Mechanical properties and acoustic emission characteristics of fissured shale under true triaxial unloading and reloading

Abstract Excavation unloading and subsequent support affect the stability of fissured rock around tunnels. True triaxial unloading and reloading tests were conducted on shale specimens with different fissure inclinations, numbers, and configurations. Acoustic emission (AE) counts and energy were monitored throughout testing. At a fissure inclination of 45°, increasing fissure number from zero to three reduced peak strength from 119.4 to 44.7 MPa and elastic modulus from 12.06 to 4.72 GPa. Three failure processes were identified: a stress drop followed by recovery and failure, failure without a distinct preceding drop, and a drop coinciding with failure. Unloading strain in the minor principal stress direction was approximately two to five times that in the major principal stress direction. AE activity increased sharply near failure, whereas cumulative counts decreased as fissure configurations became more complex. Bedding promoted interlayer cracking and influenced crack propagation. Adding a third fissure increased the failure angle from 61° to approximately 76° and intensified fragmentation. These results indicate that fissure geometry, rock bridge continuity, and bedding jointly control shale deformation and failure. The findings provide experimental evidence for stability assessment, support design, and damage monitoring in fractured shale tunnels.