黄土湿陷对基桩竖向承载性状影响的试验研究

Test study on the influence of loess collapsibility on vertical bearing capacity characteristic of foundation pile

为探究黄土湿陷对基桩承载力及沉降的影响及"先湿法"、"后湿法"两种工况下基桩负摩阻力特性的差异,在湿陷性黄土场地设置3根桩,分别对应"天然状态"、"先湿法"和"后湿法"三种工况进行现场静载荷试验。研究表明:桩周土湿陷对桩产生的下拉荷载,不仅使基桩承载力降低,引起的桩顶沉降也不可忽视,"后湿法"的湿陷沉降更为显著;从桩顶沉降趋势可以看出,"先湿法"得到的基桩承载力大于"后湿法";由于负摩阻力的产生需抵消桩身在前期加载过程中产生的正摩阻力,故"后湿法"的中性点位置浅,下拉荷载和平均负摩阻力较小;"后湿法"更符合桩基正常使用实际情况,以"先湿法"得到的平均负摩阻力及中性点深度指标作为设计参考值较为保守。

In order to explore the influence of collapsibility on the bearing capacity and settlement of foundation pile and the difference of negative skin friction properties of foundation piles under the two working conditions of “ wet first method ” and “wet after method”, 3 piles were set up in the collapsible loess site to carry out static load test under the three working conditions of “natural state”, “wet first method” and “wet after method” respectively. The results show that downdrag caused by soil collapsibility around the pile not only reduces the bearing capacity of the foundation pile, but also causes the settlement of the pile top, and the subsidence of “wet after method” is more significant. From the settlement trend of pile top, it can be seen that the bearing capacity of foundation pile obtained by “wet first method” is greater than that obtained by “wet after method”. Due to the generation of negative skin friction need to offset the positive skin friction produced by the pile in the early loading process, the neutral point of the “wet after method” is shallow, and the downdrag load and average negative friction are small. The “wet after method” is more in according with the actual situation of normal use of pile foundation, and the average negative skin friction and neutral point depth indexes obtained by the “ wet first method” are more conservative as the design reference values.