A FIELD TESTING TO ATTEST THE STATIC FRICTION CHARACTERISTIC OF NEGATIVE SKIN FRICTION ALONG PILES

摘要

One primary aim of the field testing in this paper is to observe the basic characteristics of negative skin friction (NSF) along pile in seismic subsidence caused by a short delay blasting, which simulates an earthquake event. Several results of this in-situ test, furthermore, indicate that the characteristic of NSF reported here differs from those before in-situ features of NSF in collapsing loess ground.The first result is that the maximum NSF along pile hits 86kPa and this value is much more than before field NSF data related to the loess settlement by soaking. According to a statistical data of NSF by the effective method, this great difference cannot be explained successfully by the varied strength of soil mass and pore water pressure. The second result reveals that during the NSF's increasing process, the position of natural point is almost at the same depth, approximately a 13m depth of pile body or a corresponding depth-buried of 15m, according with the bottom depth of seismic loess layer in the testing field. A fixed phenomenon of natural point is great contrast to the situation in collapsing loess ground evidently, in which the natural point changes its depth along pile with the development of NSF. The third result describes that NSF increases with the depth along pile gradually and there is an extreme value, likely being the maximum value also, of NSF on pile near/at the neutral point. Because position of natural point is relative stable on pile in the field testing,the distribution of NSF along pile merely bears on the weight of loess mass, which increases with the depth. Based on the three results of the field test and some statistical data in this text, the authors believe that this field testing observes a static friction characteristic of NSF, whereas the previous insitu tests are performance of the NSF's dynamical friction characteristic.