Asperity Degradation Characteristics and Mechanical Behavior of Rock Joints Subjected to Pre-peak Cyclic Loading

Abstract

In this study, a series of artificial rock joint specimens were subjected to a pre-peak cyclic loading test. The effects of the initial joint roughness parameters (Z₂(initial)), joint wall compressive strength (JCS), normal stress during cycle (σ_nc), cycle frequency (f), cycle amplitude (A), and cycle number (N) on the mechanical behaviors and asperity degradation were studied extensively. The results revealed that the joint surface degradation coefficient R_d and the joint surface degradation thickness T_d were positively correlated with f, A, and N and negatively correlated with the JCS. As Z₂(initial) and σ_nc increased, ${\varvec{T}}_{\varvec{d}}$ increased, and R_d decreased. It was concluded that specimens with larger Z₂(initial) and σ_nc were dominated by vertical degradation, whereas specimens with smaller Z₂(initial) and σ_nc were dominated by horizontal degradation. The joint roughness parameter Z₂(N) decreased with increasing f, A, Z₂(initial), σ_nc, and N but decreased with decreasing JCS. As N increased, the rate of decrease in Z₂(N) transitioned from fast to slow, and the degradation process was divided into a rapidly declining stage and a slowly declining stage. Based on regression analyses, a joint asperity degradation model was presented to calculate the joint roughness parameters Z₂(N) of rock joints under various influential factors in the pre-peak cyclic loading stage. Additionally, based on the constant normal load (CNL) direct shear test on the specimens after cycling, it was found that with an increase in N, the peak stress, residual stress, and peak dilation decreased, while the peak shear displacement increased. Finally, a critical condition was presented to assess the joint pre-peak cyclic loading stage, and the critical cycle number N_c of the joint from the pre-peak cyclic loading stage that was transmitted to the peak shear stress stage was proposed.