Rehabilitation of La Nerthe Tunnel on Paris-Marseille High-Speed Railway Line

The La Nerthe tunnel is located on the Paris-Marseille railway line under the Étoile mountain range. This 4638 m long double track tunnel was built at the middle of 19th century under a maximum cover of 180 m and currently is used as part of the high-speed railway line (TGV). This masonry tunnel with hard limestone blocks at the sidewalls and bricks at the crown passes through marl, gypsum and limestone zones. A major fault with accompanying extensive tectonic deformations cuts across the tunnel axis. The in-situ stress tests performed on the new high-speed rail line in the zone with intense tectonic fractures highlighted the tectonic residual stresses with very high lateral earth pressure coefficients (k0). The La Nerthe tunnel underwent ovalization deformation with excessive displacement at the springline, and pinching and bursting of the bricks at the crown. In this paper, the geological context and the damage to the tunnel lining are described in detail. The theoretical tunnel profile has been compared with the actual real section in order to determine the size and the shape of the lining deformation. The initialization of the existing stress field in the tunnel masonry lining using the original excavation and support methods and also a simplified approach for considering the time effect has been explained in detail. Finite element analyses have allowed an understanding of the possible causes of the observed damages and to propose an adequate reinforcement method. The applied rehabilitation system and the constraints of work in this busy mainline railway tunnel have been discussed in detail