Numerical simulation of large tunnel alignments under seismic loading: The Large Hadron Collider as a case study

Abstract

Three-dimensional seismic analysis of large tunnel alignments using the continuum (Finite-Element; FE) modelling technique can be computationally expensive. This is due to the extended length of tunnels compared to their diameter (which controls the local ground-structure interaction and hence, the required maximum FE element size). Such models can become more intricate as the tunnel passes through different terrain and lithological profiles and with the complex fixity conditions provided by intermediate underground stations. Furthermore, the effect of asynchronicity of ground motions on the tunnel seismic performance may be non-negligible given the length scale. So far, existing modelling techniques lack the competence to simulate the seismic performance of large tunnels along their length with confidence and computational efficiency. Aiming to bridge this gap, this study proposes an efficient numerical methodology to model the seismic response of large tunnel alignments using a dynamic Beam-on-Nonlinear-Winkler Foundation (BNWF) approach. Ground-structure interaction was modelled using parallel springs and dashpots calibrated against 2D nonlinear FE analyses. These springs/dashpots were subjected to a free-field ground displacement time-history obtained from 2D nonlinear wave propagation analyses. The method was implemented for the circular Large Hadron Collider (LHC) tunnel network at CERN (Geneva, Switzerland), which is 27 km in circumference with six large underground caverns housing the particle detectors (station analogues) with large vertical circular shafts to the ground surface. The results show the generation of seismic actions at the global scale of the tunnel alignment which are not captured by conventional 2D plane-strain analyses. The approach can identify key critical tunnel locations where more subsequent detailed local (3D) analyses should be focussed.