Shear Strength Characteristics of Soft Rock Joints Based on Constant Normal Stiffness Testing

Large-scale shear apparatus is designed to investigate the shear hehaviour of simulated soft rock joints under constant normal stifmess condition. Regular saw-teeth profiles having inclinations between 9.5" and 26.5" have been tested. The strength envelope based on these test results indicates that predictions made by Panon (1966) are accurate for the lower asperity angles. However, for higher asperity angles, Patton's model overestimates the shear strength especially in the low to medium normal stress range. In contrast, while Barton's model (1973) is realistic for Constant Normal Load (CNL) condition, it seems to be inappropriate for modelling the shear behaviour of soft joints under Constant Normal Stiffness (CNS) condition. An empirical strength envelope for the prediction of peak shear strength of joints is proposed in this study to account for the change in normal stress and surface degradation during CNS shearing.