Small strain stiffness of red mudstone fill material under anisotropic stress state

This paper investigates the effect of anisotropic stress states on the small strain stiffness of red mudstone fill material (RMF). A comprehensive experimental program was conducted, including 18 triaxial-bender element tests, 4 isotropic consolidation tests, and 6 stress-controlled loading-unloading tests. The results indicate that the normalized strength is well characterized by the nonlinear strength envelope. Under isotropic stress conditions, the small strain stiffness increases with mean stress, which can be described by a power equation. During conventional triaxial shear, small strain stiffness increases at low axial strains. When the axial strain exceeds 2%, the damage point can be identified, at which point small strain stiffness decreases by more than 25% with further axial strain. A power model has been employed to characterize the small strain stiffness and shear stress at both the damage point and peak point. Unloading at stress states below the damage point results in an increase in small strain stiffness. Conversely, due to irreversible structural disturbance, unloading at stress states above the damage point leads to a progressive reduction in small strain stiffness. The difference in small strain stiffness at various unloading points can exceed 30%. Therefore, the coupled effects of stress history and stress path should be considered for accurate determination of small strain stiffness, as the conventional monotonic model is not applicable in such coupled scenarios.