Protective coal seam mining can not only effectively prevent coal and gas outbursts in mines, but also provide stress unloading space for the protected coal seam. The coal body in the protected coal seam might undergo deformation, internal damage and structural damage, which changes its gas seepage characteristics. This study aims to explore the variations of permeability of the coal body in the protected coal seam under axial unloading. With the coal body from the outburst coal seam in the Huaibei mining area as the research object, experiments were conducted to explore the gas seepage characteristics of axially unloaded coal body under different confining pressures and gas pressures, using the TAWD-2000 coal-rock mechanics-seepage experimental system. According to the results, with respect to the gas seepage, the variations of permeabilities of axially unloaded coal samples are closely related to their deformation and damage. As the confining pressure and gas pressure rise, the difference between the permeability at the final failure point and the initial permeability rises at a decreasing rate. The experiments fully demonstrate that the protective coal seam is technically important for the unloaded gas drainage and the coal and gas outburst prevention of the protected coal seam. Under different confining pressures and gas pressures, the permeability of axially unloaded coal varies to different extents and at different rates. The mining scheme for the protective coal seam should be designed in accordance with its confining pressure and gas pressure. This study is of guiding significance for the prevention and control of coal and gas outbursts in coal seam groups.

In this study, the compression failure test of rock with prefabricated fractures under different true triaxial conditions is carried out by using the true triaxial electro-hydraulic servo test system. The traditional large number of fracture laws with prefabricated fissures are merged and attributed to the induction of intermediate principal stress. The test results show that the direction of ${\sigma }_2$ has a significant effect on the deformation characteristics of the prefabricated fractured rock. The internal crack expansion direction is more random and the crack distribution is more extensive and complex under uniaxial and conventional triaxial conditions. Under biaxial and true triaxial conditions, the crack propagation direction is clearly along the ${\sigma }_2$ direction. This shows that the development process of internal cracks in rocks tend to the direction of ${\sigma }_2$. Further, the failure mechanism of rock with prefabricated cracks is analyzed based on theory. It is found that the intermediate principal stress direction plays a very important role in inducing the direction of rock crack propagation. According to the unified idea, the fracture analysis of fractured rock is summed up as true triaxial theory, and the results are consistent with the experimental results. This provides a new perspective for the study of rock fracture mechanics, and provides an important basis for the analysis of surrounding rock fracture mechanism of underground engineering.

This study determines the safety-stability factors of the waste dump benches at different heights and analyses the deformation law, stress variation, and crack propagation characteristics of typical slope models. The meso-mechanical parameters obtained by discrete element numerical simulation are modified through slope radar monitoring data. The results show that the critical height of the dumping benches of the studied open-pit mine is 60 ~ 70 m, and the amount of bench settlement is positively correlated with the height of the benches. After the critical height is exceeded, the bench forms a three-wedge failure due to uneven settlement.

The study of deformation, strength, and other mechanical characteristics of sandstone under true triaxial compression is significant for understanding failure mechanisms in rock and evaluating the stability of underground structures. Conventional and true triaxial compression tests for sandstone are conducted for different stress states in this study using the self‐developed true triaxial electrohydraulic servo test system combined with acoustic emission (AE) testing. This study presents an in‐depth and systematic investigation of deformation, strength, and AE characteristics. The results show significant differences in deformation, strength, and acoustic emission characteristics for the rock under conventional triaxial and true triaxial compression tests, respectively. The peak strength, axial strain, lateral strain, and incremental strain (in unstable crack growth stage) increase with increasing confining pressure under conventional triaxial compression, and the AE count gradually decreases while shear crack proportion gradually increases, indicating that increasing confining pressure gradually inhibits the shear slip effect along fractures, delays perforation of the rock shear fracture surface, and enhances the ability of the rock to withstand deformation and load. Under true triaxial compression, the peak strength increases and then decreases with increasing intermediate principal stress σ₂ and the axial strain ε₁ and lateral strain ε₂ gradually decrease; besides, the lateral strain (expansion) of the rock is mainly in the minimum principal stress σ₃ direction, and lateral expansion tends to decrease before increasing. AE events first weaken and then enhance with increasing σ₂, and the proportion of shear cracks increases first and then decreases, indicating that the confining pressure gradually changes from the shear slip effect that controls crack offset to the damage effect that promotes crack tension with increasing σ₂. In addition, the protective effect of confining pressure improves when σ₃ increases.

In this study, CO₂ seepage of sandstone samples from the Taiyuan-Shanxi Formation coal seam roof in Ordos Basin, China, under temperature-stress coupling was studied with the aid of the TAWD-2000 coal rock mechanics-seepage test system. Furthermore, the evolution law and influencing factors on permeability for CO₂ in sandstone samples with temperature and axial pressure were systematically analyzed. The results disclose that the permeability of sandstone decreases with the increase in stress. The lower the stress is, the more sensitive the permeability is to stress variation. High stress results in a decrease in permeability, and when the sample is about to fail, the permeability surges. The permeability of sandstone falls first and then rises with the rise of temperature, which is caused by the coupling among the thermal expansion of sandstone, the desorption of CO_2, and the evaporation of residual water in fractures. Finally, a quadratic function mathematical model with a fitting degree of 98.2% was constructed between the temperature-stress coupling effect and the permeability for CO₂ in sandstone. The model provides necessary data support for subsequent numerical calculation and practical engineering application. The experimental study on the permeability characteristics for CO₂ in sandstone under high temperature and overburden pressure is crucial for evaluating the storage potential and predicting the CO₂ migration evolution in underground coal gasification coupling CO₂ storage projects.