Polytechnic Association Proceedings. 865 



arouiid them. The great loss of support by this action, may be 

 understood by one of the rules before given, applied to a case 

 where a scour equal to one-third of the depth of the pile, destroys 

 not one-third, but two-thirds of its supporting power.* 



"Wooden piles, alternately wet and dry, will decay rapidly, and 

 in seawater, are liable to the attack of the Teredo. In these cases 

 they should not be used. 



There are also cases where, from the great weight which it is 

 necessaiy to impose, or where the soil is so loose as not to aiibrd 

 the lateral support, with timbers of such length as are accessible, or 

 in very deep water, where the great length of the pile above the 

 earth will absorb a large part of the force of the ram, by its vibra- 

 tions, and prevent its penetration to a requisite depth, iron piles 

 should be substituted for wooden ones. 



Before entering upon the discussion of iron piles, it may be well 

 to further consider the question of placing foundations below the 

 surface of the water, by other methods. 



Such foundations are sometimes brought up to the surface of the 

 water, by means of loose rock, thrown upon the bed of the river, 

 forming an artificial island ; sometimes by cribs of timber, and 

 sometimes of such cribs filled with stone or concrete. The first of 

 these methods is rarely admissible, from the great obstruction 

 which it produces in the channel, and the other two plans, depend- 

 ing upon the durability of the timber, are seldom used for impor- 

 tant structures. 



When it becomes necessary to place the foundation at a consid- 

 erable depth below the surface of the water, resort is had to a 

 coffer-dam, caisson, or to the diving bell. 



Coffer-dams are used when it is required to drive the foundation 

 piles to great depths, or where the case demands, that the work 

 required to be built beneath the surface of the water, shall be dry 

 and exposed. 



Coffer-dams, in deep water, are so expensive that caissons of 

 wood or iron are now generally substituted. 



The French engineers have used a caisson without a bottom, and 

 filled in with a deep bed of concrete, both outside and inside of 

 the caisson, which then becomes a coffer-dam, and allows the water 

 to be pumped out and the work constructed within the space. 



* Piles are usually driven with the small end down, so that the area of the upper one- 

 third is greater in proportion to the length than the average of the whole pile. If the pile 

 was of the same diameter thr«ughout, the loss would be five-ninths. 



[Inst.1 55 



