Effect of Fissure Penetration Degrees on Spalling Process and Feature of Surrounding Rock in Rectangular Tunnel Under True Triaxial Loading

To investigate the effect of fissure penetration on the spalling process and features of rectangular tunnel sidewalls, true triaxial compression tests were conducted on cubic sandstone specimens with varying fissure penetration degrees. Acoustic emission monitoring and video equipment were used to record the failure process of the surrounding rock in real time. The experimental results show that the presence of fissures weakened the strength of the surrounding rock, and as fissure penetration increased, the initial failure stress first increased and then decreased, while the peak failure stress decreased with increasing penetration. This study demonstrates that increasing fissure penetration levels significantly escalate the extent and intensity of surrounding rock failure. At higher penetration levels, additional support and optimized excavation boundaries are recommended to reduce stress concentration and ensure tunnel stability.