The nature of the materials, the presence of water tables, and the reduced space available for installation of plants, led designers to choose the twin-tunnel solution rather than just one tunnel of a larger diameter. Consequently, the potential for risks in excavating the tunnel in a complex urban environment was considerably reduced. The particular difficulty of this tunnelling project was related to excavating through extremely heterogeneous terrains, representing limiting conditions for the application of a hydroshield tunnel boring machine, including: sticky clays; blocks "fluctuating" in marly matrix; decompression zones; and voids produced by decomposition of chalks. Ten rings in each of the two tunnels were equipped with vibrating-wire strain gauges installed during manufacturing of the segments. Soil improvement solutions were implemented where the settlements indicated potential risk of damage to the pre-existing structures. Such interventions entail improving the characteristics of the ground, and mitigating the deformation effects induced by tunnelling, by means of low-pressure cement injection.