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In order to study the dynamic evolution law of the pressure relief gas reservoir in the mining of extra-thick coal seams, the 3DEC numerical simulation software was used to analyze the stress and displacement distribution law of the overburden rock caused by mining, and the evolution characteristics of the pressure relief gas reservoir were studied, and the pressure relief gas was proposed. The method of discriminating the location of the reservoir area, and the practice of gas drainage in high-position boreholes at the test working face. The results show that: after the extra-thick coal seam is mined, the height of the overburden caving zone is 49.5 m, and the height of the fracture zone is 104 m. After the key strata in the pressure relief zone on the working face side loses stability, a pressure relief gas reservoir is formed below it. Pressure relief gas reservoir areas are divided into three categories: ①The key layer is in the bent subsidence zone, and the high-level reservoir is formed below it; ②The key layer is in the fracture zone, and the middle-level reservoir is formed under the masonry beam; ③The key layer is in the fracture zone. In the collapse zone, a low-level storage area is formed under the cantilever beam. The shape and area of ​​the reservoir are closely related to the state of the control key layer above. Before the control key layer is broken, the area continues to increase. The shape of the low-level reservoir is rectangular, and the shape of the middle and high-level reservoir is semi-elliptical, expanding to a three-dimensional shape. It is a rectangular cross-section ring body and an elliptical cross-section semi-ellipsoid body; the area decreases rapidly after breaking, and then changes periodically with the pressure of the overlying rock. The low-level reservoir is trapezoidal, the middle-level reservoir is triangular, and the high-level reservoir is shaped like a trapezoid. The shape is a semi-ellipse, and the three-dimensional shape is expanded to a rectangular cross-section ring, a triangular cross-section ring and an elliptical cross-section semi-ellipsoid. The location of the final hole of the high-level borehole was arranged in the range of the middle reservoir area, and the drainage verification was carried out. The maximum drainage concentration of a single hole was 34.5%, the average drainage concentration was 16.8%, and the upper corner gas concentration after drainage was 0.55%, The gas drainage effect is good, indicating that the arrangement of high-level drilling holes according to this method is reasonable.

In order to study the mechanical damage characteristics and energy evolution law of coal after ultrasonic fracturing, uniaxial compression experiments were carried out on fractured coal with different power by using electronic universal pressure testing machine, acoustic emission system. The mechanical damage parameters of different power ultrasonic fracturing coal were obtained, and the relationship between acoustic emission signal and damage parameters of coal mass with different power fracturing was explored. The fractal characteristics of coal surface cracks were analyzed by box fractal dimension, and the damage degradation mechanism of ultrasonic fracturing coal caused by different ultrasonic power was clarified. The experimental results show that the characteristic strength and elastic modulus of coal gradually decrease with the increase of ultrasonic fracturing power, and the damage parameters of uniaxial compressive strength and elastic modulus of coal are linearly correlated with the fracturing power. During the uniaxial compression process, the acoustic emission ringing count can be divided into three stages, which are quiet period, rising period and fluctuating period. With the increase of ultrasonic power, the incubation period of crack is shorter and the proportion of relative time is smaller. With the increase of crack growth rate, the weakening effect on the strength of coal is gradually enhanced, the sudden increase of acoustic emission signal is obvious., the final failure of coal sample is more broken, and the failure characteristics change from elastic brittleness to ductility-plasticity with the increase of fracturing power. Coal pore fissure extension, and three is a linear positive correlation between the fractal dimension damage parameters of coal surface cracks and the fracturing power. The greater the fractal dimension damage parameter is, the more complex the coal fracture morphology is. Based on the acoustic parameters, the relationship between the damage parameters and the normalized parameters of acoustic emission is analyzed, and it has a good fitting relationship. The above results show that ultrasonic fracturing causes damage to the coal structure, makes the coal seam damage and deformation, and forms a complex coal seam seepage network, thus improving the permeability of coal seam.

To explore the evolution law of overburden strata during the mining of huge thick coal seams, an experimental study on the distribution characteristics of the overburden three zones with large mining height was carried out. Based on the geological conditions and mining methods of the huge thick coal seam, a two-dimensional physical similarity simulation experiment under the condition of huge thick coal seam was carried out. The results showed that the height of the overburden strata of the huge thick coal seam in Liuhuanggou（９－１５） 06 working face was 60 m. The horizontal fracture density of the overburden strata in the gob was distributed as a parabola as the propulsion distance increased. Affected by the distribution of strata in the “horizontal three zones”, Delamination of goaf was characterized by “high on both sides and low in the middle”. By calculating physical quantities such as mining overburden subsidence, fissure density, and layer separation and other physical quantities, the formula for determining the height of overburden “three zones” under huge thick coal seams was derived.

The isolated working face is significantly impacted by the adjacent goaf and the mining activities of the working face itself, causing the overlying rock layers above the working face to exhibit far more intense activity compared to an ordinary working face. The stress levels are high, and the surrounding rock suffers severe damage, posing serious challenges to the safe and efficient extraction of the working face. Improving the service life of the retreating roadway in an isolated working face is a pressing technical issue that coal mining companies must address. Focusing on the characteristics of the strata and mining conditions of the 8213 isolated working face in the Yanjiahe Coal Mine, which features a three-soft coal seam, a combination of field investigation, theoretical analysis, on-site monitoring, and numerical simulation methods was employed. This approach aimed to analyze the fundamental laws of mine pressure behavior in the three-soft coal seam isolated working face as well as the deformation and failure mechanisms of the surrounding rock in the retreating roadway. Using elastic thin plate theory, it was determined that the basic roof periodic fracture step of the 8213 isolated face in the Yanjiahe Coal Mine is approximately 23 m. Field mine pressure monitoring on the 8213 isolated working face revealed that during non-periodic pressure events, the support resistance of the working face generally fluctuated stably below the rated working resistance. When the basic roof collapsed, the average working resistance of the support showed a significant increase with periodic pressure steps ranging from 16 to 27 m and an average of 22 m. Numerical simulations were further used to analyze the changes in stress and the plastic zone of the overlying rock on the 8213 isolated working face, clarifying the mechanism by which instability in the overlying rock structure leads to incidents. This analysis provides theoretical support for the safe mining of isolated working faces. [ABSTRACT FROM AUTHOR]

This study aims to investigate the mechanical properties and energy characteristics of coal-rock combinations during uniaxial compression, specifically focusing on variations in interface inclination angles. Each of the stress–strain curves of the CRC (Coal-Rock-Like Material Combination) with varying interface inclination angles were obtained using the DYD-10 universal testing machine and a high-definition camera system. Subsequently, the energy of the CRC was calculated using a specific formula. The experimental results demonstrated that when the interface inclination angles increased, the compressive strength and elastic modulus of the CRC showed a monotonous decreasing trend. The storage of elastic energy mainly occurred in the elastic stage and the plastic fracture stage, and the growth of dissipated energy increases obviously in the post-peak failure stage. As the inclination angles of the interface experience a rise, the energy storage limit (ESL) of the specimen decreases obviously, the CRC was more prone to fracture as the rate of dissipated energy conversion gradually increased. In order to understand the process of coal and rock mass disasters and prevent coal and gas outbursts, the mechanical features and fracture energy characteristics of the CRC under various interface dip angles were examined in this article. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

China possesses widely distributed low-permeability coal seams of various ranks, where differences in coal rank determine fracture mode selection and optimization of fracturing effects. This study investigates the response of low-rank lignite and high-rank anthracite to liquid nitrogen freeze–thaw. Using nuclear magnetic resonance technology, we examined T2 relaxation curves, porosity, and pore throat changes during freeze–thaw processes, focusing on unfrozen water from a microscopic perspective. Experimental results indicate that lignite exhibits significantly higher T2 relaxation amplitudes compared to anthracite, with a predominance of adsorption pores. Lignite shows a more pronounced response to freeze–thaw cycles, resulting in increases of 0.9% in cumulative porosity and 0.9% in pore throat count for lignite, while anthracite shows increases of 0.1% and 0.13%, respectively. The average aperture of flow pores increased by 45.2% and 49.4%. Upon returning to room temperature, lignite shows a two-stage increase in porosity loss rate after initial fluctuations, while anthracite exhibits a slightly fluctuating trend. Both lignite and anthracite demonstrate a lag effect in unfrozen water during freeze–thaw processes, with maximum lag percentages at −5 °C and 5 °C recorded as 51.07% and 67.75%, respectively. The primary factors contributing to the unfrozen water lag effect are the supercooling effect during freezing and changes in pore ice melting points due to water-ice phase transitions. The rapid temperature differential from low-temperature liquid nitrogen triggers uneven thermal stress within the coal body and freeze expansion, optimizing coal pore structure and enhancing connectivity and permeability. [ABSTRACT FROM AUTHOR]

This paper aims to address the issue of hydraulic support crushing accidents or support failures in the retracement roadway (RR) that frequently occurs when a fully mechanized mining face is retraced during the end-mining stage. The deformation and instability mechanism of surrounding rock in the RR during the end mining of a fully mechanized mining face at the Hanjiawan Coal Mine located in the northern Shaanxi mining area is explored through field measurement, theoretical analysis, similar simulation, and numerical simulation. The results reveal that the stability of the remaining coal pillar (RCP) and the fracture position of the main roof are the main factors contributing to large-scale dynamic load pressure in the RR during the end-mining stage. The plastic zone width limit of the RCP is identified to be 5.5 m. Furthermore, the stress distribution within the RCP during the end-mining stage is determined, and the linear relationship between the load borne by the RCP and the strength of the coal pillar is quantified. A similar simulation experiment is conducted to examine the collapse and instability characteristics of the overlying rock structure during the end-mining stage. UDEC (v.5.0) software is utilized to optimize the roof support parameters of the RR. A surrounding rock control technology that integrates the anchor net cable and hydraulic chock is proposed to ensure RR stability. Meanwhile, a method involving ceasing mining operations and waiting pressure is adopted to ensure a safe and smooth connection between the working face and the RR. This study provides a reference for the surrounding rock control of the RR during end mining in shallow, closely-spaced coal seams under similar conditions. [ABSTRACT FROM AUTHOR]