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.

[Ill.u.s.tration: 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 quant.i.ty 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

[Ill.u.s.tration: 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 pa.s.s 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 quant.i.ty 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 coa.r.s.e to fine in such a manner that when the gravel is compacted into a road surface the s.p.a.ces 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 pa.s.s a 3-1/2-inch circular screen opening, and for the wearing course, as large as will pa.s.s 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 pa.s.s 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 pa.s.s 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 coa.r.s.e 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 pa.s.s 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 coa.r.s.e 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 pa.s.s 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 cla.s.sification 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 coa.r.s.e. These may be mixed on the work by first placing the coa.r.s.e 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 pa.s.s 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 antic.i.p.ated, 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 antic.i.p.ated 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 princ.i.p.ally 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 sc.r.a.ping 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 quant.i.ty 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 coa.r.s.e 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.