Influence of Weak Interlayer Thickness on Mechanical Response and Failure Behavior of Rock Under True Triaxial Stress Condition

Abstract

In this study, we meticulously investigate the influence of weak interlayer thickness on mechanical response and failure behavior of rock in underground chamber excavation, with the aid of a true triaxial test system. We simulated the true triaxial stress condition of surrounding rocks containing weak interlayers proximate to the excavation boundary, where one face remains free, while others are subjected to loading stress and increasing tangential stress. The experimental results reveal a distinct change in the fracture morphology near the free surface as the weak interlayer thickness augments, concomitant with a gradual decrement in the fractal dimension. With the increase of the weak interlayer thickness, the failure characteristics of composite rock changes from “binary” feature to “unitary” feature, i.e. mixed tension-shear failure near the free surface and shear failure far away from the free surface changes to mixed tension-shear failure that only occurs near the free surface. Moreover, an inverse relationship is observed between the bearing capacity of the composite rock mass and the thickness of the weak interlayer. In opposition, there is an incremental trend in peak strain in the tangential stress direction, illustrating that the slip shear strain of the rock positioned above the weak interlayer chiefly governs the strain behavior of the composite rock mass in the tangential stress direction, and such strain intensifies with the enlargement of the weak interlayer. By employing the finite-discrete element method (FDEM), we replicated the failure process of the surrounding rock containing weak interlayer near the free surface of the tunnel, demonstrating considerable congruity with our physical test results. In light of the failure characteristics of the surrounding rock with the weak interlayer, we advocated for a localized area priority reinforcement support strategy, prioritizing regions with weak interlayers. Numerical simulations verified the effectiveness of this approach in controlling fracture proliferation in surrounding rocks and inhibiting the splitting failure of surrounding rocks containing the weak interlayer near the free surface.

Keywords: True triaxial, Weak interlayer, Mixed tension-shear failure, Slip shear strain, Finite-discrete element method