256 PRESSURE OF EARTH AGAINST WALLS. 



The greatest pressure for a bank of vegetable earth will be 

 at three-tenths the height of the wall from the bottom. 



For a bank of sand, at one-third the height of the wall. 



For a bank of rubble, at one-fifth the height of the wall. 



For a bank of unhewn stone, at one-fourteenth the height 

 of the wall. 



For a bank of ~brick, at one-twelfth the height of the wall. 



For a bank of clay, at three-thirty-seconds the height of 

 the wall. 



Walls should therefore be built proportionably strong to 

 these heights to sustain the different pressures. If the bank 

 is liable to be saturated with water the wall should be 

 doubled in strength. 



