Effect of shear rate on shear mechanical properties of rock-like joints under different boundary conditions

In order to study the effect of shear rate on the shear mechanical properties of rock joints under different boundary conditions, RDS-200 rock joint shear test system was used to carry out direct shear tests with five different shear rates under two boundary conditions including constant normal stress and constant normal stiffness on artificially cast irregular rock-like joints with the same joint morphology. The experimental results are as follows. (1) Under the constant normal stress boundary condition and a same normal stress, the pre-peak shear stiffness of rock-like joints decreases with a decreasing rate, and the peak shear strength and residual shear strength decrease logarithmically with the increase of shear rate; in addition, the cohesion of rock-like joints increases, and the internal friction angle decreases logarithmically with the increase of shear rate. (2) Under the constant normal stiffness boundary condition and a same normal stress, the pre-peak shear stiffness of rock-like joints decreases with a decreasing rate, the peak shear strength decreases logarithmically, and the residual shear strength increases first and then decreases under higher normal stress with an increase of shear rate; moreover, the cohesion of rock-like joints decreases and the internal friction angle increases logarithmically with an increase of shear rate. (3) Compared with the constant normal stress boundary condition, the cohesion force of the rock-like joint increases by 115.85% on average, and the internal friction angle decreases by 8.44% on average under the constant stiffness boundary condition; the pre-peak shear stiffness, the peak shear strength and the residual shear strength of the rock-like joint increase by 11.96%, 19.47% and 32.32% on average, respectively, and the peak normal displacement decreases by 40.12% on average under the same initial normal stress and shear rate. The research results can provide some references for the shear instability evaluation of surface and underground engineering rock joints under different shear rates.