中文摘要：

既有建筑地下增层开挖，会引起原桩-土体系加载刚度改变，从而影响桩基在增层改造后的承载性能。基于单桩模型，应用 Mindlin 附加应力解考虑开挖引起的竖向有效应力变化，分别计算开挖前后桩侧和桩端极限阻力。在建立桩侧和桩端双曲线型荷载传递函数基础上，针对某工程案例，分析开挖前后桩侧和桩端刚度以及桩顶荷载-位移曲线变化情况、讨论承载力降低系数随沉降控制值的变化规律以及初始刚度降低系数和极限阻力降低系数随开挖深度和卸荷比的变化规律。结果显示，随增层开挖深度增加，初始刚度和极限阻力降低系数均减小，土层离开挖面越近，其值越小；随卸荷比增加，初始刚度和极限阻力降低系数均减小，土层强度越低，其值越小；开挖深度一定时，桩侧初始刚度和极限阻力降低系数小于桩端，且初始刚度降低系数小于极限阻力降低系数；承载力降低系数随沉降控制值增大而增大，并趋于稳定。

英文摘要：

Further excavation beneath the basement of an existing building is likely to change the loading stiffness of the virgin pile-soil system,thus affecting the post-excavation pile behavior. In this study,Mindlin′s solution for additional stress allowing for the variation of vertical effective stresses induced by excavation around a single pile was used. The ultimate shaft and base resistances of a pile before and after excavation were calculated according to the stress solutions. A case study was carried out on the basis of the load transfer functions of hyperbola-type along the pile shaft and below the pile base. The study involved the variation of the system stiffness and the load-displacement curves for pile-head due to excavation. Discussion was focused on the variation of reduction factors of bearing capacity with the control values of settlement,and the variation of reduction factors of the initial rigidity and the ultimate resistance with excavated depth and unloading ratio. Results show that the initial rigidity and the ultimate resistance decreased with the increasing of excavated depth,varies inversely with the unloading ratio and are smaller for weaker soils. For a given excavation depth,the decreases of the initial rigidity and the ultimate resistance are more prominent along the pile shaft than beneath the pile base,particularly for the initial rigidity. The reduction factors for bearing capacity increase with the increasing of control values of settlement until a constant value is reached.