DRAINAGE OF IREIGATED SHALE LAND. 



13 



In making the examinations a small stake should be driven at 

 each hole and the stakes numbered consecutively. If a map is to be 

 made, the depth to shale should be kept in a notebook. Otherwise 

 the depth to shale should be marked on each stake in- order to facili- 

 tate locating the drains along the ridges and other higher portions 

 of shale. 



The weathered, flakelike covering of the shale ridges, mixed with 

 soil, frequently makes it difficult to determine the true depth to the 

 solid shale formation, as pockets of coarse shalelike material occur 

 in the soil and may be far from any shale formation ; consequently 

 examinations of borings from small auger holes frequently are mis- 

 leading. Where there is any doubt, holes should be continued for 

 some depth after encountering the shale to make sure that it is the 

 solid formation. Occasionally, where the exact depth to shale seems 

 uncertain, it can be ascertained best by excavating a pit. 



LOCATION OF DRAINS. 



Attempts to accomplish drainage by running tile lines across and 

 along the upper edge of the affected area (see lateral A, fig. 9, p. 35), 

 thereby cutting into a number of shale ridges and points contributing 

 water, have proved unsatisfactory; (1) because of the absence of 

 an impervious stratum that can be reached by the tile, the lack of 

 which permits some of the water to pass freely below the tile to a 

 point farther down the slope ; (2) these ridges do not discharge water 

 at their points alone, but frequently along the sides also. This is 

 due to the fact that pressure exists and the water is supplied at 

 various points from the continuous shale formation beneath the shale 

 ridge. 



These complex conditions existing in the shale make it imperative 

 that the drainage lines should follow as nearly as possible along the 

 backbone of the shale ridges or cut through the knolls and other high 

 portions. Usually the lines should extend for some distance up the 

 ridge or other formation above the affected area. Not only is it 

 necessary to run the tile lines along the shale ridges to secure the 

 best results, but trenching in the shale is far easier than in wet adobe 

 soils. Wliere quite broad shale ridges are encountered, one line will 

 be insufficient ; on such ridges a tile line should extend along each side 

 (lateral C, fig. 9, p. 35). 



Where a number of shale ridges that are supplying the seepage 

 water are encountered the main line should, if possible, run along the 

 points of these ridges, with a branch following up each ridge (lateral 

 B, fig. 9, p. 35). As not all shale ridges, or points, furnish seepage 

 water, a number of deep holes should be put down into the shale, not- 

 ing the degree of hardness, whether or not soft layers are encountered, 



