HIGHWAY CONSTRUCTION HI 



treatment. Whatever its character, it must he brought to a dry and 

 tolerably hard condition hy draining and rolling. Sand arid gravels 

 which do not hold water, present no difficulty in securing a solid and 

 secure foundation; clays and soils retentive of water are the most 

 difficult. Clay should be excavated to a depth of at least 18 inches 

 below the surface of the finished covering; and the space so excavated 

 should be filled in with sand, furnace slag, ashes, coal dust, oyster 

 shells, broken brick, or other materials which are not excessively absorl>- 

 ent of water. A clay soil or one retaining water may be cheaply and 

 effectually improved by laying cross-drains with open joints at inter- 

 vals of 50 or 100 feet. These drains should be not less than 18 inches 

 below the surface, and the trenches filled with gravel. They should 

 be 4 inches in internal diameter, and should empty into longitudinal 

 drains. 



Sand and planks, gravel, and broken stone have been successively 

 used to form the foundation for pavements; but, although eminently 

 useful materials, their application to this purpose has always been a 

 failure. Being inherently weak and possessing no cohesion, the main 

 reliance for both strength and wear must be placed upon the surface- 

 covering. This covering usually (except in case of sheet asphalt) 

 composed of small units, with joints between them varying from one- 

 half an inch to one and a-half inches possesses no elements of cohe- 

 sion; and under the blows and vibrations of traffic the independent 

 units or blocks will settle and be jarred loose. On account of their 

 porous nature, the subsoil quickly becomes saturated with urine and 

 surface waters, which percolate through the joints; winter frosts up- 

 heave them ; and the surface of the street becomes blistered and broken 

 up in dozens of places. 



Concrete. As a foundation for all classes of pavement (broken 

 stone excepted), hydraulic-cement concrete is superior to any other. 

 When properly constituted and laid, it becomes a solid, coherent mass 

 capable of bearing great weight without crushing. If it fail at all, it 

 must fail altogether. The concrete foundation is the most costly, but this 

 is balanced by its permanence and by the saving in the cost of repairs to 

 the pavement which it supports. It admits of access to subterranean 

 pipes with less injury to the neighboring pavement than any other, for 

 the concrete may be broken through at any point without unsettling 

 the foundation for a considerable distance around it, as is the case with 



