Case study on the mechanics of NPR anchor cable compensation for large deformation tunnel in soft rock in the Transverse Mountain area, China

A study was conducted to analyze the deformation mechanism of strongly weathered quartz schist in the Daliangshan Tunnel, located in the western Transverse Mountain area. A large deformation problem was experienced during the tunnel construction. To mitigate this problem, a support system was designed incorporating negative Poisson ratio (NPR) anchor cables with negative Poisson ratio effect. Physical model experiments, field experiments, and numerical simulation experiments were conducted to investigate the compensation mechanical behavior of NPR anchor cables. The large deformations of soft rocks in the Daliangshan Tunnel are caused by a high ground stress, a high degree of joint fracture development, and a high degree of surrounding rock fragmentation. A compensation mechanics support system combining long and short NPR anchor cables was suggested to provide sufficient counter-support force (approximately 350 kN) for the surrounding rock inside the tunnel. Comparing the NPR anchor cable support system with the original support system used in the Daliangshan tunnel showed that an NPR anchor cable support system, combining cables of 6.3 m and 10.3 m in length, effectively prevented convergence of surrounding rock deformation, and the integrated settlement convergence value remained below 300 mm. This study provides an effective scientific basis for resolving large deformation problems in deeply buried soft rocks in western transverse mountain areas.