DRAINAGE OF IRRIGATED SHALE LAND. 



17 



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 BELIEF 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 

 features 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°— Bun. 502—17 3 



