TILE DRAINAGE 25 



and, second, the average distance oi the ground water 

 below the surface. Commonly speaking, four feet is con- 

 sidered deep drainage, three feet medium and two to two 

 and one half feet shallow. In heavy, retentive clay soils 

 it is advisable to lay the tiles between medium and shallow 

 for two reasons: First, because the water filters through 

 them so slowly that it takes a long time to reach the drains, 

 and, second, because the cost of digging increases very 

 rapidly with the depth in a hard clay soil. In a more 

 porous, loamy soil experience has shown that the most 

 practical depth is about three feet. The second condition 

 mentioned above, which affects the depth, is important in 

 this respect, in that the level of the ground water changes 

 with the season and is usually highest in the spring. If the 

 ground water then comes too near the surface only in the 

 spring and naturally lowers itself later on in the season 

 it may be desirable to lay the drains only deep enough to 

 dry the land for plowing and cultivation at the proper 

 time. Shallow drainage will usually suffice where this 

 condition of affairs is to be met. On the other hand, when 

 the ground water is not sufficiently low at any season of 

 the year for the maximum development of the roots, it is 

 best to resort to deep drainage, and unless the soil is a 

 retentive clay the tiles may be placed from three and one 

 half to four feet deep. 



Fig. 1. Showing effect of soil and depth of drains on distance apart. Line 

 XY indicates surface of ground. Line ABCDE indicates surface of ground 

 water in porous soil with tile 2 feet deep and 50 feet apart. Line FHS indi- 

 cates the same for tile 3 feet deep and 100 feet apart, the ground water hav- 

 ing the same slope and coming equally near the surface in both cases. 

 Line AMCPE indicates surface of ground water in retentive clay soil with 

 tile 2 feet deep and 50 feet apart, while line FNS indicates the same for tile 

 3 feet deep and 100 feet apart, the slope of the ground water being the same 

 in both cases, but coming clear to the surface of the ground in the latter 

 case. 



