Numerical simulation of critical flow velocity in the covered karst soil cave formation

The covered karst collapse has the characteristics of concealment, suddenness, uncertainty, and mechanism complexity, which leads to its prevention and treatment more difficult. In the process of covered karst collapse, soil caves formed in the overlying soil, develop and expand until the surface collapse. Numerical simulation such as the finite element method is an effective method of soil cave collapse; however, the finite element method can not solve the problem of large deformation of rock mass such as particle migration and soil collapse. Based on the analysis of the geological conditions of karst collapse in Jiangshan city, Zhejiang province, this study used the particle flow code (PFC) to simulate the formation process of covered karst soil cave and then to reveal the displacement of special soil particles, the development trend of cracks, and the change of system imbalance force on a micro level. The multi-field variation characteristics of the formation of covered karst soil cave under external force conditions were reproduced. The results show that the development process of karst soil cave could be summarized into three stages including crack generation, connected surface formation, and particle peeling. As the karst soil cave began to develop, there was a good consistency among the displacement curve of soil particles, the penetration degree of cracks, and the unbalance force, and the vertical expansion speed was larger than the horizontal speed. The critical flow velocity of the initial soil cavity is 0.104 cm/s when the overburden soil is clay. This study can provide basic information for the treatment of karst collapse.