PILE FOUNDATIONS 53 



PILING AND BULKHEADS 



Pile Foundations. These are often necessary in quick- 

 sands and muck to support bridges, overflows, etc. Two 

 kinds of piles are used, timber and concrete. Timber 

 piles will answer the purpose if not too expensive and if 

 they are driven below the water table. The soil surround- 

 iny them may, however, afterwards become dry due to the 

 construction not far away of deep brick sewers. If the 

 piles dry out they are sure to rot. Wooden piles are 

 driven by the familiar " land machine/' with hammer 

 weighing from 1500 to 2000 pounds, or else by the steam 

 hammer. The latter is not so satisfactory in peaty soils, 

 due to the presence of snags, which can often be shoved 

 aside by the vigorous blows of the drop-hammer machine. 

 Wooden piles are sometimes jetted into place by the 

 hydraulic process which, it is almost needless to say, can 

 be used only hi sandy soils free from stones. Instead of 

 sawing off and capping driven piles, it is better to make a 

 foundation of concrete. This is much cheaper and is also 

 stronger, since the concrete surrounds the piles and holds 

 them rigidly in position. 



Concrete piles are also used and may be of the type that 

 are driven or jetted into position or else of the type that 

 are cast in a hole prepared by the driving of a metal form. 

 Concrete piles have the advantage of durability, and when 

 reinforced a marked advantage as columnar piles. They 

 are thus especially adapted to semi-fluid peaty soils. 



The safe load to be borne by a pile is a difficult matter 

 to determine. Arbitrary rules have been employed and 

 when used by experienced engineers give results on the 

 safe side. Considerable literature has recently been pub- 

 lished on the subject of piles and pile driving, for which 



