[Transcriber's Note: Every effort has been made to replicatethis text as faithfully as possible, including obsolete andvariant spellings and other inconsistencies. Text that has beenchanged to correct an obvious error is noted at the end of thisebook. 

Also, on pages 47-48, the Greek letter theta is represented byTHETA. In chemical and mathematical notations, a subscript isenclosed in braces and preceded by an underscore (e. G. , H_{2}O. )]

 AGRICULTURAL ENGINEERING SERIES

 E. B. MCCORMICK, CONSULTING EDITOR

 FORMERLY DEAN OF ENGINEERING DIVISION KANSAS STATE AGRICULTURAL COLLEGE

 AMERICAN

 RURAL HIGHWAYS

 _McGraw-Hill Book Co. Inc. _

 PUBLISHERS OF BOOKS FOR

 Coal Age -- Electric Railway Journal Electrical World -- Engineering News-Record American Machinist -- Ingenieria Internacional Engineering & Mining Journal -- Power Chemical & Metallurgical Engineering Electrical Merchandising

 [Illustration: _Frontispiece_]

 AMERICAN RURAL HIGHWAYS

 BY

 T. R. AGG, C. E. 

 PROFESSOR OF HIGHWAY ENGINEERING IOWA STATE COLLEGE

 FIRST EDITION

 MCGRAW-HILL BOOK COMPANY, INC. NEW YORK: 239 WEST 39TH STREET LONDON: 6 & 8 BOUVERIE ST. , E. C. 4 1920

 COPYRIGHT, 1920, BY THE McGRAW-HILL BOOK COMPANY, INC. 

PREFACE

AMERICAN RURAL HIGHWAYS was written for use as a text or reference incourses dealing with rural highways and intended for agriculturalengineers, students in agriculture and for short courses and extensioncourses. The reader is assumed to have familiarity with drawing andsurveying, but the text is adapted primarily for students who do notreceive training along the lines of the usual course in Highway orCivil Engineering. 

The text is intended to familiarize the student with the relation ofhighway improvement to national progress, to indicate the variousproblems of highway administration and to set forth the usual methodsof design and construction for rural highways in sufficient detail toestablish a clear understanding of the distinguishing characteristicsand relative serviceability of each of the common types of roadwaysurface. 

Experience with classes made up of students in agriculture oragricultural engineering and with trade school students in road makingserved as a guide in the selection and arrangement of the material. Detailed discussion of tests of materials and of the theory of designhas to a considerable extent been eliminated as being outside of thescope of the course for which the text is intended. 

In the preparation of American Rural Highways reference was had tomany books on highway subjects and to current periodical literature. Wherever direct extracts were made from such source, appropriateacknowledgment 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 inTraffic--Location or Farm to Market Traffic--Farm to FarmTraffic--Inter-City Traffic--Inter-County and Inter-StateTraffic--Rural Education--Rural Social Life--Good Roads andCommerce 1-12

CHAPTER II

HIGHWAY ADMINISTRATION

Township Administration--County Administration--StateAdministration--Federal Administration--Special Assessments--ZoneMethod of Assessing--General Taxation--Vehicle Taxes--SinkingFund Bonds--Annuity Bonds--Serial Bonds--Comparison of Methodsof Issuing Bonds--Desirability of Road Bonds 13-28

CHAPTER III

DRAINAGE OF ROADS

The Necessity for Drainage--Importance of Design--SurfaceDrainage--Run-off--Ordinary Design of Ditches--UndergroundWater--Tile Drains--Lying Tile--Culverts--Length of Culvert--Farm Entrance Culverts--Metal Pipe--Clay and Cement ConcretePipe--Concrete Pipe--Endwalls for Culverts--Reinforced ConcreteBox 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--RollingResistance--Internal Resistance--Air Resistance--Effect ofTrades--Energy Loss on Account of Grades--Undulating Roads--Guard Railing--Width of Roadway--Cross Section--Control ofErosion--Private Entrances--Æsthetics 42-62

CHAPTER V

EARTH ROADS

Variations in Soils--Variation in Rainfall--Cross SectionsElevating Grader--Maney Grader--Slip Scraper--FresnoScraper--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 SandyRoads--Sand-clay on Clay or Loam--Characteristics--NaturalGravel--The Ideal Road Gravel--Permissible Size of Pebbles--Wearing Properties--Utilizing Natural Gravels--Thicknessof Layer--Preparation of the Road--Trench Method--SurfaceMethod--Maintenance 74-88

CHAPTER VII

BROKEN STONE ROAD SURFACES

Design--Properties of the Stone--Kinds of Rocks used forMacadam--Sizes of Stone--Earth Work--Foundation for theMacadam--Telford Foundation--Placing the Broken Stone--Rolling--Spreading Screenings--Bituminous Surfaces--MaintenanceCharacteristics 89-97

CHAPTER VIII

CEMENT CONCRETE ROADS

Destructive Agencies--Design--Concrete Materials--FineAggregate--Proportions--Measuring Materials--Preparation of theEarth Foundation--Placing Concrete for Two-course Road--Curingthe Concrete--Expansion Joints--Reinforcing--Bituminous Coatingson Concrete Surfaces--Characteristics--Maintenance 98-105

CHAPTER IX

VITRIFIED BRICK ROADS

Vitrified Brick--Paving Brick--Repressed Brick--Vitrified FiberBrick--Wire-cut-lug Brick--Tests for Quality--Other Tests--Foundation--Sand Bedding Course--Sand Mortar Bedding Course--Green Concrete Bedding Course--Bituminous Fillers--MasticFillers--Marginal Curb 106-115

CHAPTER X

BITUMINOUS ROAD MATERIALS AND THEIR USE

Classes of Bituminous Materials--Coal Tar--Water Gas Tar--NaturalAsphalt--Petroleum Asphalt--Mixtures--Classification According toConsistency--Road Oils--Liquid Asphalts--Asphalt Cements--Fillers--Bitumen--Specifications--Surface Treatments--Applyingthe Bituminous Binder--Finishing the Surface--Patching--Penetration Macadam--Foundation--Upper or Wearing Course--Patching Characteristics--Hot Mixed Macadam--Foundation--Sizesof Stone--Mixing the Wearing Stone--Placing and WearingSurface--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 familiarthings, are utilized constantly with little thought of howindispensable they are to the conduct of the business of a nation orof the intimate relation they bear to the everyday life of anycommunity. The degree to which a nation or a community perfects itstransportation facilities is an index of its industrial progress andpublic highways constitute an important element in the nationaltransportation system. It is to be expected that the average citizenwill think of the public highway only when it affects his ownactivities and that he will concern himself but little with the broadproblem of highway improvement unless it be brought forcibly to hisattention through taxation or by publicity connected with theadvancement of specific projects. 

=National in Scope. =--The improvement and extension of the highwaysystem is of national importance just as is development and extensionof railways, and concerted action throughout a nation is aprerequisite to an adequate policy in regard to either. It isinconceivable that any community in a nation can prosper greatlywithout 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 manyvaried sources to supply the things that they want. Goodtransportation facilities contribute greatly to community prosperityand indirectly to national prosperity, and the benefits of highlyimproved public highways are therefore national in scope. This facthas been recognized in Europe, notably in England, France and Belgium, where the public highways are administered largely as nationalutilities. 

Until recent years, highway improvement in the United States has beensubordinated to other more pressing public improvements, but duringthe World War the inadequacy of the transportation system of theUnited States became apparent. While such an unprecedented load upontransportation facilities may not recur for many years, it has becomeapparent that more rapid progress in highway improvement is necessaryand in the United States the subject is now likely to receiveattention commensurate with its importance. 

=Development of Traffic. =--The character and extent of the highwayimprovement needed in any locality is dependent entirely on thedemands of traffic. In sparsely settled areas, particularly those thatare semi-arid or arid, the amount of traffic on local roads is likelyto be small and the unimproved trails or natural roads adequate. Butas an area develops either on account of agricultural progress or theestablishment of industrial enterprises, the use of the publichighways both for business and for pleasure increases and the oldtrails are gradually improved to meet, at least to some degree, thenew demands of traffic. In sparsely settled areas, it is possible forthe public to accommodate its use of the highways to the physicalcondition thereof, and business is more or less regulated according tothe condition of the roads. This is not always pleasant or economicalbut is the only possible arrangement. In populous districts, withdiversified activities, it becomes imperative to have year-roundusable roads in order to transact with reasonable dispatch the regularbusiness of the industries. Anything less will handicap normalcommunity progress. 

The advent of the motor driven vehicle in the United States hasresulted in a greatly increased use of the public highways ofagricultural areas, even of those that are sparsely populated, becauseof the convenience of the motor vehicle both for passenger and forfreight service. Probably in excess of 90 per cent of the tonnagepassing over the rural highways in the United States is carried bymotor vehicles. This class of traffic has really just developed and noone can predict what it will be in ten years, yet it has alreadyintroduced into the highway problem an element that has revolutionizedmethods of construction and maintenance. 

A different set of traffic conditions exists in those parts of theUnited States where large areas are devoted primarily to industrialpursuits, the agricultural development being of secondary importance. Public highways connecting the industrial centers are indispensableadjuncts to the business facilities in such communities and areordinarily subjected to a very large volume and tonnage of trafficconsisting principally of motor vehicles. The roads first selected forimprovement will not be those serving the agricultural interests ofthe district, but rather those serving the industrial centers. Inter-city roads of great durability and relatively high cost arenecessary for such traffic conditions. 

Not infrequently the transportation needs will require a system ofboth inter-city and rural highways in the same community. There arefew areas in the United States where there is no agriculturaldevelopment. It is apparent therefore that the nature of the highwaysystems and the administrative organization under which they are builtand maintained will differ in various states or areas according tothe nature of development of that area agriculturally andindustrially. In planning improvements of highway systems, it isrecognized that one or more of several groups of traffic may beencountered and that the extent and nature of the improvement must besuch as will meet the requirements of all classes of traffic, the mostimportant being first provided for, and that of lesser importance asrapidly as finances permit. 

KINDS OF TRAFFIC ON PUBLIC HIGHWAYS

=Local or Farm to Market Traffic. =--In strictly agriculturalcommunities the principal use of the highways will pertain toagricultural activities and most of it will be between the farm andthe most convenient market center. In the ordinary state, the numberof rural families will not average more than six to eight per squaremile, but in some districts it may reach twenty families per squaremile. The travel from the district around a market center willoriginate in this rather sparsely populated area and converge onto afew main roads leading to market. The outlying or feeder roads will beused by only a few families, but the density of traffic will increasenearer the market centers and consequently the roads nearer town willbe much more heavily traveled than the outlying ones. It is apparenttherefore that considerable difference may exist in the kind ofconstruction adequate for the various sections of road where farmtraffic is the principal consideration. This traffic is made up ofhorse drawn wagons, transporting farm products and of horse drawn andmotor passenger vehicles, the motor traffic comprising 80 per cent ormore of the volume of traffic and a greater per cent of the tonnage. Motor trucks are now employed to some extent for marketing farmproducts and, where surfaced highways have been provided, this classof traffic is superseding horse drawn traffic. 

=Farm to Farm Traffic. =--In the ordinary prosecution of farmingoperations, a considerable amount of neighborhood travel isinevitable. Farmers help each other with certain kinds of work, exchange commodities such as seed, machinery and farm animals andvisit back and forth both for business and pleasure. To accommodatethis traffic, it is desirable to provide good neighborhood roads. Traffic of this sort follows no particular route and can to someextent accommodate itself to the condition of the highways withoutentailing financial loss, although some discomfort and someinconvenience may result from inadequate highway facilities. Thistraffic will be partly motor and partly horse drawn, but theproportion of motor driven is large. 

=Inter-city Traffic. =--In strictly agricultural districts there is alarge amount of travel between towns, both for business and forpleasure. The pleasure travel is mostly in motor vehicles and aconsiderable part of the business traffic is the same, although horsedrawn vehicles are employed to some extent. 

In industrial districts there is a large volume of this class oftraffic consisting of motor passenger vehicles used for business andfor pleasure and of motor freight vehicles used for general businesspurposes. In addition, there is certain to be a large amount of motortruck freight traffic incident to the particular industrial pursuitsof the cities. Where adequate public highways connect industrialcenters, there is invariably a very large amount of inter-citytraffic, due in part to the needs of industry and in part toconcentration of population in industrial centers. 

=Inter-County and Inter-State Traffic. =--Automobile touring is apopular means of relaxation, especially on the part of those who livein the cities, although it is by no means confined to them. Traffic ofthis kind follows the routes where roads are best and passes entirelyacross a county, attracted by some public gathering. Often it isinter-state in character, made up of tourists who are traveling todistant pleasure resorts. Such traffic at present constitutes arelatively small part of the travel on public highways, except oncertain favorable routes, but as the wealth of the country increasesand good touring roads are numerous, long distance travel willincrease and will eventually necessitate the construction of a numberof well maintained national highways, located with reference to theconvenience of the automobile tourist. 

PUBLIC HIGHWAYS AND COMMUNITY LIFE

It is well to recognize the intimate relation public highways bear tothe economic progress of a nation. Normal development of all of thediverse activities of a people depends very largely upon the highwaypolicy that is adopted and whether the actual construction ofserviceable roads keeps pace with transportation needs. 

=Rural Education. =--It has become increasingly apparent during theWorld War that the demand upon North America for food stuffs is tobecome more and more insistent as the years pass. Already theconsumption in the United States has approached quite closely to theaverage production and yet the population is constantly increasing. The time is not far distant when greater production will be requiredof the agricultural area in North America in order to meet the homedemand for foodstuffs, and many thousands of tons will be needed forexport. This need can only be met by agricultural methods that willincrease greatly the present yield of the soil. The adoption of betteragricultural methods must of necessity be preceded by the technicaltraining of the school children who will be the farmers of the nextgeneration, which can best be accomplished in graded schools with wellequipped laboratories and with suitably trained teachers. The problemof providing such schools in rural communities has, in some instances, been solved by consolidating a number of rural school districts andconstructing a well equipped building to accommodate the students froman area several miles square. An educational system of this sort canreach its highest usefulness only when adequate public highwaysfacilitate attendance of pupils. The whole trend of rural educationalprogress is toward a system which is predicated upon a comprehensivehighway policy in the district. 

=Rural Social Life. =--Closely allied to the rural educational problemis the rural social problem. Motor cars and good roads do a great dealto eliminate the isolation and lack of social opportunity that hascharacterized rural life in the United States. A high order ofcitizenship in rural communities is essential to the solution of manyproblems of rural economics, and such citizens will not live away fromthe social opportunities of modern life. The rural school house andthe rural church may become social centers and local plays, movingpicture shows and lectures and entertainments of other kinds madeavailable to those who live in the country. Their enjoyment of thesesocial opportunities will be much more general if the public highwaysare at all times in a condition to be traveled in comfort. Good homesand good schools on good roads are prerequisites to the solution ofmany rural problems. 

If there is opportunity for those who live in the cities to get someadequate idea of rural life and the conditions under which farmingoperations are carried on it will correct many misunderstandings ofthe broad problems of food production and distribution. Reference hasfrequently been made to the seeming desire on the part of city peopleto get into the country, and, by facilitating the realization of thisdesire, a great social service is rendered. 

=Good Roads and Commerce. =--That good highways are almost as necessaryas are railroads to the commercial development of a nation isrecognized but, unlike the railroads, the highways are not operatedfor direct profit and the responsibility of securing consideration ofthe demand for improvements is not centralized. Therefore, sentimentfor road improvement has been of slow growth, and important projectsare often delayed until long after the need for them was manifest. Movements to secure financial support for highway improvement must gothrough the slow process of legislative enactment, encountering all ofthe uncertainties of political action, and the resulting financialplan 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 highwayproblem can never receive adequate consideration until public highwaysare recognized as an indispensable element in the business equipmentof a nation. 

During the World War all transportation facilities were taxed to thelimit, and motor trucks were utilized for long distance freighthaulage to an extent not previously considered practicable. As aresult, the interest in the motor truck as an addition to thetransportation 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 ormore. 

The part the motor truck will play in the future can only beestimated, but it seems clear that the most promising field is forshipments destined to or originating in a city of some size and awarehouse or store not on a railroad spur, and especially when theshipments are less than car load lots. The delays and expense incidentto handling small shipments of freight through the terminals of alarge city and carting from the unloading station to the warehouse orother destination constitute a considerable item in the cost oftransportation. 

Mr. Charles Whiting Baker, Consulting Editor of _EngineeringNews-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 motortruck transport, and possibly new types of trucks or trucks andtrailers will be developed so that freight traffic over many roadswill be of considerable tonnage and an established part of thetransportation system of the nation. In the article above referred toare given the following data relative to the cost of hauling onimproved roads by motor truck and these cost estimates are based onthe best information available at this time. They should be consideredas approximate only, but serve to indicate the limitations of thetruck 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 arecompared with the cost of hauling freight over trunk-line railwayswith heavy traffic where the cost per ton-mile, including terminalcharges, ranges from 1. 7 _mills_ per ton-mile to 4. 4 _mills_ perton-mile. 

In view of these facts it seems reasonable to suppose that motorvehicles for use on the public highways are more likely to be employedto supplement the rail transport than to compete with it. To theactual cost of operation of motor trucks given in Table 2, thereshould be added the proportionate cost of maintaining the highway forthe use of the truck, which is partly covered by the item "LicenseFee" in the table. The license fee would necessarily be considerablylarger if it were to compensate adequately for the wear on thehighways over which the trucks operate. This will still furtherincrease the cost of hauling by motor truck. 

Motor trucks are employed for many kinds of hauling where their speedand consequently their daily capacity is an advantage over teamhauling that is decidedly worth while. It probably could be shownthat for many kinds of hauling, teams are more economical than motortrucks, but when promptness and speed and the consequent effect ondependent activities are considered, the motor truck often has adistinct advantage, and the use of the truck to replace horse drawnvans is progressing rapidly. This is true not only in the cities, butalso in the smaller towns and in the country. Motor trucks have beenadopted in a great many communities for delivery of farm products tomarket, and this use of the truck is certain to increase rapidly. Buttrucks in this service will use the secondary roads as well as themain or primary roads. 

These observations emphasize the extent to which the highway policy ofthe nation must be predicated on the use of the highways by motorvehicles. 

CHAPTER II

HIGHWAY ADMINISTRATION

The systems of highway administration extant in the various politicalunits in the United States present a patchwork of overlappingauthority and undetermined responsibility. Highway laws are beingconstantly revised by state legislatures and with each revision thereis some change in administrative methods and often the changes arerevolutionary in character. In most states, the trend is away fromcounty and township administration and toward state administration, with provision for considerable participation by the federalgovernment. 

It will be pertinent to consider briefly the present functions of eachof the administrative authorities having duties in connection withhighway work in the United States, although these duties vary greatlyin the several states and change periodically with the action oflegislatures. 

=Township Administration. =--Township or "Town" authority is a survivalof the old New England town government and the town board consists ofthree or more trustees who hold office for fixed terms. The usual termis three years, but is less in some states. The incumbent is generallya man who has other responsibilities of a public or private nature andwho gives but little of his time to highway matters. In some statesthe pay is a fixed annual salary and in others a per diem with somelimitation on the amount that may be drawn in any one year, whichlimitation may be statutory or may be by common consent. 

The township highway commissioners or trustees have jurisdiction overcertain of the roads in the township, usually best described as allroads not by law placed under the jurisdiction of some otherauthority. In certain instances, the township authorities have chargeof 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 amatter of general observation that the trend of legislation is towardremoving from the jurisdiction of the township officials all roadsexcept those upon which the traffic is principally local in character. The actual mileage of roads in the United States that is at presentadministered by township officials is large, probably constituting notless than seventy per cent of the total mileage. 

In most states the township officials are responsible for themaintenance of the roads under their jurisdiction and also supervisesuch new construction as is undertaken. This includes the constructionof culverts and bridges as a rule, but in some states the county boardof supervisors is responsible for all of the bridge and culvert workon the township roads. In other states, the township board isresponsible only for bridges or culverts that cost less than a certainamount specified by law (usually about $1000) and the county boardprovides for the construction and upkeep of the more expensive bridgesand culverts. 

Funds for the work carried out by the township road officials areobtained by general taxation, the amount that may be levied beinglimited by statute and the actual levy being any amount up to themaximum that the township board deems necessary for its purposes. Itis the general observation that the tax levy is usually the maximumpermitted by law. 

In many states, township officials are permitted to issue bonds forroad construction, almost invariably, however, with the restrictionthat each issue must be approved by the voters of the township. Thereis always a provision that the total amount of bonds outstanding mustnot exceed the constitutional limit in force in the state. In severalstates, the townships have large amounts of road bonds outstanding. 

=County Administration. =--In some states the county is the smallestadministrative unit in the road system. A county board, called theboard of county supervisors or board of county commissionersconsisting of from three to fifteen members, is the administrativeauthority. Its members are elected for fixed terms which vary inlength from one to five years. The county board usually has manypublic responsibilities other than highway administration, and isgenerally made up of men with considerably more business ability thanthe average township board. 

The county board has jurisdiction over all of the highways in thecounty in some states, and in others it has charge of only the moreimportant highways. In most states, the laws set forth specificallywhat highways shall be under the jurisdiction of the countyauthorities. 

In addition to having direct supervision of the improvement andmaintenance of the roads assigned to county administration, the countyboards in some states arrange for the construction of all culverts andbridges on the roads that are under township supervision, or at leastthe more expensive bridges and culverts on such roads. Sometimes thisis accomplished by granting county aid for township bridges, underwhich system the county pays a part of the cost of the construction ofbridges on the township roads. The amount of aid varies, but isgenerally about one-half of the cost, and the township and countyofficials jointly assume the responsibility of arranging for theconstruction by contract or otherwise. 

The county board obtains funds for road work through a direct tax onall property in the county, the maximum rate being limited by statute. County boards are also authorized to issue bonds for road constructionunder statutory restrictions and limitations similar to thoseeffective in the township as to total amount issued, and manymillions of dollars' worth of highway bonds have been issued bycounty authorities in the United States. 

=State Administration. =--In a state, the administrative authority inhighway matters is vested in a board of commissioners usuallyconsisting of three or more members. In a few states, theadministrative authority is delegated to a single commissioner. Wherethe authority is vested in a board, that board is usually appointed bythe governor. In several states one or more members of the commissionhold that position _ex officio_; for example, in several states thegovernor is by law a member of the commission, in others the secretaryof state or the dean of engineering at the State University or thestate geologist is a member of the commission. Where theadministrative authority is a single commissioner he may be electedalong with other state officers, but this is the case in only a fewstates. 

The authority of the state highway department varies in the severalstates, but in general the departments serve in the dual capacity ofgeneral advisers to the county and township authorities on roadmatters and as the executive authority responsible for theconstruction of those highways that are built entirely or in part fromstate or federal funds. 

State highway departments consist of the commission or commissioner, and the technical and clerical staff required to perform the dutiesimposed on the state organization. To some extent the state highwaydepartments are able to encourage economical and correct constructionof highways by the township and county authorities by furnishing themstandard plans and specifications and by formulating regulations togovern the character of construction, but such efforts are likely tobe more or less ineffective unless the state authority has supervisionof the allotment of state or federal funds to the various counties andtownships. Nevertheless, most state highway departments do a greatdeal of advisory work in connection with the highway constructioncarried out by county and township authorities. 

State highway departments are supported by funds obtained in variousways, laws differing greatly in this respect. The necessary support isin some states appropriated from funds obtained by general taxation, and is in others obtained from automobile license fees. In stillothers, the funds are secured by a combination of the two methodsmentioned above. In addition to these support funds, a certain part ofthe money obtained as federal aid may be employed for the engineeringand inspection costs on federal aid roads. The above mentioned fundsare required to maintain the state highway department. In addition, the departments have supervision of the expenditures of constructionfunds which can be used for road construction and maintenance, and maynot be expended for salaries or other overhead expense. 

In a number of states, automobile license fees are set aside forfinancing road construction and maintenance, and the work paid forfrom the fees is carried out under the supervision of the statehighway department. 

In a number of instances, state bonds have been issued for roadconstruction, and the expenditure of the proceeds of the sale of roadbonds has usually been supervised by the state highway department. 

All federal aid funds allotted to a state must be expended under thedirection of the state highway department. 

=Federal Administration. =--Federal authority in highway work is vestedin the Bureau of Public Roads of the United States Department ofAgriculture. The official head is the Secretary of Agriculture, butthe administrative head is the Director of the Bureau. In this Bureauare the various instrumentalities needed for carrying oninvestigations and furnishing information to the various states onhighway subjects. The Bureau also supervises the construction offederal aid roads in a general way through district engineers, eachof whom looks after the work in several states. 

Funds for the support of the Bureau of Public Roads are obtained fromcongressional appropriations to the Department of Agriculture and froma percentage of the funds appropriated for federal aid. 

Federal aid is money appropriated by Congress to be distributed to thevarious states to stimulate road construction. It is granted to thestates on the condition that the states will expend at least an equalamount on the projects involved. The states in turn usually give asuitable part of the state allotment to each county. There are variouslimitations as to the amount of federal aid per mile of road and thetype of construction that may be employed, but these are matters ofregulation that change from time to time. 

It will be seen that each of the administrative authorities, exceptthe Bureau of Public Roads, is to some extent subservient to a higherauthority, and the Bureau of Public Roads is supervised by the UnitedStates Congress. Considerable diplomacy is required on the part of anyadministrative authority if his contact with other officials is to bewithout friction. This is especially true in connection with theformulation of a policy regarding the types of construction to beadopted for an improvement. The responsibility for the selection isvariously placed on the township, county or state authority, the lawsnot being uniform in this respect. If state or federal funds areallotted to an improvement, the state authority either makes theselection of the type of construction or the selection is made by somesubordinate authority subject to the approval of the state highwaydepartment. Where the improvement is paid for exclusively withtownship or county funds, the selection is often made by the townshipor county authority without review by higher authority. Many abuseshave crept into highway administration through the unscrupulousmethods of promoters of the sale of road materials or road machinery. A great deal of the selling activity of the agents for thesecommodities is entirely irreproachable, but it is well known that suchis not always the case. As a result, the tendency of legislation is torequire the state highway department to approve contracts formaterials or construction entered into by the township or countyauthorities. The state highway departments can secure the requisitetechnical experts to determine the merits of materials and equipmentand, in spite of some glaring examples of inefficiency or worse, havemade a good record for impartiality and integrity as custodians of thefunds for which they are responsible. 

HIGHWAY FINANCE

The paramount problem in highway administration is the development ofan adequate financial plan for carrying on road improvement. Thenecessary expenditures are enormous, although the money so expended isprobably much less than the actual benefit resulting from theimprovements. 

=Special Assessments. =--There is presumed to be a direct andrecognized benefit conferred on farm lands by the construction ofimproved highways adjacent thereto. Therefore, it is equitable tocharge a part of the cost against the lands so benefited. 

The principle of paying for public improvements by a specialassessment upon private property has been long established and a largeproportion of the public improvements in the cities and towns havebeen made financially possible through the medium of specialassessments on abutting and adjacent property. The same principle hasbeen applied to the financing of drainage projects for reclaiming farmlands. Recently the special assessment method has come into limiteduse in financing rural highway improvements. The policy in such casesis to assess the abutting and adjacent property in a zone along theimproved road for a percentage of the cost of the improvement. Theamount so assessed does not ordinarily exceed one-fourth of the totalcost of the improvement and may be considerably less. The assessmentis spread over an area extending back from one to six miles from theimproved road. The assessment area is generally divided into aboutfour zones parallel to the road. The zone next the road is assessed ata rate arbitrarily determined as a fair measure of the benefit, andeach succeeding zone is assessed at a somewhat lower rate. Generallyabout three-fourths of the total assessment is placed on the half ofthe assessment area lying next to the road. 

Many systems of making assessments have been proposed which aremechanical in application after the area and rate of distribution ofbenefit have been established, but in practice it is always foundnecessary to make adjustments on individual parcels of land because ofvariation in benefits received and it is impossible to eliminate theexercise of human judgment in equalizing the assessments. 

=Zone Method of Assessing. =--The area to be assessed on each side ofthe improved road is divided into zones usually four in number, but alarger or smaller number of zones may be adopted. The rate for eachzone is then arbitrarily determined. For a typical case, the first offour zones would receive an assessment of 50 per cent of the amount tobe borne by the area; the second zone 25 per cent, the third 15 percent and the fourth 10 per cent. Other percentages sometimes adoptedare 45, 25, 20 and 10 and 60, 20, 15 and 5. The set of percentagesfirst mentioned seems to insure the most equitable distribution for anarea 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 withinthe assessment area, the rate is arbitrarily reduced to relieve thatparcel of land somewhat, or the assessment is first spread as aboveoutlined and afterward equalized as judgment dictates. 

In applying the zone method some difficulty is encountered indetermining an equitable distribution on those parcels of land lyingpartly in one zone and partly in another, but the rate may be arrivedat with reasonable accuracy by pro-rating in accordance with the exactconditions. 

In. Fig. 1, let it be assumed that the assessment area is to be twomiles wide, one mile on each side of the road and the variousownerships to be indicated by the parcels of land numbered 1 to 8, asshown. Each zone for the assessment of the 3-1/4 mile section is 1/4mile wide and the rates for the several zones are 50, 25, 15 and 10per cent respectively. Let it be assumed that the portion of the costof 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 × frontage on | Amount of | Parcel| Rate | improved road = | Assessment | Assessment | | assessment units | per unit[1]| 1 | 2 | 3 | 4 | 5 ------+-------+----------------------+------------+------------- 1 | a 50 | 50 × 2640 = 132, 000 | $0. 016655 | $1558. 46 | b 75 | 75 × 1320 = 99, 000 | | 1153. 90 2 | 40 | 40 × 2640 = 105, 600 | | 1230. 77 3 | 10 | 10 × 2640 = 26, 400 | | 307. 69 4 | 25 | 25 × 1320 = 33, 000 | | 384. 66 5 | [2]85 | 85 × 5280 = 448, 800 | | 5230. 88 6 | 15 | 15 × 5280 = 79, 200 | | 923. 08 7 | [2]65 | 65 × 7920 = 514, 800 | | 6000. 00 8 | 35 | 35 × 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 ofroad is made up in like manner, and let it be assumed that the portionof 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 zoneone-fourth mile in width and the rates for each zone the same asbefore. 

 ------+-------+----------------------+------------+------------- | | Rate × 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 ofroad is especially heavy on parcels 1, 2 and 3. In order to preventunjust charges against such properties, laws usually limit the totalassessment against any parcel of land to a fixed percentage of a fairmarket value or of the assessed value. The assessment on these parcelswould be reduced as seemed expedient and the deficit would bedistributed over the remainder of the area in the same manner as theoriginal assessment was spread. In practice such re-distribution isordinarily made by the arbitrary adjustment in accordance with whatthe authorized officials consider to be fair and equitable. The methodoutlined is merely a mechanical means of securing distribution andmust not be considered as an infallible method of making theassessment. It is always necessary to review the results in the lightof the actual benefits to be presumed for each parcel of land. Nevertheless, the method outlined will prove equitable in a majorityof cases. 

=General Taxation. =--There is a general community benefit derived fromthe construction of good roads in that the actual cost of marketingfarm products is lessened with a resulting lowering of the price tothe consumer. The benefit also accrues from the greater facility withwhich all community business may be conducted. The introduction ofbetter opportunities for social, religious and educational activitiesin the rural districts which results from improved highways is also acommunity benefit of no mean importance. A part of the cost of roadimprovement may therefore be equitably paid from funds obtained bygeneral taxation. 

A considerable portion of the current expense of maintaining thetownship and county highway work and at least a part of the cost ofmaintaining state highway activities is met from funds obtained bygeneral taxation. Likewise, the funds required for the amortization ofbond issues are often obtained from general taxation although vehiclelicense fees are sometimes used for that purpose. 

General taxes are levied on all taxable property in a political unitunder statutory provisions regulating the amount of the levy and thepurpose for which the revenue is to be used. In the aggregate, theroad taxes are large but in the township or county the rate isgenerally small compared to some other taxes, such as the school tax. 

=Vehicle Taxes. =--The great direct benefit derived by those whoactually operate vehicles over the roads justifies the policy ofrequiring a vehicle to pay a license fee in lieu of other taxes, thefunds so obtained to be used for the construction and maintenance ofpublic highways. In practice, this method has already been applied tomotor vehicles in most states and has proven to be an important sourceof revenue. Its application to horse-drawn vehicles has not beenattempted, due probably to the fact that such horse-drawn vehicles asuse the public highways are also employed about the farm or in thetowns and the determination of an equitable basis for taxationinvolves many difficulties. 

The rate of the fee for motor vehicles should be based on theirdestructive effect on the road so far as that is possible. The scaleof fees should therefore take account of weight and speed of vehicleand if the license is in lieu of all other taxes, it should also begraduated with the cost of the vehicle. 

When funds are thus derived, every precaution should be taken toinsure that the money is used judiciously for construction andespecially for maintenance on those roads most useful to motortraffic. 

=Highway Bonds. =--Bond issues for road improvement afford a means ofconstructing roads and paying for them while they are being used. Avery large volume of such bonds are outstanding in the United States. Road bonds should be issued only for durable types of improvement andthe life of the bond should be well within the probable useful life ofthe road surface. It is customary and highly desirable that thegeneral nature and extent of the improvement be established before thebonds are issued. It is desirable that bond issues be subject toapproval by referendum before issue and that is provided in everyinstance. 

Highway bonds are of three classes known as Sinking Fund, Annuity andSerial Bonds, respectively. The earlier bonds issued were almost allof the sinking fund class, but in recent years the serial bond hasbeen widely employed and is probably the most satisfactory toadminister. 

=Sinking Fund Bonds. =[1]--When this type of bond is employed, theamount of the expenditure for road improvement is determined upon andthe length of the period during which tax payments shall be made issettled. To employ a concrete example, it may be assumed that $100, 000is to be expended for road work and is to be paid at the end of tenyears. The interest rate on the bonds will vary with the condition ofthe bond market and the stability of the political unit issuing thebonds, but is usually about 5 per cent. Knowing these factors, theamount to be added to the sinking fund each year is computed. In orderto 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 isset aside each year which is supposed to be invested and draw interestwhich will be added to the principle, and the principle and interestcomprise the sinking fund. The principle of the sinking fund isobtained by tax levies, a sum being added to the principle of thesinking 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 properadministration of the sinking fund. It must be invested with fidelityand the fund be kept intact. Usually the sinking fund cannot beinvested at as high a rate of interest as the bonds bear and there issome loss as a result. Road bonds bearing 5 per cent interest canusually be sold at par while the sinking fund will usually net about 3or 3-1/2 per cent interest. The total cost of a bond issue will begreater by the sinking fund method than by either of the other methodsdescribed. 

=Annuity Bonds. =--Annuity bonds are drawn in such a manner that theamount of the payment for principle and interest is the same each yearduring the life of the bond. When the amount of the issue and the rateof interest has been determined and the amount of the desired annualpayment has been determined, the number of years the bonds must run iscomputed. 

This method is convenient in that the amount of the tax to be leviedeach year remains constant. 

=Serial Bonds. =--Serial bonds are drawn so that a uniform amount ofthe principle is retired each year after retirement starts and thetotal interest payments decrease each year after the first bonds areretired. The first bond may not be retired for a number of years afterthe issue of the bonds, but when it once starts retirement proceeds ata constant rate annually. 

=Comparison of Methods of Issuing Bonds. =--The relative costs offinancing by either of the three methods depends upon the rate ofinterest in each case and the net rate secured on the sinking fundprovided 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 byeach 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 financingenables the highway authorities to construct a large mileage of roadsin a few years and spreads the cost over the period during which thepublic is being benefited. Better prices are obtained on contracts fora large mileage than for smaller jobs, and the community can receivethe benefit more quickly than where construction proceeds piecemealwith current funds. The vital consideration is to insure that the termof the bonds is well within the useful life of the road, and thatample provision is made to maintain the roads during that period. Under proper restrictions the bond method of financing is to becommended. The bonds are an attractive investment and readilymarketable on satisfactory terms. 

CHAPTER III

DRAINAGE OF ROADS

=The Necessity for Drainage. =--The importance of drainage for allroads subject to the effects of storm or underground water has alwaysbeen recognized by road builders, but during recent years constantlyincreasing attention has been given to this phase of roadconstruction. It is unfortunate that there has in the past been sometendency to consider elaborate drainage provisions less necessarywhere 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 connectionwith either earth roads or any class of surfaced roads is to inviterapid deterioration of some sections of the roadway surface and to addto maintenance costs. 

The degree to which lack of drainage provisions affect theserviceability of the road surface varies with the amount ofprecipitation in the locality and the manner in which it isdistributed throughout the year. In the humid areas of the UnitedStates, which are, roughly, those portions east of a north and southline passing through Omaha and Kansas City, together with the northernpart of the Pacific slope, precipitation is generally in excess of 30inches per year and fairly well distributed throughout the year, butwith seasonal variations in rate. In these areas, the effect of theprecipitation, both as regards its tendency to lower the stability ofsoils and as an eroding agent, must be carefully provided against inhighway design. 

Outside of the areas mentioned above, the precipitation is much lessthan 30 inches per year and its effect as an agent of erosion is ofgreatest significance, although in restricted areas there may be shortperiods when the soil is made unstable by ground water. 

=Importance of Design. =--The drainage system for a proposed roadimprovement ought to be designed with as much care as any otherelement, and, to do so, a study must be made of all factors that haveany bearing on the drainage requirements and the probableeffectiveness of the proposed drainage system. The well establishedprinciples of land drainage should be followed so far as applicable. 

The basic principle of road drainage is to minimize the effect ofwater to such an extent that there will always be a layer ofcomparatively dry soil of appreciable thickness under the traveledpart of the road. This layer should probably never be less than twofeet thick and for soils of a structure favorable to capillary actionit should be at least three feet thick. The means employed toaccomplish the requisite drainage will be as various as the conditionsencountered. 

=Surface Drainage. =--The drainage method which is by far the mostnearly general in application is that which utilizes open ditches, andthe system which employs these ditches is usually referred to assurface drainage. The full possibilities of this method of minimizingthe effects of storm water are rarely fully utilized in roadconstruction. Very frequently, deterioration of a road surface isdirectly attributable to failure to provide adequately for the removalof the storm water or water from the melting of snow that has fallenon the road, or water that flows to the road from land adjacentthereto. Surface water can usually most cheaply and expeditiously becarried away in open ditches, although special conditions areoccasionally encountered which require supplementary tile drains. Thecross section commonly adopted for roads lends itself naturally to theconstruction of drainage ditches at the sides of the traveled way, andthese are usually the principal dependence for the disposal of stormwater. 

=Run-off. =--The capacity required of side ditches to insuresatisfactory surface drainage will be affected by the amount andnature of the precipitation in the region where the road is built. Theannual rainfall in a region may amount to several feet, but may bewell distributed throughout the year with an absence of excessiverainfall for short periods, that is, flood conditions may rarelyoccur. In other areas, the annual rainfall may be comparatively smallbut the precipitation occurs at a very high rate, that is, floodconditions may be common, or it may be at a low rate extending over aconsiderable period. These peculiarities must be known before anadequate drainage system can be planned. 

It is almost universally true in the United States that precipitationat a very high rate will be for a relatively short duration, andduring these short periods, which usually do not exceed thirtyminutes, a portion of the water that falls on the areas adjacent tothe road and that drains to the road ditches will soak into the soiland therefore not reach the ditches along the road. The extent towhich the water is taken up by the soil will vary with the porosityand slope of the land and the character of the growth thereon. Cultivated land will absorb nearly all of the water from showers up tofifteen or twenty minutes duration; grass land a somewhat smallerpercentage; and hard baked or other impervious soil will absorb acomparatively small amount. Rocky ground and steep slopes will absorbvery little storm water. 

The surface of the road is designed to turn water rapidly to theditches, but when the material is the natural soil, there is alwaysconsiderable absorption of storm water. Surfaces such as sandclay, gravel and macadam do not absorb to exceed 10 per cent of theprecipitation during short showers. Bituminous surfaces, brick andconcrete pavements, do not absorb an appreciable amount of stormwater. 

Generally it is best to assume that if a rain lasts for forty-fiveminutes or more, all of the water will run off, as the soil will reacha state of saturation in that time. This is not true of deep sand, butis 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 characteristicsof the region, and upon the nature of the road surface. 

For a required capacity, the cross section area of the ditch will varyinversely as the grade, because the velocity of flow increases with anincrease in the grade of the ditch. If the surface water must becarried along the road for distances exceeding five or six hundredfeet, the ditch must be constructed of increasing capacity toward theoutlet in order to accommodate the accumulated volume of water. 

The velocity of flow varies not only with the grade, but with theshape of the cross section, cleanness of the channel, the depth of thewater in the channel, alignment of the channel and the kind ofmaterial in which the channel is formed. It is not necessary to go togreat refinement in the design of the side ditches for the ordinarycase where the water is carried along the road for only a few hundredfeet. The ditches are made of ample capacity by using the commonlyaccepted cross section for a road, which will be discussed in a laterparagraph. But where large areas must be drained by the road ditches, it is desirable carefully to design the side ditches. The basis forthat design is too lengthy to be included herein, and reference shouldbe made to a standard treatise on the subject. 

=Ordinary Design of Ditches. =--For grades of one per cent or less onroads in the humid area, the bottom of the ditch should be at leastthree and one-half feet lower than the traveled surface of the road, except for very sandy soil. For grades greater than one per cent, thisdepth may be decreased one foot, and for grades of four per cent andupward, the depth may be still less. These general rules for depth aresusceptible of variation but are believed to be the minimum except inarid or semi-arid climates. It is far better to be too liberal inditch allowance than to be too conservative. In arid or semi-aridregions, the ditch design will be based on the necessity of providingfor flood flow and preventing damage through erosion. Ordinarydrainage requirements will be satisfactory with the ditch about onefoot deep. 

If the topography is such that it is evident considerable storm waterwill flow from the adjacent land to the road ditches, the design mustbe modified to take this into account. Sometimes such water can bediverted by ditches well back from the road, and thus prevented fromflowing into the side ditches along the roadway. It is especiallydesirable to divert water, which would otherwise flow down the slopeof a cut, by means of a ditch on the hill-side above the upper edge ofthe slope of the cut. 

Ditches are not effective unless they afford a free flow throughouttheir length and have an outlet to a drainage channel of amplecapacity. Therefore, ditch grades should be established by survey, especially if the gradient is less than one per cent, and theconstruction work should be checked to insure that the ditch isactually constructed as planned. A few high places in the ditch willgreatly reduce the effectiveness, although these may appear at thetime of construction to be slight. Constricted places, such as mightbe due to a small amount of loose earth left in the ditch, are alwaysto 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 naturalsoil, five feet or more from the toe of the slope of the fill, andalong the filled portion of the road there should be a berm of threeor four feet between the toe of the slope of the fill and the nearedge of the ditch. 

=Underground Water. =--In a preceding paragraph, mention was made ofthe fact that only a part of the storm water runs off over the surfaceof the ground, the larger part being absorbed by the soil. The waterthus absorbed flows downward through the pores in the soil until it isdeflected laterally by some physical characteristic of the soilstructure. The movement of underground water is affected by manycircumstances, but only two conditions need be discussed herein. 

Underground water, like surface water, tends to attain a levelsurface, but in so doing it may need to flow long distances throughthe pores of the soil, and to overcome the resistance incident to sodoing some head will be required. That is to say, the water will behigher at some places than at others. If a cut is made in grading theroad, the road surface may actually be lower than the ground waterlevel in the land adjoining the road. As a result, the water will seepout of the side slopes in the cut and keep the ditches wet, or evenfurnish enough water to occasion a flow in the ditch. Similarly, thehigher head of the underground water near the top of a hill may resultin ground water coming quite close to the surface some distance downthe hill. The remedy in both cases is tile underdrains alongside theroad to lower the ground water level so that it cannot affect the roadsurface. 

Sometimes the ground water encounters an impervious stratum as itflows downward through the soil, or one that is less pervious than thesurface soil. When such is the case, the water will follow along thisstratum, and should there be an outcrop of the dense stratum, a springwill be found at that place. This may be on a highway. The imperviousstratum may not actually outcrop but may lie only a few feet under thesurface of the road, in which case, the road surface will be so watersoaked as to be unstable. The so-called "seepy places" so often notedalong a road are generally the result of this condition. Thiscondition can be corrected by tile laid so as to intercept the flow ata depth that precludes damage to the road. Commonly, the tile will belaid diagonally across the road some distance above the section wherethe effect of the water is noted, and will be turned parallel to theroad at the ditch line and carried under one of the side ditches to anoutlet. 

=Tile Drains. =--Where the soil and climatic conditions are such thatthe roadway at times becomes unstable because of underground waterrising to a level not far below the road surface, the ground waterlevel is lowered by means of tile underdrains. The function of thetile drains in such cases is precisely the same as when employed inland drainage; to lower the ground water level. 

=Laying Tile. =--The tile lines are usually laid in trenches parallelto the center line of the road near the ditch line and at least 4 feetdeep so as to keep the ground water level well down. They must becarefully laid to line and grade. A good outlet must be provided andthe last few joints of pipe should be bell-and-spigot sewer pipe withthe joints filled with cement mortar. The opening of the tile shouldbe covered with a coarse screen to prevent animals from nesting in thetile. 

It is frequently necessary to lay a line of tile at the toe of theslope in cuts to intercept water that will percolate under the roadfrom the banks at the sides. In some cases, it is desirable toback-fill the tile trench with gravel or broken stone to insure rapidpenetration of surface water to the tile. In other instances, it isadvantageous to place catch basins about every three or four hundredfeet. These may be of concrete or of tile placed on end or may beblind catch basins formed by filling a section of the trench withbroken stone. When a blind catch basin is used, the top should bebuilt up into a mound, and for a tile or concrete catch basin, agrating of the beehive type should be used, so that flow to the tilewill not be obstructed by weeds and other trash that is carried to thecatch basin. 

=Culverts. =--Culverts and bridges are a part of the drainage systemand the distinction between the two is merely a matter of size. Generally, structures of spans less than about eight feet are classedas culverts, but the practice is not uniform. In this discussionculverts will be defined as of spans of 8 feet or less. 

Numerous culverts are required to afford passage for storm water andsmall streams crosswise of the road, and their aggregate cost is alarge item in the cost of road improvement. The size of the waterwayof a culvert required in any location will be estimated by aninspection of the stream and existing structure, and by determiningthe extent and physical characteristics of the drainage area. Sometimes there is sufficient evidence at the site to indicate quiteclosely the size required, but this should always be checked byrun-off computations. The drainage area contributing water to thestream passing through the culvert under consideration is computedfrom contour maps or from a survey of the ground, and the size ofculvert determined by one of the empirical formulas applicable to thatpurpose. In these formulas, the solution depends upon the properselection of a factor "C" which varies in accordance with the natureof the drainage area. Two of these that are quite widely used are asfollows:

 _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 varyingfrom 1 for flat country to 4 for mountainous country or rocky soil, the exact value to be selected after an inspection of the drainagearea. 

_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 rollingagricultural country, subject to floods at times of melting snow, andwith 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_. Indistricts not affected by accumulated snow, and where the length ofvalley is several times its width, 1/5 or 1/6 or even less may beused. _C_ should be increased for steep side slopes, especially if theupper part of the valley has a much greater fall than the channel atthe culvert. The value of _C_ to be used in any case is determinedafter 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 culvertshould 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 to30 feet for main roads. The headwall to the culvert should not be amonument, but should be no higher than needed to prevent vehicles fromleaving the roadway at the culvert. 

=Farm Entrance Culverts. =--At farm entrances, culverts are required tocarry the farm driveway across the side ditch of the road. Theseculverts are usually about 16 feet along, and should be of a sizeadequate to take the flow of the side ditch. The farm entrance culvertshould be of such design that it can be easily removed to permitcleaning out the ditches with a road grader. 

TYPES OF CULVERTS

Culverts constructed of concrete and poured in place are called boxculverts because of the rectangular form of the cross section. Culverts of pre-cast pipe are known as pipe culverts. Several forms ofpipe culvert are in general use. 

[Illustration: Fig. 3. --Typical Concrete Box Culvert]

=Metal Pipe. =--These may be of cast iron, steel or wrought iron. Thecast iron pipe is very durable but expensive and heavy to handle andis not widely used in highway construction. Steel pipe has beenemployed to a limited extent but its durability is questioned. Atleast it is known that the pipe made from uncoated, light sheet steelis not very durable. Sheet iron and sheets made from alloy ironcoated with spelter have been extensively used and seem to be durable, especially when laid deep enough to eliminate possibility of damagefrom heavy loads. To insure reasonable resistance to corrosion, themetal sheets should be coated with at least one and one-half ounces ofspelter per square foot of sheet and the sheets should not be lighterthan 16 gauge for small sizes and should be heavier for the largersizes. 

=Clay and Cement Concrete Pipe. =--The ordinary burned clay bell andspigot pipe that is employed for sewer construction is sometimes usedfor culverts. It must be very carefully bedded, preferably on aconcrete cradle and the joints filled with cement mortar. Culverts ofthis type have a tendency to break under unusual loads, such astraction engines or trucks. They may be damaged by the pressure fromfreezing water, particularly when successive freezing and thawingresults in the culvert filling with mushy snow, which subsequentlyfreezes. 

=Concrete Pipe. =--Reinforced concrete pipe is a satisfactory materialfor culverts, if the pipe is properly designed. The pipe should becarefully laid on a firm earth bed with earth carefully back-filledand tamped around the pipe. The joints in the pipe should be filledwith cement mortar, or should be of a design that will be tight. 

=Endwalls for Culverts. =--A substantial retaining wall is placed ateach end of the culvert barrel, whatever the type. This is to preventthe end of the culvert from becoming choked with earth and to retainthe roadway at the culvert. It also indicates to the drivers thelocation of the end of the culvert. The endwall extends a foot or morebelow the floor of the culvert to prevent water from cutting under thebarrel. Plain concrete or stone masonry are most commonly used forculvert endwalls. 

[Illustration: Fig. 4. --Two Types of Drop Inlet Culvert]

=Reinforced Concrete Box Culverts. =--The pipe culvert is limited inapplication to the smaller waterways. Reinforced concrete isextensively used for culverts of all sizes, but especially for thelarger ones. These are usually constructed with endwalls integral withthe barrel of the culvert. Culverts of this type must be designed forthe loads anticipated to insure suitable strength and stability, andmust be constructed of a good quality of concrete. Figs. 2 and 3 showdesigns for pipe and box culverts. 

[Illustration: Fig. 5. --Drop Inlet Culvert]

=Drop Inlet Culverts. =--In some locations erosion has begun in thefields adjacent to a culvert and it will probably continue until thestream above the culvert has eroded to about the level of the floor ofthe culvert. This is a reason for placing the culvert as high as theroadway will permit, so long as the area above the culvert will beproperly drained. Considerable reclamation of land is possible if theculvert 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 ofthe top of the box at the inlet end. 

Where the stream crossing the road has eroded to considerable depth orhas considerable fall, as would sometimes be the case on side hillroads, the culvert barrel would follow the general slope of the ditchbut 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 resultof spasmodic and carelessly directed work carried out at odd times onvarious sections of a road, finally resulting in the worst placesbeing at least temporarily bettered. The grade on the steepest hillsis probably reduced somewhat and some of the worst of the low lyingsections are filled in and thereby raised. Short sections of surfacingsuch as gravel or broken stone may be placed here and there. From thestandpoint 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 thosewho use the roads. In nearly every instance such work costs more inmoney and labor that it is worth. 

Lasting improvement of public highways can be brought about onlythrough systematic and correlated construction carried on for a seriesof years. In other words, there must be a road improvement policywhich will be made effective through some agency that is so organizedthat its policies will be perpetuated and is clothed with enoughauthority to be capable of enforcing the essential features of gooddesign and of securing the proper construction of improvements. 

Details of highway construction and design must vary with many localconditions and types of surface. The limits of grades and the manyother details of design may properly be adopted for a specific pieceof work only after an adequate investigation of the local requirementsand 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 devicesand improvements in methods remain long unknown except to thecomparatively few who have the means for informing themselves of allsuch developments. 

It follows then that the logical system of conducting road improvementis through an agency of continuing personnel which will supervise thepreparation of suitable plans and direct the construction inaccordance with the most recent experience. 

=Road Plans. =--The information shown on the plans prepared for roadimprovement varies somewhat with the design and with the ideas of theengineer as to what constitutes necessary information, but in generalthe plans show the existing road and the new construction contemplatedin an amount of detail depending principally upon the character of theconstruction. Simple plans suffice for grade reduction or reshaping anearth road surface, while for the construction of paved roads, theplans must be worked out in considerable detail. The essentialrequirement is that there be given on the plans all informationnecessary to enable the construction to be carried out according tothe intentions of the engineer, that all parts of the work fittogether, that the culverts are of the proper size and located at theproper places, ditches drain properly, grades are reduced to thepredetermined rate, that excavated material is utilized and that anexact record of the work done is retained. Plans are indispensable toeconomical road construction and the preparation of the plans is thework of the expert in road design, that is, the highway engineer. 

=Problem of Design. =--The problem of road design is to prepare plansfor a road improvement with the various details so correlated as toinsure in the road constructed in accordance therewith the maximum ofsafety, convenience and economy to the users thereof. The degree towhich the design will be effective will depend to a considerableextent upon the financial limitations imposed upon the engineer, butskill and effort on the plans will do a great deal to offset financialhandicap and no pains should be spared in the preparation of theplans. Moreover, the plans must afford all of the information neededby the contractor in preparing a bid for the work. 

=Preliminary Investigation. =--The first step in road improvement is tosecure an adequate idea of the existing conditions on the road orroads involved. The detail to which this information need go willdepend entirely upon the purpose of the preliminary investigation, forbefore a definite plan is prepared, it may be necessary to choose thebest from among several available routes. For this purpose, it is notalways necessary to make an actual instrument survey of the severalroutes. A hasty reconnaissance will usually be sufficient. This ismade by walking or riding over the road and noting, in a suitable bookor upon prepared blanks, the information needed. The items ofinformation recorded will usually be as follows: distances, grades, type of soil on the road and nature of existing surface, character ofdrainage, location of bridges and culverts and the type of each withnotes as to its condition, location of railway crossings and notes asto type, location of intersecting roads, farm entrances, and allsimilar features that have a bearing on the choice of routes. Thesedata can be obtained in a comparatively short time by a skilledobserver who may drive over the road in a motor car. Sometimes it maybe desirable to make a more careful study of some certain sections ofroad and this may be done by waking over the section in question inorder to make a more deliberate survey of the features to beconsidered than is possible when riding in a motor car. 

Factors other than relative lengths of routes will obviously determinethe cost of improvement and the comparative merits of the improvedroads. Some special characteristic of a road, such as bad railroadcrossings or a few bad hills, may eliminate a route, or availabilityof materials along a route may offset disadvantages of alignment orgrade. 

In special cases, complete surveys of routes may be required finallyto select the best route, but these instances are few in number. 

=Road Surveys. =--When a road has been definitely selected forimprovement, a careful survey is made to furnish information for thepreparation of the plans. This will consist of a transit survey and alevel survey. 

The transit survey is made by running a line between establishedcorners following the recorded route of the road, or if no records areavailable or the road is irregular in alignment, by establishingarbitrary reference points and running a line along the center line ofthe existing road or parallel thereto. The topography is referenced tothis line in such completeness that it can be reproduced on the plans. The level survey consists in taking levels on cross sections of theroad at one hundred foot intervals, and oftener if there are abruptchanges in grade. Special level determinations are made at streams, railroad crossings, intersecting roads or lanes and wherever itappears some special features of the terrain should be recorded. 

From the surveys and such other information as has been assembledrelative to the project, a plan is prepared which embodies a designpresumed to provide for an improvement in accordance with the besthighway practice. 

THE PROBLEM OF DESIGN

It will be convenient to consider separately the components of a roaddesign, although in the actual design the consideration of thesecannot be separated because all parts of the plan must fit together. 

=Alignment. =--The alignment of the road is determined to aconsiderable extent by the existing right-of-way, which may followsection lines, regardless of topography, as is the case with manyroads in the prairie states, or it may follow the valleys, ridges, orother favorable location in hilly country. In many places the roads ofnecessity wind around among the hills in order to avoid excessivegrades. In designing an improvement, it is generally desirable tofollow the existing right-of-way so far as possible. But the elementof safety must not be lost sight of, and curves should not preclude aview ahead for sufficient distance to insure safety to vehicles. Thenecessary length of clear view ahead is usually assumed to be 250feet, but probably 200 feet is a satisfactory compromise distance whena greater distance cannot be obtained at reasonable cost. To securesuitable sight distance, the curves must be of long radii, and wherepossible the right-of-way on the inside of the curve should be clearedof trees or brush that will obstruct the view. Where the topographywill not permit a long radius curve and the view is obstructed by anembankment or by growing crops or other growth, it is desirable toseparate the tracks around the curve to eliminate the possibility ofaccidents on the curve. This is readily accomplished if the road issurfaced, but if it is not surfaced, the same end is accomplished bymaking the earth road of ample width at the curve. 

Relocations should be resorted to whenever they shorten distances orreduce grades sufficiently to compensate for the cost. 

=Intersections. =--At road intersections, it is always difficult todesign a curve that entirely meets the requirements of safety becausethere is not enough room in the right-of-way, and enough additionalright-of-way must be secured to permit the proper design. It is notnecessary to provide an intersection that is adapted to high speedtraffic, where main roads cross, but, on the contrary, a design thatautomatically causes traffic to slow up has distinct advantages. 

Where a main route, improved with a hard surface, crosses secondaryroads, it is satisfactory to continue the paved surface across theintersecting road at normal width and make no provision for theintersecting road traffic other than a properly graded approach at theintersection. 

=Superelevation. =--On all curved sections of road, other thanintersections, account is taken of the tendency of motor cars to skidtoward the outside of the curve. This tendency is counteracted bydesigning the cross section with superelevation. 

[Illustration: Fig. 6]

In Fig. 6, _F_ represents the tangential force that tends to causeskidding. _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 ismoving. _w_ is the component of the weight parallel to the surface_c-d_, _v_ = velocity of the vehicle in feet per second. _m_ = massof 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 ofsuperelevation 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 squareof the velocity and inversely as the radius of the curve. 

Theoretically, the amount of the superelevation should increase with adecrease in the radius of the curve and should also increase as thesquare of the speed of the vehicle. On account of the variation inspeeds of the vehicles, the superelevation for curves on a highway canonly be designed to suit the average speed. At turns approachingninety degrees, the curve is likely to be of such short radius that itis impossible to maintain the ordinary road speed around the curve, even with the maximum superelevation permissible. It is good practiceto provide the theoretical superelevation on all curves having radiigreater than 300 feet for vehicle speeds of the maximum allowed bylaw, which is generally about 25 miles per hour. Where the radii areless than 300 feet, the theoretical superelevation for the maximumvehicle speeds gives a superelevation too great for motor trucks andhorse drawn vehicles and generally no charge is made in superelevationfor radii less than 300 feet, but all such curves are constructed withthe same superelevation as the curve with 300 foot radius. 

The diagram in Fig. 7 shows the theoretical superelevation for variouscurve radii. 

[Illustration: Fig. 7. Curves showing Theoretical Superelevation forVarious Degrees of Curve for Various Speeds of Vehicle]

At the intersection of important highways, the problem is complicatedby the necessity for providing for through traffic in both directionsand for traffic which may turn in either direction and the engineermust provide safe roadways for each class of traffic. 

=Tractive Resistance. =--The adoption of a policy regarding the gradeson a road involves an understanding of the effect of variation in thecharacter of the surface and in rate of grade upon the energy requiredto transport a load over the highway. The forces that oppose themovement of a horse drawn vehicle are fairly well understood and theirmagnitude has been measured by several observers, but comparativelylittle is known about the forces opposing translation of rubber tiredself-propelled vehicles. 

The resistance to translation of a vehicle is made up of threeelements: resistance of the road surface to the rolling wheel, resistance of the air to the movement of the vehicle and internalfriction in the vehicle itself. 

=Rolling Resistance. =--When the wheel of a vehicle rolls over a roadsurface, both the wheel and the surface are distorted. If the wheelhas steel tires and the road surface is plastic, there will beconsiderable distortion of the road surface and very little of thewheel. A soft rubber tire will be distorted considerably by a brickroad surface. Between these extremes there are innumerablecombinations of tire and road surface encountered, but there is alwaysa certain amount of distortion of either road surface or wheel, or ofboth, which has the same effect upon the force necessary fortranslation as a slight upward grade. When both the tire and the roadsurface strongly resist distortion (as steel tires on vitrified brickpaving), the resistance to translation is low but the factor of impactis likely to be introduced. Where impact is present, energy is used upin the pounding and grinding of the wheels on the surface, and thisfactor increases as the speed of translation, and may be aconsiderable item. Impact is especially significant on rough roadswith motor vehicles, particularly trucks, traveling at high speed. These two factors (impact and rolling resistance) combined constitutethe major part of the resistance to translation for horse drawnvehicles. 

=Internal Resistance. =--For horse drawn vehicles, the internalresistance consists of axle friction, which is small in amount. Forself-propelled vehicles, the internal resistance consists of axlefriction and friction in the driving mechanism, of which gearfriction and the churning of oil in the gear boxes is a large item. Internal friction is of significance in all self-propelled vehiclesand especially so at high speeds. 

=Air Resistance. =--At slow speeds, the resistance of still air totranslation is small, but as the speed increases, the air resistanceincreases rapidly and at the usual speed of the passenger automobileon the road becomes a very considerable part of the total resistanceto translation. This factor has no significance in connection withhorse drawn vehicles, but is to be taken into account when dealingwith self-propelled vehicles at speeds in excess of five miles perhour. 

Many determinations of tractive resistance with horse drawn vehicleshave been made from time to time and these show values that are fairlyconsistent when the inevitable variations in surfaces of the same typeare taken into account. Table 4 is a composite made up of valuesselected from various reliable sources and Table 5 is from experimentsby 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 resistanceof motor vehicles, but the following tables are based on sufficientdata to serve to illustrate the general trend. 

These data on the tractive resistances of an electric truck with solidrubber tires on asphalt and bitulithic, wood, brick and granite block, water-bonded and tar macadam, cinder and gravel road surfaces wereobtained by A. E. Kennelly and O. R. Schurig in the research divisionof the electrical engineering department of the MassachusettsInstitute of Technology, and are published in Bulletin No. 10 of thedivision. 

An electric truck was run over measured sections, ranging from 400 to2600 feet in length, surfaced with these various materials, at certainspeeds 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 variouscombinations. 

 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 totranslation due to the fact that there is a component of the weight ofthe vehicles parallel to the road surface and opposite in direction tothe motion when the load is ascending the hill and in the samedirection when the vehicle is descending. In Fig. 8 _W_ represents theweight of the vehicle, acting vertically downward, _w_ is thecomponent 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 × per cent of grade. 

 _W_{2}_ = 0. 01 _W_ × per cent grade. 

 _W_{2}_ = 0. 01 × 2000 × 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 agrade is therefore 20 lbs. Per ton for each one per cent of grade andthis force tends either to retard or to accelerate the movement of thevehicle. 

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 andany 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 canmaterially decrease _f_{a}_ by reducing speed. _f_{g}_ varies with therate of grade. For any vehicle loaded for satisfactory operation on alevel road with the power available, the limiting condition is thefactor _f_{g}_. If the load is such as barely to permit motion on alevel road, any hill will stall the vehicle. Therefore, in practicethe load is always so adjusted that there is an excess of power on alevel road. If draft animals are employed the load is usually aboutone fourth of that which the animals could actually move by theirmaximum effort for a short period. With motor vehicles, the excesspower is provided for by gearing. 

If it be assured a load of convenient size is being moved on a levelroad by draft animals, there is a limit to the rate of grade up whichthe 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 perhour, an amount equal to 1/8 to 1/10 of its weight, and for shortintervals can pull 3/4 of its weight. The maximum effort possible istherefore six times the average pull, but this is possible for onlyshort intervals. A very short steep hill would afford a conditionwhere such effort would be utilized. But for hills of any length, thatis, one hundred feet or more but not to exceed five hundred feet, itis safe to count on the draft animal pulling three times his normalpulling power for sustained effort. 

The limiting grade for the horse drawn vehicle is therefore onerequiring, to overcome the effect of grade, or _f_{g}_, a pull inexcess of three times that exerted on the level. 

A team of draft animals weighing 1800 lbs. Each could exert acontinuous pull of about 1/10 of their weight or 360 lbs. If it beassumed that the character of the vehicle and the road surface is suchthat _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 100 lbs. Per gross ton ona level section of road, then the gross load for the team would be 3. 6tons. The same team could for a short time exert an additional pull ofthree times 360 lbs. Or 1080 lbs. For each 1 per cent of grade a pullof 20 lbs. Per ton would be required or _f_{g}_ for the 3. 6 tons loadwould be 72 lbs. For each per cent of grade. At that rate, thelimiting grade for the team would be fifteen per cent. 

If, however, the character of the vehicle and the road surface weresuch that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 60 lbs. Per grosston on a level section of road, the gross load for the team on thelevel would be 6 tons, and the limiting grade 9 per cent. 

The above discussion serves to illustrate the desirability of adoptinga low ruling or limiting grade for roads to be surfaced with amaterial having low tractive resistance and the poor economy ofadopting a low ruling grade for earth roads or roads to be surfacedwith material of high tractive resistance. 

It may be questioned whether horse drawn traffic should be thelimiting consideration for main trunk line highways, but it iscertain that for a number of years horse drawn traffic will be afactor on secondary roads. 

In the case of motor vehicles, excess power is provided by means ofgears and no difficulty is encountered in moving vehicles over gradesup to 12 or 15 per cent, so that any grade that would ordinarily betolerated on a main highway will present no obstacle to motorvehicles, but the economy of such design is yet to be investigated. 

=Energy Loss on Account of Grades. =--Whether a vehicle is horse drawnor motor driven, energy has been expended in moving it up a hill. Apart of this energy has been required to overcome the variousresistances other than grade, and that has been dissipated, but theenergy required to translate the vehicle against the resistance due tograde has been transformed into potential energy and can be partiallyor wholly recovered when the vehicle descends a grade, provided thephysical conditions permit its utilization. If the grade is so steepas to cause the vehicle to accelerate rapidly, the brakes must beapplied and loss of energy results. The coasting grade is dependentupon the character of the surface and the nature of the vehicle. Inthe cases discussed in the preceding paragraph, the coasting gradeswould be five per cent and three per cent respectively. For horsedrawn vehicles then the economical grades would be three and five percent, which again emphasizes the necessity of lower grades on roadsthat are surfaced than on roads with no wearing surface other than thenatural soil. 

The theory of grades is somewhat different when motor vehicles areconsidered, since it is allowable to permit considerably higher speedthan with horse drawn vehicles before applying the brakes and theeffect of grade can be utilized not only in translating the vehicledown the grade, but also in overcoming resistances due to mechanicalfriction and the air. On long grades, a speed might be attained thatwould require the use of the brake or the same condition might applyon very steep short grades. There is at present insufficient data onthe tractive resistance and air resistance with motor vehicles topermit the establishing of rules relative to grade, but experienceindicates a few general principles that may be accepted. 

If a hill is of such rate of grade and of such length that it is notnecessary to use the brake it may be assumed that no energy lossresults so far as motor vehicles are concerned. Where there is no turnat the bottom of the hill and the physical condition of the roadpermits speeds up to thirty-five or forty miles per hour grades offive per cent are permissible if the length does not exceed fivehundred feet and grades of three per cent one thousand feet long areallowable. It is a rather settled conviction among highway engineersthat on trunk line highways the maximum grade should be six per cent, unless a very large amount of grading is necessary to reach thatgrade. 

=Undulating Roads. =--Many hills exist upon highways, the grade ofwhich is much below the maximum permissible. If there are gradesranging from 0 to 4 per cent, with a few hills upon which it isimpracticable to reach a grade of less than six per cent, it isquestionable economy to reduce the grades that are already lower thanthe allowable maximum. It is especially unjustifiable to incur expensein reducing a grade from two per cent to one and one-half per cent ona road upon which there are also grades in excess of that amount. Theundulating road is not uneconomical unless the grades are above theallowable maximum or are exceptionally long or the alignment followsshort radius curves. 

=Safety Considerations. =--On hills it is especially desirable toprovide for safety and curves on hills are always more dangerous thanon level sections of road. Therefore, it is desirable to provide asflat grades as possible at the curves and to cut away the berm at theside of the road so as to give a view ahead for about three hundredfeet. Whether a road be level or on a hill, safety should always beconsidered and the most important safety precaution is to provide aclear view ahead for a sufficient distance to enable motor vehicledrivers to avoid accidents. 

[Illustration: Fig. 9. --Types of Guard Rails]

=Guard Railing. =--When a section of road is on an embankment, guardrails are provided at the top of the side slope to serve as warningsof danger, and to prevent vehicles from actually going over theembankment in case of skidding, or if for any reason the driver losescontrol. These are usually strongly built, but would hardly restrain avehicle which struck at high speed. But they are adequate for theprotection of a driver who uses reasonable care. A typical guard railis shown in Fig. 9, but many other designs of similar nature areemployed. At very dangerous turns a solid plank wall six or eight feethigh is sometimes built of such substantial construction as towithstand the severest shock without being displaced. 

Trees, shrubs and the berms at the side of the road in cuts areparticularly likely to obstruct the view and should be cleared or cutback so far as is necessary to provide the proper sight distance. 

=Width of Roadway. =--For roads carrying mixed traffic, 9 feet of widthis needed for a single line of vehicles and 18 feet for 2 lines ofvehicles. In accordance with the above, secondary roads, carryingperhaps 25 to 50 vehicles per day, may have an available traveled way18 feet wide. Those more heavily traveled may require room for threevehicles to pass at any place and therefore have an available traveledway 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 givethe required width of traveled way, and, in addition, provide thedrainage channels that may be needed. In regions of small rainfall theside ditches will be of small capacity or may be entirely omitted, butusually some ditch is provided. The transition from the traveled wayto ditch should be a gradual slope so as to avoid the danger incidentto abrupt change in the shape of the cross section. The depth of ditchmay be varied without changing to width or slope of the traveled partof the road as shown in Fig. 10. 

[Illustration: Fig. 10]

=Control of Erosion. =--The construction of a highway may be utilizedto control general erosion to some extent, particularly when publichighways exist every mile or two and are laid out on a gridironsystem, as is the case in many of the prairie states. The streamscross the highways at frequent intervals and the culverts can beplaced so as effectually to prevent an increase in depth of thestream. This will to some extent limit the erosion above the culvertand 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 culvertunder which they pass, it is sometimes advisable to provide tile forcarrying the water under the road, instead of the culvert, and, bycontinuing the tile into the drainage area of the culvert, eliminatethe flow of surface water and reclaim considerable areas of land. 

Erosion in the ditches along a highway can be prevented byconstructing weirs across the ditch at frequent intervals, thuseffectually preventing an increase in the depth of the ditch. 

Wherever water flows at a velocity sufficient to produce erosion orwhere the drainage channel changes abruptly from a higher to a lowerlevel, paved gutters, tile or pipe channels should be employed toprevent erosion. 

=Private Entrances. =--Entrance to private property along the highwayis by means of driveways leading off the main road. These shouldalways be provided for in the design so as to insure easy andconvenient access to the property. The driveways will usually crossthe side ditch along the road and culverts will be required to carrythe water under the driveway. Driveways that cross a gutter by meansof a pavement in the gutter are usually unsatisfactory, and to crossthe gutter without providing a pavement is to insure stoppage of theflow at the crossing. The culvert at a driveway entrance must be largeenough to take the ditch water readily or it will divert the water tothe roadway itself. Generally end walls on such culverts are notrequired as in the case of culverts across a highway. 

=Aesthetics. =--Much of the traffic on the public highways is forpleasure and relaxation and anything that tends to increase theattractiveness of the highways is to be encouraged. Usually theroadside is a mass of bloom in the fall, goldenrod, asters and otherhardy annuals being especially beautiful. In some states wild rosesand other low bushes are planted to serve the two-fold purpose ofassisting to prevent erosion and to beautify the roadside. In humidareas trees of any considerable size shade the road surface and are adistinct disadvantage to roads surfaced with the less durablematerials such as sand-clay or gravel. It is doubtful if the same istrue of paved surfaces, but the trees should be far enough back fromthe traveled way to afford a clear view ahead. Shrubs are notobjectionable from any view-point and are to be encouraged for theirbeauty, so long as they do not obstruct the view at turns. 

CHAPTER V

EARTH ROADS

Highways constructed without the addition of surfacing material to thenatural soil of the right-of-way are usually called earth roads. Butif the natural soil exhibits peculiar characteristics or is of adistinct type, the road may be referred to by some distinctive nameindicating 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 mayhave been shaped and smoothed for traffic or may be in its naturalstate except for a trackway formed by the vehicles that have used it. 

=Variations in Soils. =--The nature of the existing soil will obviouslydetermine the serviceability and physical characteristics of the roadsurface it affords. That is to say that even under the most favorableconditions some earth roads will be much more serviceable than others, due to the better stability of the natural soil. Some soils are denseand somewhat tough when dry and therefore resist to a degree thetendency of vehicles to grind away the particles and dissipate them inthe form of dust. Such soils retain a reasonably smooth trackway indry weather even when subjected to considerable traffic. Other soilsdo not possess the inherent tenacity and stability to enable them toresist the action of wheels and consequently grind away rapidly. Roadson such soils become very dusty. These are the extremes and betweenthem are many types of soils or mixtures of soils possessing varyingdegrees of stability, and, in consequence, differing rates of wear. Similarly the various soils exhibit different degrees of stabilitywhen wet. 

It is to be expected that soils will differ with the geographicallocation, for it is well known that there is a great variation insoils in the various parts of the world. But wide differences are alsoencountered in the soil on roads very near each other and even onsuccessive stretches of the same road. It is for this reason thatearth roads often exhibit great differences in serviceability even ina restricted area. 

=Variation in Rainfall. =--The stability of a soil and its ability tosupport the weight of vehicles varies greatly with the amount of waterin the soil. A certain small amount of moisture in the soil isbeneficial in that practically every soil compacts more readily whenmoist than when dry because the moisture aids in binding together theparticles. But most soils also become unstable when the amount ofwater present is in excess of that small amount referred to above andthe stability decreases very rapidly as the amount of water in thesoil increases. 

The serviceability of an earth road will change continually as themoisture content of the soil changes and consequently the generalutility of the earth road system in any locality is dependent to aconsiderable extent upon the amount and seasonal distribution ofprecipitation. The methods of maintaining earth roads appropriate toany locality must of necessity be adapted to the climatic conditions, and the amount of work required to give the highest possible degree ofserviceability will be exceedingly variable from season to season andfrom place to place. In regions of great humidity, earth roads may beexpected to have a low average of serviceability, while in aridregions they may possess sufficient durability for a considerablevolume of traffic. The design adopted for earth roads and the methodsof maintenance followed should therefore be carefully evolved to meetthe soil and climate conditions where the roads are located. Thesewill differ greatly throughout a state or even a county. 

=Cross Sections. =--The general principles of road design were setforth in Chapter IV. In Fig. 11 are shown typical cross sections forearth roads adapted to various conditions as indicated. It is notapparent that one form of ditch is particularly preferable to theother and since some engineers prefer the V section and others thetrapezoidal section both are shown. It would appear that the V shapedditch is somewhat the easier to construct with the blade grader whilethe trapezoidal is readily excavated with the slip or fresno scraper. The ditch capacity required and consequently the dimensions willdepend upon the drainage requirements, as was pointed out in ChapterIII. 

[Illustration: Fig. 11. Cross Section for Earth Roads]

EARTH ROADS IN REGIONS OF CONSIDERABLE RAINFALL

In the zones where the annual precipitation exceeds 30 inchesdistributed over several months, earth roads will be unserviceable fora considerable period each year unless they are constructed so as tominimize the effect of water. This is done by providing for the bestpossible drainage and by adopting a method of maintenance that willrestore the surface to a smooth condition as quickly as possible aftera period of rainy weather or after the "frost comes out" in thespring. 

Before the construction of the desired cross section is undertaken, all of the grade reduction should be completed, except for minor cutswhich can be handled with the elevating grader in the manner that willbe described presently. 

Where any considerable change in grade is to be effected, the earthcan be moved in several ways and of these the most economical cannotbe readily determined. Ordinarily a contractor or a county will usethe equipment that happens to be at hand even though some other mightbe more advantageous. 

=Elevating Grader. =--Where the topography is such as to permit itsuse, the elevating grader is employed in grade reduction to load theearth into dump wagons in which it is hauled to the fill or wastebank. The elevating grader consists essentially of a heavy shear plowor disc plow which loosens the earth and deposits it on a movingcanvas apron. The apron carries the material up an incline anddeposits it into a wagon which is driven along under the end of theapron. When the wagon is loaded, the grader is stopped while theloaded wagon is hauled out and an empty one drawn into position. Themotive power for the elevating grader is either a tractor or five orsix teams of mules. For many kinds of work, particularly wherefrequent turning is necessary or where the ground is yielding, mulesare preferable to a tractor. The apron is operated by gearing from therear wheels of the grader. Generally four mules are hitched to apusher in the rear of the grader and six or eight in the lead. Thismethod of grade reduction is particularly advantageous when thematerial must be hauled a distance of 500 yards or more, because wagonhauling in such cases is the most economical method to employ. Atractor may be used to draw the elevating grader and one having acommercial rating of 30 to 45 horsepower is required. 

=Maney Grader. =--If the haul is long and the nature of the cut willnot permit the use of the elevating grader because of excessive gradesor 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, suspendedfrom a four-wheel wagon gear. When loading, the scoop is let down andfilled in the same manner as a two-wheeled scraper or "wheeler. " Thepull required to fill a Maney grader is so great that a tractor isordinarily employed in place of a "snap" team. The tractor is hitchedat the end of the tongue, without interfering with the team drawingthe grader. One team readily handles the grader after it is loaded. For this service a tractor having a commercial rationing of 25 to 30horsepower is required. 

=Wheel Scraper. =--For moving earth for distances between 150 and 500yards, the wheel scraper of a capacity of about 1-1/2 yards is quitegenerally employed. The soil must be loosened with a plow before itcan conveniently be loaded into the wheeler and a heavy plow isordinarily employed for that purpose. Two furrows with the plow willloosen a strip of earth about as wide as the scoop of the scraper andif more is loosened it will be packed down by the scrapers wheeling inplace to load. A helper or "snap" team is employed to assist inloading, after which the wheel scraper is handled by one team. 

=Slip Scraper. =--The slip scraper differs from the wheel scraper inthat the scoop is not suspended from wheels but is dragged along theground. It is drawn by one team and the capacity is two to five cubicfeet, but the material spills out to some extent as the scraper isdragged along and the method is not suitable for long hauls, 100 feetbeing about the economical limit. 

=Fresno Scraper. =--The Fresno scraper is one form of slip scraperrequiring four horses or mules for efficient work. It differssomewhat from the ordinary slip scraper in shape and is of largercapacity, but is a drag type of scraper much favored in the westernstates. 

SHAPING TO PROPER CROSS SECTION

If a road has been graded so that the profile is satisfactory or ifthe existing profile of the location is satisfactory, and the surfaceis to be shaped to a prescribed cross section, either the elevatinggrader or the blade grader may be employed. 

=Elevating Grader Work. =--If the elevating grader is used in shapingthe earth road, the apron will be lowered and the material will beexcavated at the sides of the road and deposited on the middleportion. If slight changes in grade are desired, wagons will accompanythe grader and catch under the apron at the high places and haul thematerial to the low places. After the earth has been deposited it mustbe worked over to secure the correct cross section and be madepassable for vehicles. This requires that clods be broken, weeds andgrass that are mixed with the earth be removed by harrowing andforking and that the surface be carefully smoothed with a bladegrader. This latter operation will have to be repeated several timesbefore a satisfactory surface is secured. But this miscellaneous workis highly important and under no circumstances ought to be neglected. Nothing so detracts from an otherwise creditable piece of work asfailure to provide a smooth surface for the use of vehicles. It isespecially uncomfortable for the users of a highway if sods and weedsin quantity are left in the road after it has been graded. The humusthat will be left in the soil as the vegetable matter decays increasesthe porosity of the road surface making it more absorbent than soilwithout humus. This increases the susceptibility to softening fromstorm water or ground water. 

The tractor can advantageously be used to draw the elevating grader onthis class of work, but will be greatly handicapped if there are wetsections along the road, through which the tractor must be driven. Inmany cases its use is prohibited by such conditions and for all-roundservice 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 atractor are suitable for shaping the earth road. Some of these haveblades 12 feet long and excellent control for regulating the depth ofcutting. Often two such graders are operated tandem. These machineshave a device which permits the operator to steer the graderindependently of the tractor. Thus the grader can be steered off tothe side to cut out the ditches, while the tractor continues to travelon the firm part of the road. Earth moved with the blade grader isusually fairly free from large lumps and can readily be smoothed to asatisfactory surface for the use of traffic. The sods and weeds willbe drawn into the road along with the earth just as they are when theelevating grader is employed. Precaution must therefore be taken toeliminate them before the vegetable matter decays, and to smooth thesurface for the use of traffic. 

=Costs. =--The cost of shaping an earth road in the manner describedabove will vary through rather wide limits because the nature andamount of work to be done varies so greatly. Some roads can be gradedsatisfactorily 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 elevatinggrader work plus a moderate amount of grade reduction in the way ofremoving slight knolls. For the amount of grade reduction necessary inrolling country, followed by grader shaping, $1000. 00 to $1800. 00 permile will be required. The method is not adapted to rolling countrywhere the roads are undulating and require some grade reduction onevery hill. For hilly roads one of the methods described for gradereduction will be required and the cost will obviously depend upon theamount of earth moved. Averages of cost figures mean nothing in suchcases 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 hasbeen graded, it will be yielding and will readily absorb water for along time after the completion of the work. The condition of thesurface will naturally deteriorate rapidly during the first season itis used unless the road receives the constant maintenance that is aprerequisite to satisfactory serviceability. The road drag isgenerally recommended for this purpose, and if a drag is properly usedit will serve to restore the shape of the surface as fast as it isdestroyed by traffic. 

Good results with the drag depend upon choosing the proper time todrag and upon doing the work in the right way when using the drag. Thebest time to drag is as soon after a rain as the road has dried outenough to pack under traffic. If the work is done while the road istoo wet, the first vehicles traveling the road after it has beendragged will make ruts and to a considerable extent offset the gooddone by the drag. If the road is too dry, the drag will not smooth theirregularities. A little observation will be required to determine theproper time for dragging on any particular soil, but usually after arain or thaw there is a period lasting a day or two when conditionsare about right. 

[Illustration: Fig. 13. --Road Drag]

The drag is used merely to restore the shape of the surface and to doso 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 middleafter the work is completed. Some patrolmen start at one side of theroad and gradually work across the road on successive trips, finallyfinishing up at the side opposite that at which the start was made. The next dragging should start on the opposite side from the first ifthat method is followed. 

By shifting his weight on the drag, the operator can adjust thecutting edge so that very little loose material is moved crosswise ofthe road and that is the proper method to pursue. In that case noridge will remain at the middle of the road. If a slight one is leftit should be removed by a final trip with the drag. 

In addition to the dragging, weeds must be cut along the road abouttwice a year, the ditches must be kept cleaned out and culverts open. 

All of the maintenance for 10 miles of earth road can be accomplishedby one man giving his entire time to the work, and that is the onlymethod that has proven adequate to the problem. 

EARTH ROADS IN ARID REGIONS

In areas where the rainfall is less than 18 inches per year, andespecially where it is 10 inches or less, an entirely different roadproblem exists. The effect of precipitation is of significanceprimarily from the standpoint of erosion, and the design of crosssection and ditches and the culvert provisions are entirely differentfrom those necessary in humid regions. 

Frequently the rainfall in semi-arid regions will be seasonal andprovision must be made to care for a large volume of water during therainy season, but, in general, road design is adapted to prevention oferosion rather than to elimination of ground water effects, or thesoftening effects of surface water. Generally the rainy period doesnot last long enough to warrant expensive construction to eliminateits general effects. In fact, the saturation of the soil is morelikely to be a benefit than otherwise. 

Earth roads are likely to be satisfactory except where the traffic issufficient to grind the surface into dust to such an extent that anexcessive dust layer is produced. In such locations the problem is oneof providing a durable surface unaffected by long continued dryweather. 

Grade reduction will have the same importance as in humid areas andwill be carried out in the same way. 

Maintenance will consist in repairing the damage from occasionalfloods and in removing or preventing accumulations of drifting sandor dust. Crude petroleum oils have been satisfactory for maintenancein such locations when used on stable soils. 

=Value of Earth Roads. =--The serviceability of the earth road dependsto a large extent upon the care exercised in its maintenance. The onlypart of earth road construction that is permanent is the gradereduction. The cross section that is so carefully shaped atconsiderable cost may flatten out in one or two years, especially ifthe road goes through unusually wet periods. Traffic will continuallyseek a new track during the period when the road is muddy and is aslikely to cross the ditch to the sod near the fence as to use anyother part of the road. Continual and persistent maintenance istherefore essential to even reasonable serviceability. At best theearth road will be a poor facility for a considerable period each yearin the regions of year-around rainfall. In most localities, roads ofdistinctly minor importance are of necessity only earth roads and forthe comparatively small territory they serve and the small amount oftraffic, they probably serve the purpose. For roads of any importancein the humid areas of the United States, the earth road cannot carrysatisfactorily 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 ofroad surfaces composed of the natural soil existing on theright-of-way of the road. It has been found that soils of a clayeynature in which there is a considerable percentage of sand usuallyafford a serviceable road surface for light or moderate traffic, especially in areas where climatic conditions are favorable. A studyof these soils, together with the construction of experimental roadsof various mixtures of sand and clay, has led to a fairlycomprehensive understanding of the principles of construction andrange of capacity of this type of road surface, which is known as thesand-clay road. 

The sand-clay road surface consists of a natural or artificial mixtureof sand and clay, in which the amount of clay is somewhat greater thansufficient to fill the voids in the dry sand. It may be assumed thatthe sand contains 40 per cent of voids and that at least 45 per centof clay is required to fill the voids and bind the sand grainstogether, because the clay spreads the sand grains apart during themixing, thus having the effect of increasing the voids. As a matter ofexperiment, it is found to be impractical to secure by availableconstruction methods mixtures of sufficient uniformity to render itnecessary 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 considerablestudy of available materials and investigations of their behavior whencombined. Extensive and exhaustive experiments have been conductedwith sand-clay mixtures in various places where they are widely usedfor road surfaces and the following general principles have beendeduced. 

=The Binder. =--In the sand-clay road, stability is obtained byutilizing the bonding properties possessed to some degree by allsoils. Naturally this characteristic may be expected to vary widelywith the several types of soil. It is generally considered to be acommon property of clay, but the term clay is a general one that isoften applied to soils differing greatly in physical characteristicsand the term therefore loses its significance in this connection. Those soils that are properly and technically called clay aredecidedly sticky when wet and are the best materials for sand-clayconstruction. Of the clays, those that produce a tough sticky mud arebest. This can be tested by mixing a small quantity into a stiff mudand molding it into a ball and immersing in water. If the ball retainsits shape for some little time, it is likely to prove a verysatisfactory binder, but, if it becomes plastic and loses its shape, it will be an inferior binder, as a general rule. The ball clay, asthe former is called, may be of any color common to soils, notnecessarily yellow or reddish as is sometimes supposed. Likewise, balls of mixtures containing varying percentages of sand and thebinder to be used may be made up and immersed in water. The mixturethat holds its shape longest is of course the best combination of thematerials and indicates the mixture to use in the construction. 

An ideal, or even a fairly satisfactory soil for a binder may notexist in the vicinity of a proposed improvement, and consequently aninferior binder is frequently the only material available. 

Sometimes deposits of clay or gravel contain a considerable percentageof gypsum which serves as a binder and is particularly effective whenused 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 naturalmixture of sand and clay in which the ingredients happen to exist inabout the correct relative proportions for sand-clay road surfaces arefound in many localities. These mixtures are commonly referred to astop-soil. If the deposits are somewhat deficient either in sand orclay, they can be utilized if the proper corrections in theproportions are made during construction. Very satisfactory roadsurfaces are sometimes constructed with mixtures that appear to be farfrom ideal in composition, but experience and frequent trials areneeded to determine the best way in which to handle these mixtures. 

=Sand-Clay Surfaces on Sandy Roads. =--Sand-clay surfaces may beconstructed on naturally sandy roads either by adding clay and mixingit with the sand to secure the desired composition, or a layer of anatural sand-clay mixture, caliche or sand-clay-gypsum may be placedon top of the sand. 

The most widely used method is to mix clay or other binder with thesand. Since there is no need to provide for ditches to carry stormwater on a deep sand soil, the sand is graded off nearly flat acrossthe road and no ditches are provided. The clay is dumped on the roadin a layer about 8 inches thick and is then mixed into the sand. It isdesired to mix enough sand with the clay to produce a mixture composedof approximately 1/3 clay and 2/3 sand. The mixing is accomplished invarious ways, the most common being to use a heavy plow at first andto follow this with a heavy disc harrow. The mixing is a tedious anddisagreeable process, but its thorough accomplishment isindispensable. The mixing is most readily done when the materials aresaturated with water and in practice it is customary to depend uponrain for the water, although in the final stages water may be hauledand sprinkled on the road to facilitate final completion of themixing. After the mixing has been completed, the surface is smoothedwith the blade grader and is kept smooth until it dries out. Repeateddragging will be required, during the first year especially, and tosome extent each year in order to keep the surface smooth, but thedragging 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 beexpected each year, the sand-clay mixture is likely to break throughunless it is of considerable thickness and generally the surface layeris made much thicker than for regions where the annual rainfall isfairly well distributed. This is especially necessary when the binderis of inferior quality. It is not uncommon in such cases to make thesand-clay surface as much as two feet thick. 

As the mixing progresses it may appear that patches here and there aredeficient in either clay or sand and the mixture in these places iscorrected by the addition of a little sand or clay as may berequired. 

If the top-soil is used it is deposited on the sand in the requiredquantity and is remixed in place to insure uniformity. If either sandor clay is needed to give a satisfactory mixture, the proper materialis added and mixed in as the work progresses. The surface is finallysmoothed 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. Thesurface may be constructed of a natural sand-clay mixture or of a sandmixed with the natural soil. If the former, the surface of theexisting road is prepared by grading so as to insure good drainage andthe natural mixture is then deposited and the surface completed asdescribed in the preceding section. 

If the surface is formed by mixing sand with the existing soil, thesands may be deposited in a layer about six inches thick which willgradually mix with the soil as the road is used. A second applicationof sand may follow in a year or two if it is needed. Such a roadsurface will lack uniformity of composition and it seems preferable tomix the sand with the soil by plowing and discing as previouslydescribed. 

=Characteristics. =--Sand-clay road surfaces do not have sufficientdurability for heavily traveled highways, but will be satisfactory fora moderate amount of traffic. These surfaces have maximumserviceability when moist, not wet, and consequently are not asdurable in dry climates as in humid areas. They are likely to becomesticky and unstable in continued wet weather and to become friable andwear 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 consistingof a mixture of more or less rounded stones, sand and earthy material, which is found in natural deposits. These deposits exist in almostevery part of North America, being especially numerous in theglaciated areas, but by no means confined to them. Gravel depositsconsist of pieces of rock varying in size from those of a cubic yardor more in volume to the finest stone dust, but with pieces ranging insize from that which will pass a 3-inch ring down to fine sandpredominating. The larger pieces are usually more or less rounded andthe finer particles may be rounded or may be angular. Many varietiesof rocks are to be found among the gravel pebbles, but the rocks ofigneous origin and possessing a considerable degree of hardnessgenerally predominate. Intermixed with the pieces of rock there islikely to be clay or other soil, the quantity varying greatly indifferent deposits and even in various places in the same deposits. 

Often there are found deposits of material which are by the laymantermed gravel, which are really clayey sand or sand containing a fewpebbles, but which are of value to the road builder for the sand claytype of surfacing. The term gravel is exceedingly general and unlessspecifically defined, gives little indication of the exact nature ofto 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 towhich it possesses the properties of an ideal gravel for roadsurfacing. Ideal gravel is seldom encountered, but a consideration ofits characteristics serves to establish a measure by which to estimatethe probable value of any deposit. 

=The Ideal Road Gravel. =--The ideal road gravel is a mixture ofpebbles, sand and earthy material, the pieces varying from coarse tofine in such a manner that when the gravel is compacted into a roadsurface the spaces between the larger pebbles are filled with thefiner material. The pebbles are of a variety of rock that is highlyresistant to wear so that the road surface made from the gravel willhave the quality of durability. The gravel possesses good cementingproperties, insuring that the pieces will hold together in the roadsurface. The cementing property may be due to the rock powder in thedeposit or to earthy material mixed with the rock particles, or toboth. Table 7 shows the results of a number of tests made upon gravelsand indicates that the cementing property of the gravel does notalways depend upon the clay content. 

=Permissible Size of Pebbles. =--The larger pebbles in the gravel areless likely to crush under loads than smaller pebbles of the same sortof rock, but if the rock is of some of the tougher varieties such astrap, there is very little likelihood of even the smaller pebblescrushing. If the pebbles are of rock of medium toughness, the smallerpebbles might be crushed under the heavier loads. It is the usualpractice to permit gravel to be used for the foundation course inwhich the pebbles are as large as will pass a 3-1/2-inch circularscreen opening, and for the wearing course, as large as will pass a2-1/2-inch circular screen opening. If larger pebbles are allowed inthe wearing course, the surface is certain to become rough after atime. If the gravel is to be placed in a single course as is a verycommon practice, then the maximum size should not exceed that whichwill pass a 2-1/2-inch circular screen opening. 

The Wisconsin Highway Commission has constructed a very large mileageof excellent gravel roads and the sizes specified for their roads areas follows:

 "_Bottom Course Gravel_. --Bottom course shall consist of a mixture of gravel, sand and clay with the proportions and various sizes as follows:

 "All to pass a two-inch screen and to have at least sixty and not more than seventy-five per cent retained on a quarter-inch screen; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate to be retained on a one-inch screen; at least sixty-five and not more than eighty-five per cent of the total fine aggregate to be retained on a two hundred-mesh sieve. "

 "_Top Course Gravel_. --Top course shall consist of a mixture of gravel, sand and clay with the proportions of the various sizes as follows:

 "All to pass a one-inch screen and to have at least fifty and not more than seventy-five per cent retained on a quarter-inch screen; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate (material over one-fourth inch in size) to be retained on a one-half-inch screen; at least sixty-five and not more than eighty-five per cent of the total fine aggregate (material under one-fourth inch in size) to be retained on a two hundred-mesh sieve. "

 "_Screened Gravel and Sand Mixtures_. --Where it is impossible to obtain run of bank gravel containing the necessary binder in its natural state, screened gravel shall be used and the necessary sand and clay binder added as directed by the engineer. Gravel and sand shall be delivered on the work separately. Clay binder shall be obtained from approved pits and added as directed by the engineer. "

 "_Run of Bank Gravel_. --When run of bank gravel is permitted either for one course or two course work, the size shall not exceed that specified for bottom or top course. If necessary, the contractor shall pass all the material through a two-inch screen for the bottom course, and through a one-inch screen for the top course. When the work consists of only one course, the material shall be of the sizes as specified for the top course. The necessary binder shall be contained in the material in its natural state, excepting that a small percentage of clay binder may be added as directed by the Engineer. "

=Wearing Properties. =--A certain amount of grinding action takes placeon the road surface under the direct action of wheels, especiallythose with steel tires. Where rubber tired traffic predominates, thisaction is much less severe than where steel tired vehiclespredominate, but the tendency exists on all roads. In addition, thereis distortion of the layer of gravel under heavy loads which causesthe pieces of stone in the surface to rub against each other and towear away slowly. 

The gravel road in the very best condition is slightly uneven butthere is comparatively little jar imparted to vehicles, and, consequently, little impact on the surface. When somewhat worn, theimpact becomes a factor of some importance and the pounding ofvehicles has a very destructive action on the surface. Soft pebbleswill be reduced to dust in a comparatively short time. 

The degree to which any gravel resists the destructive action oftraffic depends upon the varieties of rock represented by the pebblesin the gravel. If the pebbles are mostly from rocks of good wearingproperties, that quality will be imparted to the road surface. Ifmostly from rocks of little durability, the same characteristic willbe imparted to the road surface. A very good general notion of theprobable durability of gravel can therefore be obtained by a carefulvisual examination of the material and classification of the rockvarieties represented by the pebbles. 

=Utilizing Natural Gravels. =--Gravel road construction is advantageousonly when it can be accomplished at low first cost. This usuallypresupposes a local supply of gravel that can be utilized, or at anyrate a supply that need not be shipped a long distance. In the natureof things, such deposits are likely to be deficient in some of thedesirable characteristics, and may be deficient in most of them. Byvarious means, the defects in the materials can be partially correctedwhile constructing the road. 

If the gravel deposit consists of layers of varying composition asregards size and clay content, the material may be loosened from theexposed face and allowed to fall to the bottom of the pit therebybecoming mixed to a sufficient extent to produce a reasonably uniformproduct. If deficient in clay, it often proves feasible to add a smallpart of the clay over-burden, thereby insuring enough binder. Sometimes adjoining deposits will consist one of relatively finematerial, the other of relatively coarse. These may be mixed on thework by first placing the coarse material in a layer about 5 inchesthick and adding the finer material in a similar layer. The two willmix very rapidly during the operations of spreading and shaping. 

When deposits contain pebbles larger than will pass a 3-1/2-inch ring, these larger stones will prove to be undesirable if placed on theroad, as they are almost sure to work to the surface of the gravellayer and become a source of annoyance to the users of the road. Oversize stone can be removed while loading the gravel or whilespreading it, if care is exercised and not too large a proportion isoversize. It is preferable however to remove the oversize by means ofscreens at the pit. Usually on large jobs the oversize is crushed andmixed with the supply so as to utilize what is really the best part ofthe material. 

Gravels deficient in bonding material are often encountered indeposits where there is insufficient overburden to give enoughadditional binder or where the overburden is of a material unsuitablefor binder. Such materials may be utilized by adding binder in theform of clay after the gravel has been placed on the road. 

Almost any gravel deposit can be utilized in some way if the materialis of a durable nature, regardless of other characteristics. Theserviceability of a gravel road will depend largely on how nearly thegravel approaches the ideal, but variations in the manipulations willdo much to overcome deficiencies in materials. 

=Thickness of Layer. =--The thickness of the layer of gravel requireddepends both upon the type of soil upon which it is placed and thenature of the traffic to which the road will be subjected. Gravelsurfaces should not ordinarily be constructed on highways carryingheavy truck traffic, but if gross loads of three or four tons are theheaviest anticipated, the gravel will be reasonably stable. On suchroads, a layer of well compacted gravel ten inches thick will supportthe loads if a well drained earth foundation is provided. If butlittle truck traffic is anticipated and loads up to three tons onsteel tires are the average, a layer 8 inches thick will besufficient. In dry climates, a layer six inches thick will beadequate if it can be kept from raveling. 

On secondary roads, carrying principally farm-to-market traffic, andnot a great volume of that, the above thicknesses may be reduced aboutone-fourth. 

The exact thickness needed for any particular road is a matter forspecial study on account of the variations in the gravels and in thesupporting power of the soil upon which they are placed. 

PLACING GRAVEL

=Preparation of the Road. =--The roadway that is to be surfaced withgravel is first brought to the desired grade and cross section. Itwould be advantageous if this could be done a year before the gravelis placed so that no settlement of the earth foundation would occurafter the gravel surface is completed. But if that is impractical, thegrading may be done just prior to placing the gravel, providingappropriate methods are adopted for securing compacted fills. 

=Trench Method. =--Two distinct methods of placing the gravel are ingeneral use, known as the trench method and the surface or featheredge method respectively. The method to adopt for any particular roadwill depend largely on certain conditions that will be explainedlater. 

In the trench method, a trench of the proper width and depth forreceiving the gravel is excavated in the earth road surface and thegravel is placed therein. 

The trench is formed by plowing a few furrows and scraping out theloosened earth with a blade grader. The loose material is generallymoved out laterally to build up earth berms or "shoulders" alongsidethe gravel. Into this trench the gravel is dumped in the properquantity to give the required thickness after being compacted. 

The greatest care must be exercised in spreading the gravel toeliminate unevenness where the loads were deposited. An ordinary bladegrader is one of the best and most economical implements to use forspreading the gravel. When the gravel has been deposited in the trenchfor a distance of a thousand feet or more, the spreading isaccomplished by dragging the surface repeatedly with the blade grader, the work being continued until all waviness disappears. The gravel isthen thoroughly and repeatedly harrowed with a heavy stiff toothharrow to mix thoroughly the fine and coarse gravel so as to produceas nearly a uniform mixture as may be. The gravel is then finallysmoothed with the blade grader. 

The gravel may be compacted by rolling or may be allowed to pack fromthe action of traffic. The former is greatly to be preferred wherepracticable. The rolling is performed with a three-wheeledself-propelled roller weighing about 8 tons and must be done while thegravel is wet. Generally a sprinkling wagon is used to wet down thegravel, but advantage is always taken of rains to facilitate the work. The gravel must be spread in layers not over 5 or 6 inches thick toget the desired results, which means that for an ordinary gravel roadabout 10 inches thick, the gravel will be placed in two layers ofabout equal thickness, each of which will be rolled. 

The gravel will compact slowly even if it is not rolled, but generallydoes not become stable until the material is thoroughly soaked byrains. Then it will begin to pack, but will become badly rutted anduneven during the process. During this period the surface must be keptsmooth by means of the blade grader. The drag does not suffice forthis purpose, tending to accentuate the unevenness rather than tocorrect it. 

If gravel is placed in a trench in dense soil and rainy weatherensues, sufficient water will be held in the trench to causeunevenness from foundation settlement and the gravel will become mixedwith the soil to some extent and be thereby wasted. Trenches cut fromthe road bed upon which the gravel is placed, to the side ditches, will relieve this condition by affording an outlet for the surpluswater. Nevertheless some difficulty may be expected if the trenchmethod is used and wet weather prevails. If it is possible to closethe road against traffic until the road is dry the method isapplicable. Moreover, in long-continued dry weather, the dispersionand loss of considerable gravel from the action of automobile trafficis avoided because the gravel is held between substantial earth bermsand the gravel will pack better and hold its shape longer whenconstructed by the trench method than otherwise. 

=Surface Method. =--The surface method is one in which the gravel isplaced on the graded earth road surface without earth shoulders tohold the gravel in place. It is also sometimes called the feather-edgemethod. Except for the manner of placing as just mentioned, theseveral operations are conducted in the same general manner as for thetrench method. The gravel does not compact as quickly as in the trenchmethod and a considerable loss of material is likely to result fromthe effect of automobile traffic while the gravel is loose. But it hasthe advantage of being free from difficulties in wet weather and insome locations is therefore preferable to the trench method. It isparticularly applicable to those projects on which the placing ofgravel continues throughout the winter, the gravel being dumped andspread, to be finally smoothed and finished in the early summer. 

=Bonding. =--Where gravels deficient in binder are utilized, clay forbinder is sometimes added as the gravel is placed on the road. Thismay be done by spreading the clay on top of the lower course ofgravel, placing the upper layer and sprinkling and rolling until theclay squeezes up through the surface layer. It may also beaccomplished by spreading dry clay on the upper course before it isharrowed and then harrowing to mix it with the gravel. Both methodsare practiced, but the former is believed to be preferable. A thirdmethod is to separate the sand and pebbles and to mix the clay binderwith the sand and then spread the sand on top of the pebbles and mixby harrowing. 

=Maintenance. =--Gravel surfaces require careful maintenance, especially during the first season the road is used. The gravel willcompact slowly and during the process will be rutted and otherwisedisturbed by traffic. It is important during this period to restorethe shape once a week or at least twice a month. The light bladegrader is usually employed for the purpose so long as the gravel issomewhat loose. Later a drag of the type known as the planer willprove to be the most effective. Figure 16 shows a type of drag that isvery satisfactory for use on gravel roads. 

[Illustration: Fig. 16. --Road Planer]

CHAPTER VII

BROKEN STONE ROAD SURFACES

The broken stone road surface, or macadam road as it is usuallytermed, consists of a layer of broken stone, bonded or cementedtogether by means of stone dust and water. The surface may or may notbe coated with some bituminous material. 

=Design. =--It has been an accepted assumption that the macadam roadsurface is somewhat more stable than the gravel road surface of equalthickness, and since this is probably the consensus of opinion ofengineers familiar with both types, it may be accepted untilexperimental data are available on the subject. 

The thickness of the layer of macadam required for a road will dependupon the same factors that were considered in connection with thethickness of the gravel surface, i. E. , kind of stone used, characterof earth foundation and nature of the traffic. 

The standard macadam surface where good earth foundation is to be hadand where the loads do not exceed about four tons has for years beeneight inches thick. For heavier loads or inferior foundation, asomewhat greater thickness would be employed, but the best practicewould probably provide a foundation course of the Telford type fordoubtful foundation conditions, especially for the extremely uncertaincases. For soils of very good supporting strength such as very sandyloam or deep sand or for arid regions where stable foundation isalways assured the thickness of the macadam might be reduced to sixinches. It should be borne in mind that the broken stone road is notadapted to the traffic carried by trunk line highways in populousdistricts, but is rather a type permissible on secondary roads andusually adequate for local roads. It should never be employed forroads carrying any considerable volume of passenger automobile trafficor motor truck traffic. If surfaced with a bituminous material it willcarry up to 1200 passenger automobiles per day, but not to exceedfifty trucks. 

=Properties of the Stone. =--The stone employed for the broken stoneroad should possess the qualities of hardness and toughness and shouldbe capable of resisting abrasion sufficiently well to have reasonablelife under the traffic to which it is subjected. Since the traffic mayvary from very light on some roads to far beyond the limit of theeconomical capacity of this type of pavement on others, it followsthat any particular deposit of stone might be durable enough for someroads, while for others it might be entirely inadequate. As a generalrule it has been found that stone that wears away at a moderate ratewill, when used for water-bound macadam surface, result in a smoothertrackway than one that will wear very slowly. It is not thereforealtogether certain that the most durable stone to be had should beselected for a particular road. This is especially true now that thewater-bound macadam surface has been largely superseded for trunk linehighways and other heavily traveled roads, and is employed inlocations where service conditions are not severe. 

The stone employed for the water-bound macadam surface must possessgood cementing properties, because the surface depends for stabilityprimarily upon the bonding action of the dust from the broken stone. This is in contrast to the gravel road, where little dependence isplaced upon the bonding effect of the rock dust. In preparing thestone for macadam surfaces, the ledge rock is crushed and screened, and in that way a supply of the finer particles, which are a part ofthe output of the crusher, is obtained for use in bonding thesurface. This finely broken material, usually called screenings, isessential to the construction of the water-bound type of surface. Rocks vary considerably in the cementing properties of the dust, butusually the rocks classed as "trap, " such as andesite, gabbro andrhyolite, and schist and basalt possess good cementing properties. Limestones usually possess good cementing properties, but some of thedolomitic limestones are of low cementing value. Quartz, sandstone andthe granites are of low cementing value. 

=Kinds of Rocks Used for Macadam. =--Limestone and chert are the twosedimentary rocks, employed most extensively for broken stone roads. These rocks are found in widely distributed areas and vary in physicalcharacteristics from very soft material of no use to the road builderto materials possessing considerable durability. It is desirable tocarefully test out the deposits of these materials before using toascertain the probable value of the rock, for the construction of theroad surface. 

Of the igneous rocks, those classed as trap are best known to the roadbuilder and many of the deposits of trap rock afford an excellentmaterial for broken stone roads where the severest conditions oftraffic are encountered. The trap rocks are tough and durable andgenerally possess excellent cementing properties. 

Granite and sandstone are seldom used for water-bound macadam as theypossess poor cementing properties and a binder of some kind must beadded to cement the pieces together. For this purpose clay or thescreenings from some other variety of stone may be utilized. 

Some other materials are occasionally employed for the construction ofmacadam surfaces. Of these, oyster or marine shells, burnt shale, andslag are most common. 

Shells and slag are of rather low durability but possess goodcementing properties. Shale is a makeshift suitable only for verylight traffic roads. 

=Sizes of Stone. =--The stone for the wearing course of a macadam roadshould be as large as practicable, because the larger the pieces themore durable the surface. If the individual stones are too large it isdifficult to secure a smooth surface, and large stones will be readilyloosened by tipping as the wheels roll over them. These considerationslimit the size to a maximum of that which will pass a 2-1/2-inchscreen. Stone of excellent wearing qualities may be somewhat smaller, but never less than that which will just pass a 1-1/2-inch screen. 

For the lower course, the size is not particularly important exceptwhere the earth foundation is such as to require special construction. It is not uncommon to use the same size of stone for both upper andlower course and yet in many instances stone up to that which willjust pass a 3-1/2-inch screen is used for the lower course. Stone muchsmaller in size may also be used successfully, but if the stone isbroken to a smaller size than is required, unnecessary expense isincurred. 

The bonding material is the finer portion of the product of thecrusher, which is called screenings. This material may be so finelycrushed as to pass a one-fourth inch screen, or may be so coarse as tojust pass a one-half inch screen, but in any case must contain all ofthe dust and fine material produced by the crusher. 

Where the soil and drainage conditions demand an especially stablefoundation course, the Telford type is used. The Telford foundationconsists of a layer of stones of various dimensions that can be laidso as to give a thickness of 8 inches. These large stones are placedby hand and therefore the size requirements are not rigid. Stoneshaving one dimension about 8 inches and the others not over 10 or 12inches are satisfactory. 

=Earth Work. =--A thoroughly drained and stable earth foundation isessential to success with the macadam type of surface. Before placingthe stone, the road must be shaped to the proper cross section andall grade reduction work completed. Preferably heavy fills should havea year to settle before the macadam surface is placed. Side ditches, necessary culverts and tile drains should be constructed as requiredfor drainage. The earth work is often carried out in connection withthe construction of the macadam surface, being completed just ahead ofthe surfacing. In that case, the fills must be carefully rolled asthey are placed. The road bed may be shaped in connection with theother earthwork. If the road has been brought to a satisfactory gradesome time prior to placing the macadam, the road bed for the brokenstone will be prepared as needed for placing the stone. 

=Foundation for the Macadam. =--Macadam surfaces are quite generallyplaced in a trench as described in the trench method for placinggravel. It is an almost universal practice to compact the layer ofstone by rolling with an 8- or 10-ton power roller, and if the stoneis not held between substantial earth berms or shoulders, the rollingmerely serves to spread the stone out over the road bed instead ofcompacting it. If an attempt is made to roll broken stone which hasbeen placed on a yielding foundation, no benefit results, but on thecontrary the stone is likely to be forced down into the soil. Toinsure that the layer of broken stone can be compacted by rolling, itis first necessary to roll the earth foundation until it becomes hardand unyielding. If soft or yielding places appear during the rollingthese should be corrected by tile drains or by removing the earth fromthe spongy place and back-filling with material that will compact whenrolled. 

It is not always easy to determine why these soft places exist in whatappears to be a well drained roadway, especially since they are aslikely to be found on fills as anywhere else. Apparently they are dueto local pockets of porous soil held by denser soil so that the waterdoes not readily drain away. It is usually true that such places areobserved during the season of frequent precipitation more often thanduring other seasons of the year. 

In dry climates, the difficulties of securing suitable foundations forthe broken stone road are largely eliminated, but it may be observedthat this type of surface is not suitable for such climates unlesssome sort of bituminous binder is employed to hold the stones inplace. The cementing power of the stone dust is inadequate when thesurface is continually dry. 

[Illustration: Fig. 17. --Cross Section for Macadam]

=Telford Foundation. =--When the Telford type of foundation isemployed, the earth subgrade is prepared and then the Telford stoneplaced carefully by hand. The spaces between the large stones arefilled with the spalls broken from the larger stones in fitting themin place. When completed the base is rolled with a heavy roller tosecure a firm unyielding layer. The thickness is generally about eightinches. Any fairly sound stone may be used for the Telford base. 

=Placing the Broken Stone. =--It has been found impracticable properlyto roll a greater thickness than about 5 or 6 inches of loose stone, therefore, the stone for the macadam surface is usually placed in twolayers, the first or lower layer being rolled before the next layer isplaced. The stone is hauled in dump wagons, trucks or dump cars, dumped on the road bed and spread by hand rakes or by means of a bladegrader and is then rolled. To insure the proper thickness the loadsare accurately spaced to spread to the proper thickness. 

=Rolling. =--A three-wheeled or "macadam" type of roller, of theself-propelled type, is best for compacting the broken stone road. Theweight varies from eight to fifteen tons, but for most conditions theten or twelve ton size seems to be preferable. On Telford baseconstruction, a heavier machine is desirable and for very hard stoneit may be successfully employed. 

The first trip with the roller is made along the edge of the stone andeach successive trip is made a little nearer the middle until finallyone half of the strip of stone has been rolled. The roller is thentaken to the opposite side of the roadway and the operation repeatedon the other half. The rolling is continued until the stone isthoroughly compacted, which is evidenced by the fact that the rollermakes but a slight track in the surface. 

The second layer of stone is then placed and rolled in the same manneras the first. 

=Spreading Screenings. =--After the upper course has been rolled, thescreenings are spread on it from piles alongside the road, enoughbeing used to fill the voids in the layer of stone and furnish aslight excess. As the screenings are spread they are rolled to workthem into the voids. When these are filled, the surface is sprinkledthoroughly by means of an ordinary street sprinkling cart and againrolled. In this way the dust and water are mixed into a mortar whichfills the crevices between the stones. This mortar hardens in a fewdays, giving a bond that is weak, but sufficient for the purpose ifthe traffic is not too heavy. A broken stone road finished in this wayis called a water-bound macadam, and is ready for traffic in three orfour days after completion. 

=Bituminous Surfaces. =--On account of the inadequacy of thewater-bound macadam when subjected to motor traffic and to obviate thetendency of broken stone surfaces to loosen in dry weather, there hasbeen developed a method of covering the surface with a bituminousmaterial such as tar or asphalt. This will be described in detail in alater chapter. 

=Maintenance. =--Even under favorable conditions as regards kind andamount of traffic the macadam road requires constant maintenance. Thefirst effect of traffic will be to brush away the fine materials usedfor bonding the surface, thus exposing the larger stones in such a waythat they are rather easily loosened and removed from the surface bywheels and the hoofs of animals. This finer material must be replacedas fast as it is removed so as to protect the surface. Either stonedust or clayey sand may be used, but clay if used alone is likely tobe sticky when wet and prove to be worse than the condition it wasexpected to correct. In time, ruts and depressions will appear, eitheras the gradual effect of wear, which will inevitably effect someportions of the surface more than others, or on account of subsidenceof the foundation. Uneven places are repaired by first loosening thestone, then restoring the cross section by adding new material andtamping or rolling it in place. 

If a bituminous coating has been applied, it will eventually peel offin places and these places must be recoated as soon as practicable. 

Eventually the surface will be worn to such an extent that an entirelynew wearing surface must be added. This is done by loosening theentire surface to a depth of 3 or 4 inches and then adding a new layerof broken stone. The loosening is sometimes accomplished by means ofheavy spikes inserted in the roller wheels, and at others by means ofa special tool known as a scarifier. 

The new surface is placed and rolled in precisely the same manner asthe wearing surface of the original construction, but the layer maynot be as thick as the original wearing course. A new course will notbond to the old surface unless the old macadam has been thoroughlybroken up first. 

=Characteristics. =--The water-bound macadam is a dusty, somewhat roughsurface of low durability for rubber tired vehicles. It has long beenthe standard rural highway for steel tired vehicles, but cannot carryany considerable amount of motor traffic. It is easily repaired. Whenfinished with a bituminous surface its durability is greatly increasedand the dust is eliminated. It does not seem to be sufficiently rigidfor truck traffic, unless placed on exceptionally good foundation. 

CHAPTER VIII

CEMENT CONCRETE ROADS

The cement concrete road is one of the later developments in highwayconstruction, but the type has had sufficient use to show that it isone of the satisfactory types for heavy mixed traffic, and, where theproper materials are available, it is one of the economical types ofconstruction. 

=Destructive Agencies. =--It is well to have clearly in mind at theoutset that the concrete in a road surface is subjected to certaindestructive agencies not usually significant in connection with theuse of concrete, and these are so often disregarded that the averageserviceability of the concrete road surface is sometimes much lowerthan it would be if built with due regard for the effect of traffic onconcrete surfaces. In most structural uses of concrete, its strengthin compression only is utilized, and the factor of safety is such asto eliminate to some extent failures due to inferior materials orworkmanship. 

The concrete road surface is subjected to compression under wheelloads, to bending, causing tension in the concrete, to abrasion fromwheels, and to tension and compression due to effect of temperature. The weight of the wheel loads may cause sufficient distortion of theroad slab to produce rupture. The aggregates may be crushed underwheel loads if the material is too soft. Abrasion from steel tiredvehicles wears away the concrete unless it is hard and durable. Changes in dimension due to the effect of change in temperatureintroduce tension or compression into the road slab and may result incracks. Freezing and thawing in the subgrade subjects the slab tovertical movement and discontinuous support with the result thatlongitudinal and transverse cracks occur. 

The foregoing indicates the importance of securing good concrete forroad surfacing, and that is accomplished by using suitable aggregates, by proper design of the road surface and by following establishedconstruction methods. 

=Design. =--The widths usually adopted for concrete roads are: forsingle track roads, 9 or 10 feet, and for double track roads, 18 or 20feet. The thickness is 6 to 8 inches at the middle, varying withclimatic conditions and with the kind of soil upon which the concreteis laid. The thickness at the edge is 1 inch less than at the middleexcept that 6-inch surfaces are usually of uniform thickness, thetotal crown being 2 inches. The thickness of the two course pavementis the same as would be used for a single course pavement in the samelocation. The surface of either width has a total crown of one or twoinches to insure water running off the surface. The earth foundationis often flat, the crown being obtained by making the slab thicker atthe middle than at the edge. Fig. 18 shows cross section for concreteroads. 

[Illustration: Fig. 18. --Cross Section for Concrete Highway]

In the state of California, concrete roads four or five inches thickand surfaced with a bituminous carpet mat have been successfullyconstructed. Similar designs have been used in a few other places, butfor general practice it is unsafe to depend upon such a thin slab. Climatic and soil conditions probably account for the success of thethin roads in California. 

=Concrete Materials. =--The coarse aggregate for the concrete may bebroken stone or pebbles screened from natural gravel. Durability isnecessary, but it is also important to have uniformity in the concreteso that the road surface will wear uniformly and consequently keepsmooth. Supplies of broken stone are likely to contain a smallpercentage of soft pieces and such of these as are at the surface whenthe concrete is finished will crush under traffic, leaving a pit inthe surface. Pebbles screened from gravel are also likely to bevariable in durability and should be carefully inspected if they areto be used as aggregate for concrete roads. The harder limestones, some sandstones, pebbles from many of the gravel deposits andpractically all of the igneous rocks make satisfactory aggregates forthe concrete road. 

Sometimes none of the coarse aggregates readily available aresufficiently durable or uniform for the wearing surface of theconcrete road, but a suitable aggregate may be obtained at relativelyhigh price by shipping considerable distances. In such cases what isknown as the two course type of concrete road is employed. The wearingcourse usually is about 2 inches thick and is constructed withselected aggregates of good quality shipped in for the purpose. Thelower course is constructed of aggregates which do not possess thedesired qualities for a wearing course, but which are satisfactory forconcrete not subjected to abrasion. The aggregates for the wearingcourse will be selected with the same regard for uniformity anddurability that would be the case if they were for the one coursepavement. 

Bank run gravel, or run of the crusher stone, is generally notsufficiently uniform as regards proportion of fine and coarse materialto produce uniformity in the concrete, and the use of aggregates ofthat character is not permissible for the wearing course, but underproper inspection they may be used for the lower course of two coursepavements. 

=Fine Aggregate. =--The fine aggregate is generally natural sand, but amixture of natural sand and stone screenings is sometimes employed. The fine aggregate of whatever character must be clean, free fromorganic matter and sand, must contain no appreciable amount of mica, feldspar, alkali, shale or similar deleterious substances and notexceed two and one-half per cent of clay and silt. The sand is of sucha range of sizes that all will pass the one-fourth-inch sieve and thatnot exceeding about five per cent will pass the 100-mesh sieve. 

=Proportions. =--Various mixtures for the concrete are employed becausethese may properly vary to some extent with the exact character andgrading of the aggregates. Experience seems to have shown that theconcrete used for the wearing surface should have a crushing strengthof at least 2500 pounds per square inch, and the mixture adopted isbased on the requirements that will give the desired crushingstrength. The common mixture for the one course pavement is one partcement, two parts sand and three and one-half parts coarse aggregate. For the wearing course of the two-course type of pavement, a mixtureof the same kind is very often specified. 

While these are perhaps the most widely adopted proportions, manyothers have been used, especially where the aggregates exhibitpeculiarities or the traffic conditions are unusual. It is desired toemphasize that the purpose is to obtain concrete of the desiredstrength and there can be no such thing as "standard" proportions. 

=Measuring Materials. =--In considering the methods employed formeasuring aggregates, emphasis should be placed on the futility ofrigid requirements for the aggregates, both as regards quality andrange of sizes, if the materials are carelessly proportioned at themixer. If even reasonably near uniform wearing qualities are to besecured throughout the entire area of the concrete road surface, successive batches of concrete must be alike, and to insure that, theaggregates including the water in each batch of concrete must be mixedin exactly the same proportions. The aggregates are measured invarious ways, all essentially alike in that the intent is to insureexactly the same amount of each ingredient for each batch of concrete. 

One method is to place bottomless boxes in wheelbarrows, fill theboxes level full and then lift off the box. Another is to use awheelbarrow with a bed of such shape that the contents will be amultiple of 1 cubic foot when level full. For the larger jobs, theaggregates are hauled in industrial cars, each having sufficientcapacity for a batch of concrete. The car body is provided with apartition so as to separate the fine and coarse material. 

The water is measured in a tank which automatically refills to thesame level each time it is emptied and when adjusted for a mixturewill introduce the proper amount of water for each batch. It is highlyimportant to use the least amount of water that will produce workableconcrete. 

=Preparation of the Earth Foundation. =--The concrete road is generallyplaced directly on the natural soil which has been brought to theproper cross section. Some engineers advocate that in preparing thesubgrade, the earth be thoroughly rolled; others prefer not to rollthe subgrade. If fills of considerable depth are constructed, theyshould either be rolled as built or else should be allowed to settlefor some months before the concrete road is placed, preferably thelatter. 

=Placing the Concrete. =--The concrete is placed between substantialside forms of a height equal to the thickness of the concrete roadslab at the edge, and is shaped roughly by means of shovels. 

Various methods have been developed for striking the surface to theexact shape desired and smoothing it. If hand finishing methods areemployed, a plank template is cut to the prescribed cross section andthe concrete is shaped by drawing the template along the side forms. Sometimes the template is used as a tamper, being moved along veryslowly accompanied by an up and down motion that tends to tamp theconcrete. The template is then drawn along a second time to smooth thesurface finally. 

After the surface has been struck off by hand, it is finally smoothed, first by rolling crosswise with a slight hand roller about 8 inches indiameter and 30 inches long. The final finish is effected by dragginga piece of web belting back and forth across the surface. 

Machines designed to tamp the concrete and strike it off to therequired cross section are also employed for finishing. The machine ispower operated and is carried on wheels that run on the side forms, and the machine moves slowly along as the tamping progresses. Theconcrete is tamped, struck off to shape and smoothed with the belt atone operation. This method of finishing produces denser and strongerconcrete than can be produced by hand finishing methods. 

=Placing Concrete for Two-course Road. =--The methods employed for thetwo-course concrete road are much the same as for the one-course road. The concrete for the lower course is placed and struck off by means ofhand tools, and after that course has progressed a few feet, the uppercourse is placed and finished as has been described for the one-courseroad. 

=Curing the Concrete. =--The setting action of cement is a chemicalprocess, not merely a drying out of the water introduced in mixing theconcrete. The chemical action is progressive for a long time, but ismore rapid during the first few hours than during the later periods, and the concrete reaches about three-fourths of its maximum strengthat the end of seven days. During the setting period and particularlyduring the first few days, plenty of water must be available to thecement. 

To prevent too rapid loss of water from the concrete during thesetting period, the surface must be protected from the wind and sun. This is accomplished by first covering with canvas as soon as theconcrete has hardened sufficiently and by later covering with earth, to a depth of two inches. The earth covering is kept wet for about tendays and is left in place for about one month. 

In some places the ponding method of curing is adopted. The surface isdivided into sections by earthen dikes and the space inside the dikesfilled with water to a depth of two or three inches. The watercovering is maintained for two weeks or longer. 

No traffic is permitted on the surface for one month, and in coldweather traffic may be kept off the surface for a longer period. 

=Expansion Joints. =--To permit the concrete slab to accommodate itselfto changes in dimension due to temperature changes, expansion joints1/2 inch wide are placed about every thirty feet. These consist of asheet of some prepared bituminous material placed in position as theconcrete is poured. 

Experience seems to indicate that in spite of the expansion joints, the concrete will crack more or less and many engineers think itadvisable to omit expansion joints in constructing the pavement andwhen cracks develop to pour bituminous material into them, thusforming expansion joints. 

The prevailing practice in rural highway construction is to omit theexpansion joints, but they are commonly adopted in city pavements. 

=Reinforcing. =--To minimize the cracking, either bar or wire meshreinforcing is used in the concrete. If bars are used they are placedin the concrete as it is poured so as to form a belt around eachsection about 15 feet square. If the mesh type is employed, a part ofthe layer of concrete is placed and smoothed off and a strip of themesh laid in place. Additional concrete is then poured on top of themesh to bring the slab to the required thickness. 

=Bituminous Coatings on Concrete Surfaces. =--The concrete road surfaceis sometimes coated with a layer of bituminous material and stonechips or gravel pebbles. This is particularly advisable where noreally satisfactory aggregates are available and the concrete surfacewould not possess sufficient durability. The bituminous material isapplied hot to the surface and is then covered with stone chips orgravel pebbles, ranging in size from 3/4 inch down to 1/4 inch, theresulting coating being about 3/4 inch thick. Many failures of thistype of surface have been recorded due to the difficulty of securingadhesion to the concrete. This seems to be due in part to inability toget the proper bituminous materials and in part to climatic effects. Considerable progress has been made in developing this type of surfaceand it may eventually become a satisfactory maintenance method. 

=Characteristics. =--The concrete road is of a granular texture and isnot slippery. It is of course rigid and noisy for steel tiredvehicles. It is an excellent automobile road and its low tractiveresistance makes it a desirable surface for horse drawn vehicles. Itpossesses a high degree of durability if properly constructed. It islikely to crack indiscriminately but as a general rule the cracks arenot a serious defect. 

=Maintenance. =--The cracks that appear in the concrete surface arefilled once or twice a year, tar or asphalt being employed. The dustand detritus is cleaned out of the cracks and the hot filler pouredin, with enough excess overflowing to protect the edges. 

CHAPTER IX

VITRIFIED BRICK ROADS

Vitrified brick roads consist of a foundation course of Portlandcement concrete, broken stone or slag macadam, or of brick laid flat, the first named being by far the most generally used, and a wearingcourse of vitrified brick. 

=Vitrified Brick. =--Vitrified brick are made from clay of such acharacter that when heated to the required temperature they will fuseinto a glassy texture. Brick roads are constructed on roads carryingthe severest of traffic and the brick must therefore be tough and ofhigh resistance to wear. 

Not all of the clays from which brick may be manufactured will producea product suitable for road construction, and paving brick, eventhough truly vitrified, are of different degrees of durability, depending upon the nature of the clay and the care exercised in themanufacture. 

Paving brick are manufactured by the stiff mud process, which meansthat the clay is molded into form in a relatively dry condition. Toaccomplish this, considerable pressure is exerted in forcing thecolumn of clay through the dies, which form the prism from which thebrick are cut. If the clay is unsuitable in character or is notproperly ground and mixed, the brick will possess planes of weaknessbetween the various layers of clay which have been pressed together, and these planes, called laminations, are a source of weakness if toomarked. It is usual to specify that the brick used for road surfacesshall be free from marked laminations. 

If the brick is not properly burned it will be only partly vitrifiedand therefore not of maximum durability. It is customary to specifythat the brick shall show a glassy fracture indicating completevitrification. 

Various defects of a minor nature occasionally develop in the brickduring the successive steps in the manufacturing process. Check cracksresulting from the burning or from too rapid cooling are oftenencountered, but unless these are deep, that is 3/16 inch or more, they do not impair the wearing quality of the brick, nor indicatestructural weakness. Kiln marks are formed on some of the brick due tothe weight of the brick above in the kiln. These depressions are notobjectionable unless the brick are so distorted that they will not lieevenly in the pavement. 

Spacing lugs or raised letters are formed on one face of the brick toinsure sufficient space between the brick for the filler. These lugsor letters are not less than 1/8 inch nor more than 1/4 inch high andof such design that they will not obstruct the free flow of fillerinto the joints between the brick. 

Several varieties of paving brick are to be had, the difference beingprincipally in the design or size. 

=Repressed Brick. =--In this type of brick the spacing lugs are formedby pressing the green brick, after it has been cut to size, into amold on one face of which are recessed letters or other devices intowhich the clay is pressed, thus forming the spacing lugs. 

=Vertical Fiber Brick. =--These brick are designed to be laid with onewire-cut face up and spacing is provided by two or more beads on theside of the brick. Sometimes the vertical fiber brick has no spacinglug, it being contended that the irregularities of the brick are suchas to provide all of the space required. In practice this does notalways work out, as the brick are so regular in shape that when laidthere is too little space between the brick to permit the introductionof a suitable filler. The use of brick without spacing lugs is justbeginning and is not yet a generally accepted practice. 

=Wire-cut-lug Brick. =--This is a type of non-repressed brick which hasspacing lugs provided by cutting one face in a special manner whichprovided lugs for spacing. In this type the wire cut face is the onebetween the brick as they are laid in the pavement. 

=Tests for Quality. =--The standard test for quality of paving brick isthe rattler test. The brick rattler consists of a barrel of 14 sides24 inches long, mounted so as to rotate at a speed between 29. 5 and30. 5 revolutions per minute. The duration of a test is 1800revolutions. Ten brick constitute a charge and these are placed in therattler along with 300 lbs. Of cast iron spheres. The spheres are oftwo sizes, the smaller being 1-7/8 inch in diameter when new, and thelarger 3-3/4 inches in diameter when new. Ten of the larger spheresare used and the balance of the charge is made up of the small size. 

When tested in the standard manner the loss allowable for the severalclasses of service are as follows:

 ------------+---------------+---------------- | | Maximum Loss Traffic | Average Loss | for any Brick ------------+---------------+---------------- Heavy | 20 per cent | 24 per cent Medium | 22 per cent | 26 per cent Light | 25 per cent | 28 per cent ------------+---------------+----------------

=Other Tests. =--Sometimes the absorption test is specified for pavingbrick, but it is rarely a vitrified brick that will pass the rattlertests which fails to pass a reasonable absorption test. Absorption ofwater in an amount exceeding 4 per cent indicates incompletevitrification and failure of such brick is almost certain during therattler tests. 

The cross breaking test is also sometimes employed, but generallyonly to check the general quality of the brick. Failure in servicemore frequently occurs from excessive wear than from any other causeand the cross breaking test has little significance, except for brickless than 3 inches thick, which are to be laid on a sand beddingcourse. 

=Foundation. =--The foundation for brick roads is usually of Portlandcement concrete, the thickness varying with the nature of the trafficand the kind of soil upon which the pavement is built. For welldrained soils and normal highway traffic, 5 inches is the ordinarythickness of foundation. Under favorable conditions such as locationswith sandy soils or in semi-arid or arid regions where the soil isalways stable, the foundation may be four inches thick, and aconsiderable mileage of brick road has been built with concretefoundations less than four inches thick. 

In other locations the soil and traffic conditions require a base sixinches or more in thickness, and the proper thickness can bedetermined only after all of the factors involved are known and havebeen analyzed. It is impractical to adopt a standard thickness offoundation that will be equally economical for all locations and allkinds of traffic. As the brick pavement is essentially a heavy traffictype of surface, the design cannot be varied greatly with similarfoundation conditions because the weight of individual loads is thesignificant factor and this does not vary so much as the volume oftraffic. A variation in volume of traffic may be compensated for by avariation in the quality of the brick as already set forth. 

The mixtures for the concrete foundation vary widely because of thevariation in the aggregates employed. If the fine and coarse aggregatefor the concrete are of good quality a mixture of one part cement, twoand one-half parts sand and five parts of coarse aggregate wouldinsure concrete of adequate strength. A somewhat leaner mixture issometimes employed and would be satisfactory if the aggregates were ofexceptional concrete making quality. Mixtures of sand and pebbles(unscreened gravel) may also be used if care is exercised to secure amixture of adequate strength. The proportion will of necessity varywith each particular material and the discussion of the variousconsiderations involved may be obtained from various standard works onconcrete and concrete materials. 

Broken stone macadam is sometimes utilized for the foundation courseof the brick pavement and such foundations are constructed aswater-bound, which is described in a previous chapter. The thickness, like that of the concrete foundation, varies with the soil conditionsand the weight of the loads that are expected to use the road. Themacadam is placed in a single layer and is rolled and bonded withscreenings as described in the chapter dealing with water-boundmacadam. Six inches is a common thickness for the macadam base. Thistype of foundation should be employed only where the soil is quitestable and where material costs are such as to insure that the macadambase is materially cheaper than one of concrete. This would usually bein locations where the cost of cement is high because of long haulsand where suitable macadam materials may be obtained close at hand. 

Old macadam roads are sometimes utilized for the foundation for thebrick surface, but the instances where this is permissible arecomparatively few in number. When an old macadam is to be used it isreshaped to the proper cross section and re-rolled and bonded so as toafford a stable foundation of the proper cross slope. 

BEDDING COURSE FOR BRICK SURFACES

In order to equalize the variations in size and shape of the brick, they are laid on a bedding course composed of material into which thebrick may be forced by rolling. In this way the upper surfaces of allbrick can be brought to the proper elevation to insure smoothness andeasy riding qualities. Several kinds of bedding course are nowemployed. 

=Sand Bedding Course. =--The sand bedding course has been referred toas a sand cushion, but as a matter of experience the cushion effect isslight, although sometimes pavements have become uneven because thebrick have pushed down into the sand after the pavement was used for atime. The sand for the bedding course should preferably be finegrained, all particles passing the eight mesh sieve, but ordinaryconcrete sand is satisfactory. The sand need not be clean, as acomparatively large percentage of silt or clay does not impair theusefulness of the material. 

[Illustration: Fig. 19. --Cross Sections for Brick Highways]

=Sand Mortar Bedding Course. =--In order to eliminate the tendency forthe straight sand bedding course to shift because of the impact oftraffic on the brick, a lean cement mortar is sometimes employedrather than the straight sand. Sand and cement in the ratio of onepart cement to four or five parts of sand are mixed dry, and after thebrick have been rolled, is moistened to furnish water to hydrate thecement. The sand employed is ordinary clean concrete sand. 

=Green Concrete Bedding Course. =--In the monolithic type of brickroad construction, the brick are laid directly on the green concretebase before the concrete has taken a set and the irregularities of thebrick are taken up by rolling them until bedded in concrete. 

FILLERS FOR BRICK SURFACES

The spaces between the brick are filled with some material that willprevent the brick from being displaced and prevent water getting tothe bedding course. A suitable filler must adhere to the brick andfill completely the spaces between them. It must withstand traffic soas to remain intact in the joints and when in place it must be rigidenough to prevent displacement of the brick. 

=Cement Grout Filler. =--One of the most commonly used fillers forbrick pavements consists of a grout composed of Portland cement andfine sand. When properly mixed and applied the grout filler meets allrequirements for a filler except that it is non-elastic and some meansmust be adopted for caring for pavement expansion. 

=Bituminous Fillers. =--Asphaltic materials and tars are widely used asfillers for brick pavements. Such fillers are of high melting pointand consequently solid at ordinary temperature. They are poured intothe joints hot and when they cool are firm enough to comply with therequirements for a filler. In addition, they have enough ductility toaccommodate the expansion of the pavement due to temperature changes. 

=Mastic Fillers. =--Mastic consists of a mixture of about equal volumesof fine sand and a solid bituminous material. The mixture is preparedat high temperature and is worked into the joints between the brickwhile hot. When cool it resembles the straight bituminous fillerexcept that the mastic is somewhat more resistant to wear than thestraight bituminous filler. 

EXPANSION JOINTS

It is recognized that brick will expand and contract with changes intemperature. When a bituminous or mastic filler is employed there issufficient yield to the filler to accommodate the change in dimensionin the brick, but when the grout filler is used either the expansionjoint must be provided or the pavement must be designed to withstandthe compression due to expansion of the brick. Expansion joints mayconsist of a sheet of bituminous mastic prepared for the purpose andset in place in the pavement. The sheet of joint material is simplyinserted between courses of brick at the proper place. 

Another method of forming an expansion joint consists in placing astrip of wood between courses of brick at the place where a joint isrequired. After the pavement has been grouted, the wooden strip ispulled out and the joint is filled with a suitable bituminous filler. 

=Marginal Curb. =--If the sand bedding course is employed, it isnecessary to provide curbing along the sides of the brick to hold thebedding course in place. The curb is usually constructed integral withthe base and of concrete of the same mixture as the base. The width ofthe curb is usually six inches and the top of the curb is at the sameelevation as the edge of brick surface. 

CONSTRUCTION OF THE SURFACE

Before the construction of a brick surface should be undertaken on aroad, the drainage should be provided for even more completely thanfor a less costly type of surface since it does not pay to jeopardizethe stability of the pavement by failure to provide adequately for thestability of the supporting soil. Grades should also be reduced to theeconomical limit. 

The earth subgrade is brought to the proper elevation and crosssection and is thoroughly rolled. If there are places where the soilwill not compact properly under rolling, these places are corrected bytaking out the material and back filling with new material that willproperly compact under the roller. 

The aggregates for the concrete may be distributed along on theprepared subgrade or may be stored in stock piles or bins atconvenient points. If stored on the subgrade, a traction mixer isemployed which is drawn along the road as the work progresses, thematerials being placed directly in the mixer. If stored at a centralpoint, they may be transported to the mixer on the road and dumpeddirectly into the mixer, or the mixer may be set up at the storagepiles and the concrete hauled in trucks to the road where it isdeposited and shaped. 

The concrete is spread to the proper thickness and tamped either byhand or by machinery. If the marginal curb is to be employed, it isconstructed immediately after the concrete for the base has beenfinished but before the cement begins to set. 

After the foundation concrete has set, the bedding course is spreadand struck off to the proper thickness. When the bedding courseconsists of sand-cement mortar, the sand and cement are mixed dry andspread to prescribed thickness. It is considered to be desirable toroll the sand bedding course with a light hand roller before the brickare placed, but the sand-cement bedding course is not rolled. Thebedding course must be carefully shaped by means of a templet orstrike board before the brick are placed. 

The brick are laid in straight courses across the pavement, with thespacing lugs all in the same direction if brick with spacing lugs areemployed, and with the lugs in contact with the brick of adjoiningcourses. If brick without spacing lugs are used they are laid looselyso that there will be room for the filler between the brick ofadjoining courses. 

After the brick have been laid they are rolled to bed them in the sandor sand-mortar bedding course and thus secure a smooth surface. Forthis purpose a light, power driven, tandem roller is used and therolling is continued until the brick are thoroughly bedded. Anydefective brick that are noted are removed and replaced with goodbrick and after this culling has been completed the surface is oncemore thoroughly rolled. If a cement-sand bedding course is employed, the surface is sprinkled just after the final rolling so that waterwill flow down between the brick and moisten the bedding coursesufficiently to cause the cement to set. In some cases, thesand-cement bedding course is sprinkled just before the brick are laidbut in warm weather the setting would take place before the brickcould be rolled if that were done. In cool weather the setting issufficiently slow to permit rolling before the bedding course hardens. 

The filler is applied to the surface after the rolling. If thebituminous type of filler is employed, the hot filler is poured ontothe surface and worked into the joints by means of squeegees, withcomparatively little material left on the surface. In some instancescone-shaped pouring pots are employed and the material is poureddirectly into the joints. 

The cement grout filler is applied in the same general manner as thebituminous filler. The grout, consisting of equal parts of sand andcement, is mixed to a thin consistency and poured onto the surface andis then worked into the joints with squeegees. Two or moreapplications are usually required to effect a complete filling of thejoints. The surface should be covered with sand and be kept moistuntil the cement grout has set. 

CHAPTER X

BITUMINOUS ROAD MATERIALS AND THEIR USE

Tars and asphaltic materials of various kinds are widely used for roadconstruction and maintenance, especially for road surfaces subjectedto motor traffic. Materials of this character that are employed inhighway work possess varying degrees of adhesiveness, and while theymay be semi-solid or viscous liquids at air temperature, they melt onthe application of heat and can be made sufficiently fluid to mix withthe mineral aggregates that may be used in the road surface. Uponcooling, the bituminous materials return to the previous state andimpart a certain amount of plasticity to the mixture, at the same timeserving as a binding or cementing agent, which is sufficiently stablefor many classes of road construction. 

=Classes of Bituminous Materials. =--Bituminous materials may beclassified, according to the source from which they are obtained, ascoal tars, water gas tars, native or natural asphalts and oil orpetroleum asphalts. 

=Coal Tar. =--Coal tar is obtained as a by-product in the manufactureof illuminating gas from coal. It is also obtained in the manufactureof coke from coal. The tar thus obtained is manufactured into productsthat are used for dust layers on gravel or macadam roads, binders formacadam and gravel surfaces, fillers for brick, wood block and stoneblock pavements and for expansion joints. These various materialsdiffer mainly in their consistency at air temperature. (They maydiffer widely in chemical composition, but that need not be consideredherein. )

=Water Gas Tar. =--Water gas tar is obtained as a by-product in themanufacture of illuminating gas from crude petroleum. It is used forthe same kinds of construction as coal tar, and the products utilizedfor the several purposes, like the coal tars, differ mainly inconsistency. 

=Natural Asphalt. =--Natural asphalt is found in deposits at manyplaces in the world, existing in beds or pools where it has exudedfrom the earth or as veins in cavities in the rocks. It is of varyingcomposition and consistency, but those kinds in most general use aresolid or very viscous liquids at air temperature. Of the deposits thathave been developed on a commercial scale, the Trinidad lake in theBritish West Indies and Bermudez deposit in Venezuela are best known. Both of these materials are too hard in the natural state to be usedfor road construction, and are softened, or fluxed as it is called, with fluid petroleum oil before being used. 

=Petroleum Asphalt. =--Petroleum asphalt is a residue remaining afterthe fluid products have been distilled from petroleum. Residues ofthis sort are not always suitable for road construction, but a numberof brands of road material are obtained from this source. Oil asphaltsare used for dust layers, for binders for macadam roads, for asphaltcements for sheet pavement surfaces, and for fillers for blockpavements and expansion joints. 

=Mixtures. =--Water gas tars and asphalts are sometimes mixed toproduce road materials, and likewise native asphalts and residuesobtained from petroleum are sometimes mixed to produce asphalt cementsfor paving mixtures. 

=Classification according to Consistency. =--The various bituminousmaterials may be classified according to consistency in discussing thevarious uses to which they may be put. 

=Road Oils. =--Road oils are fluid petroleum oils of such consistencythat they may be applied cold or by heating slightly. They are usedas dust layers on earth, gravel and macadam surfaces. Their efficacydepends upon the binding properties of the small amount of asphalticmaterial that is contained in the oil. 

=Liquid Asphalts. =--These are somewhat less fluid than the road oils, and must always be heated before application, but are viscous liquidsat ordinary temperature. These materials are obtained from crudepetroleum or semi-solid native bitumens, in which case they areusually called malthas. Both coal tars and water gas tars ofsemi-solid consistency are also employed for the same class ofconstruction as the liquid asphalts. 

These materials are used for carpeting mediums on macadam roads and ascementing agents in the construction of hot-mixed macadam. 

=Asphalt Cements. =--The solid asphaltic materials used for hot-mixedtypes of construction are called asphalt cements. They may bepetroleum residues or native asphalts fluxed with petroleum oils. Theyare solids at ordinary temperature and must be heated to a temperaturein excess of two hundred and fifty degrees before they aresufficiently fluid to use. Asphalt cements are used for sheet asphaltand asphaltic concrete construction and for hot-mixed bituminousmacadam. 

=Fillers. =--Fillers are solid asphalts or tars that are used forfilling expansion joints in rigid pavements and for filling the spacesbetween the blocks in brick, wood block and stone block pavements. 

=Bitumen. =--Bituminous materials are all soluble to a greater orlesser extent in carbon disulphide and the soluble portion is calledbitumen. It is the bitumen that gives to the materials the cementingproperties utilized in road construction. Mixtures of mineralaggregates and bituminous materials for various purposes areproportioned with bitumen as a basis. Therefore, less of an asphaltcontaining one hundred per cent bitumen will be used than of onecontaining less than one hundred per cent of bitumen. 

 TABLE 8

 PROPERTIES OF ASPHALTIC ROAD MATERIALS

 (A) Material (B) Specific Gravity (C) Consistency (D) Solubility in CS_2, Per Cent (E) Solubility of Bitumen in CCl_4, Per Cent (F) Solubility of Bitumen in 86° Naphtha, Per Cent (G) Fixed Carbon, Per Cent (H) Flash Point (I) Ductility -------------------------+---------+----------+----------+------------ (A) | (B) | (C) | (D) | (E) -------------------------+---------+----------+----------+------------ Mexican oil asphalts |1. 03-1. 05|As desired| 99. 5-99. 9|99. 5-99. 9 California oil asphalts |1. 02-1. 04|As desired| 99. 9 | 99. 9 Texas oil asphalts |1. 01-1. 03|As desired| 99. 9 | 99. 9 Bermudez natural asphalt|1. 07 | 25 | 95 | 99+ Trinidad natural asphalt |1. 40 | 7 | 56-57 | 100 Bermudez asphalt cement |1. 04-1. 06|Up to 135 | 95-97 |99. 5 or more -------------------------+---------+----------+----------+------------

 -------------------------+---------+----------+----------+------------ (A) | (F) | (G) | (H) | (I) -------------------------+---------+----------+----------+------------ Mexican oil asphalts | 70-80 | 13-16 | 200°C. Up| 60-100 California oil asphalts | 75-80 | 10-12 | 200°C. Up| 100+ Texas oil asphalts | 75-80 | 12-14 | 200°C. Up| 50-100 Bermudez natural asphalt| 68-70 | 13-14 | . . . | . . . Trinidad natural asphalt | 64-65 | 10-11 | . . . | . . . Bermudez asphalt cement | 77-80 | 11-12 | 175-200 | 25-50 -------------------------+---------+----------+----------+------------

=Specifications. =--Some properties of bituminous materials can bevaried in the process of manufacture, while others are inherent in thematerial and cannot be changed in the process of manufacture. Specifications must therefore be drawn with care to insure that therequirements can be met by satisfactory materials. But certainproperties, such as specific gravity, may vary greatly among materialsequally satisfactory for construction purposes. One should not bemisled by apparent differences in the characteristics of materials, because these may simply be natural peculiarities which have nobearing on the usefulness of the material. There are given in Table 8the properties of some of the commonly used bituminous materials andthe properties that can be varied in the process of manufacture areindicated with an asterisk. A variation in these properties willusually result in some change of other properties, but generally not agreat change. 

SURFACES IN WHICH BITUMINOUS MATERIALS ARE UTILIZED

I. Surface Treatments

Attention has been directed to the rapid deterioration of water-boundmacadam when subjected to passenger automobile traffic. 

In water-bound macadam the stones are held in place by a weak cementcomposed of stone dust and water, and this cement is not sufficientlystrong to hold the stones in place when they are subjected to theshear of automobile tires. In finishing the water-bound macadamsurface, the spaces between the stones are filled with screening andin addition a layer about one-fourth inch thick is left on thesurface. 

The automobile traffic first brushes aside all of the screenings andsmaller particles of rock, exposing the larger stones. These graduallyloosen as the road is used and are brushed aside. When this effectbegins, the road is said to be raveling. Various lengths of time mayelapse from the time the road is first finished until raveling begins, depending upon the character of the stone, the weather and the amountof motor traffic. During the period before raveling starts, it iscomparatively easy to restore the road surface at any time by theaddition of screenings or clay and sand. Usually there will be a fewsmall areas of the surface that, on account of faulty construction, will ravel or become rutted much earlier than the remainder of thesurface. These can be repaired by the methods described in the chapteron "Water-bound Macadam Construction. " When the surface begins toravel seriously, maintenance becomes much more difficult and in orderto prevent raveling and the difficulties of maintenance thereafter, the macadam surface is often coated with a bituminous material. 

[Illustration: Fig. 20. --Oiling a Gravel Road]

If there is any dust or screenings on the road surface, the bituminousmaterial will not adhere to the stones and will soon flake off undertraffic. The surface of the macadam must therefore be thoroughlycleaned before the bituminous material is applied. The usual practiceis to finish the road as water-bound macadam, and permit traffic on itfor a sufficient length of time to show any weak places in the surfaceand at the same time thoroughly to season the surface. If anydefective places appear, they are repaired and when the surfaceexhibits satisfactory stability, but before it begins to ravel, thebituminous surface is applied. There will ordinarily be some stonedust and some screenings remaining on the surface at the timebituminous treatment is undertaken, and there may also be some cakedmud or other foreign material. All of this must be removed so as toexpose the stones throughout. 

=Applying the Bituminous Binder. =--The bituminous binder may bedelivered in tank cars, which is desirable if the work is near arailroad siding, or ample tank wagon service is available for longhauls so that the tank will not be held up too long. Often it isdesirable to purchase the binder in barrels and haul these to the siteof the work in advance of beginning the construction of the surface. 

The bituminous material may be applied by means of hand spreading cansnot unlike an ordinary garden watering pot, except that a slottednozzle is substituted for the ordinary perforated one. If hand methodsare employed for spreading, the bituminous material is heated in openkettles and then spread on the surface, the quantity required usuallybeing about one-half gallon per square yard of surface. Thetemperature of the binder should be great enough to insure fluidityand the road should be dry at the time of the application. As soon asthe material has been spread, the surface is finished with a dressingof chips. 

=Finishing the Surface. =--For surface dressing the best material isstone chips ranging in size from about 1 inch down to one-fourth inch. But the chips must be of durable material, or they will quickly grindinto dust. They must be free from dust when applied, as the presenceof any considerable amount of dust interferes with the properfinishing of the surface. The stone chips are rolled into the surface, a sufficient quantity being used to just cover the surface. 

=Patching. =--It almost always happens that some small areas will notbe properly cleaned or that for some unknown reason the coating peelsoff the surface. Such places must be promptly patched to prevent themenlarging under the action of traffic. This work is usually done bypatrolmen, who inspect the road at frequent intervals and make thenecessary repairs. The patrolman is equipped with a small heatingkettle, a spreading can and the necessary brushes, tampers andmiscellaneous tools needed for the repair work. The place to bepatched is carefully cleaned, coated with bituminous binder and stonechips and tamped until dense and solid. Repairs made in this way areexceedingly important in that they arrest deterioration in its earlystages and maintain a high degree of serviceability. 

II. Penetration Macadam

A considerable mileage of macadam has been constructed in which anattempt was made to eliminate the difficulties of maintenance by amethod of construction that involves applying a bituminous binder insuch a manner as to permit it to penetrate two inches or more into thesurface. It is expected that the binder will coat the stones to suchan extent as to increase materially the stability of the bituminousmacadam over the surface treated one. It is also expected that lessdifficulty will be encountered in maintaining a surface of bituminousmaterial and stone chips on this type of road than on the water-boundmacadam. The extent to which these expectations have been realized hasvaried to a marked degree and although some excellent surfaces havebeen constructed by this method, the results have as a rule beenneither uniform nor entirely satisfactory. It seems to be apparentthat good results cannot be obtained unless the materials are entirelysuitable and the construction is carried out with unusual skill. 

=Foundation. =--The foundation or lower course consists of a layer ofbroken stone six inches thick placed on a well drained and thoroughlyrolled earth subgrade. In exceptional cases, the Telford type offoundation might be employed. 

The lower course of broken stone is finished in the same manner aswater-bound macadam, being bonded with stone screenings or with finegravel of high clay content. 

Since this course is in reality the foundation of the surface, it isnecessary to secure stability by appropriate construction methods, exactly as in constructing water-bound macadam. 

[Illustration: Fig. 21. --Type of Roller used on Gravel and MacadamRoads]

=Upper or Wearing Course. =--The wearing course consists of a layer ofstone about two and one-half inches thick. The stone is placed androlled and the spaces between the stones partially filled with somesuitable bituminous material. The bituminous material is usuallyapplied by means of a mechanical spreading device connected to a tankwagon. The bituminous materials employed for this class ofconstruction are semi-solid in character and must be heated to givethem sufficient fluidity for application. They may be heated in thetank wagon which is used for the application or they may be heated inseparate tanks and transferred to the distributing wagon forspreading. Some kind of a nozzle or group of nozzles is employed forspreading the material so that it can be delivered in the form of aspray or at least in a thin fan-shaped stream and can be distributedin a fairly uniform layer over the stone. The binder will cool ratherrapidly after it is applied, but meanwhile will flow into the openingsbetween the stones and will form over the surface stones a coating ofslight thickness. 

The surface of the macadam is next covered with a layer of chips oftough rock, similar to the material used for the final dressing insurface treatments. These are carefully brushed into the openingsbetween the larger stones by means of heavy brush brooms. This is anexceedingly important part of the work and often a much neglected partof the construction. 

The surface is then covered with a second application of bituminousmaterial, somewhat less in quantity than required for the firsttreatment and the surface again covered with stone chips and brushed. 

The surface is then thoroughly rolled and is ready for traffic. 

=Patching. =--As in the case of surface treatments, there are likely tobe places that, on account of defects in the construction, will failsoon after the road is placed under traffic. These will quicklyenlarge unless they are repaired promptly. The repairs are made byloosening the stone in the area affected and adding new stone asneeded and then pouring on the necessary amount of bituminous materialto coat the stones. Allowance must be made for the compression of thematerial by tamping so that a depression does not result. The stonesare carefully tamped to place and covered with chips which are alsotamped. 

=Characteristics. =--The penetration macadam is a surface well adaptedto motor traffic if the individual vehicles are not too heavy. It islikely to squeeze out of shape under motor truck traffic, becomingseriously uneven and uncomfortable for traffic. Its durability ismaterially affected by the construction methods followed. 

III. Hot Mixed Macadam

The wearing course of the mixed macadam is composed of graded brokenstone or gravel and a bituminous binder. Usually the bituminousmaterial only is heated prior to the mixing, but sometimes the stoneis also heated. 

=Foundation. =--The lower course, which serves as the foundation, iseither broken stone macadam, gravel or concrete. 

Where a foundation of broken stone is used, it is constructed of thematerials and in the manner described for the foundation of thepenetration macadam. Quite often a badly worn macadam or gravel roadis used for the foundation and a new wearing course provided by addinga mixed macadam surface. If such is the case, the old surface isworked over so as to restore the shape sufficiently and to insure thatit is everywhere of sufficient thickness. 

Portland cement concrete is sometimes used as a foundation for themixed macadam, but not often. Usually if the traffic is of a characterrequiring a concrete foundation, it is desirable to use a betterwearing course than the mixed macadam, and the asphaltic concrete orsheet asphalt type of surface is employed. It is necessary to finishthe surface of the concrete base with some device that will leave thesurface rough to prevent the macadam from creeping. A knobbed tamperwhich leaves numerous irregular depressions about 2 inches in diameterand three-fourths inch deep is often employed. 

=Sizes of Stone. =--For the wearing surface, stone ranging in size from2 inches down to one-fourth inch is usually employed. If the stone isof good quality the maximum size may be but 1-1/2 inches, but soft oreven medium stone of that size are likely to crush under traffic. Thestone for the base course should preferably be from 3 inches down, butany available size will be satisfactory if the layer is well rolledand bonded. The base course is constructed in the same manner aswater-bound macadam and any material satisfactory for the base courseof macadam will serve for the base course of mixed macadam. Screeningshaving good bonding properties will also be required for the basecourse. 

=Mixing and Wearing Surface. =--Several methods are employed in mixingthe wearing surface. The simplest is to mix by hand with shovels. Theaggregates are heated in improvised heaters which may consist ofnothing more than a metal pipe two or three feet in diameter, aroundwhich the stone is piled. The mixing platform is usually a metal platesometimes arranged so that it can be heated by means of a fireunderneath. The bituminous material is heated in kettles. For somemixtures, the stone is not heated, but the bituminous material isalways heated. The batch of stone is placed on the mixing platform, the bituminous material added and the materials mixed by hand. 

Machine mixing is practiced much more extensively than hand mixing, being both more rapid and cheaper. The mixer is similar to a concretemixer except that the drum is arranged so that it can be heated. Thehot stone and the bituminous binder are put into the drum and mixedfor the requisite length of time. Sometimes the stone is mixed cold, the bituminous material only being heated. 

=Placing the Wearing Surface. =--The hot mixture is carted to the roadand spread to such thickness that after rolling the wearing surfacewill be not less than two inches thick. The hot mixture is dumped andthen spread by means of shovels to the approximate thickness and thespreading completed by means of rakes. The surface is then rolledeither with a tandem or a three-wheeled roller until thoroughlycompressed. 

=Seal Coat. =--After the rolling has been completed, the surface iscovered with hot bituminous cement and dressed with pea gravel orstone chips and again rolled. Traffic may be permitted in twenty-fourhours. 

=Characteristics. =--The mixed macadam is a somewhat resilient surfaceof excellent riding qualities and considerable durability for mediumtraffic. It is likely to creep and become uneven when subjected toheavy loads. The seal coat will wear off in two or three years andwill require replacing. 

IV. Asphaltic Concrete

Asphaltic concrete is a name given to a road surface mixture which iscomposed of graded stone, graded sand and asphalt cement. This type isdesignated as asphaltic concrete because of the analogy of the mixtureto Portland cement concrete. 

Asphaltic concrete is of two general types known as bitulithic, orWarrenite, and Topeka asphaltic concrete, respectively, thedifferences being in the nature of the mixture. 

=Bitulithic or Warrenite. =--The stone employed for these types isgraded down from a size about equal to one-half of the thickness ofthe wearing course, and stone passing a 1-1/4 or 1-1/2-inch screen isusually specified. From the maximum size the stone is graded down tothe finest particles produced by the crusher. The range of sizes ofstone will vary with the source of the supply, and in order to securethe desired density in the mixture, varying amounts of graded sand andmineral dust, such as ground limestone or Portland cement, are addedto the broken stone. Usually the resulting mixture contains less thanfifteen per cent of voids, and to this carefully graded mineralaggregate there is added enough asphalt cement to bind together theparticles. 

=Topeka Asphaltic Concrete. =--In this type of asphaltic concrete, themineral aggregate consists of a mixture of carefully graded sand andof broken stone of such size that all will pass a one-half-inch screenand graded down to the fine dust produced by the crusher. To thismixture is added about nine per cent of Portland cement or limestonedust. The voids in the mixture are usually about twenty-five percent. 

It will be seen that the essential differences between the Bitulithicand Topeka types are these: the Topeka type contains a largerpercentage of voids and stone of a smaller maximum size than theBitulithic. Both types have been extensively employed for city paving, but the Bitulithic and Warrenite types have also been used to someextent for rural highways. The Topeka type has been used but littlefor rural highways. 

=Foundation. =--The foundation for the asphaltic concrete may be an oldmacadam road, a base course constructed of broken stone or Portlandcement concrete, the latter being used much more extensively thaneither of the other types. 

Sometimes asphaltic concrete is used for resurfacing water-boundmacadam or gravel roads when the traffic has increased to the pointwhere the cost of maintenance of the water-bound macadam has becomeexcessive. The existing surface is repaired and the cross section isrestored, or possibly flattened somewhat. 

=Placing the Surface. =--The stone, sand and asphalt cement are heatedto the required temperature and combined in the proper proportions andare then thoroughly mixed by a mechanical mixer. The mixture is hauleddirectly to the road and is dumped and spread by means of rakes. It isthen rolled thoroughly while still hot, a three-wheeled roller beingmost satisfactory. After rolling, a seal coat of hot asphalt cement isspread over the surface and covered with hot stone chips about 1/4inch in size. The surface can be opened to traffic immediately afterthe surface has been completed. 

=Characteristics. =--The asphaltic concrete surface is of excellentriding properties, is easily repaired and of moderate durability. Itis a particularly desirable surface for pleasure automobile riding andfor horse drawn traffic. 

CHAPTER XI

MAINTENANCE OF HIGHWAYS

Proper maintenance of highways is equally important with properconstruction. With nearly all types of road construction, the need formaintenance arises soon after the surface is placed under traffic andis continuous thereafter. The nature and amount of maintenance workvaries greatly among the several types of surface and the organizationsuitable for a system of highways will depend to a considerable extentupon the kinds of surfaces that are to be maintained. 

The upkeep of a road may be conveniently considered as of two kinds, viz. , (1) that which has to do with the wearing surface and earthshoulders or berms upon which there is some traffic and (2) that whichhas to do with the side ditches and drainage structures and keepingthe roadside in presentable condition. Both kinds of work are usuallycarried out by the same organization, but whereas the nature of thework indicated under (1) will vary with the type of wearing surfaceand with all variations in traffic, that which is indicated under (2)will be nearly constant in any locality. 

ORGANIZATION FOR MAINTENANCE

Maintenance of highways is preferably under the administration of thesame authority as construction and when an improvement is undertakenunder the jurisdiction of a State Highway Department, the completedimprovement is ordinarily maintained under the state authority. Ifthe improvement is made by county authorities, the maintenance is alsocarried out under county authority. 

The nature of the organization of maintenance forces is dependent uponthe kind of roads to be cared for and must of necessity be varied inany instance as conditions demand. In general, either maintenancegangs or patrolmen are employed and often both are used on the sameroad system. 

=Patrol Maintenance. =--Where this system is in operation, the highwaysystem is divided into patrol districts of from six to eighteen milesof highway and a single patrolman is placed in charge of eachdistrict. He is provided with all of the necessary tools and materialsrequired in his district and performs all of the work required in theordinary upkeep of the highway. He should work under the direction ofthe county engineer or the district engineer for the state highwaydepartment, because his work involves the use of materials andprocesses requiring technical supervision. 

=Gang Maintenance. =--The maintenance gang may be employed for sometypes of road surface in lieu of the patrolman or with other types ofsurface may be employed to supplement the work of the patrolman. Themaintenance gang consists of three to ten men and is furnished all ofthe tools and materials required for the particular kind of work theydo. Ordinarily the gang goes over the roads assigned to it once eachseason and performs those repair operations requiring more work thanthe patrolman can find time for. The work of the maintenance gang likethat of the patrolman should be under engineering supervision. 

=Maintenance of Earth, Sand-clay, Gravel and Macadam Roads. =--Theordinary upkeep of earth, sand-clay, gravel and macadam surfaces ismost readily accomplished by the patrol method, since constant care isrequired to keep the roads in a condition of maximum service ability. 

The tools required for each patrolman may include the following:

 1 shovel 1 spade 1 stone rake 1 pick 1 scythe 1 tamper 1 or more road drags 1 mowing machine for cutting weeds 1 wheelbarrow (sometimes) 1 light truck 1 small kit carpenter's tools

The work of the patrolman consists in keeping the surface of the roadsmooth by dragging, repairing chuck holes by tamping in fresh materialof the appropriate kind, keeping the ditches and culverts free fromobstruction, cutting weeds and repairing bridge floors if they are ofplank construction. Removal of snow drifts is sometimes a part of thepatrolman's duty, but more often that is done by special gangs. Usually the patrolman is authorized to hire teams for dragging andcutting weeds. 

When an earth road requires to be re-graded so as to restore thecross-section and deepen the ditches, a gang is sent in to performthat work, as it is obviously impossible for the patrolman to performwork, of that kind. 

If the gravel road is being maintained with a bituminous carpet coat, the patrolman will be furnished the necessary tools to enable him topatch the surface with bituminous material as necessity requires. 

When the surface deteriorates to such an extent that a new carpet coatis required, the gang system is employed for all work connected withresurfacing, instead of attempting to have the work done by patrolmen. 

The maintenance of the macadam road is carried out in much the samemanner as that of the gravel road. The binder of stone dust or clayeysand is renewed as often as it is swept off by traffic. Depressions orruts are repaired by first loosening the surface with a pick and thenadding broken stone and screenings to restore the surface. 

When the macadam reaches the stage where entire resurfacing isneeded, the work is performed by gangs organized and equipped for thepurpose; and likewise when the surface is being maintained with abituminous carpet, the renewal of the carpet coat is performed byspecial gangs, but the ordinary upkeep of the surface by patching ishandled by a patrolman. 

MAINTENANCE OF MIXED BITUMINOUS SURFACES

[Illustration: Fig. 22. --Scarafier used in Gravel Road Maintenance]

These types of surface can be kept in satisfactory condition if theyare carefully repaired once or twice each season. This work requiresconsiderable experience and some special equipment, not ordinarilysupplied to patrolmen. A gang is organized for the work and suppliedwith the proper equipment. They go over the roads and patch all wornplaces, generally first removing the wearing surface entirely in thearea affected. 

The wearing surface mixture is then prepared and tamped or rolled intoplace. If the area affected is small, tamping is satisfactory, andwhen the area is considerable, rolling is employed. The upkeep of theside roads may be accomplished by the same gang but is preferablytaken care of by patrolmen, who do not attempt any but minor repairsto the wearing surface. 

MAINTENANCE OF BRICK AND CONCRETE ROADS

On brick and concrete roads, the principal work on the wearing surfaceconsists in filling the cracks with a suitable bituminous material. This work is done by patrolmen or by special gangs and generally willbe done once each year. The upkeep of the side roads is cared for bypatrolmen who drag the side roads and cut the weeds as occasionrequires. 

INDEX

 Administration county; 15 federal; 17 highway; 13 state; 16 township; 13

 Aesthetics; 62

 Aggregate, fine; 101

 Aggregate, coarse; 100

 Air resistance; 51

 Alignment; 46

 Applying bituminous binder; 122

 Asphaltic concrete; 128

 Asphalt, natural; 117 liquid; 118 petroleum; 117

 Assessments, special; 19 zone method; 20

 Bedding course, green mortar; 111 sand mortar; 111 sand bedding mortar; 111

 Binder for gravel; 75

 Bitulithic or warrenite; 128

 Bitumen; 118

 Bituminous coatings on concrete; 105

 Bituminous fillers; 112

 Bituminous road materials and their use; 116

 Bituminous surfaces; 96, 120

 Blade grader; 69

 Bonding; 87

 Bonds, annuity; 26 serial; 27 sinking fund; 25

 Box culverts; 39

 Brick roads; 113

 Brick, repressed; 107 tests of 108; vertical fiber; 107 vitrified; 106 wire-cut-lug; 108

 Broken stone road surfaces; 89

 Cement, asphaltic; 118

 Cement concrete roads; 98

 Cement grout filler; 112

 Characteristics, asphaltic concrete; 129 bituminous macadam; 125 broken stone; 97 concrete; 105 mixed macadam; 128 sand clay; 78

 Classes of bituminous materials; 116

 Classification according to consistency; 117

 Clay and cement concrete pipe; 39

 Coal tar; 116

 Concrete, asphaltic; 128

 Concrete materials; 100

 Concrete pipe; 39

 Control of erosion; 61

 Costs; 70

 County administration; 15

 Cross sections; 60, 65

 Culverts; 56

 Curing concrete; 103

 Design, broken stone roads; 89 concrete roads; 99 earth roads; 42

 Desirability of road bonds; 27

 Development of traffic; 2

 Drainage, necessity of; 29

 Drainage of roads; 29

 Earth roads, in arid regions; 72 humid regions; 65 value of; 73

 Earth works; 92

 Education, rural; 6

 Effect of grades; 54

 Elevating grader; 66

 Elevating grader work; 68

 End walls for culverts; 39

 Energy loss on account of grades; 57

 Entrances, farm; 37, 61

 Expansion joints; 104

 Farm entrance culverts; 37

 Federal administration; 17

 Fillers; 118

 Finance, highway; 19

 Fine aggregate; 101

 Finishing surface of concrete; 122

 Foundation, asphaltic concrete; 129 brick; 109 macadam; 93 mixed macadam; 126 penetration macadam; 123 Telford; 94

 Gang maintenance; 131

 Grader, Maney; 67 use of; 69

 Gravel, ideal; 81 natural; 83 roads; 74

 General taxation; 24

 Good roads and commerce; 7

 Green concrete bedding course; 111

 Highway administration; 13

 Highway finance; 19 maintenance; 130

 Importance of design; 30

 Ideal road gravel; 81

 Inter-city traffic; 5

 Inter-county and inter-state traffic; 5

 Internal resistance; 50

 Intersections; 46

 Laying tile; 35

 Length of culvert; 37

 Liquid asphalt; 118

 Local farm to market traffic; 4

 Macadam; 89

 Maintenance, concrete; 105 earth roads; 70 general; 131 gravel roads; 88 macadam; 96 of highways; 130 patrol; 131

 Maney grader; 67

 Marginal curb; 113

 Measuring materials; 101

 Metal pipe; 38

 Mixing wearing surface; 127

 Mixtures; 117

 Natural asphalt; 117 gravel; 79

 Necessity for planning; 42 drainage; 29

 Patching; 122, 125

 Patrol maintenance; 131

 Pebbles, size of; 80

 Petroleum asphalt; 117

 Placing asphaltic concrete; 129

 Placing broken stone; 94

 Placing concrete; 102, 103 mixed macadam; 127

 Plans for roads; 43

 Preliminary investigation; 44

 Preparation of earth foundation; 102 of road; 85

 Private entrances; 61

 Properties of stone; 90

 Proportions for concrete roads; 101

 Purpose of highways; 1

 Reinforced concrete box culverts; 39

 Reinforcing; 104

 Repressed brick; 107

 Road oils; 117

 Road plans; 43

 Rocks, kind of, for macadam; 91

 Rolling, macadam; 95

 Rolling resistance; 50

 Run-off; 31

 Rural education; 6

 Rural social life; 7

 Safety consideration; 58

 Sand bedding course; 111

 Sand clay and gravel road; 74

 Sand mortar bedding course; 111

 Seal coat; 127

 Serial bonds; 27

 Sinking fund bond; 25

 Slip scraper; 67

 Special assessments; 19

 Specifications; 119

 Spreading screenings; 95

 State administration; 16

 Stone, use of; 92

 Surface drainage; 30

 Surfaces, bituminous; 120

 Surface method; 87

 Superelevation; 47

 Tests, brick; 108

 Tile drains; 35

 Topeka asphaltic concrete; 128

 Tractive resistance; 52

 Trench method; 85

 Truck operation costs; 9

 Types of culverts; 38

 Underground water; 34

 Undulating roads; 58

 Use of blade grader; 69

 Utilizing natural gravels; 83

 Value of earth roads; 73

 Variation in rainfall; 64

 Variation in soils; 63

 Vehicle taxes; 24

 Vertical fiber brick; 107

 Vitrified brick roads; 106

 Vitrified brick; 106

 Water gas tar; 117

 Width of roadway; 59

 Wire-cut-lug brick; 108

 Zone method of assessing; 20

 * * * * *

[Transcriber's Notes:

The transcriber made these changes to the text to correct obviouserrors:

 1. P. 5, accomodate --> accommodate 2. P. 39, guage --> gauge 3. P. 46, enbankment --> embankment 4. P. 63, tought --> tough 5. P. 68, absorbant --> absorbent 6. P. 73, persistant --> persistent 7. P. 77, indispensible --> indispensable 8. P. 119, aspealt --> asphalt 9. P. 127, repaid --> rapid 10. P. 130, Vetrified brick; 105 --> Vitrified brick; 106 11. P. 130, Virtical --> Vertical

End of Transcriber's Notes]