Experimental study on triaxial shear-seepage of red sandstone under different confining pressures and water pressures

For the study of shear cracking in red sandstone and the mechanical and seepage behaviours before and after shear failure, standard cylindrical specimens underwent shear tests in a triaxial stress environment. The tests utilized the Rock Top multi-field coupling test system and a self-made shear diversion component. Different confining pressures and permeability pressure differentials were applied to investigate red sandstone's triaxial shearseepage behaviour. Additionally, discrete element particle methods were employed to study the shear cracking process in models of different sizes and shapes. The research findings indicate: The pre-peak shear stress exhibits a linear increase with shear deformation. Post-peak shear and circumferential deformation rates accelerate, with 1 to 3 stress drops observed. Higher confining pressure leads to a steeper pre-peak deformation curve, increased final shear deformation, and a smoother circumferential deformation-shear displacement curve, with concentrated peak circumferential deformation. The shear-seepage process is categorized into stages of seepage interruption, seepage state transition , and increasing seepage velocity. Characteristic values of shear stress demonstrate a linear relationship with confining pressure. Rock samples display macroscopic single shear plane failure. Increased confining pressure results in more severe fragmentation of the fracture edge and larger shear cracks at the ends. Seepage rates before and after shear failure follow a power-law decrease with confining pressure. Seepage rates exhibit a strong linear relationship with permeability pressure differential. The slope of the flow rate-permeability pressure differential fitting curve decreases with increasing confining pressure. Post-unloading seepage rate is one order of magnitude lower than the pre-unloading rate, with the difference gradually decreasing with increasing confining pressure. Simulation results align with experiments, revealing oblique shear cracks under both confining and normal constraints. The angle and density of oblique shear cracks are influenced by the length-to-diameter ratio and volume, with the least impact at a three-dimensional size ratio of 1. Under equal size ratios, triaxial shear strength is weaker than direct shear strength. The results of this triaxial shear test demonstrate reliability in both methods and outcomes. © 2024 Academia Sinica. All rights reserved.