Abstract: To further investigate the unloading mechanical response and yield failure mechanism of tunnel surrounding rock, we analyzed the strength law and failure mechanism of thick-walled cylinders using the Mohr-Coulomb yield criterion. Unloading deformation and failure comparison tests were conducted on thick-walled cylindrical specimens made of natural sandstone and cement mortar materials, aiming to explore the unloading deformation and failure mechanism of tunnel surrounding rock. The results indicate: 1) Compared to loading and unloading experiments on rock core specimens, the surrounding rock at the most dangerous point around the roadway in thick-walled cylinder unloading tests reached the material’s elastic limit more rapidly, making it more susceptible to yielding and failure. 2) The unloading deformation of cement mortar specimens was consistent with that of natural sandstone specimens. Unloading deformation was essentially complete when the unloading ratio reached approximately 95%. 3) Under high initial stress conditions, failure occurred in the surrounding rock during unloading. The inner wall of the specimen first reached an unfavorable stress state, leading to failure. 4) Unloading induced the surrounding rock to squeeze and expand toward the free surface of the circular hole, producing extensive spalling of “fish-scale” rock flakes and rock blocks (characterized by thin edges and thick middles) from the inner wall, resulting in localized damage. These findings provide theoretical support for stability control and supporting design of tunnel surrounding rock in deep underground engineering.

Key words: thick-walled cylinder sample, surrounding rock, excavation unloading, mechanical effect, yield criterion