Long-Term Behavior of CERN Tunnel in the Molasse Region

The world-famous Large Hadron Collider (LHC) particle accelerator is housed about 100 m below ground surface in a large-scale underground tunnel network at the European Centre for Nuclear Research (CERN). Miles of deep CERN tunnels were excavated decades ago lined with spray shotcrete in a weak sedimentary rock called the red molasse, which is a rock mass comprising an irregular, alternating sequence of sandstones and marls. Such complex ground conditions have contributed to significant bending moment in the tunnel lining and consequently led to cracks, water infiltration and other structural distress years after tunnel construction. In this paper, a series of soil-fluid coupled finite element (FE) analysis was conducted to investigate the long-term behaviour of a CERN Tunnel: TT10 in the molasse region. For simplicity, the tunnel behaviour was investigated in a 2D plane strain condition at a representative horseshoe shape cross section. The complex ground strata were modelled using 8-node quadratic elements with local mesh refinement, whilst engineering properties of different layers of sandstones and marls were considered with a particular interest in ground permeabilities. Results show the development of pore water pressure with time around the tunnel when the tunnel lining is fully permeable acting as a water drainage channel. Furthermore, the long-term tunnel deformation is detected to be within only several millimeters. The computed FE results show a decrease in the horizontal diameter accompanied by an increase of the vertical one, suggesting a vertical tunnel elongation mechanism of deformation, which was also confirmed by the data collected from field measurements.