DRAINAGE 



197 



length and grade, showing how maps and specifications 

 can be made. 



Depth to make drains. The roots of nearly all cul- 

 tivated plants grow best where the surface of the ground 



Figure 85. Showing elevation of the soil above datum, also elevation of drain at 

 stations 50 feet apart, as entered from surveyor's blank. 



water is at the depth of 3 to 6 feet or more, so that they 

 can spread in soil containing only capillary water with 

 the interspaces filled with air down to at least some feet. 

 Tile drains may be laid 3 or 4 feet deep rather cheaply, 

 but deeper than this the expense increases rapidly. 

 Extensive experience has led farmers to lay tile drains 

 at the depth of 3 or 4 feet where there are no special 

 reasons for laying shallower or deeper. In some cases 

 where the water percolates through soils very slowly, it 

 is best to lay the tiles not deeper than an average of 

 2 feet. Since the drain must have a nearly uniform fall 



Figure 86. Depth of ditch at each station, secured by subtracting height of ditch 

 from height of surface on surveyor's blank. 



regardless of the contour of the surface of the ground, 

 and at all points must slope toward the outlet, the drain 

 is necessarily deeper at some points than at others. 

 In low places, it is sometimes necessary to bring the tiles 

 nearer the surface ; always, however, keeping them 

 sufficiently deep that they will not be disturbed by 

 the plow. 



