Experimental study of shear behavior of rock joints under constant normal stiffness conditions

The shear behavior of synthetic soft rock joints (regular saw-tooth) was investigated in the laboratory under constant normal stiffness condition (CNS). A large-scale shear apparatus was designed and constructed which can test joints under both constant normal load (CNL) and CNS conditions. It is observed that CNL condition overestimates joint dilation compared to CNS condition and thereby, underestimates the peak shear stress of joints. Plot of shear stress against normal stress shows that a bilinear shear strength envelope is suitable for soft rock joints subjected to CNL conditions, while linear or bilinear envelopes are acceptable for CNS testing depending on the asperity angles. The shear behavior of infilled joints was also investigated under CNS conditions, and it was found that a very small thickness of bentonite infill reduced the shear strength significantly. The shear strength of joints almost approached that of pure infill, when the infill thickness to asperity height ratio reached 1.60.