CONSTRUCTION OF UN DER-D RAINS 



255 



8' 3' 3 



3 3 3 



\ 



s 



3" 3" 3' 



textured soil the water level rises rapidly 

 away from the drain and reaches the sur- 

 face at no great distance. On sand soil 

 this gradient is much less. The aim 

 must be to have the water level reduced 

 a definite distance below the surface, 

 after a reasonable interval of time follow- 

 ing rainfall, and the drains must be 

 sufficiently frequent to accomplish this. 

 In heavy clay soil this interval may be 

 as small as twenty-five feet, while in 

 coarse-textured soil it may be 200 or 300 o 

 feet. Usually, it is best to adopt some 

 minimum interval, and 

 place the first lines of tile 

 at two or more times this 

 interval. If the drainage 

 does not prove sufficiently thorough, 

 additional drains may be installed with- 

 out affecting the general system. 



The natural or irregular system is 

 designed primarily to collect water from 

 the surface where it has accumulated, 

 or beneath the surface where it comes 

 within the range of the plant roots. 

 Large areas of land are drained by a 

 single line of tile in the low places. 

 Where land is kept wet by seepage, the 

 with ' minimum drains should tap these as near their 



number of large . , 1 1 



tile, but having source as is practicable. 



many turns and rp,, . r 1 1 j ,1 



branches. I he size ot drains depends on the 



Fig. 85. A more 

 simple system of 

 drains, but one re- 

 quiring more large 

 tile than in Fig. 86. 



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