Excavation instability with end restraint effect under high stress: insights from True-Triaxial simulation experiments

The tunnel face as an intrinsic end restraint significantly impacts the excavation stability. However, the previous studies scarcely address the end restraint effect (ERE) especially under high stress conditions. To address this gap, true-triaxial experiments equipped with the integrated acoustic-optic-mechanical (AOM) multi-physics field monitoring techniques are conducted on a laboratory simulated excavation model. Results show that the excavation instability processes under high stress can be divided into four stages: calm, crack initiation and directional growth, spalling damage and slab buckling, and post-peak collapse and failure. Due to the ERE, the hypothetical supporting force around the tunnel face is present, which renders the first macro-failure initiating far away from the tunnel face. As the continuous stress transfer, the occurrence of V-shaped notch and the collapse of the tunnel face gradually dominate the damage progression. Besides, with the aid of ultrasonic testing, the 3D field data of the P-wave velocities reliably quantify the crack damage zone, the distributions of which show good consistency with the 3D numerical simulations. The end friction effect induced by the rigid loading is also discussed. This study provides a realistic simulation of in-situ excavation instability with the ERE and helps interpret the field observations.