Failure Mechanisms and Optimization Strategies for Multi-level Pit-in-Pit Excavation in Soft Ground

Although extensive research has been conducted on the performance and stability analysis of pit-in-pit (PIP) excavations, studies focusing on multi-level pit-in-pit (MPIP) excavations remain scarce. This study investigated the failure mechanisms of asymmetric PIP and MPIP excavations using the finite element method-based shear strength reduction technique (FEM-SSRT) implemented in the ABAQUS platform. The study also evaluated the effectiveness of uplift pile system for reinforcing base soils. A parametric analysis was conducted to examine the impact of pile spacing (S), pile diameter (D), wall embedded ratio (η), and pile arrangement on excavation stability. Furthermore, a novel modified overturning stability index (MOSI) calculation method was developed as an evaluation metric for excavation stability, particularly for cases involving rotational wall movements above the excavation base (RM mode). The approach was evaluated against the data from a case study of MPIP excavations in Shenzhen, China. The findings provide insights into the failure modes and identification methods of wall movement for MPIP excavations, reveal the mechanisms by which uplift pile system enhance the stability, and offer practical recommendations for optimizing the pile arrangements. These contributions establish a theoretical foundation and deliver technical guidance for the design and implementation of MPIP excavations.