298 Ground Water, Wells and Farm Drainage. 



upper portion, the water moving as indicated by the long 

 arrows. 



370. Movement of Water where Heavy Clay Soils are 

 Underlaid with Sand. When a heavy, close soil is underlaid 

 with sand or gravel the movement of water toward the tile 

 drains will be almost entirely through the sand when the 

 conditions are like those represented in Fig. 118. In such 

 cases the rains percolate vertically down into the sand and 

 then move laterally to the tile drains, where it rises to enter 

 them, as shown by the arrows. 



31 



Fio. 118. Showing how the main flow of water to lines of tilu Luuy be 

 through a subsoil of sand when this is present and near the surface. 



It is clear that under conditions like these the heavy clay 

 soil above does not determine the distance apart drains 

 should be placed but rather the sand stratum below. 



371. Fall or Gradient for Drains. Generally drains should 

 be given as mucli fall as the conditions will permit and the 

 gradient should not be less than 2 inches in 100 feet if this 

 can be secured. Cases will occur where less must be 

 accepted and then careful leveling must be done to secure 

 the largest fall available. 



It will often happen that the line of lowest ground is 

 quite tortuous, making the distance long, and on this ac- 

 count making the fall small. Frequently in such cases cuts 

 across bends can be made by digging deeper, in this way in- 

 creasing the fall, as is sometimes done in straightening 

 streams. 



372. Uniform Fall Desirable. Effort should be made to 

 secure throughout the course of a main or lateral drain a 

 uniform fall, and never, where it can well be avoided. 



