动力打桩过程中饱和黏土地基的响应

Response of saturated clay ground during pile driving

用有限元法首次模拟了将桩锤击打入饱和黏土地基的全过程,得到地基在打桩过程中的反应规律。根据计算结果,确定打桩对地基的影响范围,并阐明地基中一点的超孔隙水压力随打桩过程的变化特性。对一定的深度,打桩引起的超孔隙水压力在径向关于桩径有很好的无量纲化特性,用桩径无量纲化后,不同桩径下计算的最大超孔隙水压力径向分布基本重合在一起。在半对数坐标中,最大超孔隙水压力与离桩心的距离近似为直线关系。有限元计算的超孔隙水压力也分别和圆柱孔扩张理论计算结果与现场实际测量结果进行了对比,有限元计算结果和实际测量结果一致,而圆柱孔扩张理论得出的超孔隙水压力分布与实际测量结果有较大差别,其主要原因是有限元分析完整地模拟了打桩过程中桩对土的动态横向挤压和竖向剪切作用,而圆柱孔扩张理论则忽略了打桩过程中桩周土所受的较大竖向剪力。

In this paper,a finite element model was developed for the first time to simulate the driving process and response of a pile into a saturated clay soil from the ground surface to an embedment depth. Based on the calculated results,the influence zone of pile driving was determined and the response of the excess pore water pressure of the soil to pile driving was presented. For a given depth,the buildup of the excess pore water pressure has good dimensionless characteristics with respect to the pile diameter,in other words,the distributions of the maximum excess porewater pressure in the radial direction for different pile diameters basically coincide with each other when the radial distance is normalized by the pile diameter. Moreover,it is found that,in a semi-logarithmic coordinate system, the normalized maximum excess pore water pressure approximately follows a linear relationship with the radial distance. Comparisons among finite element simulation,calculation by the theory of cylindrical cavity expansion and the field measurements indicate that the finite element analysis results are in a good agreement with the published field measurements while there exists a discrepancy between the finite element analysis results(or field measurements) and calculation results by the cylindrical expansion theory,which is resulted from that the shear force around the pile shaft is neglected in the cylindrical expansion theory while the actual piling process is fully simulated in the finite element analysis.