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Protective layer mining, as a dominating method for preventing coal and gas outburst, is generally adopted in highly gassy coal mines. In the absence of a suitable thickness coal seam to serve as the protective layer, the rock-coal composite protective layer was proposed in this paper. We conducted a series of simulations and engineering measurements to investigate the protective effect under the mining of the rock-coal composite protective layer of the Zhongtai coal mine located in the Hebi area of Henan, China. The numerical simulation analysis showed that, after the completion of protective layer mining, the minimum vertical stress of the No. 2-1 coal seam had been reduced to 3.46 MPa. The maximum vertical displacement of the No. 2-1 coal seam is 455.01 mm. The maximum expansion deformation of the No. 2-1 coal seam is 9.77‰; the effective pressure relief range is as long as 160 m. The similarity simulation experiment revealed that, after the completion of protective layer mining, the minimum vertical stress of the No. 2-1 coal seam is 4.0 MPa. The maximum vertical displacement of the No. 2-1 coal seam is 640 mm. The maximum expansion deformation of the No. 2-1 coal seam is 26.37‰; the effective protection range reaches 130 m. The engineering measurements demonstrated that the variation law of gas drainage parameters in the protected layer corresponds to the protected layer's vertical stress distribution law in numerical simulation and similarity simulation. With the exploitation of the composite protective layer, the protective layer’s pressure begins to release. The average gas drainage concentration is 2-3 times of that before the composite protective layer mining.

Hydraulic punching technology has recently developed into an effective pressure relief measure and permeability enhancement method for soft and low permeability coalbeds. Different coal outputs directly affect the shape and size of boreholes as well as the effective extraction radius. Taking the Zhongmacun mine as an example, the influence of different coal outputs and different extraction periods on effective extraction radius was analyzed and studied through field tests and numerical simulation. The results show that the increase in coal outputs from hydraulic punching can improve the effective extraction radius of the boreholes. For example, when the gas extraction reaches 90 days, with a coal output of 0.5 t/m, 1.0 t/m, and 1.5 t/m, the effective extraction radius is 3.08 m, 3.46 m, and 3.83 m, respectively. The difference in gas extraction effect of different coal output boreholes increases significantly with the extension of the extraction time, but the speed of growth gradually decreases, which is consistent with the conclusions obtained on-site. This result has important practical significance for optimizing the technical parameters of hydraulic punching, guiding the accurate layout of extraction and drilling, and enhancing the effect of gas control in mines.

Permeability is an essential indicator for predicting gas drainage yield and preventing mine gas disasters, which is significantly influenced by the stress paths and the integrity of coal. Conventional research on permeability mainly focused on the permeability evolution of initial undamaged or fractured (prefabricated fractures) coal under various stress paths; little attention has been paid to post-strength coal (stress-induced damage), especially for soft coal. To determine the permeability evolution and gas flow law of post-strength soft coal samples under various stress paths, we used the experimental method combined with the numerical method in this study. The results showed that when the confining pressure and axial pressure of post-strength soft coal samples were unloaded, the permeability increased by 1.25–1.32 times; when the coal samples were loaded into the secondary damage, the permeability first decreased and then increased. The simulation part in this study found that the development of the fracture of coal samples under triaxial compression was divided into four stages. Gas flow law of post-strength soft coal was significantly influenced by fracture locations, and the gas pressure and gas flow field near the fracture were disturbed.

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