For the new high speed railroad link between Lyons (France) and Torino (Italy) a 52 km-long tunnel between the Maurienne and Dora Riparia Valleys needs to be constructed. This tunnel will cross most of the tectonic units of the western Franco-Italian Alps. The maximum overburden depth will be 2 500 m under the border mountain massif of Ambin. The geologic setting is particularly varied and many geotechnical difficulties will have to be overcome due to structural complexity, hydrogeology, geothermal effects, highly variable state of stress at depth, and environmental considerations. The presentation will give a synthesis of the geological survey (including 2 deep and long directional borings) and develop the field measurements and the associated modelling performed to establish the initial conditions concerning natural temperature and actual state of stress within the rock mass : - results of thermal measurements (temperature gradients, conductivities) have been used to calibrate a finite element model based on pure conduction to predict ground temperature along the tunnel axis ; local anomalies found in some boreholes are discussed in relation with the hydrogeological context ; - results of stress measurements made in exploratory boreholes (HTPF method) globally show a shortage of vertical stress value especially marked in the heart of the Ambin massif, and an excess of horizontal stress values near the Arc river, in the Modane area ; these results are discussed in relation with the geostructural context.