Simplified numerical simulation of large tunnel systems under seismic loading: CERN infrastructures 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 over very long distances, compared to the tunnel diameter, different terrain and lithological profiles, complex fixity conditions provided by the intermediate station boxes, and ground motion asynchronicity. This paper 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 computationally intensive FE models into a lower-order, practically affordable numerical solution while still accounting for the aforementioned key features. This was achieved using a Beam-on-Non-linear Winkler Foundation (BNWF) model. The soil-structure interaction was considered using non-linear springs and frequency dependent dashpots. The springs were subjected to a free-field displacement time history obtained from 1-D wave propagation analysis. The proposed method is implemented for the case study of the circular Large Electron-Positron Collider (LEP) tunnel network at CERN in Geneva, Switzerland, the forerunner of the Large Hadron Collider (LHC). The tunnel system is 100m below the ground surface and completely embedded within a competent layered rock. The pre-LHC-upgraded tunnel complex contains four large underground cavern structures housing the particle detectors (‘station-boxes’) along its alignment. The study investigates forces developed along the circular tunnel alignment assuming a synchronous ground motion.