ABSTRACT
This paper discusses the work undertaken to explore the most geologically complex section of the 57.5 km Lyon-Turin cross-border base tunnel between Susa, Italy and Saint-Jean- de-Maurienne, France. This section, on the French side, has been subject to exploratory work in the Houiller briançonnais fields, between Saint-Martin-La-Porte and La Praz for nearly 20 years. After the realization of two access tunnels which had convergences up to 2 meters, this work, in the axis of the south tube of the base tunnel, was divided into 2 parts: 1.5 km realized using the traditional method to explore and cross the most converging terrain corresponding to the Front du Houiller and 9 km using a TBM for the other Houiller parts. The unknown and extremely difficult geological and geomechanical conditions in these carboniferous formations, under 300 to 1,200 m of covering, have made this work a real technical and human challenge that required a special and adaptive design with the testing and validation of constructive solutions before the launch of the construction of the base tunnel in this sector. This exploratory work has also made it possible to optimise the design and consolidation of economic forecasts and planning for this section.
