Retaining a waterproof coal pillar is an important measure to defend against water inrush accidents in mining areas and guarantee the safe mining of the next working face. In this paper, the mechanical model of the coal pillar is established and the calculation formula of the waterproof coal pillar width is derived. Then, the development of the water-conducting fracture zone of the overlying rock layer under different coal pillar widths is analyzed using numerical simulation and finally, the integrity of the coal pillar is detected using the geophysical survey method. The main conclusions are as follows: (1) According to the mechanical failure characteristics of the coal pillar, it can be divided into the plastic zone, elastic zone, and water pressure damage zone. The mechanical calculation model for each zone was established, and the formula for calculating the width of the waterproof coal pillar was obtained. (2) Numerical simulation was employed to investigate the development condition of the water conducting fracture zone in the overlying rock strata under the actual width of the waterproof coal pillar; the simulation results indicated that the water conducting fracture zone of two working faces was not connected, which can effectively prevent the accumulation of water in the 2303 goaf. (3) On-site geophysical surveys determined that the influence of water-logged goaf on the coal pillar is between 5 to 15 m; the integrity of the waterproof coal pillar is good, which effectively prevents water accumulation in the previous working face goaf and ensures safe mining in the next working face.

After coal mine wastewater is artificially injected into a groundwater reservoir transformed from a goaf, micron-sized suspended matter in the wastewater is purified by the broken rock mass in the goaf. Existing studies can only analyze the macroscopic changes in the content of suspended solids during the purification process, and it is difficult to explain the microscopic deposition mechanism of the suspended solids in broken rock. This paper studied the microscopic deposition behavior of micron-sized suspended solids inside the broken rock mass via numerical simulation using a coupled CFD-DEM method. In addition, indoor model tests were carried out to verify the accuracy and reliability of the model in comparison. The study results show that suspended solids’ deposition behavior varies significantly under broken rock masses’ different pore sizes (0.47 mm, 1.14 mm, 3.00 mm, and 5.33 mm). Within the goaf, the adsorption of suspended solids by the broken rock mass plays a dominant role. At the same time, suspended particles are mostly collected in the inlet area, and the difference in the number of deposited particles can reach 74% when comparing the first 50 mm range as well as the 50–100 mm range. The number of deposited particles at a flow rate of 0.02 m/s is 14% more than that at a flow rate of 0.06 m/s. This work offers new ideas for studying the purification mechanism of coal mine wastewater within a goaf.

Pipeline conveying is a crucial method for realizing gangue slurry filling. In order to avoid the blockage of gangue slurry in pipeline conveying, it is necessary to clarify the deposition behavior of gangue particles in the pipeline. This paper analyzes coal gangue’s microstructure and mineral composition in the Zhaozhuang No. 2 coal mine through electron microscope scanning and X-ray diffraction tests. We studied the viscosity characteristics of gangue slurry at different mass concentrations and particle sizes and analyzed the change rule of viscosity of gangue slurry with time. Based on determining the nature of the slurry material, a simulation analysis of the deposition behavior of the gangue slurry in a pipeline was carried out using the coupled fluid dynamics–discrete element (CFD-DEM) method. The results show that gangue slurry with a particle size larger than 1.0 mm is likely to lead to the blockage of the pipeline. A small increase in viscosity will promote a uniform distribution of particles inside the pipeline. The deposition behavior of particles is jointly influenced by gravity, fluid interaction force, inter-particle force, and the interaction between particles and the pipeline surface. The research results can be used as a reference for the design and study of gangue slurry grouting systems.