To prevent outbursts and extract the gas, it is necessary to investigate the structure of the key layer and the evolution of cracks during coal mining over a large height. A physical similarity model with a geometric similarity ratio of 1:100 and a strike length of 2 m was established on the basis of prevailing geological and mining conditions. Stress sensors and microseismic equipment were adopted to monitor the collapse characteristics of different key layers through variations in stress and energy. The research results showed that: The displacement of the overlying strata exhibited group motion characteristics marked by the key layer and there was zone of rapidly increasing stress in the goaf behind the working face before and after the fracture of the key layer. Through the analysis of microseismic events, fissure density, and fractal dimension, we found that: before and after the primary key layer fractured, the energy loss in the microseismic event accounted for 27% of the total, the fissure density also changed abruptly from 8 cracks/m to 10 cracks/m and the slope of the fractal dimension curves changed from 0.00243 to 9.94801 × 10⁻⁴. Then a gas drainage model suitable for this condition was constructed. When the primary key layer had not fractured, the cracks below fully developed to form a gas migration channel and a gas enrichment area, and gas boreholes could be arranged therein. After the primary key layer had been fractured, the rate of crack development decreased, the permeability would increase, and the gas concentration would increase. The boreholes could still be arranged in the gas migration channel. This study elucidated the control role of the primary key layer from multiple angles and provided experimental guidance for gas drainage over large‐mining heights and during complex key layer mining.

A drainage roadway was constructed in the fractured zone beside an air-return roadway of a mining site. However, drainage roadways serve only one working face and are discarded after mining, thus weakening sustainable mining practices. To improve the sustainability at mining sites, this paper proposes a new drainage roadway that can serve two adjacent working faces. A schematic layout of the proposed lateral high drainage roadway (LHDR) is presented. The LHDR can be used to extract pressurized gas from two neighboring mining zones. A theoretical study, numerical simulation, and field observations were performed to identify the mining-induced fractures and stress distribution. The layout of the LHDR and the optimal position of the extraction boreholes were derived considering the geological conditions in the Liyazhuang Coal Mine in China. A borehole and LHDR were installed at the specified mining site and the rock displacement of the LHDR and gas concentration at branch pipelines, boreholes of the LHDR, and upper part of the working face were monitored during mining. The performance of the LHDR was validated.

In the process of deep mining, the dynamic disasters of coal and rock occur frequently under the action of high stress and high seepage pressure, the essence of which is energy-driven coal rock failure. In order to explore the energy evolution law and damage mechanism of sandstone with intermittent cracks under the coupling effect of stress and seepage, in this paper, by comparing the differences in mechanical characteristics between fractured rock and intact rock, the energy evolution characteristics, crack propagation, and micro-damage mechanism of fractured rock under different confining pressures and seepage pressures are analyzed. The research shows that: (1) The local stress drop phenomenon occurs in the fractured rock during the loading process, and the stress–strain shape is ‘bimodal’. At the same time, there is stress concentration at both ends of the fracture. (2) The energy conversion of the fractured rock changes in stages during loading. As confining pressure rises, the energy storage limit and the maximum dissipation energy go up. The increase in seepage pressure reduces the energy storage limit, while the dissipation energy shows an upward trend. The energy consumption ratio curve shows ‘concave’ evolution during the loading process. (3) Based on the dissipation energy and residual stress, the damage state of the specimen is analyzed, and the proposed damage variable can reasonably explain the whole process of the damage evolution of intermittent fractured rock under stress and seepage. (4) The increase in confining pressure increases the friction between the particles inside the sample and promotes the transformation of the sample from tensile failure to shear failure. The seepage pressure reduces the friction between the particles in the sample through the air wedge effect to deepen the damage degree, thus promoting the tensile failure of the sample.

The process of underground coal mining fractures the overlying strata and may provide storage and transportation space for gas by changing the roof rock permeability, which is released by pressure after mining. This paper adopts the experimental method of physical similarity simulation and combines the fractal theory to study the permeability characteristics of the fracture network after mining, and it establishes the fractal permeability equation of pressure-relief gas. The results of the study show that the fracture opening shows a positive correlation with the overburden permeability, whereas the tortuosity of the fracture shows a negative correlation with the overburden permeability. The shape of the high permeability area in the fracture network is found to be similar to the hat-shaped elliptical parabolic zone. In the process of permeability evolution, the key layer structure of the overburden rock is considered as the main factor that affects the trend of change in permeability. Furthermore, based on the above research results, this study developed a targeted design of high-level boreholes in the experimental face and reversed the permeability changes around the drainage borehole. The average error between the actual measured value and the theoretically calculated value is found to be 8.11%. The theoretical model and the permeability evolution law obtained from the research results can provide valuable references and insights into further research on the pressure-relief gas flow model in the goaf.

The key parameters of high-level boreholes in high drainage roadways affect the gas treatment effect of the working face directly. Therefore, the layout parameters of high-level boreholes in the lateral high drainage roadway (LHDR) are determined and optimized as necessary. Based on the LHDR layout on the 2-603 working face of the Liyazhuang coal mine, the key technological requirements on high-level borehole parameters were analyzed and the distribution characteristics of the gas volume fraction in the coal roof were studied. The gas migration law in the mined-out areas was obtained and the layout locations of high-level boreholes were determined finally. The research demonstrates that the high-level boreholes lag the 2-603 working face distance and the position of the final borehole (the position of the final borehole in this paper refers to the distance between the final borehole and the roof) influence the stability of boreholes and the gas extraction effect. The distribution of the gas volume fraction from the intake airway to the return airway can be divided into the stable stage, slow growth stage, and fast growth stage. Influenced by the flow field in the mined-out areas, the mean volume fraction of the borehole-extracted gas has no obvious relationship with the gas volume fraction at the upper corner. According to the final optimization, the high-level borehole is determined as having a 15 m lag behind the working face and the position of the final borehole is 44 m away from the coal seam roof. These have been applied successfully in engineering practice.

Coal and gas outbursts are complex, dynamic phenomena in coal mining. Outburst two-phase flow (OTPF) poses a serious threat to the safety of underground equipment and operators during an outburst. In this study, by means of physical simulation test, the dynamic disaster characteristics of OTPF in roadway with and without branches were discussed. The experimental results showed that pulverized coal flow was initially shaped like an arc-shaped jet. The average migration velocity of OTPF under non-branched right-angle turning roadway (L-Rd) was larger than under branched right-angle turning roadway (F-Rd). In the straight section, the mass of pulverized coal under L-Rd was generally larger than under F-Rd, whereas in the turning section, the opposite was observed. The branch roadway enhanced the flow capacity of outburst pulverized coal in main roadway. An alternating pattern of strong and weak impact forces was observed. The impact dynamic disaster-causing intensity of F-Rd increased by 33.4% compared to L-Rd in the straight section and by 19.2% in the turning section. The impact force was generally higher under F-Rd relative to L-Rd, indicating that the branch roadway increased the disaster-causing risk of OTPF in main roadway. Moreover, the static pressure was generally lower under F-Rd relative to L-Rd, indicating that the branch roadway weakened the static pressure damage in the main roadway. The findings of this study hold significant value for the design of mine roadways and the establishment of emergency shelters.