Underground reservoirs in coal mines, consisting of goafs (By goaf, we mean the space that remains underground after the extraction of valuable minerals), are commonly utilized for mine water storage and drainage, with their primary load-bearing structures being the “roof–coal pillar” systems. Consequently, this structure must endure the repeated immersion behavior resulting from fluctuations in the mine water level, resulting in the risk of geological disasters. This paper analyzes the variation in mechanical properties of sandstone–coal composite samples after repeated immersion cycles through axial loading tests. The results indicate that the water content of the sample exhibits a notable and rapid increase with each successive immersion cycle. This corresponds to a decrease in the stress threshold and modulus parameters of the samples. Moreover, the acoustic emission signals serve as indicators of the softening characteristics of the samples. With the increase in immersion cycles, there is an augmentation in both the frequency and extent of shear cracks. The non-linear failure characteristics of the samples become more pronounced. Consequently, water significantly weakens the cementing material between rock grains. Both sandstone and coal display a decrease in deformation resistance capabilities at a macroscopic level. The constitutive model of the composite sample was improved based on the degradation characteristics of mechanical strength and strain energy parameters, which offers enhanced accuracy in analyzing the degradation process caused by water immersion. This paper offers a crucial theoretical foundation for comprehending the deterioration evolution characteristics of the “roof–coal pillar” bearing structure affected by repeated immersion.

The presence of frozen clay as a natural protective material makes it a crucial layer of defense against potential impacts in various engineering projects. Studying the strength and deformation characteristics of frozen clay is, therefore, particularly important. In this study, static mechanical tests, the Hopkinson impact test, and fly-plate tests were conducted on frozen clay to identify its mechanical properties under an impact load. The uniaxial compression strength, flexural strength, and elastic modulus displayed a linear increase with the change in temperature. The fractal dimension was used to describe the failure characteristics of frozen clay, yielding values ranging from 1.5691 to 1.8785. At the same temperature, the fractal dimension exhibited a strain rate effect as the strain rate increased. A light gas gun system was then used to conduct fly-plate tests on frozen clay at varying temperatures (−3, −20 °C, and ordinary temperature). The impact process was meticulously analyzed, considering factors such as shock wave velocity, particle velocity behind the shock wave, impact pressure, and volume strain. Moreover, our investigation plotted the D–u (volume strain–particle velocity) and P–μ shock (impact pressure–shock wave velocity) adiabatic curves. Notably, we observed that samples with a higher initial strength exhibited an increased resistance to compression under an identical initial density and moisture content, resulting in a discernible leftward shift of the P–μ curve. The results provide a theoretical basis and technical support for similar projects in the future.

In this study, a three-dimensional grain-based model based on the discrete element method is utilized to replicate the heterogeneous structure of crystalline granite, and corresponding laboratory tests are conducted to validate the numerical conclusions. A novel model and an analytical method involving a multilevel force chain network are employed to quantitatively investigate the influence of mineral content on the mechanical behavior of granites. First, a set of granite specimens with varying biotite contents is constructed, and then, uniaxial compression tests are conducted. The effects of the mineral content on the mechanical behavior, force chain network characteristics, and fracture resistance of granite specimens are quantitatively analyzed. The results indicate an inverse relationship between the biotite (V_B) content and the load-bearing capacity of granite under uniaxial compression conditions. As V_B increases, the number of contacts within the biotite structure increases, as does the force chain distribution density within the biotite structure, while the force chain distribution density in other intragranular structures correspondingly decreases. The average values and sum values of all of the force chains in the whole specimen decrease with increasing V_B. Among the various structures, intragranular structures exhibit the highest fracture resistance, whereas intergranular structures exhibit lower resistance.

Shale gas reservoirs are typical dual-porous media where complex pore structures and fracture networks significantly impact gas transport. However, accurately predicting permeability in such media, especially complex fracture networks, remains challenging. The complex fracture network is modeled as of a bundle of rough tree-like fracture networks. The proposed permeability model comprehensively describes the structural characteristics of pores and fractures in shale gas reservoirs, including the fractal distribution of pore diameters and fracture apertures, the rough surface, and branching characteristics of fracture networks. Then, the model's accuracy is validated using reliable experimental permeability data. This model accurately predicts gas permeability and effectively describes gas transport characteristics in shale gas reservoirs with rough tree-like fracture networks. Each parameter has a clear physical meaning and avoids the use of empirical constants. Finally, sensitivity analyses are conducted to explore the effects of structural parameters on the permeability of dual-porous media. The results show that the permeability K of dual-porous media decreases exponentially with the increase in tortuosity fractal dimension Dtp of pores and surface fractal dimension Df of fractures, while it increases as a power function with increasing pore diameter fractal dimension Dp and fracture aperture fractal dimension Dh. The structural parameters of rough tree-like fracture networks significantly impact the permeability of dual-porous media. Increasing the aperture ratio γ, reducing the length ratio β, branching levels m, and the branching angles θ can significantly reduce gas flow resistance, decrease fluid kinetic energy loss, and increase the permeability of dual-porous media. This theoretical model is significant for enhancing permeability models of dual-porous media in shale gas reservoirs, offering reliable theoretical support for understanding gas migration and optimizing shale gas extraction.

Given the significant impact of damage on the deformation and stability of surrounding rock in shallowly buried metro stations, understanding the behaviors of rocks, with the blasting vibration effect considered, is crucial. This study examines the blasting damage mechanism, including an analysis of damage factors and radii. The results revealed a nonlinear decreasing relationship between the damage factor and the distance from the blasting source. Hence, equations for the radii of the surrounding rock's crushing and crack zones are established under the influence of blasting shock and stress waves. On-site acoustic testing indicated that the surrounding rock suffers severe damage within a 0–0.6 m range from the blasting source. A damage disturbance zone exists within a 0.7–1.7 m range while the rock remains intact within a 1.7–4.7 m range. The on-site measured radius of the crushing zone differs by 9.59% from the theoretical value, and the crack zone's radius deviates by 16.47%. In addition, a simulation model is developed to investigate the stability of surrounding rock under different rock span ratio (RSR). For shallowly buried, large-span hard rock stations, the findings suggested that with RSR between 0.15 and 0.25, a smaller ratio correlates with increased settlement of the surrounding rock, higher horizontal stress values, and larger plastic zone areas post-excavation, confirming the evaluation index's rationality and scientific validity.