To master the overburden structure failure and fracture evolution law under multi-key stratum control during repeated mining in coal seams, this study takes the superimposed mining faces of 6^# and 7^# coal seams in Xinjiang mining area as the research background. Through comprehensive research methods including theoretical analysis, similarity simulation, numerical simulation, and field monitoring, it systematically reveals the evolution law of overburden fractures under repeated mining controlled by multiple key strata, and clarifies the crucial control effects of cumulative damage in the overburden and the superimposed effect of mining-induced stress fields on fracture morphology during repeated mining of close-distance coal seams. The research shows: (1) Based on key stratu theory, four key strata are identified above the working face, with the first sub-key stratum and the main key stratum located at 61.63 m and 174.63 m above the 7^# coal seam, respectively. (2) Similarity simulation displays that after mining the upper 6^#coal seam, the overburden failure exhibited an approximately “trapezoidal” distribution, with a fracture zone height of 40.2 m (height-to-mining ratio of 13.4); the fracture zone height in the area unaffected by repeated mining above the 7^# coal seam was 115 m (height-to-mining ratio of 12.8). Repeated mining intensifies cumulative damage in the overburden and superimposes mining-induced stress fields, leading to rapid fracture propagation. Ultimately, under the combined control of multiple key strata, the fracture zone and caving zone heights stabilizes at 139.68 m and 42.88 m, respectively. The overburden failure pattern evolves from the single “trapezoidal” structure of initial mining to a "double-trapezoidal" composite structure, with fracture evolution following the pattern of "slow expansion during initial disturbance-leap increase due to cumulative damage from repeated mining-gradual stabilization regulated by key strata". (3) Field monitoring using the double-end water plugging and borehole observation joint detection method measures the fracture zone height after 6^# coal seam mining as 38 m (height-to-mining ratio of 13.6 based on a measured seam thickness of 2.8 m), and the fracture zone height in the exclusively mined area of the 7^# coal seam as 100.33 m (height-to-mining ratio of 12.4 based on a measured seam thickness of 8.09 m). The predicted height-to-mining ratios under different coal thickness conditions show good agreement. The research results provide a theoretical basis for roof water hazard prevention and control and surface damage management in the Xinjiang mining area.

For the study of the layout of the roadway in the coal pillar and floor strata co-mining working face at the Zhaogu No.2 mine, a mechanical model of the segmental coal pillar within the working face was established through theoretical calculations. The analysis considered the stress state of the coal pillar area under different collapse conditions in the goaf after upper strata mining. Additionally, FLAC^3D numerical simulation software was used to simulate the stress distribution in the roadway for different layout positions during strata mining, thereby clarifying the impact of working face mining on the side roadway of the segmental coal pillar. The results show that the collapse of the goaf after upper strata mining significantly affects the stress distribution in the coal pillar area. To ensure safety during mining, roadway excavation and working face recovery should be conducted after the upper strata have fully collapsed during strata mining. The co-mining working face roadway should be positioned beneath the original upper strata goaf, avoiding stress concentration areas in the coal pillar location. Ultimately, it is determined that the side roadway for the lower strata working face should be arranged with an offset of 10 m outward. Practical on-site experience has demonstrated that under this offset, there is minimal deformation of the surrounding rock in the coal pillar side roadway, meeting the safety production requirements of the working face.

To investigate the deterioration of compressive property of traditional concrete in a high-temperature environment, uniaxial compression tests were conducted on concrete at various high temperatures. Combined analytical techniques—scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric-mass spectrometry (TG-MS)—were used to analyze the degradation mechanism. The experimental results indicate that high temperature has a strong temperature-dependent effect on concrete’s compressive strength. As temperature increases (400 °C, 600 °C, 800 °C), concrete’s compressive strength decreases. These decreases are 27.52%, 56.6%, and 80.76% relative to room temperature, respectively. This phenomenon is attributed to the direct link between concrete’s microstructure and its macroscopic mechanical properties—driven by thermal stresses generated during heating and the decomposition of cement hydration products. Temperatures above 400 °C trigger microcrack formation, and microcracks propagate more rapidly with increasing temperature. At temperatures further increasing to 600 °C, fewer cementitious materials are left decomposable; even stable calcium carbonate starts to decompose. At temperatures of 800 °C or more, decarburization occurs, rendering the concrete microstructure loose and porous. Partial separation of aggregates from the paste causes a near-total loss of compressive strength.

In view of the difference of the adsorption structure between soft and hard coal, there is a big difference in the critical value of the inspection index for the regional outburst risk caused by the gas content. For the coal seams with soft and hard coal stratification, the model of gas content in the equilibrium state was established first, and the microscopic parameters of different rank coals were determined by the low-temperature liquid nitrogen adsorption test and mercury intrusion test. Then, the adsorption capacity of coal samples was determined by the adsorption test. Finally, the residual gas content of the coal seam in the equilibrium state was calculated based on the adsorbed gas content, and the critical value of prediction indexes of regional outburst based on the residual gas content was studied. The results show that for the same metamorphic degree, the specific surface area of soft coal is larger than that of hard coal. However, under the same gas pressure, the residual gas content of hard coal of anthracite and lean coal is greater than that of soft coal with the same metamorphic degree, while that of meager-lean coal and gas-fat coal is opposite. It is suggested to adopt the small value (rounded) of the measured gas content of soft and hard coal at 0.74 MPa as the critical value of the residual gas content in the regional effect test from the economic perspective. It is of great significance to determine the critical standard of the residual gas content in the regional effect test according to local conditions for reducing the cost of outburst prevention work.

With the development of deep Earth engineering, the stability of surrounding rocks subjected to high temperatures from fire hazards has become an increasingly prominent issue. Therefore, studying the physical and mechanical properties of rocks under different thermal treatment modes is of great significance for the design of underground engineering. Taking red sandstone as the research object, this paper conducts physical parameter tests, uniaxial compression tests, and X-ray diffraction (XRD) on specimens under real-time high temperatures and natural cooling in the range of 600–1000 °C, to analyze the variations in specimen composition, the correlation between physical and mechanical properties and temperature, and to explore the underlying mechanisms. The results show that under both real-time high temperatures and natural cooling, the volume of sandstone increases while the mass decreases with rising temperature. At 1000 °C, the volume expansion rates are 3.30% and 3.80%, and the mass loss rates are 6.30% and 5.60%, respectively. Mechanical parameters, including peak strength, elastic modulus, and peak strain under the two treatments, all deteriorate significantly compared with those at room temperature. At 1000 °C, peak strength decreases by 54.83% and 36.26%, elastic modulus decreases by 74.55% and 67.96%, and peak strain increases by 65.63% and 43.75%, respectively. High-temperature-induced changes in the internal mineral structure and composition of sandstone are the main causes of rock mechanical property deterioration. During the cooling process, thermal shrinkage and recrystallization of mineral particles densify the rock structure; therefore, the compressive strength of naturally cooled sandstone is higher than that under real-time high temperatures. This study can provide theoretical guidance for the repair and reinforcement of rock engineering after high-temperature action.