Research on the Mechanical Behavior and Energy Evolution of Rock Joints during the Loading–Unloading Shear Process

The high degree of jointing in natural rock masses has a significant impact on their mechanical behavior. The joints are subjected to frequent loading and unloading during excavation. Therefore, this paper researches the mechanical behavior of joints during the cyclic loading–unloading shear process and proposes a constitutive model considering plastic shear displacement. The results indicate that the evolution of mechanical parameters is closely related to plastic shear displacement and the constitutive model has good applicability. During the shearing process, the shear stiffness gradually decreases with the increase of shear displacement and increases with the increasing normal stress. When the normal stress is the same, the dilatancy angle decreases rapidly and tends to stabilize with the increase of plastic shear displacement. The dilatancy effect is more pronounced under lower normal stress conditions. The energy evolution during the shearing process is closely related to the shear displacement, and the accumulated dissipated energy and damping ratio reach their maximum values when the shear strength reaches its peak. The evolution trend of the damping ratio is similar to that of accumulated dissipated energy, both showing a characteristic of first increasing and then decreasing with the increase of shear displacement. When the normal stress is the same, they rapidly increase to the peak with shear displacement, then gradually decrease, and tend to stabilize.