Multi-seam mining often leads to the retention of a significant number of coal pillars for purposes such as protection, safety, or water isolation. However, stress concentration beneath these residual coal pillars can significantly impact their strength and stability when mining below them, potentially leading to hydraulic support failure, surface subsidence, and rock bursting. To address this issue, the linkage between the failure and instability of residual coal pillars and rock strata during multi-seam mining is examined in this study. Key controls include residual pillar spalling, safety factor (f_s), local mine stiffness (LMS), and the post-peak stiffness (k_c) of the residual coal pillar. Limits separating the two forms of failure, progressive versus dynamic, are defined. Progressive failure results at lower stresses when the coal pillar transitions from indefinitely stable (f_s > 1.5) to failing (f_s < 1.5) when the coal pillar can no longer remain stable for an extended duration, whereas sudden (unstable) failure results when the strength of the pillar is further degraded and fails. The transition in mode of failure is defined by the LMS/k_c ratio. Failure transitions from quiescent to dynamic as LMS/k_c < 1, which can cause chain pillar instability propagating throughout the mine. This study provides theoretical guidance to define this limit to instability of residual coal pillars for multi-seam mining in similar mines.

In this paper, a hydro-thermo-mechanical coupling model based on the smoothed particle hydrodynamics with total Lagrangian formula (HTM-TLF-SPH) is proposed to simulate the crack propagation and instability process of fractured rock mass. TLF-SPH uses the Lagrangian kernel approximation, that is, the kernel function and its gradient need only be calculated once in the initial configuration, which is much more efficient than the smoothed particle hydrodynamics (SPH) based on the Euler kernel approximation. In TLF-SPH, particles interact with each other through virtual link, and the crack propagation path of rock mass is tracked dynamically by capturing the fracture of virtual link. Firstly, the accuracy and robustness of the HTM-TLF-SPH coupling model are verified by a reference example of drilling cold shock, and the simulation results agree well with the analytical solutions. Then, the crack propagation law of surrounding rock and the evolution characteristics of physical fields (displacement, seepage and temperature fields) after excavation and unloading of deep roadway under the coupling condition of hydro-thermo-mechanical are investigated. In addition, the seepage and heat transfer processes of the surrounding rock of Daqiang coal mine under different coupling conditions are successfully simulated. Meanwhile, the effect of the boundary water pressure difference on the temperature and seepage fields under the hydro-thermal coupling condition is quantitatively analyzed.

Soft rock is one of the common geological conditions in coal mine underground water reservoir engineering. The cross-scale correlation analysis of water erosion soft lithology deterioration is very important for the safety and stability of coal mine underground reservoir (CMUR) engineering. To address the issues of grain crowding and segmentation difficulties in cross-scale correlation analysis, as well as the limitations of traditional etching methods, this study proposes an image grain segmentation method based on deep learning algorithms, utilizing scanning electron microscopy and image processing techniques. The method successfully segments crowded grains and eliminates the interference from misplaced particles. In addition, indoor uniaxial compression tests were conducted to obtain the mechanical properties of sandstone samples with different water content. By quantitatively characterizing the macroscopic and microscopic deterioration degree of red sandstone samples with different water contents, the relationship between the strength changes of rock samples and the petrographic parameters such as grain size and grain shape is analyzed, and the influence law of soft lithology deterioration in CMUR engineering is revealed. The results indicate: (1) Water significantly weakens the mechanical properties and stability of soft rock. With increasing water content, the strength of sandstone samples continuously decreases, and the failure mode transitions from brittle to ductile failure. (2) The deterioration of micro-micro structures is the main cause of the decrease in mechanical properties of water-eroded soft rock. Grain size, grain area, and aspect ratio are negatively correlated with water content, indicating that hydrophilic minerals in soft rock dissolve under the action of water, leading to rock damage. (3) Grain size, area, and aspect ratio can serve as significant indicators for quantifying the strength changes of water-eroded soft rock. The research findings can be applied to stability assessment and disaster prevention in CMUR engineering.