660 SOILS: PPiOPMBTIES AND MANAGEMENT 



quicksand condition of soil The liaxdpan may need to 

 be partially broken up by d}/namite. The latter condi- 

 tion may require the placing of the tile on boards or the 

 use of wooden box drains to keep the alignment. 



Coupled with deep drainage, sufficient irrigation water 

 is employed to produce heavy percolation, by means of 

 which the excess salt is removed. The most alkaline 

 land can usually be reclaimed in two or three years of 

 leaching. 



562. Vertical drainage. — A gravity outlet for drainage 

 is sometimes difficult to provide. In such a case it may 

 be possible to remove the drainage water through some 

 porous stratum below the surface. There must be such 

 a porous stratum within reach below the surface, in order 

 to render the method of vertical drainage practicable. 

 Basin-shaped areas without an outlet may be wet because 

 of the accumulation of a thin layer of clay or other im- 

 pervious sediment in its lowest part, beneath which at 

 a short distance is a porous gravel or sand formation. 

 Anything that perforates this impervious layer and keeps 

 open the passage will afford drainage. Wells several 

 feet in diameter may be constructed and filled with stone. 

 Tile drains and open drains have been emptied into such 

 structures. An opening of temporary efficiency may be 

 formed by a charge of dynamite. The tendency of such 

 an opening, however, is to become clogged. 



A second condition under which vertical drainage may 

 be advisable exists in a soil that is underlaid within a few 

 hundred feet by a limestone or other porous rock forma- 

 tion into which the siirface water may be emptied. A 

 casing may be installed to protect the walls of the well 

 and to reach from the surface to the porous stratum. 

 In addition a trapped intake, coupled with a silt basin, 



