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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]

Excessive greenhouse gas emissions, primarily CO2 , are a principal cause of global warming. China's extensive abandoned mines and goafs present a unique opportunity for CO2 storage, offering a dual benefit of reducing emissions and repurposing mine assets. This study explores the intricate fluid–solid interactions among CO2 , H2O , and coal matrices, which significantly alter the coal's pore structure and surface chemistry, impacting carbon sequestration efficacy. Utilizing coal samples from the 4-2 seam of Huangling's No. 1 mine, Shaanxi, China, experiments simulated goaf conditions to examine CO2–H2O –coal interactions. Findings indicated that CO2–H2O exposure promotes metal cation dissolution and carbon fixation, favoring CO2 storage. Changes in coal's mineral and organic components were noted, intensifying with reaction magnitude. Postreaction increases in pore volume, porosity, and fractal dimension suggest enhanced structural complexity due to matrix swelling and mineral dissolution-precipitation. A conceptual model of coal pore evolution under CO2–H2O influence is proposed, elucidating pore characteristic evolution mechanisms and the CO2 storage process's impact on sequestration in goafs. This research aims to clarify CO2–H2O interaction mechanisms, assess storage safety, and support engineering projects targeting CO2 sequestration in abandoned mining sites. [ABSTRACT FROM AUTHOR]