ANALYTICAL CALCULATION OF FULL LIFE CYCLE SAFETY OF TUNNELS USING COMPOSITE YIELDING SUPPORT SYSTEM

The composite yielding support system provides a potential solution to ensure the entire life cycle safety of deep rheological soft rock tunnels. However, there is still few research on the evolution law of tunnel mechanical behavior under the action of composite yielding support system, which greatly limits the development and application of the support design theory. For this purpose, this study conducts a mechanical analysis on the entire life cycle safety of tunnels using composite yielding support system. Firstly, the deformation behaviors of both deformable primary support and compressible layer are theoretically described, and a mechanical model of the interaction between viscoelastic-plastic surrounding rock and composite yielding support system is established. In this model, the construction sequence of tunnel excavation and support, as well as the deformation characteristics of support, are fully considered. Mathematical analytical solutions for tunnel displacement and support pressures at different stages are derived, which can be used for assessment of entire life cycle safety of tunnels. Furthermore, by comparing the degraded theoretical model in this study with existing analytical results and comparing the consistency between theoretical prediction and numerical simulation results under the same condition, the reliability of the theoretical model established in this model is well verified. Finally, based on analytical solutions, the support effects of composite yielding support system and conventional composite lining are compared, and a parametric investigation is conducted on the impacts of compressible element length, compressible layer thickness, and secondary lining installation time. Results exhibit that it is necessary to adopt a composite yielding support system for tunnels with strong deformation of surrounding rock. In the composite yielding support system, the deformation capacities of the two yielding structures must be matched. The determination of the installation time for secondary lining requires comprehensive considerations of the deformation characteristics of surrounding rock and composite yielding support system. The theoretical model established in this study can provide useful reference for parameter determination in the initial design stage of composite yielding support system.