Controlling Meter‐Scale Deformations in Small‐Clearance Tunnels Through Layered Slate Using High‐Tension NPR Anchor Cables: Case Study From Tayi Tunnel

An active control method using negative Poisson′s ratio (NPR) anchor cables is proposed to tackle problems like significant deformations of surrounding rock due to the mutual disturbance between closely spaced twin tunnels in layered slate. Rock mechanics tests indicated a U‐shaped variation in the peak strength of slate across joint dip angles from 0° to 90°, with the lowest strength and elastic modulus occurring at 45°. Numerical simulations were conducted to examine the impact of lateral pressure coefficient, rock layer dip angle, and tunnel spacing on surrounding rock deformation and the plastic zone, considering the anisotropic characteristics of the rock mass. The results indicate that higher lateral pressure coefficients intensify deformation, with a marked acceleration in the rate of increase once the threshold of 1.15 is exceeded. Furthermore, reduced net distance intensifies mutual tunnel disturbance and increases the risk of plastic zone connectivity. An active control strategy was developed using the high pre‐tensioning force and constant‐resistance large‐deformation (CRLD) features of NPR anchor cables, based on the support principle of the excavation compensation method. This strategy utilizes a composite support system comprising “Short and Long NPR anchor cables ± Polyester fiber mesh ± W–shaped steel straps ± Steel arches”. Field monitoring demonstrated that this solution significantly controlled surrounding rock deformation (maximum deformation reduced from 1.7 m to 275 mm) and decreased the deformation rate. The stresses in the steel arches and the pressure between the initial support and secondary lining remained within safe limits. This method effectively resolves the large deformation problem in closely spaced tunnels, providing a valuable reference for similar layered soft rock tunnel projects.