桩基负摩阻力时间效应试验研究

由于黏土固结缓慢，桩基负摩阻力存在明显的时间效应，然而目前相关研究仍显不足。设计实施了能实现桩顶加载及较大超载值的单桩及双桩负摩阻力模型试验，桩周土采用砂土和软黏土夹层，测定了模型桩身应力、桩顶位移以及土体分层沉降随固结时间的变化。试验结果显示，沉降、负摩阻力具有明显的时间效应。土表超载作用下土体沉降带动桩沉降，桩与土体的沉降均表现出早期快、后期慢的趋势。试验加载初期，黏土夹层处的负摩阻力略小于砂层，但随土体固结而增长，其基本变化规律与沉降相同。因桩端砂土层沉降稳定迅速，中性点随桩身沉降增长略呈上移趋势。此外，相同荷载作用下桩间距较小的双桩，由于下拉力较小，其沉降较小。试验条件下，3D桩间距的负摩阻力群桩效应系数在0.71～0.77之间，6D桩间距时不存在负摩阻力群桩效应。

Experimental study of time effect of negative skin friction on pile

Due to the low rate of consolidation of clay, the time effect of negative skin friction on pile is obvious. However, the correlative research work is not deep enough and needs to be continued. The model tests of negative skin friction on pile and double pile are designed and conducted, which can apply pile load and large surcharge load on soil surface. And the soil in the test adoptes sand and clay interlayer. During the consolidation of clay, pile stress, pile settlement and settlement at each soil layer are measured. The results indicate that settlement and negative skin friction have significant time effect. The pile settlement increases with soil settlement under the surcharge load, which increases rapidly in early stage and slowly in later stage. The negative skin friction in clay is smaller than that in sand on the initial stage of loading, and the change rule is similar to the settlement. Since the settlement stability time of sand layer surrounding pile base is short, the neutral point rises slightly with the increase of pile settlement. Furthermore, the settlement of the double pile with small pile spacing is small under the action of the same load because the dragload is relatively small. The group effect coefficient of the double pile under negative skin friction is from 0.71 to 0.77 at 3D pile spacing and there is no group pile effect when the pile spacing reaches 6D under the present model test conditions.