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.