COMPOSITE PILING. 



393 



quantity of timber driven as piles amounted to 2900*50 cubic 

 .feet; therefore, the actual displacement in this case was 47 per 

 cent, of the quantity of timber driven as piles. 



Composite Piling. In positions where there is a considerable 

 depth of soft material over the harder strata which has to be 

 reached, but where, owing to the presence of the worm, or from 

 other causes, it is objectionable to have timber above the ground, 

 a composite method of construction consisting of an ordinary 

 timber pile driven through the mud, with an upper length of 



?* ----- ''5- 



FIG. 3-12. 



Fia. 343. 



iron fitted over the top of the timber piles by a socket, as shown 

 by Figs. 342, 343, may be adopted with advantage. If the top 

 of the timber pile is kept well below the surface and efficiently 

 protected by the cast-iron socket, it will last an indefinite time, 

 while it is cheaper and more expeditious than putting down 

 iron piling to a great depth. 



Screw-piles. In soft strata of indefinite depth, screw-piles are 

 preferable, as the resistance to settlement is so much increased 

 by the bearing surface of the screw ; they are also particularly 



