`Mechanical Response and Energy Evolution in Post-Peak Granite under Triaxial Confining Pressure Unloading–Reloading (CPUR)
摘要
In underground engineering, support systems act as stress compensation mechanisms for the redistributed stress field in the surrounding rock after excavation. This study focuses on post-peak granite—damaged rock strata entering the post-failure state through excavation-induced stress unloading. Triaxial compression tests under an initial confining pressure of 5 MPa were conducted using a novel cyclic confining pressure unloading–reloading (CPUR) path. Experimental results show that: (1) post-peak confining pressure unloading reduces residual strength by 65–75%; (2) confining pressure compensation restores strength with recovery rates of 24.5%–171.3%; and (3) compensation parameters correlate strongly with recovered strength (R2 = 0.96). Energy analysis reveals that 0.56–4.5% of dissipated strain energy can be recovered through optimized pressure compensation. Based on the energy residual recovery rate evolution, empirical and theoretical models for predicting residual strength enhancement were established. A case study of a shallow-buried large-span metro tunnel in Qingdao validates the compensation strategy, demonstrating effective deformation control and strength recovery. This research provides key insights into rock–support interactions under cyclic loading and proposes a novel theoretical framework for optimizing underground support design.
Highlights Post-peak granite loses most strength during pressure unloading but recovers with proper support. Strength recovery strongly depends on the final pressure, compensation rate, and initial stress level. Optimized pressure compensation sequences can recover part of the dissipated strain energy in rock. A new model based on experimental data accurately predicts the recovery of residual strength. Timely high-strength support effectively controls rock damage and improves tunnel stability.