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.

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

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

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 3½-inch circular screen opening, and for the wearing course, as large as will pass a 2½-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 2½-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 3½-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.