Seepage-induced sand-leakage karst collapse mechanism considering the soil arching effect

Sand-leakage karst collapse often occurs in the hidden karst area of the binary structure covering layer due to seepage. However, further research is needed to understand the macro and micro mechanical mechanisms of such collapse and the mechanical evaluation model for sand-leakage initiation. This study used the Lujia Street karst collapse, induced by the lowering of the karst water level in the binary structure covering layer karst area of Wuhan City, as a case study. The macro and micro mechanical mechanisms of collapse failure were investigated through physical model tests and PFC (particle flow code)-CFD (computational fluid dynamics) coupled numerical experiments. A soil arch limit equilibrium mechanical evaluation model was developed to assess sand leakage initiation under seepage conditions. The results show that the critical water level differences for the of Lujia Street karst collapse revealed by physical model experiments and numerical model experiments are 7.2 m and 8.0 m, respectively, with an error of approximately 10%. After sand leakage begins , the pore water pressure at the bottom of the sand layer drops sharply, leading to an increase in surface subsidence. Before reaching the critical water level difference, the velocity of the sand layer at the karst opening increases while the pore water pressure decreases. The horizontal stress, vertical stress, and lateral pressure coefficient of the soil arch increase, and the rate of hydraulic gradient increase is significantly greater than the lateral pressure coefficient. According to the constructed limit equilibrium model of soil arch under seepage, it is found that as the water level difference increases, although the collapse force and anti-collapse force in the soil arch increase, the increase in collapse force is more pronounced. This study is of great significance for improving the level of disaster prevention and monitoring of sand-leakage karst collapse.