Analysis of time-dependent deformation in tunnels using the Convergence-Confinement Method

During the excavation of a tunnel the accumulated wall displacement and the loading of tunnel support is the result of both the tunnel advance (round length and cycle time) and the time-dependent behaviour of the surrounding rock mass. The current approach to analyze the tunnel wall displacement increase is based on the Convergence-Confinement Method (CCM) performed with either analytical (closed form solutions) or the usage of the Longitudinal Displacement Profiles. This approach neglects the influence of time-dependency resulting in delayed deformation that may manifest even minutes or hours after excavation. Failure to consider the added displacements in the preliminary design can result in false selecting the time of installation and the type of support system causing safety issues to the working personnel, leading to cost overruns and project delivery delays. This study focuses on investigating and analyzing the total displacements around a circular tunnel in a visco-elastic medium by performing an isotropic axisymmetric finite difference modelling, proposing a new yet simplified approach that practitioners can use taking into account the effect of time.