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, (8) 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 and 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 
ehurn drill. yee 
For trenches in shale ranging from 6 to 7 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 alkal salts at or near 
the surface of the ground also increases rapidly in water-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 
