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



the crevices. Often where the overlying soil is thick, dense, and 

 compact, the resistance offered overcomes the low pressure, and the 

 water penetrates up into it but a little distance ; but where the depth 

 of soil is not great the water penetrates up through it and runs off 

 the surface. More frequently, however, the friction of the soil over- 

 comes the low pressure of the water before it quite reaches the sur- 

 face, and a stationar}^ condition would result but for a slight lateral 

 movement of the water in the soil which causes it to spread over a 

 more or less circular area larger than the outlet of the contributing 

 feature in the shale. 



The condition just described is relieved by wells reaching into and 

 tapping the water-bearing zone; for the water, seeking the path of 

 least resistance, enters the larger openings formed by the wells and 

 passes freely upward and outward. One well, or a number of wells, 

 will not remove all of the water from the water-bearing stratum or 

 area, but they tend to relieve the pressure and thus prevent the fur- 

 ther rise of water to the small area of soil above. The relief of 

 pressure by this method is based on a well-known principle of hy- 

 draulics, viz, that the pressure of flowing water in a confined medium 

 decreases with the increase of velocity. 



It is well known that in any artesian area wells too closely spaced 

 will interfere ; that is, the discharge from each well will be reduced 

 as the number of wells in the area is increased. In the drainage of 

 shale lands the relief wells should be spaced so closely that this inter- 

 ference practically overcomes the pressure between the confining 

 strata. Otherwise, since these confining strata always are more or 

 less imperfect, water will continue to rise into the subsoil and the 

 land will remain water-logged, even where immediately adjacent to 

 drains, whether open or covered. 



The relation between the tile drains and the relief wells in a 

 drainage system can be summed up in the statement that the relief 

 wells provide the desired drainage, while the tile drains merely 

 provide outlets for the water developed by the wells. 



AREA OF INFLUENCE OF RELIEF WELLS. 



The area of influence of an ordinary relief well in shale is not large. 

 A well may strike but one small crevice or water-carrying zone which 

 is responsible for only a very small spot in the seepage area and 

 which is only one among many that are contributing water. While 

 no rigid rule can be given for the spacing and location of wells, ex- 

 perience has shown that from two to six for each 100-foot length of 

 trench will be necessary ordinarily. This does not mean, however, 

 that a certain number of wells should be decided upon for a unit 

 length of trench and then spaced evenly throughout that length. 

 While the location of a well that will develop the maximum flow, or 



