Discontinuities have a significant effect on the mechanical behavior of the rock mass and understanding the mechanical behavior of discontinuities is essential for reliable design of any structure inside or on the rock mass. Predominant joint models have been developed under constant normal loading condition. However, it is widely accepted that the shear strength of rock joints is controlled by the inherent stiffness of the rock. In this paper, experimental study of the shear behavior of joint under constant normal load (CNL) and constant normal stiffness (CNS) were undertaken. Synthetic joint profiles with single and multiple asperities with base angles 20 and 30 degrees were prepared and direct shear tests under CNL and CNS with different initial normal stresses were carried out. Results of this study showed that the experiments conducted under CNS led to higher shear strength values and lower normal displacements compared to the experiments conducted under CNL condition. A comparison between the results of the experimental tests and conventional shear strength models was undertaken. Results of CNL experiments on multiple asperities were in good agreement with Ladanyi and Archambault model. However, for CNS tests only when the shearing mechanism is sliding, there is a good agreement between the experiments and the Patton sliding model.

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