Study on deformation laws of layered slate tunnels under different dip angles and water saturation

• Reveal the U-shaped variation of slate strength and modulus with bedding angle, with reductions of 67% and 59% at 0.9 saturation. • Develop a water-saturation-modified Hoek–Brown criterion and implement it in FLAC3D. • Achieve close agreement with monitoring, with a displacement error of only 3 mm versus 10 mm for the traditional model. • Demonstrate that deformation is symmetric at 0°/90° and asymmetric at 30°–60°, amplified by high water saturation. • 揭示板岩强度和模量随层理角度呈“U型”变化, 含水率0.9时分别降低67%和59%. • 建立考虑含水率的修正 Hoek–Brown 准则并嵌入 FLAC3D. • 模拟结果与监测高度一致, 位移误差仅3 mm, 优于传统模型的10 mm. • 说明围岩在0°/90°时为对称变形, 在30°–60°时为不对称变形, 高含水率放大该效应. . Slate exhibits distinct characteristics such as transverse isotropy, weak strength, and susceptibility to weathering. Under cyclic water action, it is prone to disintegration and has poor water stability, making it a representative of soft rock. Its complex mechanical properties and water-softening behavior impose numerous adverse effects on engineering construction. This study conducted uniaxial compression tests on slate under water-bedding coupling conditions to investigate its mechanical properties and softening mechanisms. A water-saturation-modified Hoek-Brown criterion was implemented in FLAC3D to simulate layered slate tunnel excavation. Case analysis of a Changsha tunnel showed that the coupled elastoplastic model better captures surrounding rock deformation than the conventional Hoek-Brown model, with validation from field monitoring. The study also systematically revealed the influences of layering angle and water saturation on deformation behavior. The results indicate that both the peak strength and elastic modulus of slate exhibit a "U-shaped" distribution with the increase of layering angle. The peak strength is the smallest at 60°, and the elastic modulus is the smallest between 45° and 60°. At a water saturation of 0.9, the elastic modulus decreases by 59% compared with the dry state. The error between the simulated displacement of the water-rock coupling model and the on-site monitoring value is only 3 mm, which is much better than that of the traditional Hoek-Brown model, and it can accurately reflect the asymmetric deformation characteristics of surrounding rock. The asymmetric deformation of surrounding rock is significant when the layering angle is between 30° and 60°, and the displacement is the largest at 90°; at a water saturation of 0.9, the vault displacement at a dip angle of 90° is 2 times that at 0% water saturation, and the left arch shoulder displacement at a dip angle of 60° increases by 1.9 times. This study provides quantitative insights for the design and construction of tunnels in slate-rich areas and can effectively guide the deformation control of similar projects.