Land Drainage and Soil Efficiency 



1827 



DEPTH OF DRAINS 



The depth of drains will range from about twenty inches to three and 

 a half or four feet. The commonest depth is two to three feet. In 

 sandy land the drains may be placed deeper than in clay land. Up to a 

 depth of four feet an increase in the depth of the drain will permit an 

 increase in the interval between drains of about two rods for each foot in 

 depth. In clay land an increase in the depth of laying the tile will usually 

 not permit an increase in the interval between the drains. 



GRADE 



Drains must always have some grade in order to insure the flow of 

 water. If the direction or extent of the natural grade is questionable, 

 a leveling instrument should be used in order to determine the conditions. 

 Often a homemade instnmient, using a carpenter's level, is very serviceable. 



A very small grade will suffice for the flow of water in a tile. Tile 

 drains will operate on a grade of a half inch per hundred feet, but a fall of 

 six inches to a foot per hundred feet is much more desirable. The less 

 the fall, the more careful all the construction must be in order to insure 

 permanency. Relatively larger tile must be used also. In soils of a 

 quicksand nature it is important that the lateral drains should have no 

 greater grade than the main, or that silt wells be constructed at intervals 

 in order to collect the 

 sediment and permit 

 its removal. 



SILT WELLS 



A silt well is a pit in 

 the course of the drain, 

 the bottom of which 

 extends two or three 

 feet below the line of 

 tile. The water comes 

 in at one side and 

 goes out at the other. 

 Any coarse material 

 will settle at the bot- 

 tom. One or two sec- 

 tions of sewer tile placed on end with junction connections make the most 

 simple and permanent construction. Surface water may be admitted to 

 advantage through the silt wells, thereby protecting the system of 

 drains from surface wash. 



Fig. 48. — Section across tile drain at joint, showing the 

 entrance oj water at the bottom of the joint, the protection 

 by burlap on top oj the joint to keep out silt, and the shape 

 of the surface of the water table in a drained field, a short 

 time after it had been in operation following a rain. TJte 

 water table, or zone of saturated soil, rises between the tile 



