Shear strength deterioration effect of rock mass joint surface under cyclic shear load.

Understanding the shear strength degradation mechanism of a rock mass joint surface under cyclic shear load and determining a corresponding analytical model is an important foundation for accurately evaluating the safety of rock mass engineering under seismic loads. It is worth noting that, to date, there has been a dearth of studies on the strength characteristics of joint surfaces that consider the number of loading cycles, normal load, and initial undulant angle of the structural plane. In this study, focused on the behaviour of sandstone, the particle flow code (PFC) modelling framework was used to simulate a joint surface cyclic shear test considering first- and second-order undulations. Based on the experimental results, the comprehensive effects of the number of cyclic shear cycles, the normal stress, first- and second-order undulation and the dilatancy angle on shear stress during cyclic shear were analysed. Formulas for the joint surface shear basic friction angle and dilatancy angle under cyclic shear were proposed, and a method for calculating the joint surface peak shear strength under cyclic shear considering the deterioration of the dilatancy angle and basic friction angle was established. The peak shear strength of a sample after five cycles of shearing was calculated using the proposed formula and compared with the results of numerical simulations, the Barton method, and the Homand method. The results showed that the calculated values have good consistency with the results of the numerical simulations, demonstrating the effectiveness and accuracy of the proposed formula. However, under a low normal stress, there could be errors in estimating the cyclic shear strength of the joint surface.