The new Col di Tenda tunnel between Italy and France: integrated geological investigations and geophysical prospections for preliminary studies on the Italian side

This paper illustrates the results of a multidisciplinary study led by the CNR (Italian National Research Council) on the behalf of ANAS (National Road Society) for the preliminary phase for the excavation of the new Col di Tenda tunnel between Italy and France. Surface geology of the Col di Tenda area (north-western Italy, Piemonte Region) is characterized by a gently north-dipping homocline composed of Triassic marls, Jurassic limestones and Eocene conglomerates, calcarenites and claystones, all belonging to the Sub-Briançonnais geological domain. These sedimentary sequences are cut by high-angle normal faults and overthrust Eocene sandstones and claystones of the Delphinese geological domain along a south-verging and north-dipping thrust surface. The sub-surface setting was investigated by wells, geoelectric sections and reflection seismic lines that pointed out a puzzling geological structure. Geoelectric sections in particular showed sharp lateral resistivity contrast and resistivity inversion at depth determined by variation in rock fracture density and in water circulation close to the main tectonic elements. Seismic lines were helpful to understand the geometric relationship between the Jurassic neritic limestones and the Eocene marly-calcareous multilayer of the Sub-Briançonnais domain and to exclude the presence of Triassic evaporites of the Delphinese domain along the tunnel alignment. The acquired information allowed us to achieve a complete 2D structural model of the area. The model demonstrated the most suitable route for the excavation of the new Col di Tenda tunnel among those proposed by the Italian- French inter-governative Committee.