Deep mining operates under complex high in-situ stress conditions where the inherent anisotropy of rock strata, caused by bedding planes and random joints. However, most existing studies on this plastic zone assume isotropic strata, which substantially deviates from real engineering conditions. Based on an improved anisotropic Mohr-Coulomb failure criterion, an analytical solution for the plastic zone under anisotropic conditions is derived. Comparative analyses-through both theoretical and numerical simulations-between anisotropic and isotropic cases confirm the considerable impact of bedding planes and joints on the plastic zone. The evolution and characteristics of the maximum principal stress and plastic zone are systematically investigated under varying in-situ stress conditions for three joint configurations: single-set joints, multi-set orthogonal joints, and multi-set non-orthogonal joints. The results reveal that for roadways with a single-set joint, the plastic zone approximates a banded distribution aligned with the joint dip direction. In multi-joint roadways, the plastic zone transitions from a hexagonal to an "X" shape as the joint dip angle increases. Furthermore, with an increasing lateral pressure coefficient, the plastic zone in orthogonally jointed strata evolves from "butterfly" to "horizontal line," "cross," "vertical line," and finally an "X" shape. A case study validating the model against field observations of roadway deformation and failure in anisotropic strata demonstrates that the extension direction of the plastic zone aligns with the joint orientation, exhibiting distinct asymmetric large deformation. The close agreement between numerical simulations and field measurements substantiates the reliability of the proposed model.

The phenomenon of progressive failure, stress and wave velocity asynchrony in rocks can inform early warning approaches for rock stability. In this study, the Geotechnical Consulting and Testing Systems rock triaxial test system was used to investigate the compression failure of sandstone from the Ningtiaota mine under confining pressures of 0, 2, 5, and 10 MPa, with synchronous ultrasonic wave velocity monitoring. Based on Martin’s crack strain theory, the variation laws of mechanical and wave velocity response characteristics during progressive failure were obtained from two replicate tests per confining pressure. The results indicate that the normalized stress at peak wave velocity ${\sigma }_{v_{\max }^P}/{\sigma }_f$ ranges from 0.84 to 0.99, whereas the normalized strain ranges from 0.73 to 0.98. With increasing confining pressure, both the strain and stress differences between the peak wave velocity and the peak stress increase. Wave velocity change results from the combined action of effective stress (promoting velocity increase) and crack strain (leading to velocity decrease), causing the wave velocity peak to occur ahead of the stress peak. The normalized crack initiation stress ${\sigma }_{ci}/{\sigma }_f$ ranges from 0.55 to 0.68, and the normalized crack damage stress ${\sigma }_{cd}/{\sigma }_f$ ranges from 0.79 to 0.91, consistent with literature values for intact sandstones. With increasing confining pressure, the proportion of the compaction stage remains unchanged, while the stable crack propagation stage decreases, and the elastic and unstable crack propagation stages increase. The stress-normalized difference between the peak wave velocity and the damage variable protrusion point is approximately $0.1{\sigma }_f$, showing a slight decreasing trend with increasing confining pressure.

Coal mining will lead to rock strata bending deformation and the redistribution of stress field. The change of principal stress magnitude and direction will influence the failure zone distribution of the coal pillar, which determines the stability and safety of roadway near goaf. In this paper, a theoretical model is established to analyze the rotation of principal stress in the coal pillar caused by the bending deformation of key strata due to coal mining. The theoretical principal stress magnitude and direction in the coal pillar are obtained and the variation of principal stress are analyzed with different coal pillar widths and different roof breaking locations. Furthermore, we derived a novel plastic zone distribution in the coal pillar, which considering mining induced principal stress rotation. The plastic zone in the coal pillar presents the trapezoidal shape with a curved waist, which is different from traditional rectangular plastic zone. This also reveals the mechanism of the upper part of the coal pillar is more prone to become instability and present large deformations. Additionally, a numerical model is established based on the geological conditions of Lingdong coal mine, which is used to verify the theoretical model. The numerical simulation of the stress rotation trend is consistent with the theoretical analysis results. The numerical plastic zone in coal pillar presents a curved waist trapezoidal shape, which match well with the onsite deformation of coal pillar. To a certain extent, this verifies the rationality of the theoretical model.

Underground coal gasification is a promising and clean technology for coal resources mining and the backfill body for controlling overburden failure will be exposure to high temperature and mine water. Diatomite is effective in increasing the thermal resistivity of cement-based matrices, but the mechanical behavior of diatomite modified concrete with different moisture contents after high temperature treatment remains unexplored. A modified coal-based solid waste backfill material is developed, by incorporating diatomite as supplementary cementitious materials to enhance the high-temperature performance. In the modified composite material, coal gangue constitutes 50% of the coarse aggregate, with an additional 0.5% basalt fiber incorporated. SEM, XRD and EDS were used to reveal compressive strength stabilization mechanism of specimens with different moisture contents after high temperature treatment. Experimental results show that the uniaxial compressive strength of specimens after exposure to 400 °C reaches its maximum, increasing by 17.1% compared with that at room temperature. Additionally, the mechanical properties of modified concrete containing 0%, 5%, 10%, 15% and 20% diatomite under the couple effect of water and high temperature is studied. The strength reduction caused by the degradation of C–S–H due to high-temperature dehydration can be offset by the formation of new, stable C–S–H through pozzolanic reactions after groundwater immersion, which fills pores and microcracks. The moisture content has little effect on uniaxial compressive strength of samples, which is very important for the stability of underground backfill body influenced by groundwater.