ROADS AND PAVEMENTS 395 



the roadbed by lifting and displacing its component parts. 

 Hence, the speedy and efficient removal of water is imperative 

 for the preservation of a road. 



The surface drainage, that is, the removal of the rain water 

 from the surface of a road, is provided by gutters connected 

 with side ditches or underground drains and by giving the 

 road a suitable cross-section and grade. Very often it is also 

 necessary to provide for the removal of the underground 

 water. A wet substratum cannot give a firm subfoundation 

 for a road, and will invariably destroy its efficiency under 

 traffic. Sandy soils, unless saturated with water, do not 

 present any difficulty in securing a dry and solid founda- 

 tion, especially if the fall of the natural drainage is away from 

 the line of the road, in which case gutters and side ditches 

 for the removal of the rainwater will generally be found suffi- 

 cient. The clay soils are naturally retentive of water, although 

 they are not readily saturated; when they reach the state of 

 saturation, they become very unstable and are incapable of 

 supporting heavy loads; it is, therefore, necessary to provide 

 a suitable system of subsoil drainage. 



Rock requires little attention to drainage, except where 

 the strata are interspersed with seams of clay and are inclined 

 toward the road, in which case means must be provided for the 

 removal of the water in order to prevent slips. 



The removal of the subsoil water is effected by constructing 

 underground drains or deep side ditches that discharge into 

 the natural streams. 



The main points to be attended to in the construction of all 

 types of drains are: 



1. The Fall or Grade. This should rarely exceed 1 in. in 5 ft. 

 Excessive inclination is likely to cause injury by washing in 

 consequence of the high velocity of the water. 



2. The Area of the Drain. This should be in proportion 

 to the amount of water to be removed. In using tile drains 

 3 in. should be the minimum size. 



3. The Filling. In filling the trenches, care must be taken 

 that the material used does not choke or stop the waterway. 



4. The Materials. In order to avoid large maintenance 

 expenses only durable materials should be employed. 



