Discrete Element Modeling of Stratified Excavation and Stability Analysis of Foundation Pits

The stability mechanism of cantilever excavation was investigated through a numerical simulation study using the Discrete Element Method (DEM), based on a specific practical foundation pit engineering project. Initially, the microscopic parameters of the Quaternary loess-like soil were calibrated. Subsequently, a foundation pit model with a height of 15 m, a width of 20 m, and an excavation depth of 5 m was constructed. The responses of the unreinforced and reinforced foundation pits during step-by-step excavation were comparatively analyzed (in 5 stages, 1 m per stage, with 10,000 calculation steps for equilibrium after each stage). The results indicate that the unreinforced foundation pit experienced instability failure after excavation. The failure scope expanded with increasing excavation depth, reaching a failure depth of 0.2 m at the pit top. During the fifth stage of excavation, soil accumulation occurred at the pit base. The failure mechanism was identified as the sliding out of particles at the base, leading to the progressive instability of the upper particles. In contrast, the reinforced foundation pit showed no significant damage. A “wok-bottom” type heave developed at the pit base starting from the third stage of excavation. As the excavation depth increased, the heave area developed further downward, and the surface settlement pattern conformed to the “triangular” distribution observed in the actual engineering project, thus verifying the model’s validity. Furthermore, the horizontal stress behind the retaining wall generally decreased due to excavation unloading. A local stress increase was observed only in the 0–5 m depth during the fifth excavation stage, attributed to retaining wall displacement. This study accurately characterizes the soil deformation and instability mechanism during foundation pit excavation. The simulation results provide direct guidance for the design optimization and construction risk management of the actual engineering project and enrich the practical application of the Discrete Element Method in foundation pit stability analysis.