Stability analysis and support design optimization of large-deformation tunnels in structural fracture zones with high in-situ stresses considering loose effect

When a tunnel passes through the deep soft-rock formation with high in-situ stresses, excavation and unloading disturbances will cause severe squeezing deformation and loose failure of the surrounding rock, resulting in large deformation of the supporting structure. This paper takes the Wanhe tunnel of Yumo railway, which goes through a fracture zone of granite with high in-situ stresses, as the research object. First, based on the classic Kastner and Caquot formulas, the characteristic curve of the surrounding rock considering loose effect was determined. Then, the stabilities of the surrounding rock and the supporting structure were analyzed using the convergence-confinement method to clarify the necessity of considering the loosening effect and controlling the degree of support deformation. Finally, through the comparison of working conditions, the support time and the support parameters of the second arch were determined. This research indicates that the surrounding rock pressure is dominated by the deformation pressure in the early stage. With the release of the stress, the proportion of the loose pressure exceeds 30% when the deformation reaches 0.8 m. At this time, the surrounding rock pressures acting on the support structure considering and without considering the loose effect are respectively 1.094 MPa and 0.765 MPa, indicating that the influence of the loose effect cannot be ignored in the stability analysis and the design of the supporting structure. After the initial support deforms to a certain degree, a second layer of steel arch should be added in time to resist the deformation of the surrounding rock. The best support time for the second arch is the time when the deformation reaches 0.45 m, and the best support parameter is 0.6 m spacing of the I22b steel arch. On-site monitoring data show that the large-deformation support scheme has achieved a good control effect. The research results can accurately reflect the large deformation and catastrophic process of the tunnel in the fracture zone with high in-situ stresses, and have clear guidance for the optimization of the support design in similar projects. © 2021, Science Press. All right reserved.