Study on the Shear Strength of Anchored Jointed Rock Mass Under Different Normal Stiffness Conditions

This study investigates the constant normal stiffness (CNS) boundary condition in deep rock engineering structures, which provides a more accurate representation of the stress environment of jointed rock masses than the constant normal load (CNL) boundary condition. Three CNS boundary conditions (0 GPa/m, 1.5 GPa/m, and 3.0 GPa/m) were designed to simulate the effects of confinement and various geological engineering conditions on different tunnel depths. Using direct shear tests on both anchored and unanchored joint samples under CNS conditions, this study incorporated the dilation curve of the joints into a model predicting joint shear strength. The model accounts for the effects of CNS boundary conditions, and combines the anchorage resistance model based on the theory of statically determinate beams. It also considers the relationship between axial and lateral displacements of anchors during shear deformation. Results demonstrate that both CNS boundary conditions and anchorage significantly influence shear mechanical properties. Anchor reinforcement exhibited a greater impact on peak shear stress than CNS boundary conditions, while both factors similarly affected peak normal displacement. The newly proposed model accurately predicts shear strength under different normal stiffness boundary conditions, aligning closely with experimental data. The study also analyzes the contribution of anchors to shear strength, revealing a 57.28% contribution under a stiffness condition of 0 GPa/m. With increasing normal stiffness, intrinsic shear resistance in jointed rock mass improves, while the relative contribution from anchors decreases.