SCIENCE IN HIGHWAY CONSTRUCTION 327 



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 gen- 

 erally 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 



