Bidirectional shear behavior of rock joints subjected to normal stress vibration: An experimental study

Active faults exhibit bidirectional slipping triggered by earthquakes and tectonic stress, often resulting in serious geological disasters. However, the friction behavior during the process remains inadequately understood. To address this gap, bidirectional shear tests were conducted on joint specimens under dynamic normal load (DNL) conditions. The effects of vibration amplitude and frequency on shear behavior and transformation of the shear mechanism were investigated. The results indicate that DNL delays the relative shear displacement needed to reach peak shear stress. As the amplitude or frequency increases, the shear stress response to normal stress vibration is significantly reduced. The shear stress-relative shear displacement curves show two-stage linear evolution before reaching peak shear stress. Dynamic normal stress intensifies the compaction of the joint surface and postpones the shear dilation, and promotes the expansion of joint surface damage. However, when the amplitude ratio is 50 %, the stress valley provides a favorable environment for critical asperities climbing, resulting in the damage area ratio close to the constant normal load condition. A critical threshold was observed. Below the critical threshold, the friction strength is stable. Once the threshold is exceeded, the increases in amplitude or frequency trigger a rapid decrease in the friction parameter. The rock fragment gradually evolves into a fault gouge, indicating that the shear mechanism shifts from an asperity-rock fragment coupled friction to fault gouge friction.