The Turin-Lyon new line Mont-Cenis base tunnel: geomechanical aspects and comparison with the important swiss base tunnels 
The construction of the new Alpine base tunnels is always a challenge, faced with increasing Rock Mechanics knowledge and innovative technologies, in comparison with those available for the designers and the contractors of the past. Two aspects are essential to ensure the safety of the excavations and ensure reasonable costs and construction time: ability in managing the tunnel convergences and the phenomena of violent decompression in hard and massive rocks. Regarding the first aspect, the difficulty in assessing the development of the deformation of the walls of the tunnels in poor rock masses at great depth is reflected in the difficulty in stabilizing the excavation with provisional supports, allowing to limit the convergence and controlling, at the same time, the confining pressure, whatever the technique of advancement used. However, all this determines the solution of the difficult problem of choosing the installation time of the final lining, which allows to avoid the onset of deformations unaffordable for the tunnel walls and to keep a reasonable and economic thickness of the concrete lining. The problems of large convergences occurred during the excavation of the Saint Martin la Porte adit tunnel on the French side of the Mont-Cenis base tunnel and during the Carboniferous crossing of the Loetschberg tunnel are examined by comparing the design methods of the provisional reinforcements and supports and the excavation strategy employed. Regarding the second aspect related to the phenomena of violent decompression in hard and massive rocks, the research in this field has produced so far some criteria for predicting such events and especially for the control of their effects if tunnel excavation proceeds with traditional method (D & B) or with mechanized systems (TBM). Although the geological conditions are specific to each work and the different tectonic units from site to site, the profiles of the Loetschberg, the Gotthard and the Mont- Cenis base tunnels show some similarities, as to the nature of the rocks traversed. Finally the paper examine the methods utilized for the geomechanical characterization of the rock mass and the constitutive laws used for the design of the section of the Turin-Lyon tunnel that was assessed to be potentially subject to rockburst phenomena in comparison with the Loetschberg and Gotthard tunnels rockburst zone project.