A Field Testing Research on the Characteristics of Negative Skin Friction along Pile Caused by Seismic Subsidence of Loess

Abstract eng:
Now pile foundation is widely used in the earthquake-prone loess area of China. However, negative skin friction (NSF) along piles induced by a sudden settlement of loess during strong earthquakes, seismic subsidence, has not been taken into account in design of pile foundation due to the lack of NSF data related to field testing. In order to research NSF along piles in loess ground during seismic subsidence and develop a method to estimate this seismic NSF, the authors perform a field test at a loess site (Q3) by means of a series of explosion (a short delay blasting), in which two piles of 20m long are grouted. The expected ground motion created by the explosions is strong enough to induce an obvious seismic subsidence in the field. There are 40 stress gages to be disposed into the two piles in average with a certain interval, 2m, to collect the data of NSF with depth during the field testing. The whole field observation continues 11 days. In this testing, the maximum NSF reaches 86kPa at the buried depth of 13m, with an average NSF of 54kPa on pile, which is much greater than before in-situ data of NSF related to loess settlement by soaking. It could be obviously revealed that, furthermore, NSF increases with the depth along pile gradually and there is an extreme value of NSF along pile, likely be the maximum value also, near/at the neutral point, where NSF is equal to zero due to absence of relative displacement between pile and soil mass; during the explosions and immediately after it, NSF increases rapidly with loess seismic subsidence, whereas in the subsequent stages, the development of NSF becomes slow and slow.

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