394 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



applicable to places where the concussion attending ordinary 

 pile-driving is objectionable. 



At the Clevedon Pier, the bases of the piles consisting of 

 solid wrought-iron stems 5 inches in diameter, shod with cast- 

 iron discs 2 feet in diameter, were screwed through mud and 

 sand into hard red clay, till a 4^-inch rope, passed round capstan 

 bars 6 feet long, failed with the strain. The screws penetrated 

 the ground to depths varying from 7 to 17 feet, and although 

 made with a 5-inch pitch, seldom descended more than 2J to 3 

 inches at a turn. 1 



Screw-piling in Unstable Ground. In putting in pile founda- 

 tions on unstable ground, consisting of shifting sand overlying 

 compact gravel 6 to 8 feet deep, with peat of an unknown depth 

 beneath, Mr. Conybere, M.I.C.E., adopted a method of screw- 



FIG. 345. 



piling shown by Figs. 344, 345. In this case, the crust of gravel 

 was all that could be depended upon to carry the weight ; the 

 screw discs were therefore placed 8 feet up the pile, so as to 

 take their bearing on the gravel which the lower portion of the 

 pile penetrated. The bearing surface was augmented by discs 

 bolted to the upper portion of the piles, and, bearing upon stone 

 filling, formed the frustum of a cone of resistance whereby the 

 weight was distributed over a larger area on the bed of gravel 

 beneath. 2 



In the construction of the pier at Huelva, 3 where by repeated 



1 M.P.LC.E., vol. xxxii. p. 132. 

 2 Ibid., vol. liii. p. HO. ' Ibid., p. 13G. 



