ABSTRACT
The Mont Cenis Base Tunnel, an emblematic work of the New Lyon-Turin Railway Line, runs for 57.5 km under the crystalline high cover rock formations of the Western Alps. Exceptions are two short sections in soft ground, among which the entrance section on the French side stands out: the tunnel runs for 550 m in a very low cover dejection conoid.
The two railway tubes start with a very close spacing, releasing a minimum 5 m pile at the entrance bulkhead, consolidated for about 40 meters with vertical jet-grouting columns near the village of Saint-Julien Montcenis, and then up to 35.5 meters at the end of the section.
Also worth noting is the delicate underpass of a hydraulic duct in operation, which is orthogonal to the railway line, with just 23 meters coverage. To overcome this difficult section, the project adopted profiles built with the umbrella arch method, including construction of the reinforced concrete invert vault behind the front.
Due to potential pre-reinforcement capabilities of this method, excavation was successfully completed in 16 months with no underground instability and limited subsidence on the surface.
