28 
BULLETIN 724, IT. S. DEPARTMENT OF AGRICULTURE. 
The cost of stone masonry for such structures varies from about 
$6 to about $12 per cubic yard, according to the amount of difficulty 
experienced in obtaining the stone. 
SUBDRAINAGE. 
In addition to the side ditches, gutters, etc., required for draining 
storm water awav from the surface of a road, it frequently is neces- 
3-4" - 
CONCRETE IT END WALL 
FOR PIPE CULVERTS 
Fig. 10a. 
sary to provide a system of subdrainage to lower the surface of the 
ground water under the road. The elevation of the ground-water sur- 
face depends on the capillary power of the soil and the hydrostatic 
pressure to which the ground water is subjected, and under some com- 
binations of soil and topography this elevation may be such as to 
destroy the stability of the roadbed unless subdrainage be provided. 
The common methods of providing subdrainage are, first, by open 
ditches ; second, by ditches filled with broken stone or gravel ; third, 
