INCREASE OF RESISTANCE. 391 



piles at once, fastened together by strong iron hoops, the gain 

 being due, it is supposed, to increased rigidity; the vibration, 

 which is a great source of loss of power, being thereby much 

 diminished. 1 



In putting down sheet-piling where the lengths are not too 

 great, and the conditions generally are favourable, a good plan 

 is to drive the piles in one bay together ; that is, to pitch all the 

 piles in the bay, and drive them down a foot or two at a time, one 

 after the other, until the whole are driven down to the proper 

 depth ; this method ensures the tongues being properly engaged 

 in the grooves. 



Increase of Resistance due to Quiescence. 2 During the pro- 

 gress of the extensive pile foundations carried out at the Ports- 

 mouth Docks, it was observed that, on the resumption of driving, 

 after an interval of some hours, the set of the pile was invariably 

 less than that observed on ceasing driving. 



In order to obtain information as to the value of such 

 increased resistance accruing from quiescence, a number of special 

 observations were made upon piles, the driving of which was 

 completed or nearly completed immediately before leaving work 

 for the night ; as many as possible of piles which gave an 

 excessive first set being included. On the following morning, 

 one test blow of the ram with the same fall was given, and the 

 resulting set compared with that recorded on the previous 

 evening. 



It was shown by these observations on thirty-nine beech 

 piles, averaging 13 076 feet- in length, 123'S63 square inches in 

 area, and an average of 0'0540-foot set, that they gave, when 

 tested, the following morning, an average set of 0'0234 foot, 

 showing an increased resistance in the ratio of 2'30 to 1. 



Seventy-four fir piles, averaging 19 '74 feet long, 162-70 

 square inches in sectional area, with an average set of 0'0366 

 foot, gave, when tested the following morning, an average set 

 of 0*0130 foot, showing an increased resistance in the ratio of 

 2-81 to 1. 



This result is to be accounted for by the fact that during 

 the driving the ground is to some extent disintegrated; the 

 vibration of the pile also causes the hole from the surface 

 downwards to be slightly enlarged, thus relieving the pile from 

 the full frictional resistance. On the cessation of driving, the 

 1 H.P.I.C.E., vol. xli. p. 260. 2 Ibid., vol. Ixiv. 164. 



