16 BULLETIlSr 502/ U. S. DEPAETMENT OF AGRICULTURE. 



ward through the soil by capillary action carries salts in solution and 

 by evaporation deposits them on the surface of the ground. The 

 height to which capillary water will rise depends on the type of soil, 

 its wetness and temperature, and, to some extent, on the kind of salts. 

 High temperatures and certain salts increase the range of capillarity. 

 In loose, sandy soils the rise is not great ; in average soils of the arid 

 West it probably ranges from 2 to 4 feet, and in clayey soils more 

 than this. Of all the types of soils, the adobes encountered in the 

 drainage of shale lands are perhaps most conducive to a high range 

 of capillarity ; thus the necessity for deep drainage. 



One of the greatest advantages of deep drainage in this type of 

 land is the increase of flow of the relief wells thus obtained. How- 

 ever, there is a practical limit to the maximum depth, and it is 

 believed that in the design of a drainage system the depth should be 

 fixed with reference to that limit rather than by setting a minimum 

 limit determined by capillarity, depth of root zone, etc. The prac- 

 tical limit of depth, of course, would vary somewhat as between 

 hand work and machine work. It would depend also upon the 

 nature of the ground encountered. In the presence of these variable 

 factors it becomes impossible to fix a maximum depth, but the mini- 

 mum should be not less than 6 feet and in many cases should be 7 or 

 8 feet. 



PUBPOSE OF BELIEF WELLS. 



In the drainage of many types of shale land the installation of 

 relief wells is absolutely necessary for the success of the drainage 

 system. A relief well is nothing more nor less than an artesian 

 well. This does not mean necessarily that the water rises to the 

 surface and overflows, since any well may be considered as artesian 

 where the water rises to some extent after having been drilled 

 through a comparatively impervious stratum into one carrying 

 water; in other words, where the water enters the well under 

 pressure. 



As mentioned before, the seepage areas in shale lands occur almost 

 invariably where pressure conditions exist and the movement of 

 the water is upward. In cases of extreme pressure this can be 

 detected at the surface by the appearance of drops of water that 

 have been forced up through the small pores of the soil. In but 

 few cases, however, is it possible to place the drains deep enough 

 to reach the supply of water that causes the saturation. Often the 

 water-carrying zones of shale have been found at depths approxi- 

 mating 30 feet. The cost of the installation of drainage lines 

 becomes prohibitive long before this depth is attained, but unless 

 the water-carrying medium is reached in some manner drains will 

 be of little service, no matter how carefully they are located and 

 constructed or how closely spaced. Cases are known where drain- 



