Diffusion law of gangue slurry during adjacent backfilling in coal mine caving zone: A DEM study

To improve the filling accuracy of gangue slurry in the caving zone of coal mine goafs, enhance the large-scale disposal and utilization efficiency of coal-based solid waste, and realize the green and low-carbon development of coal, this study innovatively adopts the particle flow discrete element method (DEM) for numerical simulation. A heterogeneous and anisotropic void structure model simulating the caving zone is established to systematically simulate and analyze the macro-mesoscopic variation laws of the entire filling and diffusion process of gangue slurry in the caving zone. Combined with fluid mechanics and energy dissipation theory, the diffusion mechanism of the filling slurry in the caving zone is deeply elaborated.The results show that the diffusion of gangue slurry presents a three-dimensional spatiotemporal evolution characteristic of "semi-ellipsoid — semi-cylinder — lower semi-circular frustum — expansion and elevation of semi-circular frustum — concave semi-circular frustum", and the main section successively undergoes three stages: "ellipse — rectangle — concave trapezoid". The filling slurry preferentially migrates along the tortuous and bifurcated dominant seepage channels, accompanied by velocity vector deflection, increased tortuosity, and energy attenuation. The newly accumulated slurry on the surface is dominated by translational diffusion with higher granular temperature, while the slurry accumulated at the bottom has increased energy consumption, weakened movement, and lower granular temperature due to particle collision and matric suction. Additionally, the lateral diffusion radius and frustum height at the bottom increase approximately linearly with time. Within the range of slurry injection and diffusion, the time required for porosity to reach the minimum state in the vertical direction increases linearly with the increase of height, while in the horizontal direction, it shows a power exponential growth characteristic of first rapid increase and then slow increase with the increase of horizontal distance. This study makes up for the deficiency of existing research in the understanding of the mesoscopic mechanism of gangue slurry filling. The research results can provide a reliable scientific basis for the design of filling process parameters and the accurate prediction of grouting effect of gangue slurry in the caving zone, and play an important supporting role in promoting the resource utilization of coal-based solid waste and the green mining of coal.