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Title: American Rural Highways
Author: Agg, T. R. (Thomas Radford), 1878-1947
Language: English
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Literature in Agriculture (CHLA), Cornell University)
[Transcriber's Note: Every effort has been made to replicate
this text as faithfully as possible, including obsolete and
variant spellings and other inconsistencies. Text that has been
changed to correct an obvious error is noted at the end of this
ebook.
Also, on pages 47-48, the Greek letter theta is represented by
THETA. In chemical and mathematical notations, a subscript is
enclosed in braces and preceded by an underscore (e.g., H_{2}O.)]
AGRICULTURAL ENGINEERING SERIES
E. B. MCCORMICK, CONSULTING EDITOR
FORMERLY DEAN OF ENGINEERING DIVISION
KANSAS STATE AGRICULTURAL COLLEGE
AMERICAN
RURAL HIGHWAYS
_McGraw-Hill Book Co. Inc._
PUBLISHERS OF BOOKS FOR
Coal Age -- Electric Railway Journal
Electrical World -- Engineering News-Record
American Machinist -- Ingenieria Internacional
Engineering & Mining Journal -- Power
Chemical & Metallurgical Engineering
Electrical Merchandising
[Illustration: _Frontispiece_]
AMERICAN
RURAL HIGHWAYS
BY
T. R. AGG, C.E.
PROFESSOR OF HIGHWAY ENGINEERING
IOWA STATE COLLEGE
FIRST EDITION
MCGRAW-HILL BOOK COMPANY, INC.
NEW YORK: 239 WEST 39TH STREET
LONDON: 6 & 8 BOUVERIE ST., E. C. 4
1920
COPYRIGHT, 1920, BY THE
McGRAW-HILL BOOK COMPANY, INC.
PREFACE
AMERICAN RURAL HIGHWAYS was written for use as a text or reference in
courses dealing with rural highways and intended for agricultural
engineers, students in agriculture and for short courses and extension
courses. The reader is assumed to have familiarity with drawing and
surveying, but the text is adapted primarily for students who do not
receive training along the lines of the usual course in Highway or
Civil Engineering.
The text is intended to familiarize the student with the relation of
highway improvement to national progress, to indicate the various
problems of highway administration and to set forth the usual methods
of design and construction for rural highways in sufficient detail to
establish a clear understanding of the distinguishing characteristics
and relative serviceability of each of the common types of roadway
surface.
Experience with classes made up of students in agriculture or
agricultural engineering and with trade school students in road making
served as a guide in the selection and arrangement of the material.
Detailed discussion of tests of materials and of the theory of design
has to a considerable extent been eliminated as being outside of the
scope of the course for which the text is intended.
In the preparation of American Rural Highways reference was had to
many books on highway subjects and to current periodical literature.
Wherever direct extracts were made from such source, appropriate
acknowledgment appears in the text.
T. R. AGG
AMES, IOWA,
AUGUST 18, 1920.
CONTENTS
PREFACE vii
CHAPTER I
THE PURPOSE AND UTILITY OF HIGHWAYS
Transportation Problem--National in Scope--Development in
Traffic--Location or Farm to Market Traffic--Farm to Farm
Traffic--Inter-City Traffic--Inter-County and Inter-State
Traffic--Rural Education--Rural Social Life--Good Roads and
Commerce 1-12
CHAPTER II
HIGHWAY ADMINISTRATION
Township Administration--County Administration--State
Administration--Federal Administration--Special Assessments--Zone
Method of Assessing--General Taxation--Vehicle Taxes--Sinking
Fund Bonds--Annuity Bonds--Serial Bonds--Comparison of Methods
of Issuing Bonds--Desirability of Road Bonds 13-28
CHAPTER III
DRAINAGE OF ROADS
The Necessity for Drainage--Importance of Design--Surface
Drainage--Run-off--Ordinary Design of Ditches--Underground
Water--Tile Drains--Lying Tile--Culverts--Length of Culvert--
Farm Entrance Culverts--Metal Pipe--Clay and Cement Concrete
Pipe--Concrete Pipe--Endwalls for Culverts--Reinforced Concrete
Box Culverts--Drop Inlet Culverts 29-41
CHAPTER IV
ROAD DESIGN
Necessity for Planning--Road Plans--Problems of Design--
Preliminary Investigations--Road Surveys--Alignment--
Intersections--Superelevation--Tractive Resistance--Rolling
Resistance--Internal Resistance--Air Resistance--Effect of
Trades--Energy Loss on Account of Grades--Undulating Roads--
Guard Railing--Width of Roadway--Cross Section--Control of
Erosion--Private Entrances--Æsthetics 42-62
CHAPTER V
EARTH ROADS
Variations in Soils--Variation in Rainfall--Cross Sections
Elevating Grader--Maney Grader--Slip Scraper--Fresno
Scraper--Elevating Grader Work--Use of Blade Grader--
Costs--Maintenance--Value of Earth Roads 63-73
CHAPTER VI
SAND-CLAY AND GRAVEL ROADS
The Binder--Top-soil or Natural Mixtures--Sand-clay on Sandy
Roads--Sand-clay on Clay or Loam--Characteristics--Natural
Gravel--The Ideal Road Gravel--Permissible Size of Pebbles--
Wearing Properties--Utilizing Natural Gravels--Thickness
of Layer--Preparation of the Road--Trench Method--Surface
Method--Maintenance 74-88
CHAPTER VII
BROKEN STONE ROAD SURFACES
Design--Properties of the Stone--Kinds of Rocks used for
Macadam--Sizes of Stone--Earth Work--Foundation for the
Macadam--Telford Foundation--Placing the Broken Stone--
Rolling--Spreading Screenings--Bituminous Surfaces--Maintenance
Characteristics 89-97
CHAPTER VIII
CEMENT CONCRETE ROADS
Destructive Agencies--Design--Concrete Materials--Fine
Aggregate--Proportions--Measuring Materials--Preparation of the
Earth Foundation--Placing Concrete for Two-course Road--Curing
the Concrete--Expansion Joints--Reinforcing--Bituminous Coatings
on Concrete Surfaces--Characteristics--Maintenance 98-105
CHAPTER IX
VITRIFIED BRICK ROADS
Vitrified Brick--Paving Brick--Repressed Brick--Vitrified Fiber
Brick--Wire-cut-lug Brick--Tests for Quality--Other Tests--
Foundation--Sand Bedding Course--Sand Mortar Bedding Course--
Green Concrete Bedding Course--Bituminous Fillers--Mastic
Fillers--Marginal Curb 106-115
CHAPTER X
BITUMINOUS ROAD MATERIALS AND THEIR USE
Classes of Bituminous Materials--Coal Tar--Water Gas Tar--Natural
Asphalt--Petroleum Asphalt--Mixtures--Classification According to
Consistency--Road Oils--Liquid Asphalts--Asphalt Cements--
Fillers--Bitumen--Specifications--Surface Treatments--Applying
the Bituminous Binder--Finishing the Surface--Patching--
Penetration Macadam--Foundation--Upper or Wearing Course--
Patching Characteristics--Hot Mixed Macadam--Foundation--Sizes
of Stone--Mixing the Wearing Stone--Placing and Wearing
Surface--Seal Coat--Characteristics--Asphaltic Concrete--
Bitulithic or Warrenite--Topeka Asphaltic Concrete--Foundation
--Placing the Surface--Characteristics 116-129
CHAPTER XI
MAINTENANCE OF HIGHWAYS
Petrol Maintenance--Gang Maintenance--Maintenance of Earth,
Sand-clay, Gravel and Macadam Roads 130-134
Index 135
AMERICAN RURAL HIGHWAYS
CHAPTER I
THE PURPOSE AND UTILITY OF HIGHWAYS
THE DEVELOPMENT OF HIGHWAY SYSTEMS
=Transportation Problem.=--Public highways, like many other familiar
things, are utilized constantly with little thought of how
indispensable they are to the conduct of the business of a nation or
of the intimate relation they bear to the everyday life of any
community. The degree to which a nation or a community perfects its
transportation facilities is an index of its industrial progress and
public highways constitute an important element in the national
transportation system. It is to be expected that the average citizen
will think of the public highway only when it affects his own
activities and that he will concern himself but little with the broad
problem of highway improvement unless it be brought forcibly to his
attention through taxation or by publicity connected with the
advancement of specific projects.
=National in Scope.=--The improvement and extension of the highway
system is of national importance just as is development and extension
of railways, and concerted action throughout a nation is a
prerequisite to an adequate policy in regard to either. It is
inconceivable that any community in a nation can prosper greatly
without some benefit accruing to many other parts of the country.
Increased consumption, which always accompanies material prosperity,
means increased production somewhere, and people purchase from many
varied sources to supply the things that they want. Good
transportation facilities contribute greatly to community prosperity
and indirectly to national prosperity, and the benefits of highly
improved public highways are therefore national in scope. This fact
has been recognized in Europe, notably in England, France and Belgium,
where the public highways are administered largely as national
utilities.
Until recent years, highway improvement in the United States has been
subordinated to other more pressing public improvements, but during
the World War the inadequacy of the transportation system of the
United States became apparent. While such an unprecedented load upon
transportation facilities may not recur for many years, it has become
apparent that more rapid progress in highway improvement is necessary
and in the United States the subject is now likely to receive
attention commensurate with its importance.
=Development of Traffic.=--The character and extent of the highway
improvement needed in any locality is dependent entirely on the
demands of traffic. In sparsely settled areas, particularly those that
are semi-arid or arid, the amount of traffic on local roads is likely
to be small and the unimproved trails or natural roads adequate. But
as an area develops either on account of agricultural progress or the
establishment of industrial enterprises, the use of the public
highways both for business and for pleasure increases and the old
trails are gradually improved to meet, at least to some degree, the
new demands of traffic. In sparsely settled areas, it is possible for
the public to accommodate its use of the highways to the physical
condition thereof, and business is more or less regulated according to
the condition of the roads. This is not always pleasant or economical
but is the only possible arrangement. In populous districts, with
diversified activities, it becomes imperative to have year-round
usable roads in order to transact with reasonable dispatch the regular
business of the industries. Anything less will handicap normal
community progress.
The advent of the motor driven vehicle in the United States has
resulted in a greatly increased use of the public highways of
agricultural areas, even of those that are sparsely populated, because
of the convenience of the motor vehicle both for passenger and for
freight service. Probably in excess of 90 per cent of the tonnage
passing over the rural highways in the United States is carried by
motor vehicles. This class of traffic has really just developed and no
one can predict what it will be in ten years, yet it has already
introduced into the highway problem an element that has revolutionized
methods of construction and maintenance.
A different set of traffic conditions exists in those parts of the
United States where large areas are devoted primarily to industrial
pursuits, the agricultural development being of secondary importance.
Public highways connecting the industrial centers are indispensable
adjuncts to the business facilities in such communities and are
ordinarily subjected to a very large volume and tonnage of traffic
consisting principally of motor vehicles. The roads first selected for
improvement will not be those serving the agricultural interests of
the district, but rather those serving the industrial centers.
Inter-city roads of great durability and relatively high cost are
necessary for such traffic conditions.
Not infrequently the transportation needs will require a system of
both inter-city and rural highways in the same community. There are
few areas in the United States where there is no agricultural
development. It is apparent therefore that the nature of the highway
systems and the administrative organization under which they are built
and maintained will differ in various states or areas according to
the nature of development of that area agriculturally and
industrially. In planning improvements of highway systems, it is
recognized that one or more of several groups of traffic may be
encountered and that the extent and nature of the improvement must be
such as will meet the requirements of all classes of traffic, the most
important being first provided for, and that of lesser importance as
rapidly as finances permit.
KINDS OF TRAFFIC ON PUBLIC HIGHWAYS
=Local or Farm to Market Traffic.=--In strictly agricultural
communities the principal use of the highways will pertain to
agricultural activities and most of it will be between the farm and
the most convenient market center. In the ordinary state, the number
of rural families will not average more than six to eight per square
mile, but in some districts it may reach twenty families per square
mile. The travel from the district around a market center will
originate in this rather sparsely populated area and converge onto a
few main roads leading to market. The outlying or feeder roads will be
used by only a few families, but the density of traffic will increase
nearer the market centers and consequently the roads nearer town will
be much more heavily traveled than the outlying ones. It is apparent
therefore that considerable difference may exist in the kind of
construction adequate for the various sections of road where farm
traffic is the principal consideration. This traffic is made up of
horse drawn wagons, transporting farm products and of horse drawn and
motor passenger vehicles, the motor traffic comprising 80 per cent or
more of the volume of traffic and a greater per cent of the tonnage.
Motor trucks are now employed to some extent for marketing farm
products and, where surfaced highways have been provided, this class
of traffic is superseding horse drawn traffic.
=Farm to Farm Traffic.=--In the ordinary prosecution of farming
operations, a considerable amount of neighborhood travel is
inevitable. Farmers help each other with certain kinds of work,
exchange commodities such as seed, machinery and farm animals and
visit back and forth both for business and pleasure. To accommodate
this traffic, it is desirable to provide good neighborhood roads.
Traffic of this sort follows no particular route and can to some
extent accommodate itself to the condition of the highways without
entailing financial loss, although some discomfort and some
inconvenience may result from inadequate highway facilities. This
traffic will be partly motor and partly horse drawn, but the
proportion of motor driven is large.
=Inter-city Traffic.=--In strictly agricultural districts there is a
large amount of travel between towns, both for business and for
pleasure. The pleasure travel is mostly in motor vehicles and a
considerable part of the business traffic is the same, although horse
drawn vehicles are employed to some extent.
In industrial districts there is a large volume of this class of
traffic consisting of motor passenger vehicles used for business and
for pleasure and of motor freight vehicles used for general business
purposes. In addition, there is certain to be a large amount of motor
truck freight traffic incident to the particular industrial pursuits
of the cities. Where adequate public highways connect industrial
centers, there is invariably a very large amount of inter-city
traffic, due in part to the needs of industry and in part to
concentration of population in industrial centers.
=Inter-County and Inter-State Traffic.=--Automobile touring is a
popular means of relaxation, especially on the part of those who live
in the cities, although it is by no means confined to them. Traffic of
this kind follows the routes where roads are best and passes entirely
across a county, attracted by some public gathering. Often it is
inter-state in character, made up of tourists who are traveling to
distant pleasure resorts. Such traffic at present constitutes a
relatively small part of the travel on public highways, except on
certain favorable routes, but as the wealth of the country increases
and good touring roads are numerous, long distance travel will
increase and will eventually necessitate the construction of a number
of well maintained national highways, located with reference to the
convenience of the automobile tourist.
PUBLIC HIGHWAYS AND COMMUNITY LIFE
It is well to recognize the intimate relation public highways bear to
the economic progress of a nation. Normal development of all of the
diverse activities of a people depends very largely upon the highway
policy that is adopted and whether the actual construction of
serviceable roads keeps pace with transportation needs.
=Rural Education.=--It has become increasingly apparent during the
World War that the demand upon North America for food stuffs is to
become more and more insistent as the years pass. Already the
consumption in the United States has approached quite closely to the
average production and yet the population is constantly increasing.
The time is not far distant when greater production will be required
of the agricultural area in North America in order to meet the home
demand for foodstuffs, and many thousands of tons will be needed for
export. This need can only be met by agricultural methods that will
increase greatly the present yield of the soil. The adoption of better
agricultural methods must of necessity be preceded by the technical
training of the school children who will be the farmers of the next
generation, which can best be accomplished in graded schools with well
equipped laboratories and with suitably trained teachers. The problem
of providing such schools in rural communities has, in some instances,
been solved by consolidating a number of rural school districts and
constructing a well equipped building to accommodate the students from
an area several miles square. An educational system of this sort can
reach its highest usefulness only when adequate public highways
facilitate attendance of pupils. The whole trend of rural educational
progress is toward a system which is predicated upon a comprehensive
highway policy in the district.
=Rural Social Life.=--Closely allied to the rural educational problem
is the rural social problem. Motor cars and good roads do a great deal
to eliminate the isolation and lack of social opportunity that has
characterized rural life in the United States. A high order of
citizenship in rural communities is essential to the solution of many
problems of rural economics, and such citizens will not live away from
the social opportunities of modern life. The rural school house and
the rural church may become social centers and local plays, moving
picture shows and lectures and entertainments of other kinds made
available to those who live in the country. Their enjoyment of these
social opportunities will be much more general if the public highways
are at all times in a condition to be traveled in comfort. Good homes
and good schools on good roads are prerequisites to the solution of
many rural problems.
If there is opportunity for those who live in the cities to get some
adequate idea of rural life and the conditions under which farming
operations are carried on it will correct many misunderstandings of
the broad problems of food production and distribution. Reference has
frequently been made to the seeming desire on the part of city people
to get into the country, and, by facilitating the realization of this
desire, a great social service is rendered.
=Good Roads and Commerce.=--That good highways are almost as necessary
as are railroads to the commercial development of a nation is
recognized but, unlike the railroads, the highways are not operated
for direct profit and the responsibility of securing consideration of
the demand for improvements is not centralized. Therefore, sentiment
for road improvement has been of slow growth, and important projects
are often delayed until long after the need for them was manifest.
Movements to secure financial support for highway improvement must go
through the slow process of legislative enactment, encountering all of
the uncertainties of political action, and the resulting financial
plan is likely to be inadequate and often inequitable.
The whole commercial structure of a nation rests upon transportation,
and the highways are a part of the transportation system. The highway
problem can never receive adequate consideration until public highways
are recognized as an indispensable element in the business equipment
of a nation.
During the World War all transportation facilities were taxed to the
limit, and motor trucks were utilized for long distance freight
haulage to an extent not previously considered practicable. As a
result, the interest in the motor truck as an addition to the
transportation equipment of the nation, has been greatly stimulated.
Many haulage companies have entered the freight transportation field,
delivering commodities by truck to distances of a hundred miles or
more.
The part the motor truck will play in the future can only be
estimated, but it seems clear that the most promising field is for
shipments destined to or originating in a city of some size and a
warehouse or store not on a railroad spur, and especially when the
shipments are less than car load lots. The delays and expense incident
to handling small shipments of freight through the terminals of a
large city and carting from the unloading station to the warehouse or
other destination constitute a considerable item in the cost of
transportation.
Mr. Charles Whiting Baker, Consulting Editor of _Engineering
News-Record_, states:[1]
[1] Engineering News Record, July 10, 1919.
"It costs today as much to haul a ton of farm produce ten miles
to a railway station as it does to haul it a thousand miles over
a heavy-traffic trunk-line railway. It often costs more today to
transport a ton of merchandise from its arrival in a long train
in the freight yard on the outskirts of a great city to its
deposit in the warehouse of a merchant four or five miles away
than it has cost to haul it over a thousand miles of railway
line."
Nevertheless it seems probable that new methods of operating the motor
truck transport, and possibly new types of trucks or trucks and
trailers will be developed so that freight traffic over many roads
will be of considerable tonnage and an established part of the
transportation system of the nation. In the article above referred to
are given the following data relative to the cost of hauling on
improved roads by motor truck and these cost estimates are based on
the best information available at this time. They should be considered
as approximate only, but serve to indicate the limitations of the
truck as a competitor of the steam railway.
TABLE 1
TRUCK OPERATION COSTS, FROM REPORTS BY SIX MOTOR TRUCK
OPERATORS, DIRECT CHARGES PER DAY
+---------+-------+-------+-------+-------+-------+-------+-----------
| A | B | C | D | E | F | Average
| | | | | | | Total
+---------+-------+-------+-------+-------+-------+-------+-----------
Driver | $5.00 | $5.20 | $5.00 | $5.00 | $5.17 | $5.50 | $5.13
Tires | 3.00 | 3.75 | 2.00 | 2.00 | 2.00 | 3.00 | 2.68
Oil, etc. | .30 | ... | .30 | .50 | .25 | .25 | .35
Gasoline | 3.00 | 4.00 | 3.50 | 4.65 | 2.08 | 3.75 | 3.50
| | | | | | | ------
| | | | | | | $11.66
+---------+-------+-------+-------+-------+-------+-------+-----------
INDIRECT CHARGES PER DAY
-------------+------+------+------+------+------+------+------------
| | | | | | | Average
| A | B | C | D E | F | Total
-------------+------+------+------+------+------+------+------------
Depreciation | $3.50| $4.19| $3.60| $3.40| $3.67| $4.00| $3.77
Interest | 1.20| 1.26| 1.08| 1.22| 1.10| 1.00| 1.15
Insurance | 1.50| 2.54| 1.26| 2.10| .86| .50| 1.47
Garage | 1.00| 1.20| 1.00| 1.00| .89| 1.00| 1.01
Maintenance | .50| ...| .50| ...| 1.00| ...| .75
Overhaul | 1.33| 2.75| 1.80| 1.60| 2.00| 3.00| 2.07
License | .17| .27| .20| .20| .20| .20| .20
Body upkeep | .25| ...| .30| .10| .40| ...| .27
| | | | | | | ----
| | | | | | | $10.69
Supervision | .50| 2.93| 2.05| 1.90| ... | ... | 1.90 1.90
Lost time | 2.20| ... | 1.67| 3.40| 2.50| 1.97| 2.57 2.57
| -----| -----| -----| -----| -----| -----| -----------
| 23.45| 28.09| 24.26| 28.07| 22.12| 24.17| 26.82
-------------+------+------+------+------+------+------+------------
TABLE 2
OVERHEAD CHARGES PER YEAR FOR A 5-TON CAPACITY GASOLINE
MOTOR TRUCK RUNNING AN AVERAGE OF 50 MILES PER DAY
FOR 240 DAYS PER YEAR
Driver's wages[1] $1500
Depreciation (20% on $6000 investment) 1200
Interest (6% on $6000 investment) 360
Insurance 450
Garage (rental, upkeep, etc.) 300
Maintenance, minor repairs and supplies, tire chains, tools,
lamps, springs, equipment, etc. (estimated) 300
Complete overhaul once a year 600
License fee 60
Body upkeep, repairs, painting, etc. 90
Supervision 696
-----
Total per annum $5556
Overhead charges per day for 240 days in the year,
actual operation $23.15
Overhead charges per mile for 50 miles per day .463
[1] In the above table the driver's wages have been placed under
overhead charges because the driver is paid by the month and his
wages continue even though the truck is idle because of repairs,
bad weather or lack of business, unless, of course, the idleness
should be of long duration, when the driver might be laid off.
DIRECT CHARGES PER DAY AND PER MILE FOR 5-TON TRUCK
OPERATED AS ABOVE
------------------------------------------------+---------+---------
| Cost | Cost
| per day | per mile
------------------------------------------------+---------+---------
Tires (based on present tire guarantee) | $3.00 | $0.06
Lubricants | .50 | .01
Gasoline (3-1/2 miles per gal., 14 gal. at 25c) | 3.50 | .07
| ----- | -----
| 7.00 | 0.14
------------------------------------------------+---------+---------
Total of overhead and direct charges for 240 days per year operation,
per day $30.15
Per mile .603
Cost per ton-mile for full loads one way and empty returning .2412
Cost per ton-mile for full loads one way and half load returning .16
The significance of these figures becomes apparent when they are
compared with the cost of hauling freight over trunk-line railways
with heavy traffic where the cost per ton-mile, including terminal
charges, ranges from 1.7 _mills_ per ton-mile to 4.4 _mills_ per
ton-mile.
In view of these facts it seems reasonable to suppose that motor
vehicles for use on the public highways are more likely to be employed
to supplement the rail transport than to compete with it. To the
actual cost of operation of motor trucks given in Table 2, there
should be added the proportionate cost of maintaining the highway for
the use of the truck, which is partly covered by the item "License
Fee" in the table. The license fee would necessarily be considerably
larger if it were to compensate adequately for the wear on the
highways over which the trucks operate. This will still further
increase the cost of hauling by motor truck.
Motor trucks are employed for many kinds of hauling where their speed
and consequently their daily capacity is an advantage over team
hauling that is decidedly worth while. It probably could be shown
that for many kinds of hauling, teams are more economical than motor
trucks, but when promptness and speed and the consequent effect on
dependent activities are considered, the motor truck often has a
distinct advantage, and the use of the truck to replace horse drawn
vans is progressing rapidly. This is true not only in the cities, but
also in the smaller towns and in the country. Motor trucks have been
adopted in a great many communities for delivery of farm products to
market, and this use of the truck is certain to increase rapidly. But
trucks in this service will use the secondary roads as well as the
main or primary roads.
These observations emphasize the extent to which the highway policy of
the nation must be predicated on the use of the highways by motor
vehicles.
CHAPTER II
HIGHWAY ADMINISTRATION
The systems of highway administration extant in the various political
units in the United States present a patchwork of overlapping
authority and undetermined responsibility. Highway laws are being
constantly revised by state legislatures and with each revision there
is some change in administrative methods and often the changes are
revolutionary in character. In most states, the trend is away from
county and township administration and toward state administration,
with provision for considerable participation by the federal
government.
It will be pertinent to consider briefly the present functions of each
of the administrative authorities having duties in connection with
highway work in the United States, although these duties vary greatly
in the several states and change periodically with the action of
legislatures.
=Township Administration.=--Township or "Town" authority is a survival
of the old New England town government and the town board consists of
three or more trustees who hold office for fixed terms. The usual term
is three years, but is less in some states. The incumbent is generally
a man who has other responsibilities of a public or private nature and
who gives but little of his time to highway matters. In some states
the pay is a fixed annual salary and in others a per diem with some
limitation on the amount that may be drawn in any one year, which
limitation may be statutory or may be by common consent.
The township highway commissioners or trustees have jurisdiction over
certain of the roads in the township, usually best described as all
roads not by law placed under the jurisdiction of some other
authority. In certain instances, the township authorities have charge
of all of the roads in the township, which would mean that no "county"
or "state" roads happened to be laid out in that township. It is a
matter of general observation that the trend of legislation is toward
removing from the jurisdiction of the township officials all roads
except those upon which the traffic is principally local in character.
The actual mileage of roads in the United States that is at present
administered by township officials is large, probably constituting not
less than seventy per cent of the total mileage.
In most states the township officials are responsible for the
maintenance of the roads under their jurisdiction and also supervise
such new construction as is undertaken. This includes the construction
of culverts and bridges as a rule, but in some states the county board
of supervisors is responsible for all of the bridge and culvert work
on the township roads. In other states, the township board is
responsible only for bridges or culverts that cost less than a certain
amount specified by law (usually about $1000) and the county board
provides for the construction and upkeep of the more expensive bridges
and culverts.
Funds for the work carried out by the township road officials are
obtained by general taxation, the amount that may be levied being
limited by statute and the actual levy being any amount up to the
maximum that the township board deems necessary for its purposes. It
is the general observation that the tax levy is usually the maximum
permitted by law.
In many states, township officials are permitted to issue bonds for
road construction, almost invariably, however, with the restriction
that each issue must be approved by the voters of the township. There
is always a provision that the total amount of bonds outstanding must
not exceed the constitutional limit in force in the state. In several
states, the townships have large amounts of road bonds outstanding.
=County Administration.=--In some states the county is the smallest
administrative unit in the road system. A county board, called the
board of county supervisors or board of county commissioners
consisting of from three to fifteen members, is the administrative
authority. Its members are elected for fixed terms which vary in
length from one to five years. The county board usually has many
public responsibilities other than highway administration, and is
generally made up of men with considerably more business ability than
the average township board.
The county board has jurisdiction over all of the highways in the
county in some states, and in others it has charge of only the more
important highways. In most states, the laws set forth specifically
what highways shall be under the jurisdiction of the county
authorities.
In addition to having direct supervision of the improvement and
maintenance of the roads assigned to county administration, the county
boards in some states arrange for the construction of all culverts and
bridges on the roads that are under township supervision, or at least
the more expensive bridges and culverts on such roads. Sometimes this
is accomplished by granting county aid for township bridges, under
which system the county pays a part of the cost of the construction of
bridges on the township roads. The amount of aid varies, but is
generally about one-half of the cost, and the township and county
officials jointly assume the responsibility of arranging for the
construction by contract or otherwise.
The county board obtains funds for road work through a direct tax on
all property in the county, the maximum rate being limited by statute.
County boards are also authorized to issue bonds for road construction
under statutory restrictions and limitations similar to those
effective in the township as to total amount issued, and many
millions of dollars' worth of highway bonds have been issued by
county authorities in the United States.
=State Administration.=--In a state, the administrative authority in
highway matters is vested in a board of commissioners usually
consisting of three or more members. In a few states, the
administrative authority is delegated to a single commissioner. Where
the authority is vested in a board, that board is usually appointed by
the governor. In several states one or more members of the commission
hold that position _ex officio_; for example, in several states the
governor is by law a member of the commission, in others the secretary
of state or the dean of engineering at the State University or the
state geologist is a member of the commission. Where the
administrative authority is a single commissioner he may be elected
along with other state officers, but this is the case in only a few
states.
The authority of the state highway department varies in the several
states, but in general the departments serve in the dual capacity of
general advisers to the county and township authorities on road
matters and as the executive authority responsible for the
construction of those highways that are built entirely or in part from
state or federal funds.
State highway departments consist of the commission or commissioner,
and the technical and clerical staff required to perform the duties
imposed on the state organization. To some extent the state highway
departments are able to encourage economical and correct construction
of highways by the township and county authorities by furnishing them
standard plans and specifications and by formulating regulations to
govern the character of construction, but such efforts are likely to
be more or less ineffective unless the state authority has supervision
of the allotment of state or federal funds to the various counties and
townships. Nevertheless, most state highway departments do a great
deal of advisory work in connection with the highway construction
carried out by county and township authorities.
State highway departments are supported by funds obtained in various
ways, laws differing greatly in this respect. The necessary support is
in some states appropriated from funds obtained by general taxation,
and is in others obtained from automobile license fees. In still
others, the funds are secured by a combination of the two methods
mentioned above. In addition to these support funds, a certain part of
the money obtained as federal aid may be employed for the engineering
and inspection costs on federal aid roads. The above mentioned funds
are required to maintain the state highway department. In addition,
the departments have supervision of the expenditures of construction
funds which can be used for road construction and maintenance, and may
not be expended for salaries or other overhead expense.
In a number of states, automobile license fees are set aside for
financing road construction and maintenance, and the work paid for
from the fees is carried out under the supervision of the state
highway department.
In a number of instances, state bonds have been issued for road
construction, and the expenditure of the proceeds of the sale of road
bonds has usually been supervised by the state highway department.
All federal aid funds allotted to a state must be expended under the
direction of the state highway department.
=Federal Administration.=--Federal authority in highway work is vested
in the Bureau of Public Roads of the United States Department of
Agriculture. The official head is the Secretary of Agriculture, but
the administrative head is the Director of the Bureau. In this Bureau
are the various instrumentalities needed for carrying on
investigations and furnishing information to the various states on
highway subjects. The Bureau also supervises the construction of
federal aid roads in a general way through district engineers, each
of whom looks after the work in several states.
Funds for the support of the Bureau of Public Roads are obtained from
congressional appropriations to the Department of Agriculture and from
a percentage of the funds appropriated for federal aid.
Federal aid is money appropriated by Congress to be distributed to the
various states to stimulate road construction. It is granted to the
states on the condition that the states will expend at least an equal
amount on the projects involved. The states in turn usually give a
suitable part of the state allotment to each county. There are various
limitations as to the amount of federal aid per mile of road and the
type of construction that may be employed, but these are matters of
regulation that change from time to time.
It will be seen that each of the administrative authorities, except
the Bureau of Public Roads, is to some extent subservient to a higher
authority, and the Bureau of Public Roads is supervised by the United
States Congress. Considerable diplomacy is required on the part of any
administrative authority if his contact with other officials is to be
without friction. This is especially true in connection with the
formulation of a policy regarding the types of construction to be
adopted for an improvement. The responsibility for the selection is
variously placed on the township, county or state authority, the laws
not being uniform in this respect. If state or federal funds are
allotted to an improvement, the state authority either makes the
selection of the type of construction or the selection is made by some
subordinate authority subject to the approval of the state highway
department. Where the improvement is paid for exclusively with
township or county funds, the selection is often made by the township
or county authority without review by higher authority. Many abuses
have crept into highway administration through the unscrupulous
methods of promoters of the sale of road materials or road machinery.
A great deal of the selling activity of the agents for these
commodities is entirely irreproachable, but it is well known that such
is not always the case. As a result, the tendency of legislation is to
require the state highway department to approve contracts for
materials or construction entered into by the township or county
authorities. The state highway departments can secure the requisite
technical experts to determine the merits of materials and equipment
and, in spite of some glaring examples of inefficiency or worse, have
made a good record for impartiality and integrity as custodians of the
funds for which they are responsible.
HIGHWAY FINANCE
The paramount problem in highway administration is the development of
an adequate financial plan for carrying on road improvement. The
necessary expenditures are enormous, although the money so expended is
probably much less than the actual benefit resulting from the
improvements.
=Special Assessments.=--There is presumed to be a direct and
recognized benefit conferred on farm lands by the construction of
improved highways adjacent thereto. Therefore, it is equitable to
charge a part of the cost against the lands so benefited.
The principle of paying for public improvements by a special
assessment upon private property has been long established and a large
proportion of the public improvements in the cities and towns have
been made financially possible through the medium of special
assessments on abutting and adjacent property. The same principle has
been applied to the financing of drainage projects for reclaiming farm
lands. Recently the special assessment method has come into limited
use in financing rural highway improvements. The policy in such cases
is to assess the abutting and adjacent property in a zone along the
improved road for a percentage of the cost of the improvement. The
amount so assessed does not ordinarily exceed one-fourth of the total
cost of the improvement and may be considerably less. The assessment
is spread over an area extending back from one to six miles from the
improved road. The assessment area is generally divided into about
four zones parallel to the road. The zone next the road is assessed at
a rate arbitrarily determined as a fair measure of the benefit, and
each succeeding zone is assessed at a somewhat lower rate. Generally
about three-fourths of the total assessment is placed on the half of
the assessment area lying next to the road.
Many systems of making assessments have been proposed which are
mechanical in application after the area and rate of distribution of
benefit have been established, but in practice it is always found
necessary to make adjustments on individual parcels of land because of
variation in benefits received and it is impossible to eliminate the
exercise of human judgment in equalizing the assessments.
=Zone Method of Assessing.=--The area to be assessed on each side of
the improved road is divided into zones usually four in number, but a
larger or smaller number of zones may be adopted. The rate for each
zone is then arbitrarily determined. For a typical case, the first of
four zones would receive an assessment of 50 per cent of the amount to
be borne by the area; the second zone 25 per cent, the third 15 per
cent and the fourth 10 per cent. Other percentages sometimes adopted
are 45, 25, 20 and 10 and 60, 20, 15 and 5. The set of percentages
first mentioned seems to insure the most equitable distribution for an
area all of which is substantially equally productive.
When a road, for the improvement of which an assessment is being made,
lies on two or more sides of a parcel of land all of which is within
the assessment area, the rate is arbitrarily reduced to relieve that
parcel of land somewhat, or the assessment is first spread as above
outlined and afterward equalized as judgment dictates.
In applying the zone method some difficulty is encountered in
determining an equitable distribution on those parcels of land lying
partly in one zone and partly in another, but the rate may be arrived
at with reasonable accuracy by pro-rating in accordance with the exact
conditions.
In. Fig. 1, let it be assumed that the assessment area is to be two
miles wide, one mile on each side of the road and the various
ownerships to be indicated by the parcels of land numbered 1 to 8, as
shown. Each zone for the assessment of the 3-1/4 mile section is 1/4
mile wide and the rates for the several zones are 50, 25, 15 and 10
per cent respectively. Let it be assumed that the portion of the cost
of the 3-1/4 miles of road to be assessed on the area shown is
$20,000. The assessment would then be as follows:
------+-------+----------------------+------------+-------------
| | Rate × 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 of
road is made up in like manner, and let it be assumed that the portion
of the cost of this road that is to be assessed on the area shown is
$5500. The assessment area will be one mile wide and each zone
one-fourth mile in width and the rates for each zone the same as
before.
------+-------+----------------------+------------+-------------
| | Rate × frontage on | Amount of |
Parcel| Rate | improved road = | Assessment | Assessment
| | assessment units | per unit |
------|-------+----------------------+------------+-------------
1 | a 75 | 75 x 1320 = 99,000 | $0.010417 | $1031.25
| b 15 | 15 x 2640 = 39,600 | | 412.49
2 | 75 | 75 x 2640 = 198,000 | | 2062.53
3 | 50 | 50 x 1320 = 66,000 | | 687.51
4 | a 25 | 25 x 1320 = 33,000 | | 756.25
| b 15 | 15 x 2640 = 39,600 | |
5 | 10 | 10 x 3300 = 33,000 | | 343.73
6 | 10 | 10 x 1980 = 19,800 | | 206.24
------|-------| | |-------------
| | 528,000 | | 5500.00
------+-------+----------------------+------------+-------------
It will be noted that the combined assessment for the two sections of
road is especially heavy on parcels 1, 2 and 3. In order to prevent
unjust charges against such properties, laws usually limit the total
assessment against any parcel of land to a fixed percentage of a fair
market value or of the assessed value. The assessment on these parcels
would be reduced as seemed expedient and the deficit would be
distributed over the remainder of the area in the same manner as the
original assessment was spread. In practice such re-distribution is
ordinarily made by the arbitrary adjustment in accordance with what
the authorized officials consider to be fair and equitable. The method
outlined is merely a mechanical means of securing distribution and
must not be considered as an infallible method of making the
assessment. It is always necessary to review the results in the light
of the actual benefits to be presumed for each parcel of land.
Nevertheless, the method outlined will prove equitable in a majority
of cases.
=General Taxation.=--There is a general community benefit derived from
the construction of good roads in that the actual cost of marketing
farm products is lessened with a resulting lowering of the price to
the consumer. The benefit also accrues from the greater facility with
which all community business may be conducted. The introduction of
better opportunities for social, religious and educational activities
in the rural districts which results from improved highways is also a
community benefit of no mean importance. A part of the cost of road
improvement may therefore be equitably paid from funds obtained by
general taxation.
A considerable portion of the current expense of maintaining the
township and county highway work and at least a part of the cost of
maintaining state highway activities is met from funds obtained by
general taxation. Likewise, the funds required for the amortization of
bond issues are often obtained from general taxation although vehicle
license fees are sometimes used for that purpose.
General taxes are levied on all taxable property in a political unit
under statutory provisions regulating the amount of the levy and the
purpose for which the revenue is to be used. In the aggregate, the
road taxes are large but in the township or county the rate is
generally small compared to some other taxes, such as the school tax.
=Vehicle Taxes.=--The great direct benefit derived by those who
actually operate vehicles over the roads justifies the policy of
requiring a vehicle to pay a license fee in lieu of other taxes, the
funds so obtained to be used for the construction and maintenance of
public highways. In practice, this method has already been applied to
motor vehicles in most states and has proven to be an important source
of revenue. Its application to horse-drawn vehicles has not been
attempted, due probably to the fact that such horse-drawn vehicles as
use the public highways are also employed about the farm or in the
towns and the determination of an equitable basis for taxation
involves many difficulties.
The rate of the fee for motor vehicles should be based on their
destructive effect on the road so far as that is possible. The scale
of fees should therefore take account of weight and speed of vehicle
and if the license is in lieu of all other taxes, it should also be
graduated with the cost of the vehicle.
When funds are thus derived, every precaution should be taken to
insure that the money is used judiciously for construction and
especially for maintenance on those roads most useful to motor
traffic.
=Highway Bonds.=--Bond issues for road improvement afford a means of
constructing roads and paying for them while they are being used. A
very large volume of such bonds are outstanding in the United States.
Road bonds should be issued only for durable types of improvement and
the life of the bond should be well within the probable useful life of
the road surface. It is customary and highly desirable that the
general nature and extent of the improvement be established before the
bonds are issued. It is desirable that bond issues be subject to
approval by referendum before issue and that is provided in every
instance.
Highway bonds are of three classes known as Sinking Fund, Annuity and
Serial Bonds, respectively. The earlier bonds issued were almost all
of the sinking fund class, but in recent years the serial bond has
been widely employed and is probably the most satisfactory to
administer.
=Sinking Fund Bonds.=[1]--When this type of bond is employed, the
amount of the expenditure for road improvement is determined upon and
the length of the period during which tax payments shall be made is
settled. To employ a concrete example, it may be assumed that $100,000
is to be expended for road work and is to be paid at the end of ten
years. The interest rate on the bonds will vary with the condition of
the bond market and the stability of the political unit issuing the
bonds, but is usually about 5 per cent. Knowing these factors, the
amount to be added to the sinking fund each year is computed. In order
to pay the interest on the bonds, a tax of suitable rate is levied,
and in order to retire the bonds at the end of the period, a sum is
set aside each year which is supposed to be invested and draw interest
which will be added to the principle, and the principle and interest
comprise the sinking fund. The principle of the sinking fund is
obtained by tax levies, a sum being added to the principle of the
sinking fund each year.
[1] For a more detailed discussion of highway bonds see Bulletin
136, U. S. Dept. of Agriculture, which is the basis of this
discussion.
The success of this method of financing depends upon the proper
administration of the sinking fund. It must be invested with fidelity
and the fund be kept intact. Usually the sinking fund cannot be
invested at as high a rate of interest as the bonds bear and there is
some loss as a result. Road bonds bearing 5 per cent interest can
usually be sold at par while the sinking fund will usually net about 3
or 3-1/2 per cent interest. The total cost of a bond issue will be
greater by the sinking fund method than by either of the other methods
described.
=Annuity Bonds.=--Annuity bonds are drawn in such a manner that the
amount of the payment for principle and interest is the same each year
during the life of the bond. When the amount of the issue and the rate
of interest has been determined and the amount of the desired annual
payment has been determined, the number of years the bonds must run is
computed.
This method is convenient in that the amount of the tax to be levied
each year remains constant.
=Serial Bonds.=--Serial bonds are drawn so that a uniform amount of
the principle is retired each year after retirement starts and the
total interest payments decrease each year after the first bonds are
retired. The first bond may not be retired for a number of years after
the issue of the bonds, but when it once starts retirement proceeds at
a constant rate annually.
=Comparison of Methods of Issuing Bonds.=--The relative costs of
financing by either of the three methods depends upon the rate of
interest in each case and the net rate secured on the sinking fund
provided for retiring sinking fund bonds.
For comparative purposes, some typical examples are given in Table 3.
These illustrate the differences in total cost of securing $100,000 by
each of the three methods at various interest rates.
TABLE 3
TOTAL COST OF A LOAN OF $100,000 FOR 20 YEARS, INTEREST
COMPOUNDED ANNUALLY
---------+---------------------------------------+---------+---------
Annual | Sinking Fund Compounded | |
Interest | Annually at | |
on Bonds +----------+----------------+-----------+ Annuity | Serial
|3 per cent| 3-1/2 per cent | 4 per cent| |
---------+----------+----------------+-----------+---------+---------
4 | $154,431 | $150,722 | $147,163 |$147,163 | $142,000
4-1/2 | 164,431 | 160,722 | 157,163 | 153,752 | 147,250
5 | 174,431 | 170,722 | 167,163 | 160,485 | 152,500
5-1/2 | 184,431 | 180,722 | 177,163 | 167,359 | 157,750
6 | 194,431 | 190,722 | 187,163 | 174,369 | 163,000
---------+----------+----------------+-----------+---------+---------
=Desirability of Road Bonds.=--In theory the bond method of financing
enables the highway authorities to construct a large mileage of roads
in a few years and spreads the cost over the period during which the
public is being benefited. Better prices are obtained on contracts for
a large mileage than for smaller jobs, and the community can receive
the benefit more quickly than where construction proceeds piecemeal
with current funds. The vital consideration is to insure that the term
of the bonds is well within the useful life of the road, and that
ample provision is made to maintain the roads during that period.
Under proper restrictions the bond method of financing is to be
commended. The bonds are an attractive investment and readily
marketable on satisfactory terms.
CHAPTER III
DRAINAGE OF ROADS
=The Necessity for Drainage.=--The importance of drainage for all
roads subject to the effects of storm or underground water has always
been recognized by road builders, but during recent years constantly
increasing attention has been given to this phase of road
construction. It is unfortunate that there has in the past been some
tendency to consider elaborate drainage provisions less necessary
where rigid types of surfaces were employed. It has become apparent,
from the nature of the defects observed in all sorts of road surfaces,
that to neglect or minimize the importance of drainage in connection
with either earth roads or any class of surfaced roads is to invite
rapid deterioration of some sections of the roadway surface and to add
to maintenance costs.
The degree to which lack of drainage provisions affect the
serviceability of the road surface varies with the amount of
precipitation in the locality and the manner in which it is
distributed throughout the year. In the humid areas of the United
States, which are, roughly, those portions east of a north and south
line passing through Omaha and Kansas City, together with the northern
part of the Pacific slope, precipitation is generally in excess of 30
inches per year and fairly well distributed throughout the year, but
with seasonal variations in rate. In these areas, the effect of the
precipitation, both as regards its tendency to lower the stability of
soils and as an eroding agent, must be carefully provided against in
highway design.
Outside of the areas mentioned above, the precipitation is much less
than 30 inches per year and its effect as an agent of erosion is of
greatest significance, although in restricted areas there may be short
periods when the soil is made unstable by ground water.
=Importance of Design.=--The drainage system for a proposed road
improvement ought to be designed with as much care as any other
element, and, to do so, a study must be made of all factors that have
any bearing on the drainage requirements and the probable
effectiveness of the proposed drainage system. The well established
principles of land drainage should be followed so far as applicable.
The basic principle of road drainage is to minimize the effect of
water to such an extent that there will always be a layer of
comparatively dry soil of appreciable thickness under the traveled
part of the road. This layer should probably never be less than two
feet thick and for soils of a structure favorable to capillary action
it should be at least three feet thick. The means employed to
accomplish the requisite drainage will be as various as the conditions
encountered.
=Surface Drainage.=--The drainage method which is by far the most
nearly general in application is that which utilizes open ditches, and
the system which employs these ditches is usually referred to as
surface drainage. The full possibilities of this method of minimizing
the effects of storm water are rarely fully utilized in road
construction. Very frequently, deterioration of a road surface is
directly attributable to failure to provide adequately for the removal
of the storm water or water from the melting of snow that has fallen
on the road, or water that flows to the road from land adjacent
thereto. Surface water can usually most cheaply and expeditiously be
carried away in open ditches, although special conditions are
occasionally encountered which require supplementary tile drains. The
cross section commonly adopted for roads lends itself naturally to the
construction of drainage ditches at the sides of the traveled way, and
these are usually the principal dependence for the disposal of storm
water.
=Run-off.=--The capacity required of side ditches to insure
satisfactory surface drainage will be affected by the amount and
nature of the precipitation in the region where the road is built. The
annual rainfall in a region may amount to several feet, but may be
well distributed throughout the year with an absence of excessive
rainfall for short periods, that is, flood conditions may rarely
occur. In other areas, the annual rainfall may be comparatively small
but the precipitation occurs at a very high rate, that is, flood
conditions may be common, or it may be at a low rate extending over a
considerable period. These peculiarities must be known before an
adequate drainage system can be planned.
It is almost universally true in the United States that precipitation
at a very high rate will be for a relatively short duration, and
during these short periods, which usually do not exceed thirty
minutes, a portion of the water that falls on the areas adjacent to
the road and that drains to the road ditches will soak into the soil
and therefore not reach the ditches along the road. The extent to
which the water is taken up by the soil will vary with the porosity
and slope of the land and the character of the growth thereon.
Cultivated land will absorb nearly all of the water from showers up to
fifteen or twenty minutes duration; grass land a somewhat smaller
percentage; and hard baked or other impervious soil will absorb a
comparatively small amount. Rocky ground and steep slopes will absorb
very little storm water.
The surface of the road is designed to turn water rapidly to the
ditches, but when the material is the natural soil, there is always
considerable absorption of storm water. Surfaces such as sandclay,
gravel and macadam do not absorb to exceed 10 per cent of the
precipitation during short showers. Bituminous surfaces, brick and
concrete pavements, do not absorb an appreciable amount of storm
water.
Generally it is best to assume that if a rain lasts for forty-five
minutes or more, all of the water will run off, as the soil will reach
a state of saturation in that time. This is not true of deep sand, but
is for nearly all other soils.
The ditch capacity needed will therefore depend upon the area drained,
the character of the soil, the slopes and the rainfall characteristics
of the region, and upon the nature of the road surface.
For a required capacity, the cross section area of the ditch will vary
inversely as the grade, because the velocity of flow increases with an
increase in the grade of the ditch. If the surface water must be
carried along the road for distances exceeding five or six hundred
feet, the ditch must be constructed of increasing capacity toward the
outlet in order to accommodate the accumulated volume of water.
The velocity of flow varies not only with the grade, but with the
shape of the cross section, cleanness of the channel, the depth of the
water in the channel, alignment of the channel and the kind of
material in which the channel is formed. It is not necessary to go to
great refinement in the design of the side ditches for the ordinary
case where the water is carried along the road for only a few hundred
feet. The ditches are made of ample capacity by using the commonly
accepted cross section for a road, which will be discussed in a later
paragraph. But where large areas must be drained by the road ditches,
it is desirable carefully to design the side ditches. The basis for
that design is too lengthy to be included herein, and reference should
be made to a standard treatise on the subject.
=Ordinary Design of Ditches.=--For grades of one per cent or less on
roads in the humid area, the bottom of the ditch should be at least
three and one-half feet lower than the traveled surface of the road,
except for very sandy soil. For grades greater than one per cent, this
depth may be decreased one foot, and for grades of four per cent and
upward, the depth may be still less. These general rules for depth are
susceptible of variation but are believed to be the minimum except in
arid or semi-arid climates. It is far better to be too liberal in
ditch allowance than to be too conservative. In arid or semi-arid
regions, the ditch design will be based on the necessity of providing
for flood flow and preventing damage through erosion. Ordinary
drainage requirements will be satisfactory with the ditch about one
foot deep.
If the topography is such that it is evident considerable storm water
will flow from the adjacent land to the road ditches, the design must
be modified to take this into account. Sometimes such water can be
diverted by ditches well back from the road, and thus prevented from
flowing into the side ditches along the roadway. It is especially
desirable to divert water, which would otherwise flow down the slope
of a cut, by means of a ditch on the hill-side above the upper edge of
the slope of the cut.
Ditches are not effective unless they afford a free flow throughout
their length and have an outlet to a drainage channel of ample
capacity. Therefore, ditch grades should be established by survey,
especially if the gradient is less than one per cent, and the
construction work should be checked to insure that the ditch is
actually constructed as planned. A few high places in the ditch will
greatly reduce the effectiveness, although these may appear at the
time of construction to be slight. Constricted places, such as might
be due to a small amount of loose earth left in the ditch, are always
to be avoided.
Where the side ditch passes from a cut to the berm alongside a fill,
the ditch should be excavated throughout in the undisturbed natural
soil, five feet or more from the toe of the slope of the fill, and
along the filled portion of the road there should be a berm of three
or four feet between the toe of the slope of the fill and the near
edge of the ditch.
=Underground Water.=--In a preceding paragraph, mention was made of
the fact that only a part of the storm water runs off over the surface
of the ground, the larger part being absorbed by the soil. The water
thus absorbed flows downward through the pores in the soil until it is
deflected laterally by some physical characteristic of the soil
structure. The movement of underground water is affected by many
circumstances, but only two conditions need be discussed herein.
Underground water, like surface water, tends to attain a level
surface, but in so doing it may need to flow long distances through
the pores of the soil, and to overcome the resistance incident to so
doing some head will be required. That is to say, the water will be
higher at some places than at others. If a cut is made in grading the
road, the road surface may actually be lower than the ground water
level in the land adjoining the road. As a result, the water will seep
out of the side slopes in the cut and keep the ditches wet, or even
furnish enough water to occasion a flow in the ditch. Similarly, the
higher head of the underground water near the top of a hill may result
in ground water coming quite close to the surface some distance down
the hill. The remedy in both cases is tile underdrains alongside the
road to lower the ground water level so that it cannot affect the road
surface.
Sometimes the ground water encounters an impervious stratum as it
flows downward through the soil, or one that is less pervious than the
surface soil. When such is the case, the water will follow along this
stratum, and should there be an outcrop of the dense stratum, a spring
will be found at that place. This may be on a highway. The impervious
stratum may not actually outcrop but may lie only a few feet under the
surface of the road, in which case, the road surface will be so water
soaked as to be unstable. The so-called "seepy places" so often noted
along a road are generally the result of this condition. This
condition can be corrected by tile laid so as to intercept the flow at
a depth that precludes damage to the road. Commonly, the tile will be
laid diagonally across the road some distance above the section where
the effect of the water is noted, and will be turned parallel to the
road at the ditch line and carried under one of the side ditches to an
outlet.
=Tile Drains.=--Where the soil and climatic conditions are such that
the roadway at times becomes unstable because of underground water
rising to a level not far below the road surface, the ground water
level is lowered by means of tile underdrains. The function of the
tile drains in such cases is precisely the same as when employed in
land drainage; to lower the ground water level.
=Laying Tile.=--The tile lines are usually laid in trenches parallel
to the center line of the road near the ditch line and at least 4 feet
deep so as to keep the ground water level well down. They must be
carefully laid to line and grade. A good outlet must be provided and
the last few joints of pipe should be bell-and-spigot sewer pipe with
the joints filled with cement mortar. The opening of the tile should
be covered with a coarse screen to prevent animals from nesting in the
tile.
It is frequently necessary to lay a line of tile at the toe of the
slope in cuts to intercept water that will percolate under the road
from the banks at the sides. In some cases, it is desirable to
back-fill the tile trench with gravel or broken stone to insure rapid
penetration of surface water to the tile. In other instances, it is
advantageous to place catch basins about every three or four hundred
feet. These may be of concrete or of tile placed on end or may be
blind catch basins formed by filling a section of the trench with
broken stone. When a blind catch basin is used, the top should be
built up into a mound, and for a tile or concrete catch basin, a
grating of the beehive type should be used, so that flow to the tile
will not be obstructed by weeds and other trash that is carried to the
catch basin.
=Culverts.=--Culverts and bridges are a part of the drainage system
and the distinction between the two is merely a matter of size.
Generally, structures of spans less than about eight feet are classed
as culverts, but the practice is not uniform. In this discussion
culverts will be defined as of spans of 8 feet or less.
Numerous culverts are required to afford passage for storm water and
small streams crosswise of the road, and their aggregate cost is a
large item in the cost of road improvement. The size of the waterway
of a culvert required in any location will be estimated by an
inspection of the stream and existing structure, and by determining
the extent and physical characteristics of the drainage area.
Sometimes there is sufficient evidence at the site to indicate quite
closely the size required, but this should always be checked by
run-off computations. The drainage area contributing water to the
stream passing through the culvert under consideration is computed
from contour maps or from a survey of the ground, and the size of
culvert determined by one of the empirical formulas applicable to that
purpose. In these formulas, the solution depends upon the proper
selection of a factor "C" which varies in accordance with the nature
of the drainage area. Two of these that are quite widely used are as
follows:
_Myers' Formula: a = CA_
Where _a_ = area of cross section of culvert in square feet. _A_ =
area in acres of the drainage area above culvert. _C_ a factor varying
from 1 for flat country to 4 for mountainous country or rocky soil,
the exact value to be selected after an inspection of the drainage
area.
_Talbot's Formula_: Area of waterway in square feet =
_C_ [Square root of] ((Drainage area in acres)^3)
Transcriber's Note: The above formula used the mathematical
square root symbol in the original. One should read it as "C
times the square root of the Drainage area in acres cubed."
_C_ being variable according to circumstances thus:
"For steep and rocky ground _C_ varies from 2/3 to 1. For rolling
agricultural country, subject to floods at times of melting snow, and
with length of valley three or four times its width, _C_ is about 1/3,
and if stream is longer in proportion to the area, decrease _C_. In
districts not affected by accumulated snow, and where the length of
valley is several times its width, 1/5 or 1/6 or even less may be
used. _C_ should be increased for steep side slopes, especially if the
upper part of the valley has a much greater fall than the channel at
the culvert. The value of _C_ to be used in any case is determined
after an inspection of the drainage area."
[Illustration: Fig. 2. Design of Pipe Culvert and Bulkhead]
=Length of Culvert.=--The clear length between end walls on a culvert
should be at least equal to the width of the roadway between ditches.
This is a minimum of 20 feet for secondary roads and ranges from 24 to
30 feet for main roads. The headwall to the culvert should not be a
monument, but should be no higher than needed to prevent vehicles from
leaving the roadway at the culvert.
=Farm Entrance Culverts.=--At farm entrances, culverts are required to
carry the farm driveway across the side ditch of the road. These
culverts are usually about 16 feet along, and should be of a size
adequate to take the flow of the side ditch. The farm entrance culvert
should be of such design that it can be easily removed to permit
cleaning out the ditches with a road grader.
TYPES OF CULVERTS
Culverts constructed of concrete and poured in place are called box
culverts because of the rectangular form of the cross section.
Culverts of pre-cast pipe are known as pipe culverts. Several forms of
pipe culvert are in general use.
[Illustration: Fig. 3.--Typical Concrete Box Culvert]
=Metal Pipe.=--These may be of cast iron, steel or wrought iron. The
cast iron pipe is very durable but expensive and heavy to handle and
is not widely used in highway construction. Steel pipe has been
employed to a limited extent but its durability is questioned. At
least it is known that the pipe made from uncoated, light sheet steel
is not very durable. Sheet iron and sheets made from alloy iron
coated with spelter have been extensively used and seem to be durable,
especially when laid deep enough to eliminate possibility of damage
from heavy loads. To insure reasonable resistance to corrosion, the
metal sheets should be coated with at least one and one-half ounces of
spelter per square foot of sheet and the sheets should not be lighter
than 16 gauge for small sizes and should be heavier for the larger
sizes.
=Clay and Cement Concrete Pipe.=--The ordinary burned clay bell and
spigot pipe that is employed for sewer construction is sometimes used
for culverts. It must be very carefully bedded, preferably on a
concrete cradle and the joints filled with cement mortar. Culverts of
this type have a tendency to break under unusual loads, such as
traction engines or trucks. They may be damaged by the pressure from
freezing water, particularly when successive freezing and thawing
results in the culvert filling with mushy snow, which subsequently
freezes.
=Concrete Pipe.=--Reinforced concrete pipe is a satisfactory material
for culverts, if the pipe is properly designed. The pipe should be
carefully laid on a firm earth bed with earth carefully back-filled
and tamped around the pipe. The joints in the pipe should be filled
with cement mortar, or should be of a design that will be tight.
=Endwalls for Culverts.=--A substantial retaining wall is placed at
each end of the culvert barrel, whatever the type. This is to prevent
the end of the culvert from becoming choked with earth and to retain
the roadway at the culvert. It also indicates to the drivers the
location of the end of the culvert. The endwall extends a foot or more
below the floor of the culvert to prevent water from cutting under the
barrel. Plain concrete or stone masonry are most commonly used for
culvert endwalls.
[Illustration: Fig. 4.--Two Types of Drop Inlet Culvert]
=Reinforced Concrete Box Culverts.=--The pipe culvert is limited in
application to the smaller waterways. Reinforced concrete is
extensively used for culverts of all sizes, but especially for the
larger ones. These are usually constructed with endwalls integral with
the barrel of the culvert. Culverts of this type must be designed for
the loads anticipated to insure suitable strength and stability, and
must be constructed of a good quality of concrete. Figs. 2 and 3 show
designs for pipe and box culverts.
[Illustration: Fig. 5.--Drop Inlet Culvert]
=Drop Inlet Culverts.=--In some locations erosion has begun in the
fields adjacent to a culvert and it will probably continue until the
stream above the culvert has eroded to about the level of the floor of
the culvert. This is a reason for placing the culvert as high as the
roadway will permit, so long as the area above the culvert will be
properly drained. Considerable reclamation of land is possible if the
culvert is constructed with a box at the inlet and as shown in Fig. 4.
The area up-stream from the culvert will not erode below the level of
the top of the box at the inlet end.
Where the stream crossing the road has eroded to considerable depth or
has considerable fall, as would sometimes be the case on side hill
roads, the culvert barrel would follow the general slope of the ditch
but should have a drop inlet. This type of culvert is shown in Fig.
5.
CHAPTER IV
ROAD DESIGN
=Necessity for Planning.=--Sometimes highway improvement is the result
of spasmodic and carelessly directed work carried out at odd times on
various sections of a road, finally resulting in the worst places
being at least temporarily bettered. The grade on the steepest hills
is probably reduced somewhat and some of the worst of the low lying
sections are filled in and thereby raised. Short sections of surfacing
such as gravel or broken stone may be placed here and there. From the
standpoint of the responsible official, the road has been "improved,"
but too often such work does not produce an improvement that lasts,
and sometimes it is not even of any great immediate benefit to those
who use the roads. In nearly every instance such work costs more in
money and labor that it is worth.
Lasting improvement of public highways can be brought about only
through systematic and correlated construction carried on for a series
of years. In other words, there must be a road improvement policy
which will be made effective through some agency that is so organized
that its policies will be perpetuated and is clothed with enough
authority to be capable of enforcing the essential features of good
design and of securing the proper construction of improvements.
Details of highway construction and design must vary with many local
conditions and types of surface. The limits of grades and the many
other details of design may properly be adopted for a specific piece
of work only after an adequate investigation of the local requirements
and in the light of wide experience in supervising road improvement.
New ideas are constantly being injected into the art of road building,
but these are disseminated somewhat slowly, so that valuable devices
and improvements in methods remain long unknown except to the
comparatively few who have the means for informing themselves of all
such developments.
It follows then that the logical system of conducting road improvement
is through an agency of continuing personnel which will supervise the
preparation of suitable plans and direct the construction in
accordance with the most recent experience.
=Road Plans.=--The information shown on the plans prepared for road
improvement varies somewhat with the design and with the ideas of the
engineer as to what constitutes necessary information, but in general
the plans show the existing road and the new construction contemplated
in an amount of detail depending principally upon the character of the
construction. Simple plans suffice for grade reduction or reshaping an
earth road surface, while for the construction of paved roads, the
plans must be worked out in considerable detail. The essential
requirement is that there be given on the plans all information
necessary to enable the construction to be carried out according to
the intentions of the engineer, that all parts of the work fit
together, that the culverts are of the proper size and located at the
proper places, ditches drain properly, grades are reduced to the
predetermined rate, that excavated material is utilized and that an
exact record of the work done is retained. Plans are indispensable to
economical road construction and the preparation of the plans is the
work of the expert in road design, that is, the highway engineer.
=Problem of Design.=--The problem of road design is to prepare plans
for a road improvement with the various details so correlated as to
insure in the road constructed in accordance therewith the maximum of
safety, convenience and economy to the users thereof. The degree to
which the design will be effective will depend to a considerable
extent upon the financial limitations imposed upon the engineer, but
skill and effort on the plans will do a great deal to offset financial
handicap and no pains should be spared in the preparation of the
plans. Moreover, the plans must afford all of the information needed
by the contractor in preparing a bid for the work.
=Preliminary Investigation.=--The first step in road improvement is to
secure an adequate idea of the existing conditions on the road or
roads involved. The detail to which this information need go will
depend entirely upon the purpose of the preliminary investigation, for
before a definite plan is prepared, it may be necessary to choose the
best from among several available routes. For this purpose, it is not
always necessary to make an actual instrument survey of the several
routes. A hasty reconnaissance will usually be sufficient. This is
made by walking or riding over the road and noting, in a suitable book
or upon prepared blanks, the information needed. The items of
information recorded will usually be as follows: distances, grades,
type of soil on the road and nature of existing surface, character of
drainage, location of bridges and culverts and the type of each with
notes as to its condition, location of railway crossings and notes as
to type, location of intersecting roads, farm entrances, and all
similar features that have a bearing on the choice of routes. These
data can be obtained in a comparatively short time by a skilled
observer who may drive over the road in a motor car. Sometimes it may
be desirable to make a more careful study of some certain sections of
road and this may be done by waking over the section in question in
order to make a more deliberate survey of the features to be
considered than is possible when riding in a motor car.
Factors other than relative lengths of routes will obviously determine
the cost of improvement and the comparative merits of the improved
roads. Some special characteristic of a road, such as bad railroad
crossings or a few bad hills, may eliminate a route, or availability
of materials along a route may offset disadvantages of alignment or
grade.
In special cases, complete surveys of routes may be required finally
to select the best route, but these instances are few in number.
=Road Surveys.=--When a road has been definitely selected for
improvement, a careful survey is made to furnish information for the
preparation of the plans. This will consist of a transit survey and a
level survey.
The transit survey is made by running a line between established
corners following the recorded route of the road, or if no records are
available or the road is irregular in alignment, by establishing
arbitrary reference points and running a line along the center line of
the existing road or parallel thereto. The topography is referenced to
this line in such completeness that it can be reproduced on the plans.
The level survey consists in taking levels on cross sections of the
road at one hundred foot intervals, and oftener if there are abrupt
changes in grade. Special level determinations are made at streams,
railroad crossings, intersecting roads or lanes and wherever it
appears some special features of the terrain should be recorded.
From the surveys and such other information as has been assembled
relative to the project, a plan is prepared which embodies a design
presumed to provide for an improvement in accordance with the best
highway practice.
THE PROBLEM OF DESIGN
It will be convenient to consider separately the components of a road
design, although in the actual design the consideration of these
cannot be separated because all parts of the plan must fit together.
=Alignment.=--The alignment of the road is determined to a
considerable extent by the existing right-of-way, which may follow
section lines, regardless of topography, as is the case with many
roads in the prairie states, or it may follow the valleys, ridges, or
other favorable location in hilly country. In many places the roads of
necessity wind around among the hills in order to avoid excessive
grades. In designing an improvement, it is generally desirable to
follow the existing right-of-way so far as possible. But the element
of safety must not be lost sight of, and curves should not preclude a
view ahead for sufficient distance to insure safety to vehicles. The
necessary length of clear view ahead is usually assumed to be 250
feet, but probably 200 feet is a satisfactory compromise distance when
a greater distance cannot be obtained at reasonable cost. To secure
suitable sight distance, the curves must be of long radii, and where
possible the right-of-way on the inside of the curve should be cleared
of trees or brush that will obstruct the view. Where the topography
will not permit a long radius curve and the view is obstructed by an
embankment or by growing crops or other growth, it is desirable to
separate the tracks around the curve to eliminate the possibility of
accidents on the curve. This is readily accomplished if the road is
surfaced, but if it is not surfaced, the same end is accomplished by
making the earth road of ample width at the curve.
Relocations should be resorted to whenever they shorten distances or
reduce grades sufficiently to compensate for the cost.
=Intersections.=--At road intersections, it is always difficult to
design a curve that entirely meets the requirements of safety because
there is not enough room in the right-of-way, and enough additional
right-of-way must be secured to permit the proper design. It is not
necessary to provide an intersection that is adapted to high speed
traffic, where main roads cross, but, on the contrary, a design that
automatically causes traffic to slow up has distinct advantages.
Where a main route, improved with a hard surface, crosses secondary
roads, it is satisfactory to continue the paved surface across the
intersecting road at normal width and make no provision for the
intersecting road traffic other than a properly graded approach at the
intersection.
=Superelevation.=--On all curved sections of road, other than
intersections, account is taken of the tendency of motor cars to skid
toward the outside of the curve. This tendency is counteracted by
designing the cross section with superelevation.
[Illustration: Fig. 6]
In Fig. 6, _F_ represents the tangential force that tends to cause
skidding. _W_ represents the weight of the vehicle in pounds, THETA
= the angle of superelevated surface _c-d_, with the horizontal _c-a_.
_R_ represents the radius of the curve upon which the vehicle is
moving. _w_ is the component of the weight parallel to the surface
_c-d_, _v_ = velocity of the vehicle in feet per second. _m_ = mass
of vehicle = _W/g THETA_
_w_ = _W_ tan _THETA_
_mv^2_ _wv^2_
_F_ = ------- = ------
_R_ _gR_
If _F_ = _w_ there will be no tendency to skid; hence the rate of
superelevation necessary in any case is as follows:
_Wv^2_
_W_ tan _THETA_ = -------
_gR_
_v^2_
tan _THETA_ = -------
_gR_
The amount of superelevation required, therefore, varies as the square
of the velocity and inversely as the radius of the curve.
Theoretically, the amount of the superelevation should increase with a
decrease in the radius of the curve and should also increase as the
square of the speed of the vehicle. On account of the variation in
speeds of the vehicles, the superelevation for curves on a highway can
only be designed to suit the average speed. At turns approaching
ninety degrees, the curve is likely to be of such short radius that it
is impossible to maintain the ordinary road speed around the curve,
even with the maximum superelevation permissible. It is good practice
to provide the theoretical superelevation on all curves having radii
greater than 300 feet for vehicle speeds of the maximum allowed by
law, which is generally about 25 miles per hour. Where the radii are
less than 300 feet, the theoretical superelevation for the maximum
vehicle speeds gives a superelevation too great for motor trucks and
horse drawn vehicles and generally no charge is made in superelevation
for radii less than 300 feet, but all such curves are constructed with
the same superelevation as the curve with 300 foot radius.
The diagram in Fig. 7 shows the theoretical superelevation for various
curve radii.
[Illustration: Fig. 7. Curves showing Theoretical Superelevation for
Various Degrees of Curve for Various Speeds of Vehicle]
At the intersection of important highways, the problem is complicated
by the necessity for providing for through traffic in both directions
and for traffic which may turn in either direction and the engineer
must provide safe roadways for each class of traffic.
=Tractive Resistance.=--The adoption of a policy regarding the grades
on a road involves an understanding of the effect of variation in the
character of the surface and in rate of grade upon the energy required
to transport a load over the highway. The forces that oppose the
movement of a horse drawn vehicle are fairly well understood and their
magnitude has been measured by several observers, but comparatively
little is known about the forces opposing translation of rubber tired
self-propelled vehicles.
The resistance to translation of a vehicle is made up of three
elements: resistance of the road surface to the rolling wheel,
resistance of the air to the movement of the vehicle and internal
friction in the vehicle itself.
=Rolling Resistance.=--When the wheel of a vehicle rolls over a road
surface, both the wheel and the surface are distorted. If the wheel
has steel tires and the road surface is plastic, there will be
considerable distortion of the road surface and very little of the
wheel. A soft rubber tire will be distorted considerably by a brick
road surface. Between these extremes there are innumerable
combinations of tire and road surface encountered, but there is always
a certain amount of distortion of either road surface or wheel, or of
both, which has the same effect upon the force necessary for
translation as a slight upward grade. When both the tire and the road
surface strongly resist distortion (as steel tires on vitrified brick
paving), the resistance to translation is low but the factor of impact
is likely to be introduced. Where impact is present, energy is used up
in the pounding and grinding of the wheels on the surface, and this
factor increases as the speed of translation, and may be a
considerable item. Impact is especially significant on rough roads
with motor vehicles, particularly trucks, traveling at high speed.
These two factors (impact and rolling resistance) combined constitute
the major part of the resistance to translation for horse drawn
vehicles.
=Internal Resistance.=--For horse drawn vehicles, the internal
resistance consists of axle friction, which is small in amount. For
self-propelled vehicles, the internal resistance consists of axle
friction and friction in the driving mechanism, of which gear
friction and the churning of oil in the gear boxes is a large item.
Internal friction is of significance in all self-propelled vehicles
and especially so at high speeds.
=Air Resistance.=--At slow speeds, the resistance of still air to
translation is small, but as the speed increases, the air resistance
increases rapidly and at the usual speed of the passenger automobile
on the road becomes a very considerable part of the total resistance
to translation. This factor has no significance in connection with
horse drawn vehicles, but is to be taken into account when dealing
with self-propelled vehicles at speeds in excess of five miles per
hour.
Many determinations of tractive resistance with horse drawn vehicles
have been made from time to time and these show values that are fairly
consistent when the inevitable variations in surfaces of the same type
are taken into account. Table 4 is a composite made up of values
selected from various reliable sources and Table 5 is from experiments
by Professor J. B. Davidson on California highways.
TABLE 4
AVERAGE TRACTIVE RESISTANCE OF ROAD SURFACES TO STEEL TIRED
VEHICLES
Surface Tractive force per ton
Earth packed and dry 100
Earth dusty 106
Earth muddy 190
Sand loose 320
Gravel good 51
Gravel loose 147
Cinders well-packed 92
Oiled road--dry 61
Oiled road--wet 108
Macadam--very good 38
Macadam--average 46
Sheet asphalt 38
Asphaltic concrete 40
Vitrified brick--new 56
Wood block--good 33
Wood block--poor 42
Cobblestone 54
Granite tramway 27
Asphalt block 52
Granite block 47
TABLE 5
TRACTIVE RESISTANCES TO STEEL TIRED VEHICLES[1]
----------+-----------------+-----------------+-----------+-----------
| | Condition | Tractive | Resistance
Test No. | Kind of Road | of Road | Total lb. | per ton lb.
----------+-----------------+-----------------+-----------+-----------
29-30-31 | Concrete |Good, excellent | 83.0 | 27.6
| (unsurfaced) | | |
[2]11-12 | Concrete |Good, excellent | 90.0 | 30.0
| (unsurfaced) | | |
26-27-28 | Concrete 3/8-in.|Good, excellent | 147.6 | 49.2
| surface | | |
| asphaltic oil | | |
| and screenings| | |
13-14 | Concrete 3/8-in.|Good, excellent | 155.0 | 51.6
| surface | | |
| asphaltic oil | | |
| and screenings| | |
9-10 | Macadam, |Good, excellent | 193.0 | 64.3
| water-bound | | |
22-23 | Topeka on |Good, excellent | 205.5 | 68.5
| concrete | | |
8 | Gravel |Compact, good | 225.0 | 75.0
| | condition | |
[3]45-48 | Oil macadam |Good, new | 234.5 | 78.2
[4]46-47 | Oil macadam |Good, new | 244.0 | 81.3
38 | Gravel |Packed, in | 247.0 | 82.3
| | good condition | |
18-19-20 | Topeka on plank |Good condition, | 265.0 | 88.3
| | soft, wagon | |
| | left marks | |
34 | Earth road |Firm, 1-1/2-in. | 276.0 | 92.0
| | fine loose dust| |
24-25 | Topeka on plank |Good condition, | 278.0 | 92.6
| | but soft | |
1-2-5 | Earth road |Dust 3/4 to 2 in.| 298.0 | 99.3
3-3 | Earth |Mud, stiff, firm | 654.0 | 218.0
| | underneath | |
6-7 | Gravel |Loose, not packed| 789.0 | 263.0
----------+-----------------+-----------------+-----------+-----------
[1] Prof. J. B. Davidson in _Engineering News-Record_, August 17,
1918.
[2] Graphic record indicates that the load was being accelerated
when test was started.
[3] Drawn with motor truck at 2-1/2 miles per hour.
[4] Drawn with motor truck at 5 miles per hour.
Comparatively few data are available showing the tractive resistance
of motor vehicles, but the following tables are based on sufficient
data to serve to illustrate the general trend.
These data on the tractive resistances of an electric truck with solid
rubber tires on asphalt and bitulithic, wood, brick and granite block,
water-bonded and tar macadam, cinder and gravel road surfaces were
obtained by A. E. Kennelly and O. R. Schurig in the research division
of the electrical engineering department of the Massachusetts
Institute of Technology, and are published in Bulletin No. 10 of the
division.
An electric truck was run over measured sections, ranging from 400 to
2600 feet in length, surfaced with these various materials, at certain
speeds per hour, ranging from about 8 to about 15.5 miles per hour.
The result of the observations of speeds, tractive resistances,
conditions of surfaces, etc., were collected and studied in various
combinations.
TABLE 6
----------------------+-----------------------+-----------+----------
| | Tractive | Tractive
| |Resistance |Resistance
Type of Surface | Condition of Surface | in lbs. | in lbs.
| | per ton | per ton
| | 10 miles |12.4 miles
| | per hr. | per hr.
----------------------+-----------------------+-----------+----------
Asphalt | Good | 20.4 |
Asphalt | Poor | 22.6 | 25.5
Wood block | Good | 24.2 | 25.3
Brick block | Good | 24.6 | 26.6
Granite block | Good | 40.3 | 45.75
Brick block | Slightly worn | 25.1 | 28.0
Granite block with | | |
cement joints | Good | 25.5 | 30.2
Macadam, water bonded | Dry and hard | 23.3 | 25.8
Macadam, water bonded | Fair, heavily oiled | 35.9 | 38.7
Macadam, water bonded | Poor, damp, some holes| 36.3 | 41.6
Tar macadam | Good | 25.7 | 28.0
Tar macadam | Very soft | 36.8 | 38.7
Tar macadam | Many holes, soft, | |
| extremely poor | 52.4 | 60.6
Cinder | Fair, hard | 27.5 | 30.6
Gravel | Fair, dusty | 30.4 | 33.0
----------------------+-----------------------+-----------+----------
[Illustration: Fig. 8]
=Effect of Grades.=--Grades increase or decrease the resistance to
translation due to the fact that there is a component of the weight of
the vehicles parallel to the road surface and opposite in direction to
the motion when the load is ascending the hill and in the same
direction when the vehicle is descending. In Fig. 8 _W_ represents the
weight of the vehicle, acting vertically downward, _w_ is the
component of the weight perpendicular to the road surface and _W_{2}_
is the component parallel to the road surface.
_W_{2}_ = _W_ tan _THETA_.
tan _THETA_ = 0.01 × 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 a
grade is therefore 20 lbs. per ton for each one per cent of grade and
this force tends either to retard or to accelerate the movement of the
vehicle.
Let _F_ = the sum of all forces opposing the translation of a vehicle.
_F = f_{r} + f_{i} + f_{p} + f_{a} + f_{g}_ (1)
where
_f_{r}_ = rolling resistance of road surface.
_f_{i}_ = resistance due to internal friction in the vehicle.
_f_{p}_ = resistance due to impact of the road surface.
_f_{a}_ = resistance due to air.
_f_{g}_ = resistance due to grade, which is positive when
ascending and negative when descending.
All of the above in pounds per ton of 2000 lbs.
Let _T_ = the tractive effort applied to the vehicle by any means.
_T_ >= must be greater than _F_ in order to move the vehicle.
By an inspection of (1), it will be seen that for a given vehicle and
any type of road surface, all terms are constant except _f_{a}_ and
_f_{g}_. _f_{a}_ varies as the speed of the vehicle and the driver can
materially decrease _f_{a}_ by reducing speed. _f_{g}_ varies with the
rate of grade. For any vehicle loaded for satisfactory operation on a
level road with the power available, the limiting condition is the
factor _f_{g}_. If the load is such as barely to permit motion on a
level road, any hill will stall the vehicle. Therefore, in practice
the load is always so adjusted that there is an excess of power on a
level road. If draft animals are employed the load is usually about
one fourth of that which the animals could actually move by their
maximum effort for a short period. With motor vehicles, the excess
power is provided for by gearing.
If it be assured a load of convenient size is being moved on a level
road by draft animals, there is a limit to the rate of grade up which
the load can be drawn by the maximum effort of the animals.
Tests indicate that the horse can pull at a speed of 2-1/2 miles per
hour, an amount equal to 1/8 to 1/10 of its weight, and for short
intervals can pull 3/4 of its weight. The maximum effort possible is
therefore six times the average pull, but this is possible for only
short intervals. A very short steep hill would afford a condition
where such effort would be utilized. But for hills of any length, that
is, one hundred feet or more but not to exceed five hundred feet, it
is safe to count on the draft animal pulling three times his normal
pulling power for sustained effort.
The limiting grade for the horse drawn vehicle is therefore one
requiring, to overcome the effect of grade, or _f_{g}_, a pull in
excess of three times that exerted on the level.
A team of draft animals weighing 1800 lbs. each could exert a
continuous pull of about 1/10 of their weight or 360 lbs. If it be
assumed that the character of the vehicle and the road surface is such
that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 100 lbs. per gross ton on
a level section of road, then the gross load for the team would be 3.6
tons. The same team could for a short time exert an additional pull of
three times 360 lbs. or 1080 lbs. For each 1 per cent of grade a pull
of 20 lbs. per ton would be required or _f_{g}_ for the 3.6 tons load
would be 72 lbs. for each per cent of grade. At that rate, the
limiting grade for the team would be fifteen per cent.
If, however, the character of the vehicle and the road surface were
such that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 60 lbs. per gross
ton on a level section of road, the gross load for the team on the
level would be 6 tons, and the limiting grade 9 per cent.
The above discussion serves to illustrate the desirability of adopting
a low ruling or limiting grade for roads to be surfaced with a
material having low tractive resistance and the poor economy of
adopting a low ruling grade for earth roads or roads to be surfaced
with material of high tractive resistance.
It may be questioned whether horse drawn traffic should be the
limiting consideration for main trunk line highways, but it is
certain that for a number of years horse drawn traffic will be a
factor on secondary roads.
In the case of motor vehicles, excess power is provided by means of
gears and no difficulty is encountered in moving vehicles over grades
up to 12 or 15 per cent, so that any grade that would ordinarily be
tolerated on a main highway will present no obstacle to motor
vehicles, but the economy of such design is yet to be investigated.
=Energy Loss on Account of Grades.=--Whether a vehicle is horse drawn
or motor driven, energy has been expended in moving it up a hill. A
part of this energy has been required to overcome the various
resistances other than grade, and that has been dissipated, but the
energy required to translate the vehicle against the resistance due to
grade has been transformed into potential energy and can be partially
or wholly recovered when the vehicle descends a grade, provided the
physical conditions permit its utilization. If the grade is so steep
as to cause the vehicle to accelerate rapidly, the brakes must be
applied and loss of energy results. The coasting grade is dependent
upon the character of the surface and the nature of the vehicle. In
the cases discussed in the preceding paragraph, the coasting grades
would be five per cent and three per cent respectively. For horse
drawn vehicles then the economical grades would be three and five per
cent, which again emphasizes the necessity of lower grades on roads
that are surfaced than on roads with no wearing surface other than the
natural soil.
The theory of grades is somewhat different when motor vehicles are
considered, since it is allowable to permit considerably higher speed
than with horse drawn vehicles before applying the brakes and the
effect of grade can be utilized not only in translating the vehicle
down the grade, but also in overcoming resistances due to mechanical
friction and the air. On long grades, a speed might be attained that
would require the use of the brake or the same condition might apply
on very steep short grades. There is at present insufficient data on
the tractive resistance and air resistance with motor vehicles to
permit the establishing of rules relative to grade, but experience
indicates a few general principles that may be accepted.
If a hill is of such rate of grade and of such length that it is not
necessary to use the brake it may be assumed that no energy loss
results so far as motor vehicles are concerned. Where there is no turn
at the bottom of the hill and the physical condition of the road
permits speeds up to thirty-five or forty miles per hour grades of
five per cent are permissible if the length does not exceed five
hundred feet and grades of three per cent one thousand feet long are
allowable. It is a rather settled conviction among highway engineers
that on trunk line highways the maximum grade should be six per cent,
unless a very large amount of grading is necessary to reach that
grade.
=Undulating Roads.=--Many hills exist upon highways, the grade of
which is much below the maximum permissible. If there are grades
ranging from 0 to 4 per cent, with a few hills upon which it is
impracticable to reach a grade of less than six per cent, it is
questionable economy to reduce the grades that are already lower than
the allowable maximum. It is especially unjustifiable to incur expense
in reducing a grade from two per cent to one and one-half per cent on
a road upon which there are also grades in excess of that amount. The
undulating road is not uneconomical unless the grades are above the
allowable maximum or are exceptionally long or the alignment follows
short radius curves.
=Safety Considerations.=--On hills it is especially desirable to
provide for safety and curves on hills are always more dangerous than
on level sections of road. Therefore, it is desirable to provide as
flat grades as possible at the curves and to cut away the berm at the
side of the road so as to give a view ahead for about three hundred
feet. Whether a road be level or on a hill, safety should always be
considered and the most important safety precaution is to provide a
clear view ahead for a sufficient distance to enable motor vehicle
drivers to avoid accidents.
[Illustration: Fig. 9.--Types of Guard Rails]
=Guard Railing.=--When a section of road is on an embankment, guard
rails are provided at the top of the side slope to serve as warnings
of danger, and to prevent vehicles from actually going over the
embankment in case of skidding, or if for any reason the driver loses
control. These are usually strongly built, but would hardly restrain a
vehicle which struck at high speed. But they are adequate for the
protection of a driver who uses reasonable care. A typical guard rail
is shown in Fig. 9, but many other designs of similar nature are
employed. At very dangerous turns a solid plank wall six or eight feet
high is sometimes built of such substantial construction as to
withstand the severest shock without being displaced.
Trees, shrubs and the berms at the side of the road in cuts are
particularly likely to obstruct the view and should be cleared or cut
back so far as is necessary to provide the proper sight distance.
=Width of Roadway.=--For roads carrying mixed traffic, 9 feet of width
is needed for a single line of vehicles and 18 feet for 2 lines of
vehicles. In accordance with the above, secondary roads, carrying
perhaps 25 to 50 vehicles per day, may have an available traveled way
18 feet wide. Those more heavily traveled may require room for three
vehicles to pass at any place and therefore have an available traveled
way 30 feet wide. Greater width is seldom required on rural highways,
and 20 feet is the prevailing width for main highways.
=Cross Section.=--The cross section of the road is designed to give
the required width of traveled way, and, in addition, provide the
drainage channels that may be needed. In regions of small rainfall the
side ditches will be of small capacity or may be entirely omitted, but
usually some ditch is provided. The transition from the traveled way
to ditch should be a gradual slope so as to avoid the danger incident
to abrupt change in the shape of the cross section. The depth of ditch
may be varied without changing to width or slope of the traveled part
of the road as shown in Fig. 10.
[Illustration: Fig. 10]
=Control of Erosion.=--The construction of a highway may be utilized
to control general erosion to some extent, particularly when public
highways exist every mile or two and are laid out on a gridiron
system, as is the case in many of the prairie states. The streams
cross the highways at frequent intervals and the culverts can be
placed so as effectually to prevent an increase in depth of the
stream. This will to some extent limit the erosion above the culvert
and if such culverts are built every mile or two along the stream,
considerable effect is produced.
Where small streams have their origin a short distance from a culvert
under which they pass, it is sometimes advisable to provide tile for
carrying the water under the road, instead of the culvert, and, by
continuing the tile into the drainage area of the culvert, eliminate
the flow of surface water and reclaim considerable areas of land.
Erosion in the ditches along a highway can be prevented by
constructing weirs across the ditch at frequent intervals, thus
effectually preventing an increase in the depth of the ditch.
Wherever water flows at a velocity sufficient to produce erosion or
where the drainage channel changes abruptly from a higher to a lower
level, paved gutters, tile or pipe channels should be employed to
prevent erosion.
=Private Entrances.=--Entrance to private property along the highway
is by means of driveways leading off the main road. These should
always be provided for in the design so as to insure easy and
convenient access to the property. The driveways will usually cross
the side ditch along the road and culverts will be required to carry
the water under the driveway. Driveways that cross a gutter by means
of a pavement in the gutter are usually unsatisfactory, and to cross
the gutter without providing a pavement is to insure stoppage of the
flow at the crossing. The culvert at a driveway entrance must be large
enough to take the ditch water readily or it will divert the water to
the roadway itself. Generally end walls on such culverts are not
required as in the case of culverts across a highway.
=Aesthetics.=--Much of the traffic on the public highways is for
pleasure and relaxation and anything that tends to increase the
attractiveness of the highways is to be encouraged. Usually the
roadside is a mass of bloom in the fall, goldenrod, asters and other
hardy annuals being especially beautiful. In some states wild roses
and other low bushes are planted to serve the two-fold purpose of
assisting to prevent erosion and to beautify the roadside. In humid
areas trees of any considerable size shade the road surface and are a
distinct disadvantage to roads surfaced with the less durable
materials such as sand-clay or gravel. It is doubtful if the same is
true of paved surfaces, but the trees should be far enough back from
the traveled way to afford a clear view ahead. Shrubs are not
objectionable from any view-point and are to be encouraged for their
beauty, so long as they do not obstruct the view at turns.
CHAPTER V
EARTH ROADS
Highways constructed without the addition of surfacing material to the
natural soil of the right-of-way are usually called earth roads. But
if the natural soil exhibits peculiar characteristics or is of a
distinct type, the road may be referred to by some distinctive name
indicating that fact. Hence, roads are referred to as clay, gumbo,
sandy or caliche roads as local custom may elect. In each case,
however, the wearing surface consists of the natural soil, which may
have been shaped and smoothed for traffic or may be in its natural
state except for a trackway formed by the vehicles that have used it.
=Variations in Soils.=--The nature of the existing soil will obviously
determine the serviceability and physical characteristics of the road
surface it affords. That is to say that even under the most favorable
conditions some earth roads will be much more serviceable than others,
due to the better stability of the natural soil. Some soils are dense
and somewhat tough when dry and therefore resist to a degree the
tendency of vehicles to grind away the particles and dissipate them in
the form of dust. Such soils retain a reasonably smooth trackway in
dry weather even when subjected to considerable traffic. Other soils
do not possess the inherent tenacity and stability to enable them to
resist the action of wheels and consequently grind away rapidly. Roads
on such soils become very dusty. These are the extremes and between
them are many types of soils or mixtures of soils possessing varying
degrees of stability, and, in consequence, differing rates of wear.
Similarly the various soils exhibit different degrees of stability
when wet.
It is to be expected that soils will differ with the geographical
location, for it is well known that there is a great variation in
soils in the various parts of the world. But wide differences are also
encountered in the soil on roads very near each other and even on
successive stretches of the same road. It is for this reason that
earth roads often exhibit great differences in serviceability even in
a restricted area.
=Variation in Rainfall.=--The stability of a soil and its ability to
support the weight of vehicles varies greatly with the amount of water
in the soil. A certain small amount of moisture in the soil is
beneficial in that practically every soil compacts more readily when
moist than when dry because the moisture aids in binding together the
particles. But most soils also become unstable when the amount of
water present is in excess of that small amount referred to above and
the stability decreases very rapidly as the amount of water in the
soil increases.
The serviceability of an earth road will change continually as the
moisture content of the soil changes and consequently the general
utility of the earth road system in any locality is dependent to a
considerable extent upon the amount and seasonal distribution of
precipitation. The methods of maintaining earth roads appropriate to
any locality must of necessity be adapted to the climatic conditions,
and the amount of work required to give the highest possible degree of
serviceability will be exceedingly variable from season to season and
from place to place. In regions of great humidity, earth roads may be
expected to have a low average of serviceability, while in arid
regions they may possess sufficient durability for a considerable
volume of traffic. The design adopted for earth roads and the methods
of maintenance followed should therefore be carefully evolved to meet
the soil and climate conditions where the roads are located. These
will differ greatly throughout a state or even a county.
=Cross Sections.=--The general principles of road design were set
forth in Chapter IV. In Fig. 11 are shown typical cross sections for
earth roads adapted to various conditions as indicated. It is not
apparent that one form of ditch is particularly preferable to the
other and since some engineers prefer the V section and others the
trapezoidal section both are shown. It would appear that the V shaped
ditch is somewhat the easier to construct with the blade grader while
the trapezoidal is readily excavated with the slip or fresno scraper.
The ditch capacity required and consequently the dimensions will
depend upon the drainage requirements, as was pointed out in Chapter
III.
[Illustration: Fig. 11. Cross Section for Earth Roads]
EARTH ROADS IN REGIONS OF CONSIDERABLE RAINFALL
In the zones where the annual precipitation exceeds 30 inches
distributed over several months, earth roads will be unserviceable for
a considerable period each year unless they are constructed so as to
minimize the effect of water. This is done by providing for the best
possible drainage and by adopting a method of maintenance that will
restore the surface to a smooth condition as quickly as possible after
a period of rainy weather or after the "frost comes out" in the
spring.
Before the construction of the desired cross section is undertaken,
all of the grade reduction should be completed, except for minor cuts
which can be handled with the elevating grader in the manner that will
be described presently.
Where any considerable change in grade is to be effected, the earth
can be moved in several ways and of these the most economical cannot
be readily determined. Ordinarily a contractor or a county will use
the equipment that happens to be at hand even though some other might
be more advantageous.
=Elevating Grader.=--Where the topography is such as to permit its
use, the elevating grader is employed in grade reduction to load the
earth into dump wagons in which it is hauled to the fill or waste
bank. The elevating grader consists essentially of a heavy shear plow
or disc plow which loosens the earth and deposits it on a moving
canvas apron. The apron carries the material up an incline and
deposits it into a wagon which is driven along under the end of the
apron. When the wagon is loaded, the grader is stopped while the
loaded wagon is hauled out and an empty one drawn into position. The
motive power for the elevating grader is either a tractor or five or
six teams of mules. For many kinds of work, particularly where
frequent turning is necessary or where the ground is yielding, mules
are preferable to a tractor. The apron is operated by gearing from the
rear wheels of the grader. Generally four mules are hitched to a
pusher in the rear of the grader and six or eight in the lead. This
method of grade reduction is particularly advantageous when the
material must be hauled a distance of 500 yards or more, because wagon
hauling in such cases is the most economical method to employ. A
tractor may be used to draw the elevating grader and one having a
commercial rating of 30 to 45 horsepower is required.
=Maney Grader.=--If the haul is long and the nature of the cut will
not permit the use of the elevating grader because of excessive grades
or lack of room for turning, a grader of the Maney type may be used.
This consists of a scoop of about one cubic yard capacity, suspended
from a four-wheel wagon gear. When loading, the scoop is let down and
filled in the same manner as a two-wheeled scraper or "wheeler." The
pull required to fill a Maney grader is so great that a tractor is
ordinarily employed in place of a "snap" team. The tractor is hitched
at the end of the tongue, without interfering with the team drawing
the grader. One team readily handles the grader after it is loaded.
For this service a tractor having a commercial rationing of 25 to 30
horsepower is required.
=Wheel Scraper.=--For moving earth for distances between 150 and 500
yards, the wheel scraper of a capacity of about 1-1/2 yards is quite
generally employed. The soil must be loosened with a plow before it
can conveniently be loaded into the wheeler and a heavy plow is
ordinarily employed for that purpose. Two furrows with the plow will
loosen a strip of earth about as wide as the scoop of the scraper and
if more is loosened it will be packed down by the scrapers wheeling in
place to load. A helper or "snap" team is employed to assist in
loading, after which the wheel scraper is handled by one team.
=Slip Scraper.=--The slip scraper differs from the wheel scraper in
that the scoop is not suspended from wheels but is dragged along the
ground. It is drawn by one team and the capacity is two to five cubic
feet, but the material spills out to some extent as the scraper is
dragged along and the method is not suitable for long hauls, 100 feet
being about the economical limit.
=Fresno Scraper.=--The Fresno scraper is one form of slip scraper
requiring four horses or mules for efficient work. It differs
somewhat from the ordinary slip scraper in shape and is of larger
capacity, but is a drag type of scraper much favored in the western
states.
SHAPING TO PROPER CROSS SECTION
If a road has been graded so that the profile is satisfactory or if
the existing profile of the location is satisfactory, and the surface
is to be shaped to a prescribed cross section, either the elevating
grader or the blade grader may be employed.
=Elevating Grader Work.=--If the elevating grader is used in shaping
the earth road, the apron will be lowered and the material will be
excavated at the sides of the road and deposited on the middle
portion. If slight changes in grade are desired, wagons will accompany
the grader and catch under the apron at the high places and haul the
material to the low places. After the earth has been deposited it must
be worked over to secure the correct cross section and be made
passable for vehicles. This requires that clods be broken, weeds and
grass that are mixed with the earth be removed by harrowing and
forking and that the surface be carefully smoothed with a blade
grader. This latter operation will have to be repeated several times
before a satisfactory surface is secured. But this miscellaneous work
is highly important and under no circumstances ought to be neglected.
Nothing so detracts from an otherwise creditable piece of work as
failure to provide a smooth surface for the use of vehicles. It is
especially uncomfortable for the users of a highway if sods and weeds
in quantity are left in the road after it has been graded. The humus
that will be left in the soil as the vegetable matter decays increases
the porosity of the road surface making it more absorbent than soil
without humus. This increases the susceptibility to softening from
storm water or ground water.
The tractor can advantageously be used to draw the elevating grader on
this class of work, but will be greatly handicapped if there are wet
sections along the road, through which the tractor must be driven. In
many cases its use is prohibited by such conditions and for all-round
service of this character, mules are preferred for motive power.
[Illustration: Fig. 12.--Tractor-grader Outfit]
=Use of Blade Grader.=--Heavy blade graders designed to be drawn by a
tractor are suitable for shaping the earth road. Some of these have
blades 12 feet long and excellent control for regulating the depth of
cutting. Often two such graders are operated tandem. These machines
have a device which permits the operator to steer the grader
independently of the tractor. Thus the grader can be steered off to
the side to cut out the ditches, while the tractor continues to travel
on the firm part of the road. Earth moved with the blade grader is
usually fairly free from large lumps and can readily be smoothed to a
satisfactory surface for the use of traffic. The sods and weeds will
be drawn into the road along with the earth just as they are when the
elevating grader is employed. Precaution must therefore be taken to
eliminate them before the vegetable matter decays, and to smooth the
surface for the use of traffic.
=Costs.=--The cost of shaping an earth road in the manner described
above will vary through rather wide limits because the nature and
amount of work to be done varies so greatly. Some roads can be graded
satisfactorily for $300.00 per mile, while others will cost $700.00.
But $425.00 per mile may be taken as an average for blade or elevating
grader work plus a moderate amount of grade reduction in the way of
removing slight knolls. For the amount of grade reduction necessary in
rolling country, followed by grader shaping, $1000.00 to $1800.00 per
mile will be required. The method is not adapted to rolling country
where the roads are undulating and require some grade reduction on
every hill. For hilly roads one of the methods described for grade
reduction will be required and the cost will obviously depend upon the
amount of earth moved. Averages of cost figures mean nothing in such
cases as the cost may reach $10,000.00 per mile, or may be as low as
$2000.00 per mile.
=Maintenance.=--Regardless of the care with which an earth road has
been graded, it will be yielding and will readily absorb water for a
long time after the completion of the work. The condition of the
surface will naturally deteriorate rapidly during the first season it
is used unless the road receives the constant maintenance that is a
prerequisite to satisfactory serviceability. The road drag is
generally recommended for this purpose, and if a drag is properly used
it will serve to restore the shape of the surface as fast as it is
destroyed by traffic.
Good results with the drag depend upon choosing the proper time to
drag and upon doing the work in the right way when using the drag. The
best time to drag is as soon after a rain as the road has dried out
enough to pack under traffic. If the work is done while the road is
too wet, the first vehicles traveling the road after it has been
dragged will make ruts and to a considerable extent offset the good
done by the drag. If the road is too dry, the drag will not smooth the
irregularities. A little observation will be required to determine the
proper time for dragging on any particular soil, but usually after a
rain or thaw there is a period lasting a day or two when conditions
are about right.
[Illustration: Fig. 13.--Road Drag]
The drag is used merely to restore the shape of the surface and to do
so a small amount of material is drawn toward the middle of the road.
But there must not be a ridge of loose material left in the middle
after the work is completed. Some patrolmen start at one side of the
road and gradually work across the road on successive trips, finally
finishing up at the side opposite that at which the start was made.
The next dragging should start on the opposite side from the first if
that method is followed.
By shifting his weight on the drag, the operator can adjust the
cutting edge so that very little loose material is moved crosswise of
the road and that is the proper method to pursue. In that case no
ridge will remain at the middle of the road. If a slight one is left
it should be removed by a final trip with the drag.
In addition to the dragging, weeds must be cut along the road about
twice a year, the ditches must be kept cleaned out and culverts open.
All of the maintenance for 10 miles of earth road can be accomplished
by one man giving his entire time to the work, and that is the only
method that has proven adequate to the problem.
EARTH ROADS IN ARID REGIONS
In areas where the rainfall is less than 18 inches per year, and
especially where it is 10 inches or less, an entirely different road
problem exists. The effect of precipitation is of significance
primarily from the standpoint of erosion, and the design of cross
section and ditches and the culvert provisions are entirely different
from those necessary in humid regions.
Frequently the rainfall in semi-arid regions will be seasonal and
provision must be made to care for a large volume of water during the
rainy season, but, in general, road design is adapted to prevention of
erosion rather than to elimination of ground water effects, or the
softening effects of surface water. Generally the rainy period does
not last long enough to warrant expensive construction to eliminate
its general effects. In fact, the saturation of the soil is more
likely to be a benefit than otherwise.
Earth roads are likely to be satisfactory except where the traffic is
sufficient to grind the surface into dust to such an extent that an
excessive dust layer is produced. In such locations the problem is one
of providing a durable surface unaffected by long continued dry
weather.
Grade reduction will have the same importance as in humid areas and
will be carried out in the same way.
Maintenance will consist in repairing the damage from occasional
floods and in removing or preventing accumulations of drifting sand
or dust. Crude petroleum oils have been satisfactory for maintenance
in such locations when used on stable soils.
=Value of Earth Roads.=--The serviceability of the earth road depends
to a large extent upon the care exercised in its maintenance. The only
part of earth road construction that is permanent is the grade
reduction. The cross section that is so carefully shaped at
considerable cost may flatten out in one or two years, especially if
the road goes through unusually wet periods. Traffic will continually
seek a new track during the period when the road is muddy and is as
likely to cross the ditch to the sod near the fence as to use any
other part of the road. Continual and persistent maintenance is
therefore essential to even reasonable serviceability. At best the
earth road will be a poor facility for a considerable period each year
in the regions of year-around rainfall. In most localities, roads of
distinctly minor importance are of necessity only earth roads and for
the comparatively small territory they serve and the small amount of
traffic, they probably serve the purpose. For roads of any importance
in the humid areas of the United States, the earth road cannot carry
satisfactorily the traffic of a prosperous and busy community.
CHAPTER VI
SAND-CLAY AND GRAVEL ROADS
In Chapter IV, mention was made of the variation in serviceability of
road surfaces composed of the natural soil existing on the
right-of-way of the road. It has been found that soils of a clayey
nature in which there is a considerable percentage of sand usually
afford a serviceable road surface for light or moderate traffic,
especially in areas where climatic conditions are favorable. A study
of these soils, together with the construction of experimental roads
of various mixtures of sand and clay, has led to a fairly
comprehensive understanding of the principles of construction and
range of capacity of this type of road surface, which is known as the
sand-clay road.
The sand-clay road surface consists of a natural or artificial mixture
of sand and clay, in which the amount of clay is somewhat greater than
sufficient to fill the voids in the dry sand. It may be assumed that
the sand contains 40 per cent of voids and that at least 45 per cent
of clay is required to fill the voids and bind the sand grains
together, because the clay spreads the sand grains apart during the
mixing, thus having the effect of increasing the voids. As a matter of
experiment, it is found to be impractical to secure by available
construction methods mixtures of sufficient uniformity to render it
necessary to exercise great exactness in proportioning the components,
but reasonable care in proportioning the materials is desirable.
Successful utilization of this type of surface requires considerable
study of available materials and investigations of their behavior when
combined. Extensive and exhaustive experiments have been conducted
with sand-clay mixtures in various places where they are widely used
for road surfaces and the following general principles have been
deduced.
=The Binder.=--In the sand-clay road, stability is obtained by
utilizing the bonding properties possessed to some degree by all
soils. Naturally this characteristic may be expected to vary widely
with the several types of soil. It is generally considered to be a
common property of clay, but the term clay is a general one that is
often applied to soils differing greatly in physical characteristics
and the term therefore loses its significance in this connection.
Those soils that are properly and technically called clay are
decidedly sticky when wet and are the best materials for sand-clay
construction. Of the clays, those that produce a tough sticky mud are
best. This can be tested by mixing a small quantity into a stiff mud
and molding it into a ball and immersing in water. If the ball retains
its shape for some little time, it is likely to prove a very
satisfactory binder, but, if it becomes plastic and loses its shape,
it will be an inferior binder, as a general rule. The ball clay, as
the former is called, may be of any color common to soils, not
necessarily yellow or reddish as is sometimes supposed. Likewise,
balls of mixtures containing varying percentages of sand and the
binder to be used may be made up and immersed in water. The mixture
that holds its shape longest is of course the best combination of the
materials and indicates the mixture to use in the construction.
An ideal, or even a fairly satisfactory soil for a binder may not
exist in the vicinity of a proposed improvement, and consequently an
inferior binder is frequently the only material available.
Sometimes deposits of clay or gravel contain a considerable percentage
of gypsum which serves as a binder and is particularly effective when
used in combination with clay and sand or gravel.
In many places a soil of the type used for adobe and called "caliche"
may be found and this is an excellent binder for sand or gravel.
=Top-Soil or Natural Mixtures.=--Deposits consisting of a natural
mixture of sand and clay in which the ingredients happen to exist in
about the correct relative proportions for sand-clay road surfaces are
found in many localities. These mixtures are commonly referred to as
top-soil. If the deposits are somewhat deficient either in sand or
clay, they can be utilized if the proper corrections in the
proportions are made during construction. Very satisfactory road
surfaces are sometimes constructed with mixtures that appear to be far
from ideal in composition, but experience and frequent trials are
needed to determine the best way in which to handle these mixtures.
=Sand-Clay Surfaces on Sandy Roads.=--Sand-clay surfaces may be
constructed on naturally sandy roads either by adding clay and mixing
it with the sand to secure the desired composition, or a layer of a
natural sand-clay mixture, caliche or sand-clay-gypsum may be placed
on top of the sand.
The most widely used method is to mix clay or other binder with the
sand. Since there is no need to provide for ditches to carry storm
water on a deep sand soil, the sand is graded off nearly flat across
the road and no ditches are provided. The clay is dumped on the road
in a layer about 8 inches thick and is then mixed into the sand. It is
desired to mix enough sand with the clay to produce a mixture composed
of approximately 1/3 clay and 2/3 sand. The mixing is accomplished in
various ways, the most common being to use a heavy plow at first and
to follow this with a heavy disc harrow. The mixing is a tedious and
disagreeable process, but its thorough accomplishment is
indispensable. The mixing is most readily done when the materials are
saturated with water and in practice it is customary to depend upon
rain for the water, although in the final stages water may be hauled
and sprinkled on the road to facilitate final completion of the
mixing. After the mixing has been completed, the surface is smoothed
with the blade grader and is kept smooth until it dries out. Repeated
dragging will be required, during the first year especially, and to
some extent each year in order to keep the surface smooth, but the
dragging can be successfully accomplished only when the road is wet.
[Illustration: Fig. 14.--Cross Sections for Sand-Clay Roads]
In regions where several months of continued hot, dry weather is to be
expected each year, the sand-clay mixture is likely to break through
unless it is of considerable thickness and generally the surface layer
is made much thicker than for regions where the annual rainfall is
fairly well distributed. This is especially necessary when the binder
is of inferior quality. It is not uncommon in such cases to make the
sand-clay surface as much as two feet thick.
As the mixing progresses it may appear that patches here and there are
deficient in either clay or sand and the mixture in these places is
corrected by the addition of a little sand or clay as may be
required.
If the top-soil is used it is deposited on the sand in the required
quantity and is remixed in place to insure uniformity. If either sand
or clay is needed to give a satisfactory mixture, the proper material
is added and mixed in as the work progresses. The surface is finally
smoothed by means of the grader and drag.
=Sand-Clay on Clay or Loam.=--If the existing road is of clay or loam,
ample drainage will be required as discussed in Chapter IV. The
surface may be constructed of a natural sand-clay mixture or of a sand
mixed with the natural soil. If the former, the surface of the
existing road is prepared by grading so as to insure good drainage and
the natural mixture is then deposited and the surface completed as
described in the preceding section.
If the surface is formed by mixing sand with the existing soil, the
sands may be deposited in a layer about six inches thick which will
gradually mix with the soil as the road is used. A second application
of sand may follow in a year or two if it is needed. Such a road
surface will lack uniformity of composition and it seems preferable to
mix the sand with the soil by plowing and discing as previously
described.
=Characteristics.=--Sand-clay road surfaces do not have sufficient
durability for heavily traveled highways, but will be satisfactory for
a moderate amount of traffic. These surfaces have maximum
serviceability when moist, not wet, and consequently are not as
durable in dry climates as in humid areas. They are likely to become
sticky and unstable in continued wet weather and to become friable and
wear into chuck holes in long continued dry weather. At their best,
they are dustless, somewhat resilient and of low tractive resistance.
GRAVEL ROAD SURFACES
[Illustration: Fig. 15.--Cross Sections for Gravel Highways]
=Natural Gravel.=--Gravel is the name given to a material consisting
of a mixture of more or less rounded stones, sand and earthy material,
which is found in natural deposits. These deposits exist in almost
every part of North America, being especially numerous in the
glaciated areas, but by no means confined to them. Gravel deposits
consist of pieces of rock varying in size from those of a cubic yard
or more in volume to the finest stone dust, but with pieces ranging in
size from that which will pass a 3-inch ring down to fine sand
predominating. The larger pieces are usually more or less rounded and
the finer particles may be rounded or may be angular. Many varieties
of rocks are to be found among the gravel pebbles, but the rocks of
igneous origin and possessing a considerable degree of hardness
generally predominate. Intermixed with the pieces of rock there is
likely to be clay or other soil, the quantity varying greatly in
different deposits and even in various places in the same deposits.
Often there are found deposits of material which are by the layman
termed gravel, which are really clayey sand or sand containing a few
pebbles, but which are of value to the road builder for the sand clay
type of surfacing. The term gravel is exceedingly general and unless
specifically defined, gives little indication of the exact nature of
to which it is applied.
TABLE 7
SHOWING CEMENTING PROPERTIES OF SEVERAL SAMPLES OF
GRAVEL
-----------------+----------------------------
| Cementing Value
Per Cent Clay by +---------------+------------
Weight | As Received | Washed
-----------------+---------------+------------
4.4 | 276 | 43
6.4 | 105 | 285
5.1 | 241 | 70
14.5 | 500 | 279
8.5 | 500 | 112
10.1 | 300 | 267
14.8 | 500 | 107
7.5 | 184 | 198
16.5 | 500 | 428
2.0 | 185 | 239
1.5 | 500 | 500
4.5 | 212 | 204
2.5 | 116 | 363
-----------------+---------------+------------
The value of any gravel for road surfacing depends upon the degree to
which it possesses the properties of an ideal gravel for road
surfacing. Ideal gravel is seldom encountered, but a consideration of
its characteristics serves to establish a measure by which to estimate
the probable value of any deposit.
=The Ideal Road Gravel.=--The ideal road gravel is a mixture of
pebbles, sand and earthy material, the pieces varying from coarse to
fine in such a manner that when the gravel is compacted into a road
surface the spaces between the larger pebbles are filled with the
finer material. The pebbles are of a variety of rock that is highly
resistant to wear so that the road surface made from the gravel will
have the quality of durability. The gravel possesses good cementing
properties, insuring that the pieces will hold together in the road
surface. The cementing property may be due to the rock powder in the
deposit or to earthy material mixed with the rock particles, or to
both. Table 7 shows the results of a number of tests made upon gravels
and indicates that the cementing property of the gravel does not
always depend upon the clay content.
=Permissible Size of Pebbles.=--The larger pebbles in the gravel are
less likely to crush under loads than smaller pebbles of the same sort
of rock, but if the rock is of some of the tougher varieties such as
trap, there is very little likelihood of even the smaller pebbles
crushing. If the pebbles are of rock of medium toughness, the smaller
pebbles might be crushed under the heavier loads. It is the usual
practice to permit gravel to be used for the foundation course in
which the pebbles are as large as will pass a 3-1/2-inch circular
screen opening, and for the wearing course, as large as will pass a
2-1/2-inch circular screen opening. If larger pebbles are allowed in
the wearing course, the surface is certain to become rough after a
time. If the gravel is to be placed in a single course as is a very
common practice, then the maximum size should not exceed that which
will pass a 2-1/2-inch circular screen opening.
The Wisconsin Highway Commission has constructed a very large mileage
of excellent gravel roads and the sizes specified for their roads are
as follows:
"_Bottom Course Gravel_.--Bottom course shall consist of a
mixture of gravel, sand and clay with the proportions and various
sizes as follows:
"All to pass a two-inch screen and to have at least sixty and not
more than seventy-five per cent retained on a quarter-inch
screen; at least twenty-five and not more than seventy-five per
cent of the total coarse aggregate to be retained on a one-inch
screen; at least sixty-five and not more than eighty-five per
cent of the total fine aggregate to be retained on a two
hundred-mesh sieve."
"_Top Course Gravel_.--Top course shall consist of a mixture of
gravel, sand and clay with the proportions of the various sizes
as follows:
"All to pass a one-inch screen and to have at least fifty and not
more than seventy-five per cent retained on a quarter-inch
screen; at least twenty-five and not more than seventy-five per
cent of the total coarse aggregate (material over one-fourth inch
in size) to be retained on a one-half-inch screen; at least
sixty-five and not more than eighty-five per cent of the total
fine aggregate (material under one-fourth inch in size) to be
retained on a two hundred-mesh sieve."
"_Screened Gravel and Sand Mixtures_.--Where it is impossible to
obtain run of bank gravel containing the necessary binder in its
natural state, screened gravel shall be used and the necessary
sand and clay binder added as directed by the engineer. Gravel
and sand shall be delivered on the work separately. Clay binder
shall be obtained from approved pits and added as directed by the
engineer."
"_Run of Bank Gravel_.--When run of bank gravel is permitted
either for one course or two course work, the size shall not
exceed that specified for bottom or top course. If necessary, the
contractor shall pass all the material through a two-inch screen
for the bottom course, and through a one-inch screen for the top
course. When the work consists of only one course, the material
shall be of the sizes as specified for the top course. The
necessary binder shall be contained in the material in its
natural state, excepting that a small percentage of clay binder
may be added as directed by the Engineer."
=Wearing Properties.=--A certain amount of grinding action takes place
on the road surface under the direct action of wheels, especially
those with steel tires. Where rubber tired traffic predominates, this
action is much less severe than where steel tired vehicles
predominate, but the tendency exists on all roads. In addition, there
is distortion of the layer of gravel under heavy loads which causes
the pieces of stone in the surface to rub against each other and to
wear away slowly.
The gravel road in the very best condition is slightly uneven but
there is comparatively little jar imparted to vehicles, and,
consequently, little impact on the surface. When somewhat worn, the
impact becomes a factor of some importance and the pounding of
vehicles has a very destructive action on the surface. Soft pebbles
will be reduced to dust in a comparatively short time.
The degree to which any gravel resists the destructive action of
traffic depends upon the varieties of rock represented by the pebbles
in the gravel. If the pebbles are mostly from rocks of good wearing
properties, that quality will be imparted to the road surface. If
mostly from rocks of little durability, the same characteristic will
be imparted to the road surface. A very good general notion of the
probable durability of gravel can therefore be obtained by a careful
visual examination of the material and classification of the rock
varieties represented by the pebbles.
=Utilizing Natural Gravels.=--Gravel road construction is advantageous
only when it can be accomplished at low first cost. This usually
presupposes a local supply of gravel that can be utilized, or at any
rate a supply that need not be shipped a long distance. In the nature
of things, such deposits are likely to be deficient in some of the
desirable characteristics, and may be deficient in most of them. By
various means, the defects in the materials can be partially corrected
while constructing the road.
If the gravel deposit consists of layers of varying composition as
regards size and clay content, the material may be loosened from the
exposed face and allowed to fall to the bottom of the pit thereby
becoming mixed to a sufficient extent to produce a reasonably uniform
product. If deficient in clay, it often proves feasible to add a small
part of the clay over-burden, thereby insuring enough binder.
Sometimes adjoining deposits will consist one of relatively fine
material, the other of relatively coarse. These may be mixed on the
work by first placing the coarse material in a layer about 5 inches
thick and adding the finer material in a similar layer. The two will
mix very rapidly during the operations of spreading and shaping.
When deposits contain pebbles larger than will pass a 3-1/2-inch ring,
these larger stones will prove to be undesirable if placed on the
road, as they are almost sure to work to the surface of the gravel
layer and become a source of annoyance to the users of the road.
Oversize stone can be removed while loading the gravel or while
spreading it, if care is exercised and not too large a proportion is
oversize. It is preferable however to remove the oversize by means of
screens at the pit. Usually on large jobs the oversize is crushed and
mixed with the supply so as to utilize what is really the best part of
the material.
Gravels deficient in bonding material are often encountered in
deposits where there is insufficient overburden to give enough
additional binder or where the overburden is of a material unsuitable
for binder. Such materials may be utilized by adding binder in the
form of clay after the gravel has been placed on the road.
Almost any gravel deposit can be utilized in some way if the material
is of a durable nature, regardless of other characteristics. The
serviceability of a gravel road will depend largely on how nearly the
gravel approaches the ideal, but variations in the manipulations will
do much to overcome deficiencies in materials.
=Thickness of Layer.=--The thickness of the layer of gravel required
depends both upon the type of soil upon which it is placed and the
nature of the traffic to which the road will be subjected. Gravel
surfaces should not ordinarily be constructed on highways carrying
heavy truck traffic, but if gross loads of three or four tons are the
heaviest anticipated, the gravel will be reasonably stable. On such
roads, a layer of well compacted gravel ten inches thick will support
the loads if a well drained earth foundation is provided. If but
little truck traffic is anticipated and loads up to three tons on
steel tires are the average, a layer 8 inches thick will be
sufficient. In dry climates, a layer six inches thick will be
adequate if it can be kept from raveling.
On secondary roads, carrying principally farm-to-market traffic, and
not a great volume of that, the above thicknesses may be reduced about
one-fourth.
The exact thickness needed for any particular road is a matter for
special study on account of the variations in the gravels and in the
supporting power of the soil upon which they are placed.
PLACING GRAVEL
=Preparation of the Road.=--The roadway that is to be surfaced with
gravel is first brought to the desired grade and cross section. It
would be advantageous if this could be done a year before the gravel
is placed so that no settlement of the earth foundation would occur
after the gravel surface is completed. But if that is impractical, the
grading may be done just prior to placing the gravel, providing
appropriate methods are adopted for securing compacted fills.
=Trench Method.=--Two distinct methods of placing the gravel are in
general use, known as the trench method and the surface or feather
edge method respectively. The method to adopt for any particular road
will depend largely on certain conditions that will be explained
later.
In the trench method, a trench of the proper width and depth for
receiving the gravel is excavated in the earth road surface and the
gravel is placed therein.
The trench is formed by plowing a few furrows and scraping out the
loosened earth with a blade grader. The loose material is generally
moved out laterally to build up earth berms or "shoulders" alongside
the gravel. Into this trench the gravel is dumped in the proper
quantity to give the required thickness after being compacted.
The greatest care must be exercised in spreading the gravel to
eliminate unevenness where the loads were deposited. An ordinary blade
grader is one of the best and most economical implements to use for
spreading the gravel. When the gravel has been deposited in the trench
for a distance of a thousand feet or more, the spreading is
accomplished by dragging the surface repeatedly with the blade grader,
the work being continued until all waviness disappears. The gravel is
then thoroughly and repeatedly harrowed with a heavy stiff tooth
harrow to mix thoroughly the fine and coarse gravel so as to produce
as nearly a uniform mixture as may be. The gravel is then finally
smoothed with the blade grader.
The gravel may be compacted by rolling or may be allowed to pack from
the action of traffic. The former is greatly to be preferred where
practicable. The rolling is performed with a three-wheeled
self-propelled roller weighing about 8 tons and must be done while the
gravel is wet. Generally a sprinkling wagon is used to wet down the
gravel, but advantage is always taken of rains to facilitate the work.
The gravel must be spread in layers not over 5 or 6 inches thick to
get the desired results, which means that for an ordinary gravel road
about 10 inches thick, the gravel will be placed in two layers of
about equal thickness, each of which will be rolled.
The gravel will compact slowly even if it is not rolled, but generally
does not become stable until the material is thoroughly soaked by
rains. Then it will begin to pack, but will become badly rutted and
uneven during the process. During this period the surface must be kept
smooth by means of the blade grader. The drag does not suffice for
this purpose, tending to accentuate the unevenness rather than to
correct it.
If gravel is placed in a trench in dense soil and rainy weather
ensues, sufficient water will be held in the trench to cause
unevenness from foundation settlement and the gravel will become mixed
with the soil to some extent and be thereby wasted. Trenches cut from
the road bed upon which the gravel is placed, to the side ditches,
will relieve this condition by affording an outlet for the surplus
water. Nevertheless some difficulty may be expected if the trench
method is used and wet weather prevails. If it is possible to close
the road against traffic until the road is dry the method is
applicable. Moreover, in long-continued dry weather, the dispersion
and loss of considerable gravel from the action of automobile traffic
is avoided because the gravel is held between substantial earth berms
and the gravel will pack better and hold its shape longer when
constructed by the trench method than otherwise.
=Surface Method.=--The surface method is one in which the gravel is
placed on the graded earth road surface without earth shoulders to
hold the gravel in place. It is also sometimes called the feather-edge
method. Except for the manner of placing as just mentioned, the
several operations are conducted in the same general manner as for the
trench method. The gravel does not compact as quickly as in the trench
method and a considerable loss of material is likely to result from
the effect of automobile traffic while the gravel is loose. But it has
the advantage of being free from difficulties in wet weather and in
some locations is therefore preferable to the trench method. It is
particularly applicable to those projects on which the placing of
gravel continues throughout the winter, the gravel being dumped and
spread, to be finally smoothed and finished in the early summer.
=Bonding.=--Where gravels deficient in binder are utilized, clay for
binder is sometimes added as the gravel is placed on the road. This
may be done by spreading the clay on top of the lower course of
gravel, placing the upper layer and sprinkling and rolling until the
clay squeezes up through the surface layer. It may also be
accomplished by spreading dry clay on the upper course before it is
harrowed and then harrowing to mix it with the gravel. Both methods
are practiced, but the former is believed to be preferable. A third
method is to separate the sand and pebbles and to mix the clay binder
with the sand and then spread the sand on top of the pebbles and mix
by harrowing.
=Maintenance.=--Gravel surfaces require careful maintenance,
especially during the first season the road is used. The gravel will
compact slowly and during the process will be rutted and otherwise
disturbed by traffic. It is important during this period to restore
the shape once a week or at least twice a month. The light blade
grader is usually employed for the purpose so long as the gravel is
somewhat loose. Later a drag of the type known as the planer will
prove to be the most effective. Figure 16 shows a type of drag that is
very satisfactory for use on gravel roads.
[Illustration: Fig. 16.--Road Planer]
CHAPTER VII
BROKEN STONE ROAD SURFACES
The broken stone road surface, or macadam road as it is usually
termed, consists of a layer of broken stone, bonded or cemented
together by means of stone dust and water. The surface may or may not
be coated with some bituminous material.
=Design.=--It has been an accepted assumption that the macadam road
surface is somewhat more stable than the gravel road surface of equal
thickness, and since this is probably the consensus of opinion of
engineers familiar with both types, it may be accepted until
experimental data are available on the subject.
The thickness of the layer of macadam required for a road will depend
upon the same factors that were considered in connection with the
thickness of the gravel surface, i.e., kind of stone used, character
of earth foundation and nature of the traffic.
The standard macadam surface where good earth foundation is to be had
and where the loads do not exceed about four tons has for years been
eight inches thick. For heavier loads or inferior foundation, a
somewhat greater thickness would be employed, but the best practice
would probably provide a foundation course of the Telford type for
doubtful foundation conditions, especially for the extremely uncertain
cases. For soils of very good supporting strength such as very sandy
loam or deep sand or for arid regions where stable foundation is
always assured the thickness of the macadam might be reduced to six
inches. It should be borne in mind that the broken stone road is not
adapted to the traffic carried by trunk line highways in populous
districts, but is rather a type permissible on secondary roads and
usually adequate for local roads. It should never be employed for
roads carrying any considerable volume of passenger automobile traffic
or motor truck traffic. If surfaced with a bituminous material it will
carry up to 1200 passenger automobiles per day, but not to exceed
fifty trucks.
=Properties of the Stone.=--The stone employed for the broken stone
road should possess the qualities of hardness and toughness and should
be capable of resisting abrasion sufficiently well to have reasonable
life under the traffic to which it is subjected. Since the traffic may
vary from very light on some roads to far beyond the limit of the
economical capacity of this type of pavement on others, it follows
that any particular deposit of stone might be durable enough for some
roads, while for others it might be entirely inadequate. As a general
rule it has been found that stone that wears away at a moderate rate
will, when used for water-bound macadam surface, result in a smoother
trackway than one that will wear very slowly. It is not therefore
altogether certain that the most durable stone to be had should be
selected for a particular road. This is especially true now that the
water-bound macadam surface has been largely superseded for trunk line
highways and other heavily traveled roads, and is employed in
locations where service conditions are not severe.
The stone employed for the water-bound macadam surface must possess
good cementing properties, because the surface depends for stability
primarily upon the bonding action of the dust from the broken stone.
This is in contrast to the gravel road, where little dependence is
placed upon the bonding effect of the rock dust. In preparing the
stone for macadam surfaces, the ledge rock is crushed and screened,
and in that way a supply of the finer particles, which are a part of
the output of the crusher, is obtained for use in bonding the
surface. This finely broken material, usually called screenings, is
essential to the construction of the water-bound type of surface.
Rocks vary considerably in the cementing properties of the dust, but
usually the rocks classed as "trap," such as andesite, gabbro and
rhyolite, and schist and basalt possess good cementing properties.
Limestones usually possess good cementing properties, but some of the
dolomitic limestones are of low cementing value. Quartz, sandstone and
the granites are of low cementing value.
=Kinds of Rocks Used for Macadam.=--Limestone and chert are the two
sedimentary rocks, employed most extensively for broken stone roads.
These rocks are found in widely distributed areas and vary in physical
characteristics from very soft material of no use to the road builder
to materials possessing considerable durability. It is desirable to
carefully test out the deposits of these materials before using to
ascertain the probable value of the rock, for the construction of the
road surface.
Of the igneous rocks, those classed as trap are best known to the road
builder and many of the deposits of trap rock afford an excellent
material for broken stone roads where the severest conditions of
traffic are encountered. The trap rocks are tough and durable and
generally possess excellent cementing properties.
Granite and sandstone are seldom used for water-bound macadam as they
possess poor cementing properties and a binder of some kind must be
added to cement the pieces together. For this purpose clay or the
screenings from some other variety of stone may be utilized.
Some other materials are occasionally employed for the construction of
macadam surfaces. Of these, oyster or marine shells, burnt shale, and
slag are most common.
Shells and slag are of rather low durability but possess good
cementing properties. Shale is a makeshift suitable only for very
light traffic roads.
=Sizes of Stone.=--The stone for the wearing course of a macadam road
should be as large as practicable, because the larger the pieces the
more durable the surface. If the individual stones are too large it is
difficult to secure a smooth surface, and large stones will be readily
loosened by tipping as the wheels roll over them. These considerations
limit the size to a maximum of that which will pass a 2-1/2-inch
screen. Stone of excellent wearing qualities may be somewhat smaller,
but never less than that which will just pass a 1-1/2-inch screen.
For the lower course, the size is not particularly important except
where the earth foundation is such as to require special construction.
It is not uncommon to use the same size of stone for both upper and
lower course and yet in many instances stone up to that which will
just pass a 3-1/2-inch screen is used for the lower course. Stone much
smaller in size may also be used successfully, but if the stone is
broken to a smaller size than is required, unnecessary expense is
incurred.
The bonding material is the finer portion of the product of the
crusher, which is called screenings. This material may be so finely
crushed as to pass a one-fourth inch screen, or may be so coarse as to
just pass a one-half inch screen, but in any case must contain all of
the dust and fine material produced by the crusher.
Where the soil and drainage conditions demand an especially stable
foundation course, the Telford type is used. The Telford foundation
consists of a layer of stones of various dimensions that can be laid
so as to give a thickness of 8 inches. These large stones are placed
by hand and therefore the size requirements are not rigid. Stones
having one dimension about 8 inches and the others not over 10 or 12
inches are satisfactory.
=Earth Work.=--A thoroughly drained and stable earth foundation is
essential to success with the macadam type of surface. Before placing
the stone, the road must be shaped to the proper cross section and
all grade reduction work completed. Preferably heavy fills should have
a year to settle before the macadam surface is placed. Side ditches,
necessary culverts and tile drains should be constructed as required
for drainage. The earth work is often carried out in connection with
the construction of the macadam surface, being completed just ahead of
the surfacing. In that case, the fills must be carefully rolled as
they are placed. The road bed may be shaped in connection with the
other earthwork. If the road has been brought to a satisfactory grade
some time prior to placing the macadam, the road bed for the broken
stone will be prepared as needed for placing the stone.
=Foundation for the Macadam.=--Macadam surfaces are quite generally
placed in a trench as described in the trench method for placing
gravel. It is an almost universal practice to compact the layer of
stone by rolling with an 8- or 10-ton power roller, and if the stone
is not held between substantial earth berms or shoulders, the rolling
merely serves to spread the stone out over the road bed instead of
compacting it. If an attempt is made to roll broken stone which has
been placed on a yielding foundation, no benefit results, but on the
contrary the stone is likely to be forced down into the soil. To
insure that the layer of broken stone can be compacted by rolling, it
is first necessary to roll the earth foundation until it becomes hard
and unyielding. If soft or yielding places appear during the rolling
these should be corrected by tile drains or by removing the earth from
the spongy place and back-filling with material that will compact when
rolled.
It is not always easy to determine why these soft places exist in what
appears to be a well drained roadway, especially since they are as
likely to be found on fills as anywhere else. Apparently they are due
to local pockets of porous soil held by denser soil so that the water
does not readily drain away. It is usually true that such places are
observed during the season of frequent precipitation more often than
during other seasons of the year.
In dry climates, the difficulties of securing suitable foundations for
the broken stone road are largely eliminated, but it may be observed
that this type of surface is not suitable for such climates unless
some sort of bituminous binder is employed to hold the stones in
place. The cementing power of the stone dust is inadequate when the
surface is continually dry.
[Illustration: Fig. 17.--Cross Section for Macadam]
=Telford Foundation.=--When the Telford type of foundation is
employed, the earth subgrade is prepared and then the Telford stone
placed carefully by hand. The spaces between the large stones are
filled with the spalls broken from the larger stones in fitting them
in place. When completed the base is rolled with a heavy roller to
secure a firm unyielding layer. The thickness is generally about eight
inches. Any fairly sound stone may be used for the Telford base.
=Placing the Broken Stone.=--It has been found impracticable properly
to roll a greater thickness than about 5 or 6 inches of loose stone,
therefore, the stone for the macadam surface is usually placed in two
layers, the first or lower layer being rolled before the next layer is
placed. The stone is hauled in dump wagons, trucks or dump cars,
dumped on the road bed and spread by hand rakes or by means of a blade
grader and is then rolled. To insure the proper thickness the loads
are accurately spaced to spread to the proper thickness.
=Rolling.=--A three-wheeled or "macadam" type of roller, of the
self-propelled type, is best for compacting the broken stone road. The
weight varies from eight to fifteen tons, but for most conditions the
ten or twelve ton size seems to be preferable. On Telford base
construction, a heavier machine is desirable and for very hard stone
it may be successfully employed.
The first trip with the roller is made along the edge of the stone and
each successive trip is made a little nearer the middle until finally
one half of the strip of stone has been rolled. The roller is then
taken to the opposite side of the roadway and the operation repeated
on the other half. The rolling is continued until the stone is
thoroughly compacted, which is evidenced by the fact that the roller
makes but a slight track in the surface.
The second layer of stone is then placed and rolled in the same manner
as the first.
=Spreading Screenings.=--After the upper course has been rolled, the
screenings are spread on it from piles alongside the road, enough
being used to fill the voids in the layer of stone and furnish a
slight excess. As the screenings are spread they are rolled to work
them into the voids. When these are filled, the surface is sprinkled
thoroughly by means of an ordinary street sprinkling cart and again
rolled. In this way the dust and water are mixed into a mortar which
fills the crevices between the stones. This mortar hardens in a few
days, giving a bond that is weak, but sufficient for the purpose if
the traffic is not too heavy. A broken stone road finished in this way
is called a water-bound macadam, and is ready for traffic in three or
four days after completion.
=Bituminous Surfaces.=--On account of the inadequacy of the
water-bound macadam when subjected to motor traffic and to obviate the
tendency of broken stone surfaces to loosen in dry weather, there has
been developed a method of covering the surface with a bituminous
material such as tar or asphalt. This will be described in detail in a
later chapter.
=Maintenance.=--Even under favorable conditions as regards kind and
amount of traffic the macadam road requires constant maintenance. The
first effect of traffic will be to brush away the fine materials used
for bonding the surface, thus exposing the larger stones in such a way
that they are rather easily loosened and removed from the surface by
wheels and the hoofs of animals. This finer material must be replaced
as fast as it is removed so as to protect the surface. Either stone
dust or clayey sand may be used, but clay if used alone is likely to
be sticky when wet and prove to be worse than the condition it was
expected to correct. In time, ruts and depressions will appear, either
as the gradual effect of wear, which will inevitably effect some
portions of the surface more than others, or on account of subsidence
of the foundation. Uneven places are repaired by first loosening the
stone, then restoring the cross section by adding new material and
tamping or rolling it in place.
If a bituminous coating has been applied, it will eventually peel off
in places and these places must be recoated as soon as practicable.
Eventually the surface will be worn to such an extent that an entirely
new wearing surface must be added. This is done by loosening the
entire surface to a depth of 3 or 4 inches and then adding a new layer
of broken stone. The loosening is sometimes accomplished by means of
heavy spikes inserted in the roller wheels, and at others by means of
a special tool known as a scarifier.
The new surface is placed and rolled in precisely the same manner as
the wearing surface of the original construction, but the layer may
not be as thick as the original wearing course. A new course will not
bond to the old surface unless the old macadam has been thoroughly
broken up first.
=Characteristics.=--The water-bound macadam is a dusty, somewhat rough
surface of low durability for rubber tired vehicles. It has long been
the standard rural highway for steel tired vehicles, but cannot carry
any considerable amount of motor traffic. It is easily repaired. When
finished with a bituminous surface its durability is greatly increased
and the dust is eliminated. It does not seem to be sufficiently rigid
for truck traffic, unless placed on exceptionally good foundation.
CHAPTER VIII
CEMENT CONCRETE ROADS
The cement concrete road is one of the later developments in highway
construction, but the type has had sufficient use to show that it is
one of the satisfactory types for heavy mixed traffic, and, where the
proper materials are available, it is one of the economical types of
construction.
=Destructive Agencies.=--It is well to have clearly in mind at the
outset that the concrete in a road surface is subjected to certain
destructive agencies not usually significant in connection with the
use of concrete, and these are so often disregarded that the average
serviceability of the concrete road surface is sometimes much lower
than it would be if built with due regard for the effect of traffic on
concrete surfaces. In most structural uses of concrete, its strength
in compression only is utilized, and the factor of safety is such as
to eliminate to some extent failures due to inferior materials or
workmanship.
The concrete road surface is subjected to compression under wheel
loads, to bending, causing tension in the concrete, to abrasion from
wheels, and to tension and compression due to effect of temperature.
The weight of the wheel loads may cause sufficient distortion of the
road slab to produce rupture. The aggregates may be crushed under
wheel loads if the material is too soft. Abrasion from steel tired
vehicles wears away the concrete unless it is hard and durable.
Changes in dimension due to the effect of change in temperature
introduce tension or compression into the road slab and may result in
cracks. Freezing and thawing in the subgrade subjects the slab to
vertical movement and discontinuous support with the result that
longitudinal and transverse cracks occur.
The foregoing indicates the importance of securing good concrete for
road surfacing, and that is accomplished by using suitable aggregates,
by proper design of the road surface and by following established
construction methods.
=Design.=--The widths usually adopted for concrete roads are: for
single track roads, 9 or 10 feet, and for double track roads, 18 or 20
feet. The thickness is 6 to 8 inches at the middle, varying with
climatic conditions and with the kind of soil upon which the concrete
is laid. The thickness at the edge is 1 inch less than at the middle
except that 6-inch surfaces are usually of uniform thickness, the
total crown being 2 inches. The thickness of the two course pavement
is the same as would be used for a single course pavement in the same
location. The surface of either width has a total crown of one or two
inches to insure water running off the surface. The earth foundation
is often flat, the crown being obtained by making the slab thicker at
the middle than at the edge. Fig. 18 shows cross section for concrete
roads.
[Illustration: Fig. 18.--Cross Section for Concrete Highway]
In the state of California, concrete roads four or five inches thick
and surfaced with a bituminous carpet mat have been successfully
constructed. Similar designs have been used in a few other places, but
for general practice it is unsafe to depend upon such a thin slab.
Climatic and soil conditions probably account for the success of the
thin roads in California.
=Concrete Materials.=--The coarse aggregate for the concrete may be
broken stone or pebbles screened from natural gravel. Durability is
necessary, but it is also important to have uniformity in the concrete
so that the road surface will wear uniformly and consequently keep
smooth. Supplies of broken stone are likely to contain a small
percentage of soft pieces and such of these as are at the surface when
the concrete is finished will crush under traffic, leaving a pit in
the surface. Pebbles screened from gravel are also likely to be
variable in durability and should be carefully inspected if they are
to be used as aggregate for concrete roads. The harder limestones,
some sandstones, pebbles from many of the gravel deposits and
practically all of the igneous rocks make satisfactory aggregates for
the concrete road.
Sometimes none of the coarse aggregates readily available are
sufficiently durable or uniform for the wearing surface of the
concrete road, but a suitable aggregate may be obtained at relatively
high price by shipping considerable distances. In such cases what is
known as the two course type of concrete road is employed. The wearing
course usually is about 2 inches thick and is constructed with
selected aggregates of good quality shipped in for the purpose. The
lower course is constructed of aggregates which do not possess the
desired qualities for a wearing course, but which are satisfactory for
concrete not subjected to abrasion. The aggregates for the wearing
course will be selected with the same regard for uniformity and
durability that would be the case if they were for the one course
pavement.
Bank run gravel, or run of the crusher stone, is generally not
sufficiently uniform as regards proportion of fine and coarse material
to produce uniformity in the concrete, and the use of aggregates of
that character is not permissible for the wearing course, but under
proper inspection they may be used for the lower course of two course
pavements.
=Fine Aggregate.=--The fine aggregate is generally natural sand, but a
mixture of natural sand and stone screenings is sometimes employed.
The fine aggregate of whatever character must be clean, free from
organic matter and sand, must contain no appreciable amount of mica,
feldspar, alkali, shale or similar deleterious substances and not
exceed two and one-half per cent of clay and silt. The sand is of such
a range of sizes that all will pass the one-fourth-inch sieve and that
not exceeding about five per cent will pass the 100-mesh sieve.
=Proportions.=--Various mixtures for the concrete are employed because
these may properly vary to some extent with the exact character and
grading of the aggregates. Experience seems to have shown that the
concrete used for the wearing surface should have a crushing strength
of at least 2500 pounds per square inch, and the mixture adopted is
based on the requirements that will give the desired crushing
strength. The common mixture for the one course pavement is one part
cement, two parts sand and three and one-half parts coarse aggregate.
For the wearing course of the two-course type of pavement, a mixture
of the same kind is very often specified.
While these are perhaps the most widely adopted proportions, many
others have been used, especially where the aggregates exhibit
peculiarities or the traffic conditions are unusual. It is desired to
emphasize that the purpose is to obtain concrete of the desired
strength and there can be no such thing as "standard" proportions.
=Measuring Materials.=--In considering the methods employed for
measuring aggregates, emphasis should be placed on the futility of
rigid requirements for the aggregates, both as regards quality and
range of sizes, if the materials are carelessly proportioned at the
mixer. If even reasonably near uniform wearing qualities are to be
secured throughout the entire area of the concrete road surface,
successive batches of concrete must be alike, and to insure that, the
aggregates including the water in each batch of concrete must be mixed
in exactly the same proportions. The aggregates are measured in
various ways, all essentially alike in that the intent is to insure
exactly the same amount of each ingredient for each batch of concrete.
One method is to place bottomless boxes in wheelbarrows, fill the
boxes level full and then lift off the box. Another is to use a
wheelbarrow with a bed of such shape that the contents will be a
multiple of 1 cubic foot when level full. For the larger jobs, the
aggregates are hauled in industrial cars, each having sufficient
capacity for a batch of concrete. The car body is provided with a
partition so as to separate the fine and coarse material.
The water is measured in a tank which automatically refills to the
same level each time it is emptied and when adjusted for a mixture
will introduce the proper amount of water for each batch. It is highly
important to use the least amount of water that will produce workable
concrete.
=Preparation of the Earth Foundation.=--The concrete road is generally
placed directly on the natural soil which has been brought to the
proper cross section. Some engineers advocate that in preparing the
subgrade, the earth be thoroughly rolled; others prefer not to roll
the subgrade. If fills of considerable depth are constructed, they
should either be rolled as built or else should be allowed to settle
for some months before the concrete road is placed, preferably the
latter.
=Placing the Concrete.=--The concrete is placed between substantial
side forms of a height equal to the thickness of the concrete road
slab at the edge, and is shaped roughly by means of shovels.
Various methods have been developed for striking the surface to the
exact shape desired and smoothing it. If hand finishing methods are
employed, a plank template is cut to the prescribed cross section and
the concrete is shaped by drawing the template along the side forms.
Sometimes the template is used as a tamper, being moved along very
slowly accompanied by an up and down motion that tends to tamp the
concrete. The template is then drawn along a second time to smooth the
surface finally.
After the surface has been struck off by hand, it is finally smoothed,
first by rolling crosswise with a slight hand roller about 8 inches in
diameter and 30 inches long. The final finish is effected by dragging
a piece of web belting back and forth across the surface.
Machines designed to tamp the concrete and strike it off to the
required cross section are also employed for finishing. The machine is
power operated and is carried on wheels that run on the side forms,
and the machine moves slowly along as the tamping progresses. The
concrete is tamped, struck off to shape and smoothed with the belt at
one operation. This method of finishing produces denser and stronger
concrete than can be produced by hand finishing methods.
=Placing Concrete for Two-course Road.=--The methods employed for the
two-course concrete road are much the same as for the one-course road.
The concrete for the lower course is placed and struck off by means of
hand tools, and after that course has progressed a few feet, the upper
course is placed and finished as has been described for the one-course
road.
=Curing the Concrete.=--The setting action of cement is a chemical
process, not merely a drying out of the water introduced in mixing the
concrete. The chemical action is progressive for a long time, but is
more rapid during the first few hours than during the later periods,
and the concrete reaches about three-fourths of its maximum strength
at the end of seven days. During the setting period and particularly
during the first few days, plenty of water must be available to the
cement.
To prevent too rapid loss of water from the concrete during the
setting period, the surface must be protected from the wind and sun.
This is accomplished by first covering with canvas as soon as the
concrete has hardened sufficiently and by later covering with earth,
to a depth of two inches. The earth covering is kept wet for about ten
days and is left in place for about one month.
In some places the ponding method of curing is adopted. The surface is
divided into sections by earthen dikes and the space inside the dikes
filled with water to a depth of two or three inches. The water
covering is maintained for two weeks or longer.
No traffic is permitted on the surface for one month, and in cold
weather traffic may be kept off the surface for a longer period.
=Expansion Joints.=--To permit the concrete slab to accommodate itself
to changes in dimension due to temperature changes, expansion joints
1/2 inch wide are placed about every thirty feet. These consist of a
sheet of some prepared bituminous material placed in position as the
concrete is poured.
Experience seems to indicate that in spite of the expansion joints,
the concrete will crack more or less and many engineers think it
advisable to omit expansion joints in constructing the pavement and
when cracks develop to pour bituminous material into them, thus
forming expansion joints.
The prevailing practice in rural highway construction is to omit the
expansion joints, but they are commonly adopted in city pavements.
=Reinforcing.=--To minimize the cracking, either bar or wire mesh
reinforcing is used in the concrete. If bars are used they are placed
in the concrete as it is poured so as to form a belt around each
section about 15 feet square. If the mesh type is employed, a part of
the layer of concrete is placed and smoothed off and a strip of the
mesh laid in place. Additional concrete is then poured on top of the
mesh to bring the slab to the required thickness.
=Bituminous Coatings on Concrete Surfaces.=--The concrete road surface
is sometimes coated with a layer of bituminous material and stone
chips or gravel pebbles. This is particularly advisable where no
really satisfactory aggregates are available and the concrete surface
would not possess sufficient durability. The bituminous material is
applied hot to the surface and is then covered with stone chips or
gravel pebbles, ranging in size from 3/4 inch down to 1/4 inch, the
resulting coating being about 3/4 inch thick. Many failures of this
type of surface have been recorded due to the difficulty of securing
adhesion to the concrete. This seems to be due in part to inability to
get the proper bituminous materials and in part to climatic effects.
Considerable progress has been made in developing this type of surface
and it may eventually become a satisfactory maintenance method.
=Characteristics.=--The concrete road is of a granular texture and is
not slippery. It is of course rigid and noisy for steel tired
vehicles. It is an excellent automobile road and its low tractive
resistance makes it a desirable surface for horse drawn vehicles. It
possesses a high degree of durability if properly constructed. It is
likely to crack indiscriminately but as a general rule the cracks are
not a serious defect.
=Maintenance.=--The cracks that appear in the concrete surface are
filled once or twice a year, tar or asphalt being employed. The dust
and detritus is cleaned out of the cracks and the hot filler poured
in, with enough excess overflowing to protect the edges.
CHAPTER IX
VITRIFIED BRICK ROADS
Vitrified brick roads consist of a foundation course of Portland
cement concrete, broken stone or slag macadam, or of brick laid flat,
the first named being by far the most generally used, and a wearing
course of vitrified brick.
=Vitrified Brick.=--Vitrified brick are made from clay of such a
character that when heated to the required temperature they will fuse
into a glassy texture. Brick roads are constructed on roads carrying
the severest of traffic and the brick must therefore be tough and of
high resistance to wear.
Not all of the clays from which brick may be manufactured will produce
a product suitable for road construction, and paving brick, even
though truly vitrified, are of different degrees of durability,
depending upon the nature of the clay and the care exercised in the
manufacture.
Paving brick are manufactured by the stiff mud process, which means
that the clay is molded into form in a relatively dry condition. To
accomplish this, considerable pressure is exerted in forcing the
column of clay through the dies, which form the prism from which the
brick are cut. If the clay is unsuitable in character or is not
properly ground and mixed, the brick will possess planes of weakness
between the various layers of clay which have been pressed together,
and these planes, called laminations, are a source of weakness if too
marked. It is usual to specify that the brick used for road surfaces
shall be free from marked laminations.
If the brick is not properly burned it will be only partly vitrified
and therefore not of maximum durability. It is customary to specify
that the brick shall show a glassy fracture indicating complete
vitrification.
Various defects of a minor nature occasionally develop in the brick
during the successive steps in the manufacturing process. Check cracks
resulting from the burning or from too rapid cooling are often
encountered, but unless these are deep, that is 3/16 inch or more,
they do not impair the wearing quality of the brick, nor indicate
structural weakness. Kiln marks are formed on some of the brick due to
the weight of the brick above in the kiln. These depressions are not
objectionable unless the brick are so distorted that they will not lie
evenly in the pavement.
Spacing lugs or raised letters are formed on one face of the brick to
insure sufficient space between the brick for the filler. These lugs
or letters are not less than 1/8 inch nor more than 1/4 inch high and
of such design that they will not obstruct the free flow of filler
into the joints between the brick.
Several varieties of paving brick are to be had, the difference being
principally in the design or size.
=Repressed Brick.=--In this type of brick the spacing lugs are formed
by pressing the green brick, after it has been cut to size, into a
mold on one face of which are recessed letters or other devices into
which the clay is pressed, thus forming the spacing lugs.
=Vertical Fiber Brick.=--These brick are designed to be laid with one
wire-cut face up and spacing is provided by two or more beads on the
side of the brick. Sometimes the vertical fiber brick has no spacing
lug, it being contended that the irregularities of the brick are such
as to provide all of the space required. In practice this does not
always work out, as the brick are so regular in shape that when laid
there is too little space between the brick to permit the introduction
of a suitable filler. The use of brick without spacing lugs is just
beginning and is not yet a generally accepted practice.
=Wire-cut-lug Brick.=--This is a type of non-repressed brick which has
spacing lugs provided by cutting one face in a special manner which
provided lugs for spacing. In this type the wire cut face is the one
between the brick as they are laid in the pavement.
=Tests for Quality.=--The standard test for quality of paving brick is
the rattler test. The brick rattler consists of a barrel of 14 sides
24 inches long, mounted so as to rotate at a speed between 29.5 and
30.5 revolutions per minute. The duration of a test is 1800
revolutions. Ten brick constitute a charge and these are placed in the
rattler along with 300 lbs. of cast iron spheres. The spheres are of
two sizes, the smaller being 1-7/8 inch in diameter when new, and the
larger 3-3/4 inches in diameter when new. Ten of the larger spheres
are used and the balance of the charge is made up of the small size.
When tested in the standard manner the loss allowable for the several
classes of service are as follows:
------------+---------------+----------------
| | Maximum Loss
Traffic | Average Loss | for any Brick
------------+---------------+----------------
Heavy | 20 per cent | 24 per cent
Medium | 22 per cent | 26 per cent
Light | 25 per cent | 28 per cent
------------+---------------+----------------
=Other Tests.=--Sometimes the absorption test is specified for paving
brick, but it is rarely a vitrified brick that will pass the rattler
tests which fails to pass a reasonable absorption test. Absorption of
water in an amount exceeding 4 per cent indicates incomplete
vitrification and failure of such brick is almost certain during the
rattler tests.
The cross breaking test is also sometimes employed, but generally
only to check the general quality of the brick. Failure in service
more frequently occurs from excessive wear than from any other cause
and the cross breaking test has little significance, except for brick
less than 3 inches thick, which are to be laid on a sand bedding
course.
=Foundation.=--The foundation for brick roads is usually of Portland
cement concrete, the thickness varying with the nature of the traffic
and the kind of soil upon which the pavement is built. For well
drained soils and normal highway traffic, 5 inches is the ordinary
thickness of foundation. Under favorable conditions such as locations
with sandy soils or in semi-arid or arid regions where the soil is
always stable, the foundation may be four inches thick, and a
considerable mileage of brick road has been built with concrete
foundations less than four inches thick.
In other locations the soil and traffic conditions require a base six
inches or more in thickness, and the proper thickness can be
determined only after all of the factors involved are known and have
been analyzed. It is impractical to adopt a standard thickness of
foundation that will be equally economical for all locations and all
kinds of traffic. As the brick pavement is essentially a heavy traffic
type of surface, the design cannot be varied greatly with similar
foundation conditions because the weight of individual loads is the
significant factor and this does not vary so much as the volume of
traffic. A variation in volume of traffic may be compensated for by a
variation in the quality of the brick as already set forth.
The mixtures for the concrete foundation vary widely because of the
variation in the aggregates employed. If the fine and coarse aggregate
for the concrete are of good quality a mixture of one part cement, two
and one-half parts sand and five parts of coarse aggregate would
insure concrete of adequate strength. A somewhat leaner mixture is
sometimes employed and would be satisfactory if the aggregates were of
exceptional concrete making quality. Mixtures of sand and pebbles
(unscreened gravel) may also be used if care is exercised to secure a
mixture of adequate strength. The proportion will of necessity vary
with each particular material and the discussion of the various
considerations involved may be obtained from various standard works on
concrete and concrete materials.
Broken stone macadam is sometimes utilized for the foundation course
of the brick pavement and such foundations are constructed as
water-bound, which is described in a previous chapter. The thickness,
like that of the concrete foundation, varies with the soil conditions
and the weight of the loads that are expected to use the road. The
macadam is placed in a single layer and is rolled and bonded with
screenings as described in the chapter dealing with water-bound
macadam. Six inches is a common thickness for the macadam base. This
type of foundation should be employed only where the soil is quite
stable and where material costs are such as to insure that the macadam
base is materially cheaper than one of concrete. This would usually be
in locations where the cost of cement is high because of long hauls
and where suitable macadam materials may be obtained close at hand.
Old macadam roads are sometimes utilized for the foundation for the
brick surface, but the instances where this is permissible are
comparatively few in number. When an old macadam is to be used it is
reshaped to the proper cross section and re-rolled and bonded so as to
afford a stable foundation of the proper cross slope.
BEDDING COURSE FOR BRICK SURFACES
In order to equalize the variations in size and shape of the brick,
they are laid on a bedding course composed of material into which the
brick may be forced by rolling. In this way the upper surfaces of all
brick can be brought to the proper elevation to insure smoothness and
easy riding qualities. Several kinds of bedding course are now
employed.
=Sand Bedding Course.=--The sand bedding course has been referred to
as a sand cushion, but as a matter of experience the cushion effect is
slight, although sometimes pavements have become uneven because the
brick have pushed down into the sand after the pavement was used for a
time. The sand for the bedding course should preferably be fine
grained, all particles passing the eight mesh sieve, but ordinary
concrete sand is satisfactory. The sand need not be clean, as a
comparatively large percentage of silt or clay does not impair the
usefulness of the material.
[Illustration: Fig. 19.--Cross Sections for Brick Highways]
=Sand Mortar Bedding Course.=--In order to eliminate the tendency for
the straight sand bedding course to shift because of the impact of
traffic on the brick, a lean cement mortar is sometimes employed
rather than the straight sand. Sand and cement in the ratio of one
part cement to four or five parts of sand are mixed dry, and after the
brick have been rolled, is moistened to furnish water to hydrate the
cement. The sand employed is ordinary clean concrete sand.
=Green Concrete Bedding Course.=--In the monolithic type of brick
road construction, the brick are laid directly on the green concrete
base before the concrete has taken a set and the irregularities of the
brick are taken up by rolling them until bedded in concrete.
FILLERS FOR BRICK SURFACES
The spaces between the brick are filled with some material that will
prevent the brick from being displaced and prevent water getting to
the bedding course. A suitable filler must adhere to the brick and
fill completely the spaces between them. It must withstand traffic so
as to remain intact in the joints and when in place it must be rigid
enough to prevent displacement of the brick.
=Cement Grout Filler.=--One of the most commonly used fillers for
brick pavements consists of a grout composed of Portland cement and
fine sand. When properly mixed and applied the grout filler meets all
requirements for a filler except that it is non-elastic and some means
must be adopted for caring for pavement expansion.
=Bituminous Fillers.=--Asphaltic materials and tars are widely used as
fillers for brick pavements. Such fillers are of high melting point
and consequently solid at ordinary temperature. They are poured into
the joints hot and when they cool are firm enough to comply with the
requirements for a filler. In addition, they have enough ductility to
accommodate the expansion of the pavement due to temperature changes.
=Mastic Fillers.=--Mastic consists of a mixture of about equal volumes
of fine sand and a solid bituminous material. The mixture is prepared
at high temperature and is worked into the joints between the brick
while hot. When cool it resembles the straight bituminous filler
except that the mastic is somewhat more resistant to wear than the
straight bituminous filler.
EXPANSION JOINTS
It is recognized that brick will expand and contract with changes in
temperature. When a bituminous or mastic filler is employed there is
sufficient yield to the filler to accommodate the change in dimension
in the brick, but when the grout filler is used either the expansion
joint must be provided or the pavement must be designed to withstand
the compression due to expansion of the brick. Expansion joints may
consist of a sheet of bituminous mastic prepared for the purpose and
set in place in the pavement. The sheet of joint material is simply
inserted between courses of brick at the proper place.
Another method of forming an expansion joint consists in placing a
strip of wood between courses of brick at the place where a joint is
required. After the pavement has been grouted, the wooden strip is
pulled out and the joint is filled with a suitable bituminous filler.
=Marginal Curb.=--If the sand bedding course is employed, it is
necessary to provide curbing along the sides of the brick to hold the
bedding course in place. The curb is usually constructed integral with
the base and of concrete of the same mixture as the base. The width of
the curb is usually six inches and the top of the curb is at the same
elevation as the edge of brick surface.
CONSTRUCTION OF THE SURFACE
Before the construction of a brick surface should be undertaken on a
road, the drainage should be provided for even more completely than
for a less costly type of surface since it does not pay to jeopardize
the stability of the pavement by failure to provide adequately for the
stability of the supporting soil. Grades should also be reduced to the
economical limit.
The earth subgrade is brought to the proper elevation and cross
section and is thoroughly rolled. If there are places where the soil
will not compact properly under rolling, these places are corrected by
taking out the material and back filling with new material that will
properly compact under the roller.
The aggregates for the concrete may be distributed along on the
prepared subgrade or may be stored in stock piles or bins at
convenient points. If stored on the subgrade, a traction mixer is
employed which is drawn along the road as the work progresses, the
materials being placed directly in the mixer. If stored at a central
point, they may be transported to the mixer on the road and dumped
directly into the mixer, or the mixer may be set up at the storage
piles and the concrete hauled in trucks to the road where it is
deposited and shaped.
The concrete is spread to the proper thickness and tamped either by
hand or by machinery. If the marginal curb is to be employed, it is
constructed immediately after the concrete for the base has been
finished but before the cement begins to set.
After the foundation concrete has set, the bedding course is spread
and struck off to the proper thickness. When the bedding course
consists of sand-cement mortar, the sand and cement are mixed dry and
spread to prescribed thickness. It is considered to be desirable to
roll the sand bedding course with a light hand roller before the brick
are placed, but the sand-cement bedding course is not rolled. The
bedding course must be carefully shaped by means of a templet or
strike board before the brick are placed.
The brick are laid in straight courses across the pavement, with the
spacing lugs all in the same direction if brick with spacing lugs are
employed, and with the lugs in contact with the brick of adjoining
courses. If brick without spacing lugs are used they are laid loosely
so that there will be room for the filler between the brick of
adjoining courses.
After the brick have been laid they are rolled to bed them in the sand
or sand-mortar bedding course and thus secure a smooth surface. For
this purpose a light, power driven, tandem roller is used and the
rolling is continued until the brick are thoroughly bedded. Any
defective brick that are noted are removed and replaced with good
brick and after this culling has been completed the surface is once
more thoroughly rolled. If a cement-sand bedding course is employed,
the surface is sprinkled just after the final rolling so that water
will flow down between the brick and moisten the bedding course
sufficiently to cause the cement to set. In some cases, the
sand-cement bedding course is sprinkled just before the brick are laid
but in warm weather the setting would take place before the brick
could be rolled if that were done. In cool weather the setting is
sufficiently slow to permit rolling before the bedding course hardens.
The filler is applied to the surface after the rolling. If the
bituminous type of filler is employed, the hot filler is poured onto
the surface and worked into the joints by means of squeegees, with
comparatively little material left on the surface. In some instances
cone-shaped pouring pots are employed and the material is poured
directly into the joints.
The cement grout filler is applied in the same general manner as the
bituminous filler. The grout, consisting of equal parts of sand and
cement, is mixed to a thin consistency and poured onto the surface and
is then worked into the joints with squeegees. Two or more
applications are usually required to effect a complete filling of the
joints. The surface should be covered with sand and be kept moist
until the cement grout has set.
CHAPTER X
BITUMINOUS ROAD MATERIALS AND THEIR USE
Tars and asphaltic materials of various kinds are widely used for road
construction and maintenance, especially for road surfaces subjected
to motor traffic. Materials of this character that are employed in
highway work possess varying degrees of adhesiveness, and while they
may be semi-solid or viscous liquids at air temperature, they melt on
the application of heat and can be made sufficiently fluid to mix with
the mineral aggregates that may be used in the road surface. Upon
cooling, the bituminous materials return to the previous state and
impart a certain amount of plasticity to the mixture, at the same time
serving as a binding or cementing agent, which is sufficiently stable
for many classes of road construction.
=Classes of Bituminous Materials.=--Bituminous materials may be
classified, according to the source from which they are obtained, as
coal tars, water gas tars, native or natural asphalts and oil or
petroleum asphalts.
=Coal Tar.=--Coal tar is obtained as a by-product in the manufacture
of illuminating gas from coal. It is also obtained in the manufacture
of coke from coal. The tar thus obtained is manufactured into products
that are used for dust layers on gravel or macadam roads, binders for
macadam and gravel surfaces, fillers for brick, wood block and stone
block pavements and for expansion joints. These various materials
differ mainly in their consistency at air temperature. (They may
differ widely in chemical composition, but that need not be considered
herein.)
=Water Gas Tar.=--Water gas tar is obtained as a by-product in the
manufacture of illuminating gas from crude petroleum. It is used for
the same kinds of construction as coal tar, and the products utilized
for the several purposes, like the coal tars, differ mainly in
consistency.
=Natural Asphalt.=--Natural asphalt is found in deposits at many
places in the world, existing in beds or pools where it has exuded
from the earth or as veins in cavities in the rocks. It is of varying
composition and consistency, but those kinds in most general use are
solid or very viscous liquids at air temperature. Of the deposits that
have been developed on a commercial scale, the Trinidad lake in the
British West Indies and Bermudez deposit in Venezuela are best known.
Both of these materials are too hard in the natural state to be used
for road construction, and are softened, or fluxed as it is called,
with fluid petroleum oil before being used.
=Petroleum Asphalt.=--Petroleum asphalt is a residue remaining after
the fluid products have been distilled from petroleum. Residues of
this sort are not always suitable for road construction, but a number
of brands of road material are obtained from this source. Oil asphalts
are used for dust layers, for binders for macadam roads, for asphalt
cements for sheet pavement surfaces, and for fillers for block
pavements and expansion joints.
=Mixtures.=--Water gas tars and asphalts are sometimes mixed to
produce road materials, and likewise native asphalts and residues
obtained from petroleum are sometimes mixed to produce asphalt cements
for paving mixtures.
=Classification according to Consistency.=--The various bituminous
materials may be classified according to consistency in discussing the
various uses to which they may be put.
=Road Oils.=--Road oils are fluid petroleum oils of such consistency
that they may be applied cold or by heating slightly. They are used
as dust layers on earth, gravel and macadam surfaces. Their efficacy
depends upon the binding properties of the small amount of asphaltic
material that is contained in the oil.
=Liquid Asphalts.=--These are somewhat less fluid than the road oils,
and must always be heated before application, but are viscous liquids
at ordinary temperature. These materials are obtained from crude
petroleum or semi-solid native bitumens, in which case they are
usually called malthas. Both coal tars and water gas tars of
semi-solid consistency are also employed for the same class of
construction as the liquid asphalts.
These materials are used for carpeting mediums on macadam roads and as
cementing agents in the construction of hot-mixed macadam.
=Asphalt Cements.=--The solid asphaltic materials used for hot-mixed
types of construction are called asphalt cements. They may be
petroleum residues or native asphalts fluxed with petroleum oils. They
are solids at ordinary temperature and must be heated to a temperature
in excess of two hundred and fifty degrees before they are
sufficiently fluid to use. Asphalt cements are used for sheet asphalt
and asphaltic concrete construction and for hot-mixed bituminous
macadam.
=Fillers.=--Fillers are solid asphalts or tars that are used for
filling expansion joints in rigid pavements and for filling the spaces
between the blocks in brick, wood block and stone block pavements.
=Bitumen.=--Bituminous materials are all soluble to a greater or
lesser extent in carbon disulphide and the soluble portion is called
bitumen. It is the bitumen that gives to the materials the cementing
properties utilized in road construction. Mixtures of mineral
aggregates and bituminous materials for various purposes are
proportioned with bitumen as a basis. Therefore, less of an asphalt
containing one hundred per cent bitumen will be used than of one
containing less than one hundred per cent of bitumen.
TABLE 8
PROPERTIES OF ASPHALTIC ROAD MATERIALS
(A) Material
(B) Specific Gravity
(C) Consistency
(D) Solubility in CS_2, Per Cent
(E) Solubility of Bitumen in CCl_4, Per Cent
(F) Solubility of Bitumen in 86° 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 be
varied in the process of manufacture, while others are inherent in the
material and cannot be changed in the process of manufacture.
Specifications must therefore be drawn with care to insure that the
requirements can be met by satisfactory materials. But certain
properties, such as specific gravity, may vary greatly among materials
equally satisfactory for construction purposes. One should not be
misled by apparent differences in the characteristics of materials,
because these may simply be natural peculiarities which have no
bearing on the usefulness of the material. There are given in Table 8
the properties of some of the commonly used bituminous materials and
the properties that can be varied in the process of manufacture are
indicated with an asterisk. A variation in these properties will
usually result in some change of other properties, but generally not a
great change.
SURFACES IN WHICH BITUMINOUS MATERIALS ARE UTILIZED
I. Surface Treatments
Attention has been directed to the rapid deterioration of water-bound
macadam when subjected to passenger automobile traffic.
In water-bound macadam the stones are held in place by a weak cement
composed of stone dust and water, and this cement is not sufficiently
strong to hold the stones in place when they are subjected to the
shear of automobile tires. In finishing the water-bound macadam
surface, the spaces between the stones are filled with screening and
in addition a layer about one-fourth inch thick is left on the
surface.
The automobile traffic first brushes aside all of the screenings and
smaller particles of rock, exposing the larger stones. These gradually
loosen as the road is used and are brushed aside. When this effect
begins, the road is said to be raveling. Various lengths of time may
elapse from the time the road is first finished until raveling begins,
depending upon the character of the stone, the weather and the amount
of motor traffic. During the period before raveling starts, it is
comparatively easy to restore the road surface at any time by the
addition of screenings or clay and sand. Usually there will be a few
small areas of the surface that, on account of faulty construction,
will ravel or become rutted much earlier than the remainder of the
surface. These can be repaired by the methods described in the chapter
on "Water-bound Macadam Construction." When the surface begins to
ravel seriously, maintenance becomes much more difficult and in order
to prevent raveling and the difficulties of maintenance thereafter,
the macadam surface is often coated with a bituminous material.
[Illustration: Fig. 20.--Oiling a Gravel Road]
If there is any dust or screenings on the road surface, the bituminous
material will not adhere to the stones and will soon flake off under
traffic. The surface of the macadam must therefore be thoroughly
cleaned before the bituminous material is applied. The usual practice
is to finish the road as water-bound macadam, and permit traffic on it
for a sufficient length of time to show any weak places in the surface
and at the same time thoroughly to season the surface. If any
defective places appear, they are repaired and when the surface
exhibits satisfactory stability, but before it begins to ravel, the
bituminous surface is applied. There will ordinarily be some stone
dust and some screenings remaining on the surface at the time
bituminous treatment is undertaken, and there may also be some caked
mud or other foreign material. All of this must be removed so as to
expose the stones throughout.
=Applying the Bituminous Binder.=--The bituminous binder may be
delivered in tank cars, which is desirable if the work is near a
railroad siding, or ample tank wagon service is available for long
hauls so that the tank will not be held up too long. Often it is
desirable to purchase the binder in barrels and haul these to the site
of the work in advance of beginning the construction of the surface.
The bituminous material may be applied by means of hand spreading cans
not unlike an ordinary garden watering pot, except that a slotted
nozzle is substituted for the ordinary perforated one. If hand methods
are employed for spreading, the bituminous material is heated in open
kettles and then spread on the surface, the quantity required usually
being about one-half gallon per square yard of surface. The
temperature of the binder should be great enough to insure fluidity
and the road should be dry at the time of the application. As soon as
the material has been spread, the surface is finished with a dressing
of chips.
=Finishing the Surface.=--For surface dressing the best material is
stone chips ranging in size from about 1 inch down to one-fourth inch.
But the chips must be of durable material, or they will quickly grind
into dust. They must be free from dust when applied, as the presence
of any considerable amount of dust interferes with the proper
finishing of the surface. The stone chips are rolled into the surface,
a sufficient quantity being used to just cover the surface.
=Patching.=--It almost always happens that some small areas will not
be properly cleaned or that for some unknown reason the coating peels
off the surface. Such places must be promptly patched to prevent them
enlarging under the action of traffic. This work is usually done by
patrolmen, who inspect the road at frequent intervals and make the
necessary repairs. The patrolman is equipped with a small heating
kettle, a spreading can and the necessary brushes, tampers and
miscellaneous tools needed for the repair work. The place to be
patched is carefully cleaned, coated with bituminous binder and stone
chips and tamped until dense and solid. Repairs made in this way are
exceedingly important in that they arrest deterioration in its early
stages and maintain a high degree of serviceability.
II. Penetration Macadam
A considerable mileage of macadam has been constructed in which an
attempt was made to eliminate the difficulties of maintenance by a
method of construction that involves applying a bituminous binder in
such a manner as to permit it to penetrate two inches or more into the
surface. It is expected that the binder will coat the stones to such
an extent as to increase materially the stability of the bituminous
macadam over the surface treated one. It is also expected that less
difficulty will be encountered in maintaining a surface of bituminous
material and stone chips on this type of road than on the water-bound
macadam. The extent to which these expectations have been realized has
varied to a marked degree and although some excellent surfaces have
been constructed by this method, the results have as a rule been
neither uniform nor entirely satisfactory. It seems to be apparent
that good results cannot be obtained unless the materials are entirely
suitable and the construction is carried out with unusual skill.
=Foundation.=--The foundation or lower course consists of a layer of
broken stone six inches thick placed on a well drained and thoroughly
rolled earth subgrade. In exceptional cases, the Telford type of
foundation might be employed.
The lower course of broken stone is finished in the same manner as
water-bound macadam, being bonded with stone screenings or with fine
gravel of high clay content.
Since this course is in reality the foundation of the surface, it is
necessary to secure stability by appropriate construction methods,
exactly as in constructing water-bound macadam.
[Illustration: Fig. 21.--Type of Roller used on Gravel and Macadam
Roads]
=Upper or Wearing Course.=--The wearing course consists of a layer of
stone about two and one-half inches thick. The stone is placed and
rolled and the spaces between the stones partially filled with some
suitable bituminous material. The bituminous material is usually
applied by means of a mechanical spreading device connected to a tank
wagon. The bituminous materials employed for this class of
construction are semi-solid in character and must be heated to give
them sufficient fluidity for application. They may be heated in the
tank wagon which is used for the application or they may be heated in
separate tanks and transferred to the distributing wagon for
spreading. Some kind of a nozzle or group of nozzles is employed for
spreading the material so that it can be delivered in the form of a
spray or at least in a thin fan-shaped stream and can be distributed
in a fairly uniform layer over the stone. The binder will cool rather
rapidly after it is applied, but meanwhile will flow into the openings
between the stones and will form over the surface stones a coating of
slight thickness.
The surface of the macadam is next covered with a layer of chips of
tough rock, similar to the material used for the final dressing in
surface treatments. These are carefully brushed into the openings
between the larger stones by means of heavy brush brooms. This is an
exceedingly important part of the work and often a much neglected part
of the construction.
The surface is then covered with a second application of bituminous
material, somewhat less in quantity than required for the first
treatment and the surface again covered with stone chips and brushed.
The surface is then thoroughly rolled and is ready for traffic.
=Patching.=--As in the case of surface treatments, there are likely to
be places that, on account of defects in the construction, will fail
soon after the road is placed under traffic. These will quickly
enlarge unless they are repaired promptly. The repairs are made by
loosening the stone in the area affected and adding new stone as
needed and then pouring on the necessary amount of bituminous material
to coat the stones. Allowance must be made for the compression of the
material by tamping so that a depression does not result. The stones
are carefully tamped to place and covered with chips which are also
tamped.
=Characteristics.=--The penetration macadam is a surface well adapted
to motor traffic if the individual vehicles are not too heavy. It is
likely to squeeze out of shape under motor truck traffic, becoming
seriously uneven and uncomfortable for traffic. Its durability is
materially affected by the construction methods followed.
III. Hot Mixed Macadam
The wearing course of the mixed macadam is composed of graded broken
stone or gravel and a bituminous binder. Usually the bituminous
material only is heated prior to the mixing, but sometimes the stone
is also heated.
=Foundation.=--The lower course, which serves as the foundation, is
either broken stone macadam, gravel or concrete.
Where a foundation of broken stone is used, it is constructed of the
materials and in the manner described for the foundation of the
penetration macadam. Quite often a badly worn macadam or gravel road
is used for the foundation and a new wearing course provided by adding
a mixed macadam surface. If such is the case, the old surface is
worked over so as to restore the shape sufficiently and to insure that
it is everywhere of sufficient thickness.
Portland cement concrete is sometimes used as a foundation for the
mixed macadam, but not often. Usually if the traffic is of a character
requiring a concrete foundation, it is desirable to use a better
wearing course than the mixed macadam, and the asphaltic concrete or
sheet asphalt type of surface is employed. It is necessary to finish
the surface of the concrete base with some device that will leave the
surface rough to prevent the macadam from creeping. A knobbed tamper
which leaves numerous irregular depressions about 2 inches in diameter
and three-fourths inch deep is often employed.
=Sizes of Stone.=--For the wearing surface, stone ranging in size from
2 inches down to one-fourth inch is usually employed. If the stone is
of good quality the maximum size may be but 1-1/2 inches, but soft or
even medium stone of that size are likely to crush under traffic. The
stone for the base course should preferably be from 3 inches down, but
any available size will be satisfactory if the layer is well rolled
and bonded. The base course is constructed in the same manner as
water-bound macadam and any material satisfactory for the base course
of macadam will serve for the base course of mixed macadam. Screenings
having good bonding properties will also be required for the base
course.
=Mixing and Wearing Surface.=--Several methods are employed in mixing
the wearing surface. The simplest is to mix by hand with shovels. The
aggregates are heated in improvised heaters which may consist of
nothing more than a metal pipe two or three feet in diameter, around
which the stone is piled. The mixing platform is usually a metal plate
sometimes arranged so that it can be heated by means of a fire
underneath. The bituminous material is heated in kettles. For some
mixtures, the stone is not heated, but the bituminous material is
always heated. The batch of stone is placed on the mixing platform,
the bituminous material added and the materials mixed by hand.
Machine mixing is practiced much more extensively than hand mixing,
being both more rapid and cheaper. The mixer is similar to a concrete
mixer except that the drum is arranged so that it can be heated. The
hot stone and the bituminous binder are put into the drum and mixed
for the requisite length of time. Sometimes the stone is mixed cold,
the bituminous material only being heated.
=Placing the Wearing Surface.=--The hot mixture is carted to the road
and spread to such thickness that after rolling the wearing surface
will be not less than two inches thick. The hot mixture is dumped and
then spread by means of shovels to the approximate thickness and the
spreading completed by means of rakes. The surface is then rolled
either with a tandem or a three-wheeled roller until thoroughly
compressed.
=Seal Coat.=--After the rolling has been completed, the surface is
covered with hot bituminous cement and dressed with pea gravel or
stone chips and again rolled. Traffic may be permitted in twenty-four
hours.
=Characteristics.=--The mixed macadam is a somewhat resilient surface
of excellent riding qualities and considerable durability for medium
traffic. It is likely to creep and become uneven when subjected to
heavy loads. The seal coat will wear off in two or three years and
will require replacing.
IV. Asphaltic Concrete
Asphaltic concrete is a name given to a road surface mixture which is
composed of graded stone, graded sand and asphalt cement. This type is
designated as asphaltic concrete because of the analogy of the mixture
to Portland cement concrete.
Asphaltic concrete is of two general types known as bitulithic, or
Warrenite, and Topeka asphaltic concrete, respectively, the
differences being in the nature of the mixture.
=Bitulithic or Warrenite.=--The stone employed for these types is
graded down from a size about equal to one-half of the thickness of
the wearing course, and stone passing a 1-1/4 or 1-1/2-inch screen is
usually specified. From the maximum size the stone is graded down to
the finest particles produced by the crusher. The range of sizes of
stone will vary with the source of the supply, and in order to secure
the desired density in the mixture, varying amounts of graded sand and
mineral dust, such as ground limestone or Portland cement, are added
to the broken stone. Usually the resulting mixture contains less than
fifteen per cent of voids, and to this carefully graded mineral
aggregate there is added enough asphalt cement to bind together the
particles.
=Topeka Asphaltic Concrete.=--In this type of asphaltic concrete, the
mineral aggregate consists of a mixture of carefully graded sand and
of broken stone of such size that all will pass a one-half-inch screen
and graded down to the fine dust produced by the crusher. To this
mixture is added about nine per cent of Portland cement or limestone
dust. The voids in the mixture are usually about twenty-five per
cent.
It will be seen that the essential differences between the Bitulithic
and Topeka types are these: the Topeka type contains a larger
percentage of voids and stone of a smaller maximum size than the
Bitulithic. Both types have been extensively employed for city paving,
but the Bitulithic and Warrenite types have also been used to some
extent for rural highways. The Topeka type has been used but little
for rural highways.
=Foundation.=--The foundation for the asphaltic concrete may be an old
macadam road, a base course constructed of broken stone or Portland
cement concrete, the latter being used much more extensively than
either of the other types.
Sometimes asphaltic concrete is used for resurfacing water-bound
macadam or gravel roads when the traffic has increased to the point
where the cost of maintenance of the water-bound macadam has become
excessive. The existing surface is repaired and the cross section is
restored, or possibly flattened somewhat.
=Placing the Surface.=--The stone, sand and asphalt cement are heated
to the required temperature and combined in the proper proportions and
are then thoroughly mixed by a mechanical mixer. The mixture is hauled
directly to the road and is dumped and spread by means of rakes. It is
then rolled thoroughly while still hot, a three-wheeled roller being
most satisfactory. After rolling, a seal coat of hot asphalt cement is
spread over the surface and covered with hot stone chips about 1/4
inch in size. The surface can be opened to traffic immediately after
the surface has been completed.
=Characteristics.=--The asphaltic concrete surface is of excellent
riding properties, is easily repaired and of moderate durability. It
is a particularly desirable surface for pleasure automobile riding and
for horse drawn traffic.
CHAPTER XI
MAINTENANCE OF HIGHWAYS
Proper maintenance of highways is equally important with proper
construction. With nearly all types of road construction, the need for
maintenance arises soon after the surface is placed under traffic and
is continuous thereafter. The nature and amount of maintenance work
varies greatly among the several types of surface and the organization
suitable for a system of highways will depend to a considerable extent
upon the kinds of surfaces that are to be maintained.
The upkeep of a road may be conveniently considered as of two kinds,
viz., (1) that which has to do with the wearing surface and earth
shoulders or berms upon which there is some traffic and (2) that which
has to do with the side ditches and drainage structures and keeping
the roadside in presentable condition. Both kinds of work are usually
carried out by the same organization, but whereas the nature of the
work indicated under (1) will vary with the type of wearing surface
and with all variations in traffic, that which is indicated under (2)
will be nearly constant in any locality.
ORGANIZATION FOR MAINTENANCE
Maintenance of highways is preferably under the administration of the
same authority as construction and when an improvement is undertaken
under the jurisdiction of a State Highway Department, the completed
improvement is ordinarily maintained under the state authority. If
the improvement is made by county authorities, the maintenance is also
carried out under county authority.
The nature of the organization of maintenance forces is dependent upon
the kind of roads to be cared for and must of necessity be varied in
any instance as conditions demand. In general, either maintenance
gangs or patrolmen are employed and often both are used on the same
road system.
=Patrol Maintenance.=--Where this system is in operation, the highway
system is divided into patrol districts of from six to eighteen miles
of highway and a single patrolman is placed in charge of each
district. He is provided with all of the necessary tools and materials
required in his district and performs all of the work required in the
ordinary upkeep of the highway. He should work under the direction of
the county engineer or the district engineer for the state highway
department, because his work involves the use of materials and
processes requiring technical supervision.
=Gang Maintenance.=--The maintenance gang may be employed for some
types of road surface in lieu of the patrolman or with other types of
surface may be employed to supplement the work of the patrolman. The
maintenance gang consists of three to ten men and is furnished all of
the tools and materials required for the particular kind of work they
do. Ordinarily the gang goes over the roads assigned to it once each
season and performs those repair operations requiring more work than
the patrolman can find time for. The work of the maintenance gang like
that of the patrolman should be under engineering supervision.
=Maintenance of Earth, Sand-clay, Gravel and Macadam Roads.=--The
ordinary upkeep of earth, sand-clay, gravel and macadam surfaces is
most readily accomplished by the patrol method, since constant care is
required to keep the roads in a condition of maximum service ability.
The tools required for each patrolman may include the following:
1 shovel 1 spade
1 stone rake 1 pick
1 scythe 1 tamper
1 or more road drags 1 mowing machine for cutting weeds
1 wheelbarrow (sometimes)
1 light truck 1 small kit carpenter's tools
The work of the patrolman consists in keeping the surface of the road
smooth by dragging, repairing chuck holes by tamping in fresh material
of the appropriate kind, keeping the ditches and culverts free from
obstruction, cutting weeds and repairing bridge floors if they are of
plank construction. Removal of snow drifts is sometimes a part of the
patrolman's duty, but more often that is done by special gangs.
Usually the patrolman is authorized to hire teams for dragging and
cutting weeds.
When an earth road requires to be re-graded so as to restore the
cross-section and deepen the ditches, a gang is sent in to perform
that work, as it is obviously impossible for the patrolman to perform
work, of that kind.
If the gravel road is being maintained with a bituminous carpet coat,
the patrolman will be furnished the necessary tools to enable him to
patch the surface with bituminous material as necessity requires.
When the surface deteriorates to such an extent that a new carpet coat
is required, the gang system is employed for all work connected with
resurfacing, instead of attempting to have the work done by patrolmen.
The maintenance of the macadam road is carried out in much the same
manner as that of the gravel road. The binder of stone dust or clayey
sand is renewed as often as it is swept off by traffic. Depressions or
ruts are repaired by first loosening the surface with a pick and then
adding broken stone and screenings to restore the surface.
When the macadam reaches the stage where entire resurfacing is
needed, the work is performed by gangs organized and equipped for the
purpose; and likewise when the surface is being maintained with a
bituminous carpet, the renewal of the carpet coat is performed by
special gangs, but the ordinary upkeep of the surface by patching is
handled by a patrolman.
MAINTENANCE OF MIXED BITUMINOUS SURFACES
[Illustration: Fig. 22.--Scarafier used in Gravel Road Maintenance]
These types of surface can be kept in satisfactory condition if they
are carefully repaired once or twice each season. This work requires
considerable experience and some special equipment, not ordinarily
supplied to patrolmen. A gang is organized for the work and supplied
with the proper equipment. They go over the roads and patch all worn
places, generally first removing the wearing surface entirely in the
area affected.
The wearing surface mixture is then prepared and tamped or rolled into
place. If the area affected is small, tamping is satisfactory, and
when the area is considerable, rolling is employed. The upkeep of the
side roads may be accomplished by the same gang but is preferably
taken care of by patrolmen, who do not attempt any but minor repairs
to the wearing surface.
MAINTENANCE OF BRICK AND CONCRETE ROADS
On brick and concrete roads, the principal work on the wearing surface
consists in filling the cracks with a suitable bituminous material.
This work is done by patrolmen or by special gangs and generally will
be done once each year. The upkeep of the side roads is cared for by
patrolmen who drag the side roads and cut the weeds as occasion
requires.
INDEX
Administration county; 15
federal; 17
highway; 13
state; 16
township; 13
Aesthetics; 62
Aggregate, fine; 101
Aggregate, coarse; 100
Air resistance; 51
Alignment; 46
Applying bituminous binder; 122
Asphaltic concrete; 128
Asphalt, natural; 117
liquid; 118
petroleum; 117
Assessments, special; 19
zone method; 20
Bedding course, green mortar; 111
sand mortar; 111
sand bedding mortar; 111
Binder for gravel; 75
Bitulithic or warrenite; 128
Bitumen; 118
Bituminous coatings on concrete; 105
Bituminous fillers; 112
Bituminous road materials and their use; 116
Bituminous surfaces; 96, 120
Blade grader; 69
Bonding; 87
Bonds, annuity; 26
serial; 27
sinking fund; 25
Box culverts; 39
Brick roads; 113
Brick, repressed; 107
tests of 108;
vertical fiber; 107
vitrified; 106
wire-cut-lug; 108
Broken stone road surfaces; 89
Cement, asphaltic; 118
Cement concrete roads; 98
Cement grout filler; 112
Characteristics, asphaltic concrete; 129
bituminous macadam; 125
broken stone; 97
concrete; 105
mixed macadam; 128
sand clay; 78
Classes of bituminous materials; 116
Classification according to consistency; 117
Clay and cement concrete pipe; 39
Coal tar; 116
Concrete, asphaltic; 128
Concrete materials; 100
Concrete pipe; 39
Control of erosion; 61
Costs; 70
County administration; 15
Cross sections; 60, 65
Culverts; 56
Curing concrete; 103
Design, broken stone roads; 89
concrete roads; 99
earth roads; 42
Desirability of road bonds; 27
Development of traffic; 2
Drainage, necessity of; 29
Drainage of roads; 29
Earth roads, in arid regions; 72
humid regions; 65
value of; 73
Earth works; 92
Education, rural; 6
Effect of grades; 54
Elevating grader; 66
Elevating grader work; 68
End walls for culverts; 39
Energy loss on account of grades; 57
Entrances, farm; 37, 61
Expansion joints; 104
Farm entrance culverts; 37
Federal administration; 17
Fillers; 118
Finance, highway; 19
Fine aggregate; 101
Finishing surface of concrete; 122
Foundation, asphaltic concrete; 129
brick; 109
macadam; 93
mixed macadam; 126
penetration macadam; 123
Telford; 94
Gang maintenance; 131
Grader, Maney; 67
use of; 69
Gravel, ideal; 81
natural; 83
roads; 74
General taxation; 24
Good roads and commerce; 7
Green concrete bedding course; 111
Highway administration; 13
Highway finance; 19
maintenance; 130
Importance of design; 30
Ideal road gravel; 81
Inter-city traffic; 5
Inter-county and inter-state traffic; 5
Internal resistance; 50
Intersections; 46
Laying tile; 35
Length of culvert; 37
Liquid asphalt; 118
Local farm to market traffic; 4
Macadam; 89
Maintenance, concrete; 105
earth roads; 70
general; 131
gravel roads; 88
macadam; 96
of highways; 130
patrol; 131
Maney grader; 67
Marginal curb; 113
Measuring materials; 101
Metal pipe; 38
Mixing wearing surface; 127
Mixtures; 117
Natural asphalt; 117
gravel; 79
Necessity for planning; 42
drainage; 29
Patching; 122, 125
Patrol maintenance; 131
Pebbles, size of; 80
Petroleum asphalt; 117
Placing asphaltic concrete; 129
Placing broken stone; 94
Placing concrete; 102, 103
mixed macadam; 127
Plans for roads; 43
Preliminary investigation; 44
Preparation of earth foundation; 102
of road; 85
Private entrances; 61
Properties of stone; 90
Proportions for concrete roads; 101
Purpose of highways; 1
Reinforced concrete box culverts; 39
Reinforcing; 104
Repressed brick; 107
Road oils; 117
Road plans; 43
Rocks, kind of, for macadam; 91
Rolling, macadam; 95
Rolling resistance; 50
Run-off; 31
Rural education; 6
Rural social life; 7
Safety consideration; 58
Sand bedding course; 111
Sand clay and gravel road; 74
Sand mortar bedding course; 111
Seal coat; 127
Serial bonds; 27
Sinking fund bond; 25
Slip scraper; 67
Special assessments; 19
Specifications; 119
Spreading screenings; 95
State administration; 16
Stone, use of; 92
Surface drainage; 30
Surfaces, bituminous; 120
Surface method; 87
Superelevation; 47
Tests, brick; 108
Tile drains; 35
Topeka asphaltic concrete; 128
Tractive resistance; 52
Trench method; 85
Truck operation costs; 9
Types of culverts; 38
Underground water; 34
Undulating roads; 58
Use of blade grader; 69
Utilizing natural gravels; 83
Value of earth roads; 73
Variation in rainfall; 64
Variation in soils; 63
Vehicle taxes; 24
Vertical fiber brick; 107
Vitrified brick roads; 106
Vitrified brick; 106
Water gas tar; 117
Width of roadway; 59
Wire-cut-lug brick; 108
Zone method of assessing; 20
* * * * *
[Transcriber's Notes:
The transcriber made these changes to the text to correct obvious
errors:
1. p. 5, accomodate --> accommodate
2. p. 39, guage --> gauge
3. p. 46, enbankment --> embankment
4. p. 63, tought --> tough
5. p. 68, absorbant --> absorbent
6. p. 73, persistant --> persistent
7. p. 77, indispensible --> indispensable
8. p. 119, aspealt --> asphalt
9. p. 127, repaid --> rapid
10. p. 130, Vetrified brick; 105 --> Vitrified brick; 106
11. p. 130, Virtical --> Vertical
End of Transcriber's Notes]
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