravel roads when the traffic has increased to the point +where the cost of maintenance of the water-bound macadam has become +excessive. The existing surface is repaired and the cross section is +restored, or possibly flattened somewhat. + +=Placing the Surface.=--The stone, sand and asphalt cement are heated +to the required temperature and combined in the proper proportions and +are then thoroughly mixed by a mechanical mixer. The mixture is hauled +directly to the road and is dumped and spread by means of rakes. It is +then rolled thoroughly while still hot, a three-wheeled roller being +most satisfactory. After rolling, a seal coat of hot asphalt cement is +spread over the surface and covered with hot stone chips about 1/4 +inch in size. The surface can be opened to traffic immediately after +the surface has been completed. + +=Characteristics.=--The asphaltic concrete surface is of excellent +riding properties, is easily repaired and of moderate durability. It +is a particularly desirable surface for pleasure automobile riding and +for horse drawn traffic. + + + + +CHAPTER XI + +MAINTENANCE OF HIGHWAYS + + +Proper maintenance of highways is equally important with proper +construction. With nearly all types of road construction, the need for +maintenance arises soon after the surface is placed under traffic and +is continuous thereafter. The nature and amount of maintenance work +varies greatly among the several types of surface and the organization +suitable for a system of highways will depend to a considerable extent +upon the kinds of surfaces that are to be maintained. + +The upkeep of a road may be conveniently considered as of two kinds, +viz., (1) that which has to do with the wearing surface and earth +shoulders or berms upon which there is some traffic and (2) that which +has to do with the side ditches and drainage structures and keeping +the roadside in presentable condition. Both kinds of work are usually +carried out by the same organization, but whereas the nature of the +work indicated under (1) will vary with the type of wearing surface +and with all variations in traffic, that which is indicated under (2) +will be nearly constant in any locality. + + +ORGANIZATION FOR MAINTENANCE + +Maintenance of highways is preferably under the administration of the +same authority as construction and when an improvement is undertaken +under the jurisdiction of a State Highway Department, the completed +improvement is ordinarily maintained under the state authority. If +the improvement is made by county authorities, the maintenance is also +carried out under county authority. + +The nature of the organization of maintenance forces is dependent upon +the kind of roads to be cared for and must of necessity be varied in +any instance as conditions demand. In general, either maintenance +gangs or patrolmen are employed and often both are used on the same +road system. + +=Patrol Maintenance.=--Where this system is in operation, the highway +system is divided into patrol districts of from six to eighteen miles +of highway and a single patrolman is placed in charge of each +district. He is provided with all of the necessary tools and materials +required in his district and performs all of the work required in the +ordinary upkeep of the highway. He should work under the direction of +the county engineer or the district engineer for the state highway +department, because his work involves the use of materials and +processes requiring technical supervision. + +=Gang Maintenance.=--The maintenance gang may be employed for some +types of road surface in lieu of the patrolman or with other types of +surface may be employed to supplement the work of the patrolman. The +maintenance gang consists of three to ten men and is furnished all of +the tools and materials required for the particular kind of work they +do. Ordinarily the gang goes over the roads assigned to it once each +season and performs those repair operations requiring more work than +the patrolman can find time for. The work of the maintenance gang like +that of the patrolman should be under engineering supervision. + +=Maintenance of Earth, Sand-clay, Gravel and Macadam Roads.=--The +ordinary upkeep of earth, sand-clay, gravel and macadam surfaces is +most readily accomplished by the patrol method, since constant care is +required to keep the roads in a condition of maximum service ability. + +The tools required for each patrolman may include the following: + + 1 shovel 1 spade + 1 stone rake 1 pick + 1 scythe 1 tamper + 1 or more road drags 1 mowing machine for cutting weeds + 1 wheelbarrow (sometimes) + 1 light truck 1 small kit carpenter's tools + +The work of the patrolman consists in keeping the surface of the road +smooth by dragging, repairing chuck holes by tamping in fresh material +of the appropriate kind, keeping the ditches and culverts free from +obstruction, cutting weeds and repairing bridge floors if they are of +plank construction. Removal of snow drifts is sometimes a part of the +patrolman's duty, but more often that is done by special gangs. +Usually the patrolman is authorized to hire teams for dragging and +cutting weeds. + +When an earth road requires to be re-graded so as to restore the +cross-section and deepen the ditches, a gang is sent in to perform +that work, as it is obviously impossible for the patrolman to perform +work, of that kind. + +If the gravel road is being maintained with a bituminous carpet coat, +the patrolman will be furnished the necessary tools to enable him to +patch the surface with bituminous material as necessity requires. + +When the surface deteriorates to such an extent that a new carpet coat +is required, the gang system is employed for all work connected with +resurfacing, instead of attempting to have the work done by patrolmen. + +The maintenance of the macadam road is carried out in much the same +manner as that of the gravel road. The binder of stone dust or clayey +sand is renewed as often as it is swept off by traffic. Depressions or +ruts are repaired by first loosening the surface with a pick and then +adding broken stone and screenings to restore the surface. + +When the macadam reaches the stage where entire resurfacing is +needed, the work is performed by gangs organized and equipped for the +purpose; and likewise when the surface is being maintained with a +bituminous carpet, the renewal of the carpet coat is performed by +special gangs, but the ordinary upkeep of the surface by patching is +handled by a patrolman. + + +MAINTENANCE OF MIXED BITUMINOUS SURFACES + +[Illustration: Fig. 22.--Scarafier used in Gravel Road Maintenance] + +These types of surface can be kept in satisfactory condition if they +are carefully repaired once or twice each season. This work requires +considerable experience and some special equipment, not ordinarily +supplied to patrolmen. A gang is organized for the work and supplied +with the proper equipment. They go over the roads and patch all worn +places, generally first removing the wearing surface entirely in the +area affected. + +The wearing surface mixture is then prepared and tamped or rolled into +place. If the area affected is small, tamping is satisfactory, and +when the area is considerable, rolling is employed. The upkeep of the +side roads may be accomplished by the same gang but is preferably +taken care of by patrolmen, who do not attempt any but minor repairs +to the wearing surface. + + +MAINTENANCE OF BRICK AND CONCRETE ROADS + +On brick and concrete roads, the principal work on the wearing surface +consists in filling the cracks with a suitable bituminous material. +This work is done by patrolmen or by special gangs and generally will +be done once each year. The upkeep of the side roads is cared for by +patrolmen who drag the side roads and cut the weeds as occasion +requires. + + + + +INDEX + + + Administration county; 15 + federal; 17 + highway; 13 + state; 16 + township; 13 + + Aesthetics; 62 + + Aggregate, fine; 101 + + Aggregate, coarse; 100 + + Air resistance; 51 + + Alignment; 46 + + Applying bituminous binder; 122 + + Asphaltic concrete; 128 + + Asphalt, natural; 117 + liquid; 118 + petroleum; 117 + + Assessments, special; 19 + zone method; 20 + + + Bedding course, green mortar; 111 + sand mortar; 111 + sand bedding mortar; 111 + + Binder for gravel; 75 + + Bitulithic or warrenite; 128 + + Bitumen; 118 + + Bituminous coatings on concrete; 105 + + Bituminous fillers; 112 + + Bituminous road materials and their use; 116 + + Bituminous surfaces; 96, 120 + + Blade grader; 69 + + Bonding; 87 + + Bonds, annuity; 26 + serial; 27 + sinking fund; 25 + + Box culverts; 39 + + Brick roads; 113 + + Brick, repressed; 107 + tests of 108; + vertical fiber; 107 + vitrified; 106 + wire-cut-lug; 108 + + Broken stone road surfaces; 89 + + + Cement, asphaltic; 118 + + Cement concrete roads; 98 + + Cement grout filler; 112 + + Characteristics, asphaltic concrete; 129 + bituminous macadam; 125 + broken stone; 97 + concrete; 105 + mixed macadam; 128 + sand clay; 78 + + Classes of bituminous materials; 116 + + Classification according to consistency; 117 + + Clay and cement concrete pipe; 39 + + Coal tar; 116 + + Concrete, asphaltic; 128 + + Concrete materials; 100 + + Concrete pipe; 39 + + Control of erosion; 61 + + Costs; 70 + + County administration; 15 + + Cross sections; 60, 65 + + Culverts; 56 + + Curing concrete; 103 + + + Design, broken stone roads; 89 + concrete roads; 99 + earth roads; 42 + + Desirability of road bonds; 27 + + Development of traffic; 2 + + Drainage, necessity of; 29 + + Drainage of roads; 29 + + + Earth roads, in arid regions; 72 + humid regions; 65 + value of; 73 + + Earth works; 92 + + Education, rural; 6 + + Effect of grades; 54 + + Elevating grader; 66 + + Elevating grader work; 68 + + End walls for culverts; 39 + + Energy loss on account of grades; 57 + + Entrances, farm; 37, 61 + + Expansion joints; 104 + + + Farm entrance culverts; 37 + + Federal administration; 17 + + Fillers; 118 + + Finance, highway; 19 + + Fine aggregate; 101 + + Finishing surface of concrete; 122 + + Foundation, asphaltic concrete; 129 + brick; 109 + macadam; 93 + mixed macadam; 126 + penetration macadam; 123 + Telford; 94 + + + Gang maintenance; 131 + + Grader, Maney; 67 + use of; 69 + + Gravel, ideal; 81 + natural; 83 + roads; 74 + + General taxation; 24 + + Good roads and commerce; 7 + + Green concrete bedding course; 111 + + + Highway administration; 13 + + Highway finance; 19 + maintenance; 130 + + + Importance of design; 30 + + Ideal road gravel; 81 + + Inter-city traffic; 5 + + Inter-county and inter-state traffic; 5 + + Internal resistance; 50 + + Intersections; 46 + + + Laying tile; 35 + + Length of culvert; 37 + + Liquid asphalt; 118 + + Local farm to market traffic; 4 + + + Macadam; 89 + + Maintenance, concrete; 105 + earth roads; 70 + general; 131 + gravel roads; 88 + macadam; 96 + of highways; 130 + patrol; 131 + + Maney grader; 67 + + Marginal curb; 113 + + Measuring materials; 101 + + Metal pipe; 38 + + Mixing wearing surface; 127 + + Mixtures; 117 + + + Natural asphalt; 117 + gravel; 79 + + Necessity for planning; 42 + drainage; 29 + + + Patching; 122, 125 + + Patrol maintenance; 131 + + Pebbles, size of; 80 + + Petroleum asphalt; 117 + + Placing asphaltic concrete; 129 + + Placing broken stone; 94 + + Placing concrete; 102, 103 + mixed macadam; 127 + + Plans for roads; 43 + + Preliminary investigation; 44 + + Preparation of earth foundation; 102 + of road; 85 + + Private entrances; 61 + + Properties of stone; 90 + + Proportions for concrete roads; 101 + + Purpose of highways; 1 + + + Reinforced concrete box culverts; 39 + + Reinforcing; 104 + + Repressed brick; 107 + + Road oils; 117 + + Road plans; 43 + + Rocks, kind of, for macadam; 91 + + Rolling, macadam; 95 + + Rolling resistance; 50 + + Run-off; 31 + + Rural education; 6 + + Rural social life; 7 + + + Safety consideration; 58 + + Sand bedding course; 111 + + Sand clay and gravel road; 74 + + Sand mortar bedding course; 111 + + Seal coat; 127 + + Serial bonds; 27 + + Sinking fund bond; 25 + + Slip scraper; 67 + + Special assessments; 19 + + Specifications; 119 + + Spreading screenings; 95 + + State administration; 16 + + Stone, use of; 92 + + Surface drainage; 30 + + Surfaces, bituminous; 120 + + Surface method; 87 + + Superelevation; 47 + + + Tests, brick; 108 + + Tile drains; 35 + + Topeka asphaltic concrete; 128 + + Tractive resistance; 52 + + Trench method; 85 + + Truck operation costs; 9 + + Types of culverts; 38 + + + Underground water; 34 + + Undulating roads; 58 + + Use of blade grader; 69 + + Utilizing natural gravels; 83 + + + Value of earth roads; 73 + + Variation in rainfall; 64 + + Variation in soils; 63 + + Vehicle taxes; 24 + + Vertical fiber brick; 107 + + Vitrified brick roads; 106 + + Vitrified brick; 106 + + + Water gas tar; 117 + + Width of roadway; 59 + + Wire-cut-lug brick; 108 + + + Zone method of assessing; 20 + + * * * * * + +[Transcriber's Notes: + +The transcriber made these changes to the text to correct obvious +errors: + + 1. p. 5, accomodate --> accommodate + 2. p. 39, guage --> gauge + 3. p. 46, enbankment --> embankment + 4. p. 63, tought --> tough + 5. p. 68, absorbant --> absorbent + 6. p. 73, persistant --> persistent + 7. p. 77, indispensible --> indispensable + 8. p. 119, aspealt --> asphalt + 9. p. 127, repaid --> rapid + 10. p. 130, Vetrified brick; 105 --> Vitrified brick; 106 + 11. p. 130, Virtical --> Vertical + +End of Transcriber's Notes] + + + + + +End of the Project Gutenberg EBook of American Rural Highways, by T. 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