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Abstract:The key to the safe mining of thick coal seams under rivers is understanding the development patterns of fractured water-conducting zones (FWCZ) when various mining methods are used. To solve this problem, we employed numerical simulation to investigate FWCZ when slice mining and longwall caving mining are performed. When slice mining was carried out, the maximum height of the FWCZ in the higher slice was 88 m. When the lower slice was mined, the maximum height became 95 m, and the ratio of the fracture height to the coal seam thickness was 15.8. For longwall caving mining, the height of the FWCZ reached 126 m, which was 31 m more than that of slice mining, and the ratio of the fracture height to the coal seam thickness was 21. Through field measurements, the height of the FWCZ during longwall caving mining of thick coal seams was verified. The measured height was 108.87-112.57 m, and the measured ratio was 19. 08-19.28. Under the same stratigraphic conditions, changes in the bulking factor and structural stability of key strata were the dominant factors that determined how mining methods affected the height of the FWCZ. These development patterns can provide significant theoretical insights for effectively preventing water hazards on mine roofs.

The geomechanical and seepage evolution characteristics of coal masses during mining are the key factors that affect the drainage of coalbed methane and the safety of coal mining. Nevertheless, the influence of mining paths on coal seam permeability is rarely investigated given the complexity of mining-induced stress experiments. To study the effect of mining-induced stress on coal mining, the mechanical properties, acoustic emission characteristics and energy evolution of coal masses were experimentally evaluated through mining-induced stress experiments. Experimental results indicated that at peak intensity, the deviatoric stress and axial strain of coal samples under the stress path of protective coal-seam mining are lower than those of coal samples under the non-pillar stress path. The unloading ratio of confining pressure is large under a stress path of non-pillar mining, and the elastic energy, the absorbed energy, and the dissipated energy of coal mass are low during destruction. The effect of high confining pressure on AE events is pronounced under the non-pillar mining path. The overall b value under high confining pressure is smaller than that under low confining pressure, and AE events generally have high energy. The fracture structure of coal mass is complex, and the fractal size of coal is large under high unloading rates of confining pressure, which induce the increase of permeability after coal destruction.

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