Abstract: The joints in deep rock masses often contain weak filling materials to varying degrees, which leads to more complex mechanical properties of the rock mass. A shear test on the anchored and filled jointed rock mass is carried out under the constant normal stiffness (CNS) boundary condition, considering the combination mode of high initial normal stress and different joint roughness coefficient (JRC) - filling degree Δ. The microstructure evolution characteristics of the filled joint part are analyzed by combining the microscopic scanning electron microscope (SEM), and the calculation method related to the peak dilatancy angle under the CNS boundary condition is derived. The research results show that when Δ＜0.5, the shear stress strength of the specimen presents stress hardening; when 1.0＜Δ ≤1.5, the shear stress strength evolves from basically constant to stress softening. When Δ is less than critical filling degree Δ_cr , JRC becomes the main influencing factor of the peak shear strength. Δ controls the normal deformation of the joint. With the increase of Δ , three evolution laws of shear dilation, first shear dilation and then shear contraction, and shear contraction emerge, and JRC affects the degree of shear dilation - shear contraction change of the specimen. The failure mode of the filled joint part mainly undergoes three stages with the increase of Δ : flattening of rough points, friction of filling materials, and grinding of filling materials. From a microscopic perspective, it evolves from a loose and porous structure to a granular debris-like structure. Affected by the mutual evolution mechanism of the extrusion crushing zone and the extrusion stress concentration zone, the shear deformation mode of the anchor gradually evolves from the “approximate” tensile-shear deformation at Δ = 0 to the tensile-bending deformation mode at Δ = 1.5. On this basis, a calculation formula for the peak dilatancy angle of the anchored and filled jointed rock mass under the CNS boundary condition is proposed, and the test verification and the sensitivity analysis of the influencing parameters of the boundary condition are carried out.

Key words: constant normal stiffness boundary, joint surface roughness, joint filling degree, deformation mode of bolt, peak dilatancy angle