Field tests on negative skin friction along piles caused by seismic settlement of loess

Pile foundation has been widely used in the earthquake-prone loess area of China.However,the negative skin friction(NSF) along piles caused by a sudden settlement of loess during strong earthquakes have not been taken account into the design of pile foundation due to the lack of NSF data from field tests.In order to investigate NSF along piles in loess during the seismic settlement and to develop a method to estimate it,field tests were performed by means of a series of explosions(short delay blasting) at a loess site where two piles were grouted.The ground motion created by the explosions was strong enough to induce an obvious settlement in the site.There were 40 stress gages amounted on the two piles with an interval of 2 m to collect the data of NSF along depth during the tests.At the same time,the ground motion and settlement of loess site created by the explosions were respectively observed by accelerographes and a level gauge.During field tests,the maximum seismic settlement of loess field reached 33 mm while the average NSF along piles attained 54 kPa and the corresponding total NSF was about 1654 kN.It was shown by these test data that the NSF along piles induced by seismic settlement of loess was significant and should not be ignored in the design of pile foundation.