长短桩组合路堤桩三维复合拱土拱效应分析

基于等长桩路堤桩土拱效应的Hewlett分析方法,提出均布荷载作用下长短桩组合路堤桩的三维复合拱土拱效应分析方法。采用有限元法形象描绘土拱效应形成的应力集中形态随长桩与短桩的桩长比不同时的变化,基于此提出三维复合拱的基本理论框架。引入土拱效应系数来表征土拱的强度状态,建立桩帽顶部平面的应力平衡微分方程,结合边界条件及土拱效应系数之间的关系计算长桩和短桩的桩-土应力比;当加筋体存在时,分区域对加筋体进行变形假设,建立考虑加筋体拉力的桩帽顶部应力平衡微分方程。与离心模型试验和现场实测结果及有限元方法的模拟结果进行比较,验证了此方法的合理性,进一步开展参数分析对主要影响因素进行等级评价。分析表明,加筋体的存在明显增大桩-土应力比,尤其当桩帽覆盖率比较小时效果更为明显。

Numerical investigation on arching effect surrounding deep cylindrical shaft during excavation process

Predicting the inner displacements of deep vertical shafts during the excavation process has been a difficult task considering the geological,structural,and constructional influences.In fact,the two-dimensional(2D)analytical solution based on the retaining wall model remains insufficient for understanding the actual behavior during an excavation.This is because the deformation of vertical shafts becomes complicated due to the unexpected arching effect brought about by the three-dimensional(3D)flexible displacements occurring in the excavation process.Previous analytical solutions only considered the limit equilibrium.Therefore,the present study deals with a 3D soil-structure simulation by considering the displacements of a cylindrical shaft and the mechanical behavior of the surrounding soil as well as the geometry of the cylindrical structure.Moreover,this mechanical behaviors of the surrounding soil and shaft are controlled by the shaft stiffness;hence,the relationships among the shaft stiffness,mechanical behavior of the surrounding soil(in terms of earth pressure coefficient),and shaft displacement were investigated.A cylindrical model,120 m in depth and 20 m in diameter,was positioned at the center of a sand domain,and each excavation step was performed at an interval depth of 20 m.A 3D finite difference method analysis was applied using the modified Cam-Clay(MCC)model to represent the soil behavior.As a result,the present study provides a new normalized lateral earth pressure theory for excavated shafts by considering the 3D arching effect obtained from parametric studies using various levels of shaft stiffness.From a comparison with the analytical solutions of previous studies(Terzaghi,1943a;Prater,1977;Cheng&Hu,2005),it is found that the previous studies underestimated the earth pressure acting on the cylindrical shaft because they did not consider the accurate arching effect.

基于混合试验的桩承式加筋路堤参数影响分析

桩承式加筋路堤的路堤土拱效应、加筋体张拉膜效应以及桩土地基承载效应之间存在着较为复杂的耦合效应,由于土拱效应随路堤加载与桩间土下沉的演化,目前的计算理论与方法难以真实地评价桩承式路堤荷载调节与沉降稳定的全过程。引入混合试验的技术思路,开发一套阵列式多活动门试验装置及桩土地基混合试验方法,实现加筋垫层路堤物理模型与桩土地基数值模型的适时数据交换,开展9组不同路堤高度与加筋体拉伸模量的参数影响试验。试验结果表明:所建立的混合试验方法能够较好地实现桩承式加筋路堤的全组件参与、全效应耦合工作性能模拟,大大地节省了试验成本和时间,为桩承式加筋路堤的长期承载性能演化以及沉降预测提供了一种新的试验手段。同心圆力学模型能够较为准确地评价路堤的最大土拱效应。当路堤高度较低时,采用拉伸模量较高的加筋体能够提高桩的荷载分担比。采用拉伸模量较高的加筋体,在不同的加筋高度条件下均能够显著地降低桩间土沉降与桩土差异沉降。随着路堤高度的增加,不同加筋体模量试验的桩、土荷载分担比趋于一致,采用地基反力系数法不能可靠地反映桩间土的真实承载机制。