Large Deformation Failure Characteristics and Control Measures for Squeezing Tunnels with Steeply Inclined and Layered Rock in Strong and Active Fault Zones: A Case Study

Abstract

The complex mechanical behaviours of steeply inclined and layered surrounding rock in strong and active fault zones result in control measures that cannot adapt to asymmetric squeezing tunnel and are still unsolved. Hence, the Yuntunbao Tunnel was taken as an example to study this issue based on geological survey and indoor and outdoor tests. The results showed that strong geological structures and abundant groundwater undoubtedly deteriorate the mechanical properties of rocks containing many water-sensitive minerals, approximately 45%. The stepwise growth of deformation characteristics before reaching the rock peak strength and the gradient to abrupt failure characteristics after reaching the rock peak strength are determined via triaxial cyclic and static load tests. According to field test results, the unilateral squeezing deformation is severe and greater than 1.5 m, the average extent of the excavation loosening zone is approximately 10 m, and the highest deformation rate reaches 12 cm/d. The gradual and sudden changes in tunnel deformation are demonstrated to be consistent with the postpeak deformation characteristics of layered rock in indoor tests. Moreover, the steel arch exhibits composite failure characteristics of bending and torsion. Finally, reliable and practical controlling measures are suggested, including the optimised three-bench excavation method with reserved core soil, advanced parallel pilot tunnel, long and short rock bolts, and large lock-foot anchor pipe. Compared with tunnel deformation before taking measures, the maximum convergence deformation is reduced from 2.7 to 0.9 m, and the bearing force of the primary support is also reasonable and stable.