Crack Evolution Mechanisms and Acoustic Emission Characteristics in Rock Masses with Closed Flaw under True Triaxial Loading

Deep rock masses are subjected to complex true triaxial stress environments. Concurrently, natural rock masses contain widely distributed closed flaws. These flaws remain in tight contact due to in-situ stress confinement. Consequently, their mechanical response distinctly differs from that of open flaws. To investigate the effects of closed flaws on the mechanical properties and failure modes of rock masses, true triaxial compression tests were conducted on red sandstone specimens containing a single closed flaw with various inclination angles. The results indicate that the inclination angle of the closed flaw exerts a significant influence on the mechanical parameters of the specimens under true triaxial loading. The entire loading process can be divided into four distinct stages: compaction, elastic deformation, plastic yielding and hardening, and post-peak softening. As the flaw inclination angle increases, the true triaxial compressive strength of the specimens exhibits a "V"-shaped variation trend, decreasing initially and then increasing. Three categories of failure modes were observed in the specimens: combined tensile-shear failure, conjugate shear failure, and slip-shear failure. For the A30, A45, and A60 specimens, because the angle between the flaw and the maximum principal stress is close to the optimal shear angle of the rock mass, shear weak planes are easily formed, leading to lower specimen strength. Conversely, for specimens with 0° and 90° flaws, the shear stress component along the flaw is small, preventing the formation of an effective shear slip plane, which results in higher overall strength. Acoustic emission activity can be divided into three stages: the initial stage, the stable stage, and the final burst stage. Furthermore, the AE counts and cumulative AE counts of specimens with different inclination angles exhibit high consistency with their true triaxial peak compressive strengths. The closed-flawed rock mass is dominated by macro shear failure as a whole, whereas tensile micro-cracks merely serve as localized secondary damage forms. The rock fracturing process is governed by the energy conversion mechanism from elastic strain energy storage to dissipated energy release. In the pre-peak stage, the closed flaw inclination angle significantly affects the accumulation process of microstructural damage. Beyond the peak stress, due to the strong confinement of the constant bidirectional lateral pressures under true triaxial conditions, the instability of specimens with various inclination angles is effectively suppressed. The residual stage is primarily characterized by energy dissipation through stable shear sliding along the fracture plane, and the damage accumulation rate eventually tends to converge.