Layered phyllite, characterized by its distinct and complex mechanical properties, often presents engineering challenges during tunnel construction, including rock mass collapse and asymmetric structural failure. These issues are particularly severe under unloading conditions, where the mechanical properties of the rock deteriorate further, posing significant threats to engineering safety. This study integrates laboratory experiments, numerical simulations, and field monitoring to conduct a comprehensive investigation into the unloading mechanical properties of layered phyllite, applying the findings to real-world engineering projects. The study reveals that layered phyllite exhibits significant differences between conventional triaxial loading tests and unloading tests, with the rock exhibiting a tendency toward brittle failure and larger stress drops during unloading. The mechanical properties of layered phyllite specimens are highly sensitive to changes in the unloading rate; increased unloading rate can intensify stress concentration and energy release, which in turn reduces the peak strength and aggravates failure. However, the sensitivity of layered phyllite to initial confining pressure and bedding angle remains consistent across different unloading rates. Furthermore, numerical simulations of the unloading tests based on the SUBI (Bilinear Strain-Softening/Hardening Ubiquitous-Joint) constitutive model align closely with laboratory test results, although further parameter adjustments are required for improved accuracy. The excellent agreement between the field monitoring data and the numerical predictions from the SUBI constitutive model validates its effectiveness and accuracy in simulating the tunnel excavation response in layered phyllite. Furthermore, a comparative analysis using this validated model against the UBI-J(Ubiquitous-Joint) model, which ignores unloading effects, quantifies the significant impact of excavation-induced unloading at an engineering scale.
Yang et al. (Tue,) studied this question.