Abstract: Deep rock mass is a block-hierarchical structure. Dynamic events cause rock mass to be separated from each other during wave propagation, resulting in a low friction effect, which can easily induce the large dynamic deformation of the rock mass in the equilibrium state. Based on the previous studies, the vibration system of rock block is simplified as the equivalent mass-viscoelastic model, and the slip rate weakening mode of the rock friction is introduced. Finally, the calculation model of rock block slip is obtained. The effects of viscoelasticity property of soft medium between rock blocks and the external loads on the relative displacement of the working rock blocks are analyzed. Calculation results show that when the elastic coefficient increases or the cohesive coefficient decreases, the low friction slipping is more likely to occur. With the increase of the external disturbance or horizontal force, the horizontal residual displacement of the rock block increases, and when their amplitudes exceed a critical value, the sustained sliding instability occurs. The critical energy triggering irreversible displacement and dynamic sliding instability is closely related to the value of shear force. With large enough shear force, extremely weak disturbances can induce large irreversible displacement or even dynamic sliding instability between rock blocks, while a greater impact disturbance is required with smaller shear force. When the shear force drops to a value lower than the dynamic friction strength, a single impact load cannot induce dynamic sliding instability but only irreversible displacement. The horizontal dynamic response of the purple sandstone block under the combined effect of vertical impact load and horizontal static force is measured. The experimental results are basically consistent with the theoretical calculation, which proves the feasibility of the model.

Key words: blocky rock mass, low friction effect, velocity weakening, slip instability