Mechanical shear properties of rock joints under pre-peak cyclic shear loadings

Rock joints may undergo damage and failure under dynamic disturbances such as earthquakes. Understanding their mechanical behavior of rock joints under pre-peak cyclic shear loading is crucial for preventing dynamic disasters in underground engineering. Current studies predominantly use analog materials, often neglecting the effects of rock matrix and in-situ stress effects, which leads to a poor understanding of the evolution mechanisms of rock joints under cyclic shearing loads. This study innovatively generated natural rough structural surfaces through Brazilian splitting tests and reconstructed rock specimens with natural morphological features using 3D scanning and engraving technologies. Cyclic loading experiments were conducted to analyze the influences of normal stress, loading frequency, static and dynamic load amplitudes on mechanical indicators, including ultimate strength, cumulative displacement, hysteresis average stiffness, and damping ratio, etc. Random forest analysis was used to quantify the relative significance of these four controlling factors. Experimental findings reveal that with an increasing number of cyclic, the local strain on the structural surface increases, transforming initially banded low-strain zones into distinct high-strain clusters. Cumulative displacement and mean stiffness exhibit positive correlation with the number of cycles, in contrast to the reduction in the damping ratio. Increased normal stress and higher loading frequencies suppress displacement accumulation, while enhancing mean stiffness and damping. An increase in dynamic load amplitude reduces cumulative displacement and mean stiffness but amplifies the damping ratio, whereas an increase in static load reduces mean stiffness and the damping ratio while promoting cumulative displacement. The hierarchy of influencing factors is as follows: normal stress, loading frequency, dynamic amplitude, and static amplitude. These results provide a data foundation for assessing dynamic stability and preventing disasters in underground engineering.