40 BULLETIN 502, U. S. DEPARTMENT OF AGRICULTURE. 



this condition of the seepage water the soils of shale lands that have 

 become water-logged develop a severe alkali problem rapidly. 



The drainage of shale lands can not be accomplished by ordinary 

 methods of drainage, due to the movement of the water through the 

 shale under pressure and also to the extreme retentiveness of the 

 overlying adobe soil. 



The three essential factors for successful drainage of shale lands 

 are: (1) Proper location of drains, (2) sufficient depth, (3) relief 

 wells. 



Drains must be located so as to tap the contributing shale features, 

 such as ridges, points, knolls, etc. To so locate drains necessitates 

 careful and complete preliminary examinations. 



The amount of shale reached arid the amount of water developed 

 are augmented by increasing the depth of the drains. These depths 

 never should be less than 6 feet, and generally depths of 7 and 8 

 feet and greater are essential to success. 



A system of drainage in many of the shales will be incomplete 

 and unsuccessful without relief wells. 



The area of influence of relief wells is small; this necessitates 

 that they be closely spaced — in many cases 5 or 6 to 100 feet of 

 trench. 



The most efficient depth for the wells has been found to range 

 from 6 to 20 feet below the bottom of the tile drain. 



The major portion of the water developed by most of the drainage 

 systems in shale comes from the relief wells. 



A diameter of 2 inches has been found to be sufficient for the relief 

 wells, and in most of the shales they have been installed with the 

 soil auger. Frequently, however, hard strata require the use of a 

 churn drill. 



For trenches in shale ranging from 6 to T feet in depth, and with 

 labor at $0.25 per hour, unit costs for excavating, laying tile, and 

 back-filling, together with the cost of installing the relief wells, 

 have ranged from $0.12 to $0.25 per linear foot of trench. This 

 does not include the cost of any material for the drains. 



The acreage costs of drainage of the lands referred to in this 

 bulletin have ranged from $13 to $100 per acre for the area actually 

 affected. 



Once seepage trouble has developed in shale lands, the affected 

 area increases rapidly. The quantity of the alkali salts at or near 

 the surface of the ground also increases rapidly in Avater-logged 

 lands of this type. As a result of these conditions, the drainage 

 problem and the one of removing the excessive salts are simplest, the 

 construction most economical, and the results most satisfactory if 

 the drains are installed at the first indication of trouble, 



o 



