Mechanism and countermeasures of water and sand gushing in shield tunnel based on CEL method integrating seepage analysis

Water and sand gushing accidents of shield tunnel in silty (fine) sand aquifer frequently occur, leading to severe consequences such as tunnel damage, road collapse, and building destruction, threatening people's lives and property. Since it is difficult to record the accident evolution process on site, it is necessary to reproduce it through full-scale engineering simulations. This is to reveal the mechanism of the accidents, and further guide the overall safety performance design of the tunnel and emergency rescue. However, traditional numerical methods face technical bottlenecks when simulating water and sand gushing in shield tunnels due to challenges such as large deformation analysis, fluid-structure coupling, three-dimensional modeling at full-scale, and refined simulation of tunnel structural deformation and damage behaviors, making it difficult to simulate the disaster evolution process. To address this issue, a Coupled Eulerian-Lagrangian (CEL) method incorporating seepage analysis, referred to as the S-CEL method was proposed to simulate the interaction between water, soil, and shield tunnel during the accident. First, three experiments were used to validate the proposed method. Subsequently, a refined three-dimensional numerical model was developed, utilizing the S-CEL method to simulate the water and sand gushing process in an accident. The method was further validated by the measured data. The sequence of leakage point evolution at segment joints, as well as the soil loss mechanism and tunnel structural response mechanism associated with the evolution of leakage joints was revealed. The study showed that leakage joints progressively evolved along the longitudinal direction of the tunnel. As the number of leakage joints increased, the amount of soil loss rapidly escalated, resulting in the formation of erosion void at the interface between clay and sand layers. This led to severe uneven settlement and dislocation deformation of the tunnel, causing damage to the segment concrete. Finally, according to the numerical simulation results, the suggestions of disaster rescue measures were put forward.