Surrounding rocks of underground engineering are subjected to long-term seepage pressure, which can deteriorate the mechanical properties and cause serious disasters. In order to understand the impact of seepage pressure on the mechanical property of sandstone, uniaxial compression tests, P-wave velocity measurements, and nuclear magnetic resonance (NMR) tests were conducted on saturated sandstone samples with varied seepage pressures (i.e. 0 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa). The results demonstrate that the mechanical parameters (uniaxial compressive strength, peak strain, elastic modulus, and brittleness index), total energy, elastic strain energy, as well as elastic strain energy ratio, decrease with increasing seepage pressure, while the dissipation energy and dissipation energy ratio increase. Moreover, as seepage pressure increases, the micro-pores gradually transform into meso-pores and macro-pores. This increases the cumulative porosity of sandstone and decreases P-wave velocity. The numerical results indicate that as seepage pressure rises, the number of tensile cracks increases progressively, the angle range of microcracks is basically from 50 degrees-120 degrees to 80 degrees-100 degrees, and as a result, the failure mode transforms to the tensile-shear mixed failure mode. Finally, the effects of seepage pressure on mechanical properties were discussed. The results show that decrease in the effective stress and cohesion under the action of seepage pressure could lead to deterioration of strength behaviors of sandstone. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

In the context of repositories for nuclear waste, understanding the behavior of gas migration through clayey rocks with inherent anisotropy is crucial for assessing the safety of geological disposal facilities. The primary mechanism for gas breakthrough is the opening of micro-fractures due to high gas pressure. This occurs at gas pressures lower than the combined strength of the rock and its minimum principal stress under external loading conditions. To investigate the mechanism of microscale mode-I ruptures, it is essential to incorporate a multiscale approach that includes subcritical microcracks in the modeling framework. In this contribution, we derive the model from microstructures that contain periodically distributed microcracks within a porous material. The damage evolution law is coupled with the macroscopic poroelastic system by employing the asymptotic homogenization method and considering the inherent hydro-mechanical (HM) anisotropy at the microscale. The resulting permeability change induced by fracture opening is implicitly integrated into the gas flow equation. Verification examples are presented to validate the developed model step by step. An analysis of local macroscopic response is undertaken to underscore the influence of factors such as strain rate, initial damage, and applied stress, on the gas migration process. Numerical examples of direct tension tests are used to demonstrate the model's efficacy in describing localized failure characteristics. Finally, the simulation results for preferential gas flow reveal the robustness of the two-scale model in explicitly depicting gas-induced fracturing in anisotropic clayey rocks. The model successfully captures the common behaviors observed in laboratory experiments, such as a sudden drop in gas injection pressure, rapid build-up of downstream gas pressure, and steady-state gas flow following gas breakthrough. O 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

In this study, the axial swelling strain of red-bed mudstone under different vertical stresses are measured by swell-under-load method, and the microstructure of mudstone after hygroscopic swelling is studied by mercury intrusion porosimetry (MIP). The weakening coefficient and Weibull distribution function are introduced into the coupling model of mudstone moisture diffusion-swelling deformation-fracture based on finite-discrete element method (FDEM). The weakening effect of moisture on mudstone's mechanical parameters, as well as the heterogeneity of swelling deformation and stress distribution, is considered. The microcrack behavior and energy evolution of mudstone during hygroscopic swelling deformation under different vertical stresses are studied. The results show that the axial swelling strain of mudstone decreases with increase of the vertical stress. At low vertical stresses, moisture absorption in mudstone leads to formation of cracks caused by hydration-induced expansion. Under high vertical stresses, a muddy sealing zone forms on the mudstone surface, preventing further water infiltration. The simulation results of mudstone swelling deformation also demonstrate that it involves both swelling of the mudstone matrix and swelling caused by crack expansion. Notably, crack expansion plays a dominant role in mudstone swelling. With increasing vertical stress, the cracks in mudstone change from tensile cracks to shear cracks, resulting in a significant reduction in the total number of cracks. While the evolution of mudstone kinetic energy shows similarities under different vertical stresses, the evolution of strain energy varies significantly due to the presence of different types of cracks in the mudstone. The findings provide a theoretical basis for understanding the hygroscopic swelling deformation mechanism of red-bed mudstone at various depths. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).