不同围压与水压下红砂岩三轴剪切–渗流试验研究

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

为研究红砂岩剪切开裂过程中及剪切破坏前后力学与渗流规律,基于Rock Top多场耦合试验仪及自制剪切导流组件,实现三轴应力环境下的标准圆柱形试件剪切试验条件,开展不同围压、渗透压差作用下红砂岩三轴剪切–渗流试验研究,并利用离散元颗粒方法研究不同尺寸、形状模型的剪切开裂过程。研究表明:(1)峰前剪应力随剪切变形整体呈线性上升趋势,峰后剪切及环向变形速率加快,剪应力出现1~3次应力降;围压越大,峰前变形曲线斜率越大,剪切变形终值递增,环向变形–剪切位移曲线则越趋于平缓;峰值环向变形较为集中。(2)剪切–渗流过程可分为渗流中断阶段、渗流状态转变阶段、渗流速度逐增阶段,剪应力特征值与围压具有良好的线性关系。(3)岩样表现为宏观单一剪切面破坏,围压越大,断口边缘破碎越严重,端部剪切裂缝越大。(4)剪切破坏前后渗流量与围压服从幂函数递减规律;渗流量均随渗透压差升高而呈现极好的线性关系,且流量–渗透压差拟合曲线斜率随着围压的增大而减小;剪应力卸荷后的渗流量相比于卸荷前降低一个数量级,且随着围压增大,两者之间的差值逐渐减小。(5)模拟结果与试验相似,显示围压约束和法向约束条件下均出现斜向剪切裂纹,斜剪裂纹发育角度及密度受长径比、体积影响,三维尺寸比例为1时其影响程度最小,等尺寸比时三轴剪切强度弱于直剪强度。以上研究结果表明此次三轴剪切试验无论在方法、结果方面均具有一定的可靠性。

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 shear-seepage 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.