Road coverings consist of some foreign material as gravel, broken stone, clay, etc., placed on the surface of the earth road. The object of this covering, whatever its nature, is (1) to protect the natural soil from the effect of weather and travel, and (2) to furnish a smooth surface on which the resistance to traction will be reduced to the least possible amount, and over which vehicles may pass with safety and expedition at all seasons of the year. Where an artificial covering is employed, the wheel loads coming upon its surface are distributed over a greater area of the roadbed than if the loads come directly upon the earth itself. The loads are not sustained by the covering as a rigid structure, but are transferred through it to the roadbed, which must support both the weight of the covering and the load coining upon it.
Gravel Roads. Gravel is an accumulation of small more or less rounded stones which usually vary from the size ora small pea to a walnut. It is often intermixed with other substances, such as sand, clay, loam, etc., from each of which it derives a distinctive name. In selecting gravel for road purposes the chief quality to be sought for is the property of binding.
Gravel in general is unserviceable for roadmaking. This is due mainly to the fact that the surface of the pebbles is smooth, so that they will not bind together in the manner of broken stone. There is also an absence of dust or other material to serve as a binder, and even if such binding material is furnished it is difficult to effectively hold the rounded and polished surface of the pebbles together.
In certain deposits of gravel, particularly where the pebbly matter is to a greater or less extent composed of limestone, a con siderable amount of iron oxide has been gathered in the mass.
This effect is due to the tendency of water which contains iron to lay down that substance and to take lime in its place when the opportunity for so doing occurs. Such gravels are termed ferru ginous. They are commonly found in a somewhat cemented state, and when broken up and placed upon roads they again cement, even more firmly than in the original state, often forming a roadway of very good quality.
When no gravel but that found in rivers or on the seashore can be obtained, one-half of the stone should be broken and mixed with the other half; to the stone so mixed a small quantity of clay or loam, about one-eighth of the bulk of the gravel, must be added: an excess is injurious. Sand is unsuitable. It prevents packing in proportion to the amount added.
Preparing the Gravel. Pit gravel usually contains too much earth, and should be screened before being used. Two sieves should be provided, one with meshes of one and one-half inches, so that all pebbles above that size may be rejected, the other with meshes of three quarters of an inch, and the material which passes through it should be thrown away. The expense of screening will be more than repaid by the superior condition of the road formed by the cleaned material, and by the diminution of labor in keeping it in order. The pebbles Larger than one and a half inches may be broken to that size and mixed with clean material.
Laying the Gravel. On the roadbed properly prepared a layer of the prepared gravel four inches thick is uniformly spread over the whole width, then compacted with a roller weighing not less than two tons, and having a length of not less than thirty inches. The rolling must be continued until the pebbles cease to rise or creep in front of the roller. The surface must be moistened by sprinkling in advance of the roller, but too much water must not be used. Successive layers follow, each being treated in the above described manner until the requisite depth and form has been attained.
The gravel in the bottom layer must be no larger than that in the top layer; it must be uniformly mixed, large and small together, for if not, the vibration of the traffic and the action of frost will cause the larger pebbles to rise to the surface and the smaller ones to descend, and the road will never be smooth or firm.