Experimental mechanical strength and deformation characteristics of deep damaged–fractured rock

Mechanical and structural properties of rocks after failure are different from those of intact rock. The stability of surrounding rock in deep underground engineering is closely related to the post-peak state. By using the MTS815 rock mechanics testing system, rock specimens at different damage–fracture degrees were obtained through triaxial unloading testing of eight stress levels, including before peak stress, at the peak stress point, after peak stress, and at the residual stress stage. The damage–fracture degree was measured through changes in the elastic moduli of the specimens before and after damage. Considering the influence of irreversible plastic deformation on the damage of rock specimens, an apparent damage–fracture degree adaptive for describing the damage behaviours of rocks was proposed. The triaxial reloading test of damaged–fractured rock specimens under five confining pressure conditions was performed to reveal the influence rules of confining pressure and the damage–fracture degree of the peak stress, deformation parameters, strength parameters, and volume dilatancy characteristics of the specimens. The stress–strain curve of the damaged–fractured rock specimens with three-axis reloading after unloading at different stress levels exhibited five stages. A high damage–fracture degree of the rock specimens showed certain plastic deformation characteristics. The dilatancy phenomenon occurred easily in the volumetric strain of the specimens reloaded after unloading. The peak stress, residual strength, and elastic modulus of the specimens decreased, whereas Poisson’s ratio increased. The rock specimens with different damage–fracture degrees mainly exhibited shear failure. The greater the damage–fracture degree and the higher the confining pressure are, the relatively smaller the fracture angles of the rock specimens are.