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Thickness of Road-Covering

road-bed, covering, road, roads, material, inches and traffic


The thickness necessary for a road-covering depends upon the amount of the traffic it is to bear and upon the nature of the foundation afforded by the road-bed. Under a heavy traffic it is advisable to make the road covering heavier than might be allowable for lighter traffic, in order to provide for wear and lessen cost of renewals.

When the road-bed is firm, well drained, and not likely to soften at a wet season, it will always afford a firm bearing, upon which the covering may rest. The loads coming upon the road are then simply transmitted through the covering to the road-bed beneath, and there is no tendency on the part of the loads to break through the covering other than by direct crushing of its material. If, however, the road-bed may become soft in wet weather, it will then lose its power to firmly sustain the covering at all points, and the covering must possess sufficient strength to bridge over places where it is not supported from beneath, or a load com ing upon it may break through by bending it down ward at such point. The thickness of road-covering, therefore, must be greater where the road-bed is less perfect.

The intensity of freezing that may be expected also has an influence upon the necessary thickness of the road-covering. The effect of frost upon the road will depend in large measure upon the condition of the road-bed, and thus make the thickness depend in still greater measure upon its nature. Freezing will not injure a dry road-bed, but if it be damp and have but a thin covering the road is likely to blow or be thrown up by the action of frost.

For roads on considerable grades the thickness of the road-covering is often reduced below what is used on flat ones, because of the better drainage afforded by the slopes. It is to be remarked, however, that if the slopes are very steep the wear of the surface becomes so great, due to the horses' efforts. to obtain foothold and to the washing of surface-waters during rains, that the thickness of the coating should be increased.

Macadam roads are commonly made from 4 to 12 inches thick, and telford roads from 8 to 12 inches, of which 5 to 8 inches may be foundation pavement.

A covering 6 to 8 inches thick is usually sufficient for nearly any case of a country road, unless laid upon bad foundation, or to carry exceptionally heavy traffic.

When the road-bed is formed of firm material and well drained, a covering of 4 or 5 inches of broken stone or gravel may give good service under considerable traffic.

A thin road to be effective must have its interstices well filled with binding material and be thoroughly compacted by rolling. It will then present no voids to be filled by the soil pressing upward from below, and at the same time it will be practically impervious and prevent surface-water from reaching the road-bed, thus keeping the material in good condition to sustain the loads. The 4-inch roads of Bridgeport, Conn., which are often cited as examples of successful work, are con structed in this manner of exceptionally good mate rial. In other cases where thin roads have proved fail ures the trouble may often be traced to dampness in the subsoil or to lack of thorough construction.

Instances will frequently be met in practice where a road must be constructed over material which is likely to be unstable and cannot be made .firm by drainage. In such cases, thick roads must be built. Where the conditions are unfavorable, a road 12 to i6 inches thick may be necessary.

In many cases the problem to decide, in determining the thickness of a covering, is whether to use heavy construction or thorough drainage. It is easier to get good results with thick road-coverings, and they are in general safer to use; but skillful adaptation of less material may often save expense in construction with good results. The peculiar conditions of each case must decide what is best for that case.

On country roads the macadam surface should be given a crown of from one-thirtieth to one-twenty fourth of the width in order to provide good drainage. In many instances a considerable saving in road material may be effected by making the road thinner at the edges than in the middle. The Massachusetts Highway Commission in some instances reduce the thickness of their 6-inch roads to 21 or 31 inches at the edges. Some engineers grade the road-bed without leaving a bench at the side, and reduce the stone to thin edges. It is doubtful if there is any economy in this practice, as it is wasteful in the use of stone, although it effects a small saving in the cost of grading the road-bed.