Shear behavior model of rock joints applicable to CNL and CNS conditions

Mechanical properties of rock mass and its stability are highly affected by the mechanical and geometrical characteristics of discontinuities. One of the most important characteristics of discontinuities is shear strength and shear behavior observed in direct shear test. The direct shear test is divided into constant normal load (CNL) test and constant normal stiffness (CNS) test according to loading condition. The CNL condition is applicable to rock slope while the CNS condition corresponds to underground rock mass where the shear behavior of discontinuities depends on the stiffness of rock mass. Reliable prediction of shear behavior of rock joints for both conditions has been an important issue in rock engineering field. In this study, an empirical model of shear behavior of rock joints was proposed from direct shear tests using a servo-controlled shear testing machine. Cement mortar replicas of rock joints were tested with various normal stiffnesses (K_n), initial normal stresses (s₀), joint roughness coefficients (JRC) and joint wall compressive strengths (JCS). Results of the tests were analyzed to investigate the effects of loading condition and material properties on friction coefficient and normal displacement of the joints, and empirical models of the friction coefficient and normal displacement were suggested. In the empirical models, dimensionless terms were adopted to avoid the scale effect and therefore to enhance the applicability of the suggested models. The suggested models can be applied to predicting the shear behavior of rock joints including pre-peak and post-peak shear stress levels regardless of the loading condition. To verify the suggested models, additional shear tests of rock joints were carried out. In overall stress levels, the prediction of suggested models showed good agreement with the experimental results.