Within the first of four lots for the construction of the new metro line 17 North, underground section that crosses the villages of Bourget, Blanc Mesnil, Dugny, Bonneuil-en-France and Gonesse, the AVENIR group entrusted CIPA with the excavation, temporary and final lining of the emergency and ventilation cross-over. This batch provides for the construction of the tunnel, monocane and double track, excavated in mechanized, the southern sector of about 6km long with the creation of two new stations : Le Bourget Aéroport (LBA) and Triangle de Gonesse (TDG). All the Line 17 project covers about 20 km, with two sections underground, north and south, with tunnels and underground stations, and an aerial section, which includes a viaduct and an outdoor station, and will connect the villages between the two airports, Le Bourget and Charles de Gaulle. Following the direction and the excavation methodology choseen and completed the demolition of the opening concrete wall, CIPA starts the excavation of the cross-over in two phases, the first from the shaft to 2 meters from the future tunnel and the second, through the tunnel after the passage of the MBT and achèvement of the filling of the tunnel itself. The excavation restarted on the tunnel side by the construction of a pilot gallery of reduced dimensions, until intercepting the already excavated section and allowing access to the equipment inside the cross-over. The temporary lining is characterized by steel rib and reinforced concrete, that carried out over the entire perimeter, including the invert, at each excavation pass of 1 meter. At the end of the excavation, CIPA proceeds to the realization of the final lining, characterized by the phases of reinforcement, formwork and concreting of the invert, pedestals and vault.