American Rural Highways

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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 o