SURFACE FRICTION ON PILES. 



389 



holding on by the friction of the sides. It was then suggested 

 by a native well-sinker that by cutting away the layer of clay 

 the cylinder would go down at once, as the clay had s woollen 

 during the rain, and gripped the cylinder. The clay was accord- 

 ingly removed, and the cylinder dropped down, which showed 

 that the whole weight was hanging on 14 inches of clay with a 

 temporary friction, owing to the expansion of the clay equal to 

 3 '80 tons per square foot. 1 



Mr. Hurtzeg, M.I.C.E., found, as the result of some three 

 hundred direct observations made when drawing the piles of 

 a dam, that the gross frictional resistance of ordinary rough 

 Memel timber driven into clay amounted to 0*837 ton per square 

 foot of surface, or deducting the weight of the pile and the 

 power required to overcome the resistance of suction under the 

 worst conditions, viz. a perfect vacuum, the coefficient of fric- 

 tion was Q'837 ton per square foot of surface in contact with 

 the soil. 2 



The friction of a pile in sand is estimated by the Dutch 

 engineers at from 3 to 4'5 Ibs. per square inch, or, say, 614 Ibs. 

 per square foot. This co-efficient was adopted by Sir J. Alleyne, 

 Bart., M.I.C.E., in designing the timber staging for fixing the 

 Dordrecht Bridge in Holland. 3 



At the Portsmouth Docks a very marked alteration was 

 observed in the condition of the hard clay, after the piles had 

 been driven into it. The clay, when not affected by the piles, 

 on being excavated freely fell to pieces, with a fracture closely 

 resembling that of coal ; but when altered by the driving of the 

 piles, it was rendered extremely tough and difficult to excavate, 

 having lost all its tendency to break, and only coming away as 

 absolutely cut down by the pick. This toughening of the clay 

 materially increased the friction on the pile surface. 4 



Piles driven in the soft clay at La Rochelle and Rochfort, 

 support 164 Ibs. per square foot of lateral contact, and in the 

 silt of Loicent, 123 Ibs. per foot super. 5 



In localities where the winter weather is severe, and thick 

 ice is likely to form, a timber structure on piles is exposed to 

 an element of destruction that has no parallel in temperate 

 climates, that is, the upward pressure of the ice as the water 

 rises tending to lift the piles, unless they have a sufficient 



1 M.P.I.C.R, vol. xxxiv. p. 34. * Ibid., vol. Ixiv. p. 313. 



8 Ibid., vol. xlii. p 218. * Hid., vol. Ixiv. p. 199. * JWd., vol. 1. p. 122. 



