The grades of the road are carefully studied and laid out in the most favorable manner by an engineer so as to make it as level and straight as possible with due regard to the economics of the problem. Corresponding to these grades the subsoil foundation or subgrade of the road is constructed either by cuts or fills, so that its surface is at a depth below the surface of the road as it is to be finished corresponding to the thickness of the compressed material to be built up thereon. The subgrade must be so prepared, especially in fills, by the use of proper material and thorough rolling with a steam roller, that it is absolutely stable and rigid, and will not be thrown out of shape by frost. The preparation of the subgrade is one of the most important points in good road construction and, although it is purely a structural problem, it is too often neglected or passed over without sufficient consideration and care. It can be readily understood that the rigidity and wearing character of a road can be no greater than that of the subgrade which supports its surface.
Upon the subgrade the road itself is built, or a further foundation may be constructed upon it, if the conditions seem to demand it, that is to say, there are two types of broken-stone roads, one commonly called a macadam and the other a telford road. In the former the broken stone is placed directly on the subgrade and in the latter, from considerations of the character of the subsoil or of that of the traffic on the road, on a further foundation constructed, as described by Mr. Austin B. Fletcher, as follows:
A satisfactory telford foundation may be made by placing vertically on a layer of gravel, 2 or more inches in depth, stones of fairly uniform size, not exceeding 10 inches in width, 6 inches in depth, and varying in length from 6 to 20 inches. The stones should be set on their broadest edges, lengthwise across the road, and wedged rigidly into position by smaller stones driven by mauls into the interstices between the telford stones. The projecting points should be broken off with stone hammers, the depressions filled with chips, and the telford rolled with a steam roller until it is true to the desired cross section.
The foundation, whether of the macadam or telford type, should be properly drained, since the presence of water softens the subsoil so that the broken stone is forced into it under pressure, weakening the road and destroying the shape of the surface. This provision is very generally neglected in the United States. In addition ditches or channels must be provided on each side of the road to remove the ground water collected by the drainage system and to take care of the surface water which is thrown off the road by the crown, or camber, and the grades.
Upon the subsoil or telford foundation is placed broken stone between shoulders of soil or other suitable material, to prevent its lateral displacement. According to Macadam the stone consisted of pieces of uniform size, about two inches in diameter spread to a depth of ten inches and then compacted by the traffic which passed over the
