Underground Construction at High Altitudes: a Case History on Mont Blanc in Italy

As a result of reaching its design life the existing Monte Bianco aerial cableway, operating on the Italian side, had to be replaced by a new one, which is currently under construction. The new aerial cable way will connect the lower station in Pontal d'Entreves (1300 m.a.s.l.) with the intermediate station at Pavillon de Mont Frety (2160 m.a.s.l.), and with the top station at Helbronner Peak (3460 m.a.s.l.). A new underground link between Helbronner Peak station to New Torino Hut (3370m.a.s.l.) had also to be built. The link is basically composed by two sections: the first, which is a 80 meters deep vertical pit, descending underground from Helbronner Peak station, and the second which is a 160 meters long sub horizontal tunnel emerging at Torino Refuge. The vertical pit will also perform as a deep shaft foundation for the heavy loads required by rope anchors. On the top of Helbronner peak the presence of two adjacent stations (Italian and French) has led to the realization of vertical structures consisting in a series of micropiles connected in head by concrete curbs, to allow the excavation works on the Italian side without causing displacements to the existing French station. After a first introduction focused on the project general description, the paper will present the intensive geomechanical in situ and laboratory tests. Due to the necessity of ensuring coexistence between the construction works and the existing cableway serviceability (at least for the main part of the works), a series of 3D numerical analysis of the construction processes have been performed in order evaluate the force variation in the pre-tensioned anchors of the French cableway. The entire construction process has been modeled and the results was useful to support the final design of the new station leaving in service the French cableway.