A karst collapse column (KCC), a type of vertical structure that is common in the Carboniferous-Permian coal fields of North China, contains many graded broken rocks and often functions as a channel for groundwater inrush. This study used a custom-built apparatus to investigate the effects of the particle-size distribution on the seepage behavior of a sand particle mixture subjected to a high hydraulic gradient. Three different flow regimes were identified: (1) the Darcy regime, (2) the Forchheimer regime, and (3) the turbulent regime. When flow transition begins, the critical flow velocity, critical Reynolds number, and Forchheimer number increase as the nonuniformity coefficient $C_u$ (or the coefficient of curvature $C_c$ ) increases when the porosity remains constant, and the critical hydraulic gradient decreases. The permeability increases linearly, and the inertia factor decreases exponentially as $C_u\times C_c$ increases. Using new experimental data, the validity of five widely used empirical formulas for permeability and the inertia factor were evaluated. The results indicated that the empirically corrected formulas for permeability and the inertia factor yielded high prediction accuracy and can be used to predict the permeability and inertia factor of KCCs in practical engineering.

Mining activities, while promoting social and economic development, alter the environments and ecosystems of mining areas. Some of the alterations have negative impacts on the environment, while some have positive impacts. By understanding the positive and negative impacts related to closed mines in Inner Mongolia, this research aims to seek advantages of the positive impacts on the environment and reduce the harm of the negative impacts, extending the mining value chain. The paper reviewed four main negative environmental impacts (NEIs) of closed mines and proposed four positive environmental impacts (PEIs) which include ecosystem services, cultural services, land resources and underground space resources, and energy sources and mineral resources. The paper focused on the environmental problems and the PEI utilization of closed mines in Inner Mongolia with 20 representative cases. It carefully studied the Minda Mine which is a good example to combine several PEI utilization objectives. Multiple PEI objectives, as well as the correlations among different objectives, can benefit closed mines ecologically, economically, and socially. In addition, a three-step framework for closed mine revitalization and the factors affecting the compound PEI objectives are mentioned in the paper.

The failure mechanism of heterogeneous rocks (geological materials), especially under hydraulic conditions, is important in geological engineering. The coupled mechanism of flow-stress-damage should be determined for the stability of rock mass engineering under triaxial stress states. Based on poroelasticity and damage theory, a three-dimensional coupled model of the flow-stress-damage failure process is studied, focusing mainly on the coupled characteristics of permeability evolution and damage in nonhomogeneous rocks. The influences of numerous mesoscale mechanical and hydraulic properties, including homogeneity, residual strength coefficient, loading rates, and strength criteria, on the macro mechanical response are analyzed. Results reveal that the stress sensitive factor and damage coefficient are key variables for controlling the progress of permeability evolution, and these can reflect the hydraulic properties under pre-peak and post-peak separately. Moreover, several experiments are conducted to evaluate the method in terms of permeability evolution and failure process and to verify the proposed two-stage permeability evolution model. This model can be used to illustrate the failure mechanics under hydraulic conditions and match different rock types. The relation of permeability with strain can also help confirm appropriate rock mass hydraulic parameters, thereby enhancing our understanding of the coupled failure mechanism in rock mass engineering.

Shallow coal mining in gully regions has resulted in significant subsidence hazards and increased the risk of surface water inflow into mining panels, compromising the sustainability of surface water management and underground resource exploitation. In this study, the chain disaster process caused by shallow coal seam mining and heavy rainfall is quantitatively analyzed. The findings reveal that shallow coal seam mining leads to the formation of caved and fractured zones in the vertical direction of the overlying rock. The fractured zone can be further classified into a compression subsidence zone and a shear subsidence zone in the horizontal direction. The shear subsidence zone is responsible for generating compression and shear deformations, intercepting rainfall runoff, and potentially triggering landslides, necessitating crack landfill treatments, which are critical for promoting sustainable mining practices. The HEC-RAS program was utilized to integrate annual maximum daily rainfall data across different frequencies, enabling the establishment of a dynamic risk assessment model for barrier lakes. Numerical simulations based on unsaturated seepage theory provide insights into the infiltration and seepage behavior of rainfall in the study area, indicating a significant increase in saturation within lower gully terrain. Precipitation infiltration was found to enhance the saturation of the shallow rock mass, reducing matric suction in unsaturated areas. Finally, the disaster chain is discussed, and recommendations for managing different stages of risk are proposed. This study offers a valuable reference for the prevention and control of surface water damage under coal mining conditions in gully regions.

In the present paper, a numerical model for deformation and fracturing of brittle rocks is used to study the damage evolution and localized fracturing of brittle rocks based on statistical mesoscopic damage mechanics. In the model, material heterogeneity and the mesoscopic renormalization concept are introduced to consider the interaction among microcracks in rock. The temporal and spatial evolution of acoustic emissions in the stressed rock are also described. Correspondingly, numerical tests on rock specimens with different heterogeneities, porosities, and scales are performed to investigate the damage evolution and localized fracturing of brittle rocks. The localized damage and fracturing of brittle rocks induced by microstructural damage is investigated. The study shows that heterogeneity, porosities, and scales of rock specimen have significant impact on damage evolution and localized fracturing mode. In addition, the study reveals that the postpeak response of stress-strain curve is due to the deformation of the structural elements, and localization means that the descending branch of the stress-strain curve of the rock specimen is size dependent, and the stress-strain curve can therefore not be regarded as a pure material property.