DRAINAGE OF IRRIGATED SHALE LAND. 7 
age systems had been constructed in shale to a depth of 7 feet and 
had developed considerable quantities of water, yet seepage water 
rose to the top of the soil within a few feet of the line and ran off 
the edge of the bank into the trench. Flowing springs have been 
found within 10 feet of a 7-foot trench. Such results indicate clearly 
that ordinary methods of drainage will not relieve seepage conditions 
where the water is supplied under pressure. 
The purpose, then, of the relief well is to connect the tile line 
with the deeper strata which are the sources of the seepage water 
and thus, by permitting the water to pass out freely, to relieve the 
pressure. 
ACTION OF RELIEF WELLS. 
The pressure of water in a well depends upon the height of the 
source, the quantity of water supplied, and the extent of leakage and 
amount of friction encountered between the source and the well. In 
speaking of this pressure the term “ static head” is used to designate 
the pressure at the point where the flow is measured, when the well is 
closed. The static head is expressed in terms of the height above the 
point of measurement to which the water will rise in the well. The 
discharge of a well is directly proportional to the static head. The 
flow of a relief well is measured at the point where it discharges into 
the tile line. If the point of measurement were at the surface of the 
ground, many relief wells would have no static head and conse- 
quently would not flow. A well in which the water rose to within 1 
foot of the surface would have a static head of 6 feet if measured 
at its connection with a tile line 7 feet deep. Thus the advantage of 
deep drainage can be seen readily, as increasing the depth increases 
the flow from the relief wells. A well that has a static head of 3 
feet in a ditch 4 feet deep would discharge twice as much in a ditch 
7 feet deep. A better appreciation of this advantage of depth will 
be gained when it is realized that in this class of drainage work the 
wells supply the major portion of the drainage water. 
As explained in the description of the shale formations, the water- 
carrying zones are not continuous or regular, and several different 
crevices and zones of close jointing probably furnish the flow to one 
small seepage area. These water-carrying zones may not be freely 
connected—at least in the immediate vicinity of the seepage area— 
and each one may have a pressure slightly different from those of 
the others. The area affected by each of these small contributing 
féatures probably is quite small. The pressure of the water, although 
it may have a high source, usually is very low, owing to friction 
- encountered in the small fissures of the shale. When these crevices 
or closely jointed zones disappear or pinch out, the water has a tend- 
ency to move upward because of the more or less vertical nature of 
~ 70250°—Bull. 502—17 ——3 
