=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 sc.r.a.per.

The ditch capacity required and consequently the dimensions will depend upon the drainage requirements, as was pointed out in Chapter III.

[Ill.u.s.tration: 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 ap.r.o.n. The ap.r.o.n carries the material up an incline and deposits it into a wagon which is driven along under the end of the ap.r.o.n. 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 ap.r.o.n 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 sc.r.a.per 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. .h.i.tched 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 Sc.r.a.per.=--For moving earth for distances between 150 and 500 yards, the wheel sc.r.a.per 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 sc.r.a.per and if more is loosened it will be packed down by the sc.r.a.pers wheeling in place to load. A helper or "snap" team is employed to a.s.sist in loading, after which the wheel sc.r.a.per is handled by one team.

=Slip Sc.r.a.per.=--The slip sc.r.a.per differs from the wheel sc.r.a.per 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 sc.r.a.per is dragged along and the method is not suitable for long hauls, 100 feet being about the economical limit.

=Fresno Sc.r.a.per.=--The Fresno sc.r.a.per is one form of slip sc.r.a.per requiring four horses or mules for efficient work. It differs somewhat from the ordinary slip sc.r.a.per in shape and is of larger capacity, but is a drag type of sc.r.a.per 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 ap.r.o.n 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 ap.r.o.n 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 pa.s.sable for vehicles. This requires that clods be broken, weeds and gra.s.s 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 circ.u.mstances 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 quant.i.ty 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 cla.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 acc.u.mulations 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 a.s.sumed 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 quant.i.ty 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, b.a.l.l.s 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.