Abstract
Active support with the prestressed anchorage system as the core can improve the physical and mechanical properties of a squeezed large-deformation tunnel by increasing the confining pressure of the surrounding rock in time, which has a positive effect on the tunnel excavation and displacement control. In this study, triaxial tests were conducted to investigate the properties of carbonaceous-slate samples obtained from the typical large-deformation section of Muzhailing highway tunnel. The results indicated that the rock samples’ elastic modulus E and cohesion c increased with the confining pressure within a certain range. Considering the realization of the above rules of E and c, the secondary development of the FLAC3D built-in strain-softening model was achieved to establish a soft-rock constitutive model considering the changes in the surrounding rock parameters with respect to the confining pressure. After validating the proposed constitutive model, the effects of the prestressed anchorage system with different parameters on the mechanical behaviors of squeezed soft rock in Muzhailing tunnel were studied via numerical simulations, and the results indicated that with the increases in the prestress, anchor-cable length, and density, the displacement of the surrounding rock decreased and the disturbance ranges of E and c narrowed, with a slowing trend. In addition, the combination form of short and long anchor cables also affected the displacement of the surrounding rock. After comparing the effects on the displacement control and the parameters improvement of surrounding rock, the key parameters of prestressed anchorage system applied in Muzhailing highway tunnel were determined. Subsequently, the proposed prestressed birdcage cable system with the suggested parameters were applied in the typical large-deformation section of Muzhailing highway tunnel. Monitoring data indicated that the maximum displacement of the tunnel section decreased from 529 mm under passive support to 229 mm under active support, confirming that the prestressed birdcage cable system had a good displacement-control effect. This study could help popularize the prestressed anchorage system in tunnelling engineering and provide guidance for the design of key parameters when combating large-deformation disasters in squeezed soft rock highway tunnels with a high geo-stress.
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