High concentration and large flow flux of gas drainage from underground coal seams is the precondition of reducing emission and large-scale use of gas. However, the layered occurrence of coal seams with tectonically deformed sub-layers and intact sub-layers makes it difficult to effectively drain gas through commonly designed boreholes. In this study, the gas drainage performance in coal seams with different combinations of tectonically deformed sub-layers and intact sub-layers was numerically analyzed. The analysis results show that the gas drainage curve changes from a singlestage line to a dual-stage curve as the permeability ratios of Zone II (k II) and Zone I (k I) increase, raising the difficulty in gas drainage. Furthermore, a dual-system pressure decay model based on the first-order kinetic model was developed to describe the dual-stage characteristics of pressure decay curves with different permeability ratios. In the end, the simulation results were verified with reference to in-situ drainage data from literature. The research results are helpful for mines, especially those with layered coal seams comprising tectonically deformed sub-layers and intact sub-layers, to choose appropriate gas drainage methods and develop the original drainage designs for achieving better gas drainage performance.

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Predrainage of gas by cross‐boreholes is an important method to prevent coal and gas outbursts in the process of a coal roadway excavation. Limitations of technologies and poor effects of preventing coal and gas outbursts occur in the coal seam with a soft layer. In this study, a new hydraulic caving technology for cross‐boreholes has been proposed to solve this problem. A CT equipment was used to observe the development of coal fissures around cross‐boreholes in the coal seam with a soft layer. The plastic zone range and permeability distribution of ordinary hydraulic slotting technologies and the new hydraulic caving technology also were compared by numerical simulation. The field test of hydraulic caving was taken in the Xuehu Mine (China). The results show that the fracture development degree of soft coal is greater than that of hard coal at the same distance from cross‐boreholes. The fracture development degree in the plastic zone is greater than that in the elastic zone. The hydraulic caving technique can achieve the same effect as the ordinary hydraulic slotting with less workload. The mass of coal discharged from a cross‐borehole was 1.15 t/m and the average methane concentration of single drainage borehole increased by 1.09‐4.1 times, and the average methane extraction amount of single drainage borehole increased by 6.06 times. Elimination of coal and gas outburst had been completed after 110 days of extraction in the area of hydraulic caving, and Q and S values are lower than the critical value during roadway excavation. It is conclusion that hydraulic caving technique can effectively improve the permeability and rapidly eliminate coal and gas outburst in the coal seam with a soft layer.

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