Shear Behavior of Rock Joints Under CNS Boundary Condition

Abstract

To improve our understanding of the shear behavior of rock joints under constant normal stiffness (CNS) boundary condition, 48 groups of shear tests on plaster specimens replicated from three rough rock joints were conducted. The effects of joint roughness, normal stiffness, and initial normal stress on mechanical characteristics of joints were studied comprehensively. Test results indicate that compared to constant normal load (CNL) condition, CNS condition is more reasonable to describe the shear behavior of rock joints in underground engineering. The existence of normal stiffness restrains joint dilation, and this constraint effect is strengthened with the increase of normal stiffness. The shear strength increase with the increase of joint roughness and initial normal stress, while the influence of normal stiffness to shear strength is more obvious with the improvement of initial normal stress and the roughness of joint. Meanwhile, three typical shear curves were proposed under CNS boundary condition where both the strain softening and strain hardening behaviors are shown depending on the ratio of initial normal stress to normal stiffness. Moreover, 0.625 of \( {{\sigma_{n0} } \mathord{\left/ {\vphantom {{\sigma_{n0} } {k_{n} }}} \right. \kern-0pt} {k_{n} }} \) is a critical value for plaster mixture used in this study, there will certain be phenomenon of stress hardening when the value of \( {{\sigma_{n0} } \mathord{\left/ {\vphantom {{\sigma_{n0} } {k_{n} }}} \right. \kern-0pt} {k_{n} }} \) is lower than 0.625, and whether the phenomenon of stress hardening exists depends on the roughness and other conditions when the value of \( {{\sigma_{n0} } \mathord{\left/ {\vphantom {{\sigma_{n0} } {k_{n} }}} \right. \kern-0pt} {k_{n} }} \) is 0.625.