ABSTRACT
The paper describes the extreme geological conditions encountered during the excavation of the first stretch of the Mont-Cenis base tunnel in the new high speed railway Lyon-Turin, SMP4 site, between Saint Martin La Porte and La Praz in France. The total length of the tunnel is approximately 8700 m realized using the mechanized boring through a single shield hard rock TBM. The geology is characterized by the presence of various geological formations, fault zones with high coal content, high fractured rock mass and constant lack of a vertical and stable face. This heterogeneity has affected in a severe way the TBM advance. The aim of the paper is to highlight all TBM requirements foreseen by the contract and all needed improvements applied to overpassed or cope with different geological accidents. All installed equipment on the TBM was designed with special devices assuring to manage particular exceptional conditions. Eventually, the TBM performance is detailed referring to a series of milestones where a series of technical improvements were introduced to regain a normal excavation trend.
