Numerical simulation of large tunnel systems under seismic loading conditions: CERN infrastructure as a case study

Seismic analysis of large tunnel systems using the continuum (Finite Element; FE) approach can be complex and computationally expensive. The inefficiency stems from the extended length of tunnels, compared to the tunnel diameter that controls the local ground-structure interaction, passing through different terrain and lithological profiles, with complex fixity conditions provided by the intermediate station boxes, and the increased significance of asynchronous ground motion effects given the length scale. This study proposes an uncoupled numerical methodology to model and analyse the seismic response of large tunnel systems that is able to consider various tunnel alignments. The method is capable of simplifying the complete tunnel system at global scale into a lower-order, practically affordable numerical approach while still retaining the ability to account for the aforementioned key features. This was achieved using a Beam-on-Nonlinear-Winkler Foundation (BNWF) approach. The (dynamic) ground-structure interaction was considered using springs and dashpots calibrated against 2D nonlinear plane strain FE analyses under quasi-static and dynamic conditions, respectively. The springs were subjected to a free-field displacement time history obtained from 1-D nonlinear wave propagation analyses. The proposed method was implemented for the case study of the circular Large Hadron Collider (LHC) tunnel network at CERN (in French: Conseil Européen pour la Recherche Nucléaire) in Geneva, Switzerland, which is 27km in circumference and runs 100m below the ground surface, where the tunnel is completely embedded within a competent layered rock. The tunnel complex contains six large underground cavern structures housing the particle detectors (similar to ‘station-boxes’) along its alignment. The study investigated the seismic actions developed through the circular tunnel alignment considering various ground motion schemes and examined the effect of alignment geometry on the magnitude of these forces by a comparison with a straight alignment possessing the same tunnel-ground interaction properties. The actions generated from the low-order BNWF model of the tunnel alignment were then utilised as time-varying boundary conditions in full 3D models of critical structural locations at local scale, specifically the tunnel-cavern connection where there is a change of fixity and stiffness and the largest induced seismic actions. The work was extended by using the methodology developed to comparatively study the co-seismic behaviour of common tunnel alignments that may be utilised in underground mass-transit systems, which are of a similar size to the LHC system. The effects of various degrees of curvature in plan and change in elevation and running depth were considered, assuming a ground profile, structure and interaction characteristics based on the LHC tunnel.