Analysis of the formation process of the covered karst ground collapse induced by groundwater changes based on the coupled LBM-DEM numerical simulation at micro scale

To investigate the hydraulic characteristics and soil behaviors during the formation of covered karst ground collapse induced by the groundwater changes, a 2D fluid-solid coupling model was developed based on discrete element method and lattice Boltzmann method. This model utilizes the linearly interpolated bounce-back scheme of Bouzidi and the momentum exchange method, allowing for the simulation of the formation of covered karst ground collapse from a microscopic perspective. Using the fluid-solid coupling model, an exploratory study was conducted to simulate the formation of covered karst ground collapse induced by a decrease in the hydraulic head of confined aquifers. Simulation results indicate that when the water level of a confined aquifer declines, the groundwater flow mainly affects the particles located above a cave opening and produces a downward force on the surrounding soil. When soil particles spall, the hydraulic heads at the positions of the spalled soil particles drop sharply. This results in a significant increase in the hydraulic gradient, causing the groundwater force on internal particles to sharply increase as well. As a result, the upper particles lose stability due to the combined force of groundwater dragging and gravitational force, which can lead to a gradual acceleration process of collapse. The research results provide valuable insights into the understanding of covered karst ground collapse formation induced by the groundwater changes.