Investigation of permeability evolution in the lower slice during thick seam slicing mining and gas drainage: A case study from the Dahuangshan coalmine in China

During thick seam slicing, the permeability distribution of the lower slice has important significance for guiding the gas drainage. To investigate the characteristics of the permeability evolution in the lower slice during the upper slice mining, a stress-induced permeability evolution model was proposed, and then, based on the top slice mining of the Zhongdacao coal seam in the Dahuangshan coal mine, a series of numerical simulation was performed using the Universal Distinct Element Code (UDEC) software program. The results show that during top slicing, a compacted zone appears in the collapsed overlying strata, which causes more intense compaction in the lower slice. Permeability distribution of the lower slice develops regularly, and after a certain distance of top slicing, the regional distribution characteristic of permeability becomes significant. The lower slice can be divided horizontally into six zones: the original permeability zone with low permeability, the stress concentration zone with permeability decreasing, the initial pressure relief zone with permeability increasing, the full pressure relief zone with high permeability, the pressure recovery zone with permeability decreasing, and the compacted zone with low permeability. The length of each zone is not uniform in the depth direction. Furthermore, the lower slice can be divided vertically into a plastic zone with high permeability and an elastic zone with low permeability. The results are in good agreement with field investigations. Meanwhile, lower slice gas drainage was carried out in the Dahuangshan coal mine, which shows that gas drainage in the full pressure relief zone with high permeability is crucial to gas control during top slicing.