FDEM-CFD coupling analysis of spatiotemporal evolution of mining-induced overburden fracture-water inflow in shallow and thick coal seam under river

The distribution of overlying rock fractures and water inflow during mining in shallow and thick coal seams under the river is one of the decisive factors for the safe production of working faces. Numerical simulation is an important prediction method for both, and the key to its rationality lies in the establishment of a rock mass failure fracture fluid coupling theory and corresponding simulation methods. Taking the 15404 working face of Lujiacun Mine as the research background, the normal and tangential constitutive relationships of non through crack cracking and through crack under tensile/shear stress are constructed. Based on the conservation of mass, momentum, and state equations of two-phase flow, the enhanced immersion boundary algorithm is combined to identify the fluid solid interface. The fluid volume method is used to track and reconstruct the fluid free surface inside the crack. On this basis, a coupling program of FDEM-CFD numerical model for predicting overlying rock fractures and water inflow in coal mining under the river is formed. The development height of the water conducting fracture zone is verified through the observation of the consumption of flushing fluid in adjacent working faces, and the results of water inflow are compared using the large well method theory. The results indicate that the coupling theory of mining rock mass failure and fracture fluid, as well as the corresponding FDEM-CFD program, can numerically achieve the formation of overlying rock fractures in shallow and thick coal seams under rivers during mining, as well as the fluid transport process within the fractures. When the working face advances to 80-120 m, a water conducting crack that runs through the surface is formed within the overlying rock. The main discharge path of the Zhaoshan River is the water diversion fissure located about 8-20 m behind the working face, which is inclined towards the goaf and has an inclination angle of about 65°-72°. The simulated water inflow in the goaf is 18.78 m³/h, which is close to the calculation results of the large well method. The above achievements have been preliminarily applied in the Lujiacun mining area, providing theoretical support for further carrying out water prevention and control projects in shallow and thick coal seams under the river.