Discrete Element Method–Computational Fluid Dynamics Modeling and Microscopic Failure Mechanism Study of Engineering with Complex Boundary Conditions: A Case Study of a Foundation Pit Project

The coupled fluid–particle modeling and study of microscopic failure mechanisms for engineering projects with complex boundary conditions present significant challenges. A three‐dimensional numerical model for foundation pits considering fluid–particle coupling was successfully established by partitioning the fluid domain for excavation into several subdomains to individually construct computational grids and then assembling them into the actual fluid domain. The macroscopic and microscopic characteristics of seepage failure in excavations under two scenarios with and without displacement of retaining structure were studied, and the critical hydraulic gradient of seepage failure and the earth pressures on the inside and outside of the retaining structure were compared with the theoretical values. The results show that whether or not the retaining structure was allowed to displace, the seepage failure in foundation pits could be divided into four stages: internal erosion stage, upward erosion stage, reverse erosion stage, and shear failure stage. As the hydraulic gradient increased, when the displacement of retaining structure was allowed, the ground surface settlement outside the pit would first occur, followed by the uplift at the pit base, whereas the reverse would occur when the retaining structure was not allowed to displace. This study not only proposes an innovative method for establishing a fluid–particle coupled model for foundation pits but also reveals the microscopic mechanism of seepage failure in foundation pits, providing a reference for the establishment of three‐dimensional fluid–particle models for other projects and the prevention and control of seepage instability in foundation pits.