Requisites for Road Gravel

REQUISITES FOR ROAD GRAVEL. To be suitable for road-buil( ling purposes, gravel should fulfill the following conditions: 1. The fragments should be so hard and tough as not easily to be ground into the dust by the impact of wheels and hoofs. 2. The pebbles should be of different sizes, each in the proper propor tion. 3. There should be intermixed with the coarser particles some material which will cement and bind the whole into a solid MSS.

Durability.

From the nature of their origin, it is apparent that gravel may differ widely in the nature of the stones composing it. Not only do different gravels differ from each other, but any particular gravel may be composed of fragments of a variety of rocks. Having been transported a considerable distance by water and ice, gravel is usually fairly durable, since the softer and more friable fragments have been worn away. Although gravel is not equal to the best crushed stone for road building, in many parts of the country the rocky fragments transported by water and ice are more durable than any of the native rocks.* Sizes. If the pebbles are too large, the road will not be homogeneous, and the large stones will work to the surface under the action of traffic and frost; but, on the other hand, if the pebbles are too small, the gravel will partake too much of the character of sand, and will be difficult to bind properly. The best results are obtained when the largest pebbles are not more than to 1 inch, or at most inches, in greatest dimension. With stones larger than 1 inch, it is difficult to keep the surface from breaking up when dry. Small gravel makes a pleasanter road and one that is easier to keep in order. If stones larger than 1 or 2 inches are present, they may be screened out and used in the foundation (§ 257).

It is desirable that the several sizes should be so proportioned that the smaller ones are just sufficient to fill the interstices between the larger ones, since then less binder is required. The binder is usually the least durable ingredient, and hence the less there is of it the better. Gravel can often be improved by screening--either to remove an undesirable size or to separate it into several sizes afterward to be combined in new proportions. The proper pro portion depends upon the nature of the gravel—whether the binding material is already present in the form of dust, or whether some of the pebbles must be crushed to produce the binder.

Binder.

The most important requisite for good road building gravel is that it shall bind or pack well. If it does not pack

well, the wheels will sink into the gravel and increase the force of traction, and the rain water will penetrate the road-bed and soften it. To bind well, the several fragments should be in contact with one another at as many points as possible, in order that they may be firmly supported, and that friction may act to the best advantage to resist displacement. To secure contact at every point, all the interstices between the fragments should be filled—those between the large pebbles, with small pebbles; those between the small pebbles, with sand grains; and, finally, those between the sand grains, with some finer material, called a binder. The binding material must be very finely divided, so that it can be worked into the smallest interstices; and for this reason, it is the least durable part of the gravel, being easily washed out or blown away. For the best results, then, the sizes of the coarser particles should be so adjusted as to require a minimum amount of binder.

The binding material may consist of clay, loam, silica, stone dust, iron oxide, etc., or some ingredient which will crush under traffic and furnish a fine dust.

Clay is by far the most common binding material; but the only recommendations for it are (1) that it is easily reduced to an im palpable powder by the action of wheels or by water, and (2) that it is often found already mixed with the gravel, and (3) that if it must be artificially mixed, it is plentiful and cheap. Clay is an un desirable binder, since its binding action depends in a large measure upon t Ile state of the weather. During a rainy period it absorbs water and loses its binding power, and the road becomes soft and muddy; while in dry weather it contracts and cracks, thus releasing the pebbles and giving a loose surface. Clay is also very susceptible to the action of frost; and consequently when the frost is going out, a gravel road with a clay binder ruts up badly and frequently breaks entirely through. When the weather is neither too damp nor too dry, a gravel road with clay binder is very satisfactory. The clay should be no more than enough to fill the voids in the pebbles and sand, and for a good road-gravel should not exceed 15 to 20 per cent of the mass. Not infrequently much greater quantities of clay are present. This surplus may sometimes be removed by screening; but often it can be removed only by washing—a process which is usually expensive as to be prohibitive.