36 



HIGHWAY CONSTRUCTION 



proportionate to the amount of water reaching them. The width 

 of the bed should not be less than 18 inches; the depth will vary with 

 circumstances, but should be such that the water surface shall not 

 reach the subgrade, but remain at least 12 inches below the crown 

 of the road. The sides should slope at least 1J to 1. 



The longitudinal inclination of the ditch follows the configura- 

 tion of the general topography, that is, the lines of natural drainage. 

 When the latter has to be aided artificially, grades from 1 in 500 to 

 1 in 800 will usually answer. 



Fig. 16. 



In absorbent soil less fall is sufficient, and in certain cases level 

 ditches are permissible. The slopes of the ditches must be protected 

 where the grade is considerable. This can be accomplished by sod 

 revetments, riprapping, or paving. 



These ditches may be placed either on the road or land side of the 

 fence. In localities where open ditches are undesirable they may be 

 constructed as shown in Figs. 13 to 17, and may be formed of stone 



Fig. 17. 



or tile pipe, according to the availability of either material. If for 

 any reason two can not be built, build one. 



Springs found in the roadbed should be. tapped and led into the 

 side ditches. 



Drainage of the Surface. The drainage of the roadway 

 surface depends upon the preservation of the cross-section, with 

 regular and uninterrupted fall to the sides, without hollows or ruts^ 

 in which the water can lie, and also upon the longitudinal fall of the 



