Analysing Tri-arch Cavern Supporting Mechanism with 3D Numerical Modelling

Multi-arch caverns and tunnels are commonly adapted in underground metro station projects to meet the required cross-sectional span. The supporting mechanisms of such caverns/tunnels is very different from single-arch or circular tunnels/caverns, which have been studied by some researchers and engineers with numerical, analytical and experimental approaches. However, many of these works were only based on the cross-sectional analysis, and not able to provide a comprehensive support mechanism including all the related structures. In this paper, using the example of tri-arch underground station in Melbourne Metro project, the supporting mechanism is studied in detail, including the interactions between surrounding rockmass and key supporting structures (i.e. lining, crown, downstand beam & column). 3D numerical modelling with FLAC3D has been utilised for the study, hence the supporting mechanism at both transverse and longitudinal direction are analysed and proposed. Parametric studies have also been carried out to identify the role of key supporting elements. The key findings of this paper include identifying the central cavern crown as the critical structure, proposing a load transferring mechanism between different structures and suggesting the major supporting mechanism to be providing tensile restrain to the surrounding rock mass. The findings from this paper provide supplements to the supporting mechanism of such caverns/tunnels, which can be used as design theoretical basis for future projects.