The high porosity and high specific surface area of the broken rock mass in abandoned mine goaf make it an excellent thermal storage space. The void structure is an important factor that affects the permeability characteristics of broken rock mass, which determines the efficiency of extracting geothermal water from abandoned mine shafts. To accurately describe the void structure of broken rock mass, the effect of particle erosion on the fracture of rock blocks is considered in this study, based on which an impact-induced strength corrosion calculation model was proposed. Then, this calculation model was embedded into the three-dimensional numerical simulation of broken rock mass for secondary development. A discrete element numerical calculation model was established for broken rock masses with different size grading distributions under water immersion and lateral compression conditions. On this basis, considering the strength erosion effect of impacts, this study investigated the deformation and fracture characteristics of broken rock masses with different size grading distributions and analyzed the evolution laws of porosity in the broken rock masses. The main findings are as follows: The impact effect has a significant influence on the growth of microcracks and the breakage rate of broken rock mass. When the particle size of the broken rock mass differs significantly (size grading as G3), impact-induced strength erosion exerts the greatest impact on the growth of microcracks and the breakage rate. When the particle size of the broken rock mass is uniform (size grading as G1), impact-induced strength erosion minimally impacts the secondary fracturing of the broken rock mass. When the strain of the broken rock sample is less than 0.175, the distribution of microcracks is scattered; when the strain reaches 0.275, microcrack propagation accelerates and exhibits a clustered distribution; and when the strain reaches 0.375, microcracks exhibit a reticular distribution and their connectivity is enhanced. With the increase in deformation, the broken rock mass porosity decreases, and the porosity curve fluctuates along the z-axis with a decreasing trend and gradually becomes more uniform. This study provides a theoretical foundation for assessing the efficiency of extracting and storing mine water with heat in abandoned mine geothermal mining projects.

Grouting technology in overburden separation is recognized as an effective method to prevent surface subsidence and reuse solid waste. This study used mechanical analysis to explore deflection characteristics of key strata and accurately predict and control surface subsidence. Conceptualizing the coal–rock mass beneath the key strata as an elastic foundation, we developed a method to calculate the elastic foundation coefficients for various regions and established an equation for key strata deflection, validated through discrete element numerical simulations. This simulation also examined subsidence behavior under different grout injection–extraction ratios. Additionally, combining the equivalent mining height theory with the probability integral method, we formulated a predictive model for surface subsidence during grouting. Applied to the 8006 working face of the Wuyang Coal Mine, this model was supported by numerical simulations and field data, which showed a maximum surface subsidence of 546 mm at a 33% injection–extraction ratio, closely matching the theoretical value of 557 mm and demonstrating a nominal error of 2%. Post-grouting, the surface tilt was reduced to below 3 mm/m, meeting regulatory standards and eliminating the need for ongoing surface structure maintenance. These results confirm the model’s effectiveness in forecasting and controlling surface subsidence with grouting. The study can provide a basis for determining the grouting injection–extraction ratios and evaluating the effectiveness of surface subsidence control in grouting into overburden separation projects.

The freezing of capillary and adsorbed water exerts a substantial influence on rock seepage and frost heave. In order to investigate the influence of pore structure and water–mineral interactions on water freezing of sandstones, x-ray diffraction, nitrogen gas adsorption (N2GA), and nuclear magnetic resonance (NMR) at various temperatures are conducted. The NMR surface relaxivity of various sandstones is determined by upscaling the specific surface area per unit pore volume of N2GA based on fractal theory. The freezing characteristic curve of capillary water is established based on the accumulated pore distribution and the Gibbs–Thomson equation. The relationship between adsorbed unfrozen water content and temperature is derived from the perspective of statistical thermodynamics. The findings indicate that RS sample contains a higher hydrophilic clay content, demonstrates increased NMR surface relaxivity, and exhibits larger absorbed pore diameters compared to GS and YS samples. The freezing characteristics of capillary water are influenced by the cumulative pore distributions and contact angles. Although exhibiting similar distributions of cumulative pore volume ratios, the YS and RS samples demonstrate markedly different behaviors in capillary water freezing due to variations in contact angles. The absorbed water content and absorbed water film thickness show a consistent correlation with the hydrophilic clay mineral content. Owing to the stronger water–mineral interaction effects, the freezing rate of absorbed water in the RS sample is slower compared to GS and YS samples.