The aim of this study is to investigate the coupling effects of re-excavation and hydraulic uplift on base instability of pit-in-pit (PIP) braced excavations. The numerical model of PIP braced excavation in Shanghai soft clay overlying a confined aquifer was established by upper-bound finite element limit analysis (UBFELA) method. The effects of the sensitive design parameters (i.e., the artesian pressure, thickness and undrained shear strength of the aquitard and excavation width of inner pit) on failure mechanisms and upper-bound safety factor (FS) against hydraulic uplift were analyzed. The results show that the value of FS increases with an increase in the thickness and undrained shear strength of the aquitard, but decreases with increasing the artesian pressure and excavation width of inner pit. The failure modes can be typically classified into three categories: circular slip surface in outer pit (M1), hydraulic uplift combined with circular slip surface in entire PIP system (M2), and basal hydraulic uplift in inner pit (M3); then the corresponding critical artesian pressure is determined. Finally, the average value of critical artesian pressure used to distinguish the three types of failure modes is recommended as the design value against hydraulic uplift in the PIP system, and validity is verified by the comparison with the current design methods. The proposed stability design by UBFELA contributes to ensure the serviceability and performance of PIP system.

这项研究的目的是研究再开挖和水力提升对基坑（PIP）支撑基坑失稳的耦合作用。利用上限有限元分析（UBFELA）方法建立了上海软黏土在承压含水层上的PIP支撑开挖数值模型。分析了敏感设计参数（即自流压力，阿奎塔尔的厚度和不排水抗剪强度以及内坑的开挖宽度）对破坏机理和抵抗水力抬升的上限安全系数（FS）的影响。结果表明，FS的价值随着厚度的增加和不透水的抗剪强度的增加而增加，但随着自流压力和内部坑的开挖宽度的增加而减小。失效模式通常可分为三类：外坑（M1）的圆形滑动面，整个PIP系统中的液压隆起与圆形滑动面组合（M2）和内坑的基础水力隆起（M3）。然后确定相应的临界自流压力。最后，推荐用于区分三种失效模式的临界自流压力平均值作为PIP系统中抗水力提升的设计值，并通过与当前设计方法的比较来验证其有效性。UBFELA提出的稳定性设计有助于确保PIP系统的可维护性和性能。