Excavations in the Neapolitan Subsoil: the experience of the Toledo Station service tunnel

ABSTRACT: A Metro Line 1 extension is being executed in Naples from the terminal of Dante Station. The new underground section is composed of five new stations and two twin rail tunnels with a length of 5 km. Toledo Station has the shaft located laterally and a large size service tunnel (13 m wide, 17 m high and 40 m long) connects it to the rail and pedestrian access tunnels, starting from the access shaft. The tunnel is located in the historical centre, under a deeply urbanized area in volcanic soil (loose pozzolana silty sand and tuff) with a hydrostatic head of 27 m. For stabilizing the tunnel crown in loose volcanic soil, it was decided to adopt the Artificial Ground Freezing method among the various options of ground improvement techniques. In this paper, the observed behaviour is presented and discussed, allowing to throw a light on the effects of the rather complex execution steps of the underground excavation