388 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



balanced by the upward pressure of the air in the chamber added 

 to the friction of the ground. The total weight in the cylinder, 

 including all masonry and brickwork, was 916 tons, and the 

 pressure exerted by the compressed air 638 tons ; the difference, 

 278 tons, represents the part of the load taken up by friction. 

 This friction was developed in a layer of 13 feet in thickness, 

 composed of 9 feet 9 inches of sand and 3 feet 3 inches of coarse 

 gravel irregularly stratified. The surface of the ground bearing 

 against the cylinder was 183*2 square metres, therefore the 

 friction per square metre was 1*5 tons, or about 2 '8 cwt. per 

 square foot. 1 



Experiments made by Messrs. Gerber and Balzano during the 

 construction of the cylinder foundations of the Seinbach and 

 Lech Bridges gave a surface friction of 3 cwt. and 3'35 cwt. per 

 square foot. 2 



The pressure on the London clay at the base of the cylinders 

 carrying the Charing Cross Railway -bridge is about 8 tons per 

 square foot. This pressure is estimated on the assumption that 

 no relief is afforded by the friction of the sides against the 

 material through which the cylinders pass. 8 



Sir B. Leslie, K.C.I.E., has expressed an opinion that in 

 estimating the weight on a cylinder foundation, it is not safe to 

 rely on lateral friction of the earth through which the cylinder 

 passes, as a permanent source of support, although it may exert 

 a considerable temporary retarding influence. When the earth 

 is compact and dense, it can only exert a small amount of lateral 

 pressure, but when the earth is soft and loose, although it may 

 jamb the cylinder for a time, the supporting power of its lateral 

 pressure soon fails, as its cohesion is destroyed by vibration, and 

 the cylinder derives but little support. 4 



It is recorded that in sinking an old-fashioned rectangular- 

 foundation well 18 feet by 10 feet (so common in India), where 

 the ground was sandy at 10 feet from the surface, a thin layer of 

 clay 14 inches thick was encountered. This clay, and a depth of 

 3 feet of sand below it, was easily penetrated. It then rained, an. 1 

 the cylinder stuck fast, and only went down after a great deal of 

 loading. Again it rained, and again the cylinder stuck. It WMS 

 found that 3 feet of sand had been cleared away from under the 

 curb and that the whole weight of the cylinder 250 tons \\ ;H 



1 M.P.I.C.E., vol. li. p. 290. 2 Ibid. Ibid., vol. xxii. p. MG. 



4 Ibid., vol. xxxiv. pp. 41,42. 



