Monitoring a deep excavation in pyroclastic soil and soft rock

The completion of the downtown stretch of Line 1 of the Napoli (Italy) underground involved the construction of a number of stations in the historic centre of the city, including the Universit`a station. This station consists of two shafts excavated with the protection of reinforced concrete diaphragms, and two platform tunnels. The main shaft reaches the line tunnels excavated with TBMs in the formation of the Neapolitan Yellow Tuff, at a depth of more than 36 m below ground level and more than 33 m below the groundwater table; excavations of the shaft involved loose materials for a depth of 23 m, and the Neapolitan Yellow Tuff formation for the remaining. The two platform tunnels, each of them consisting in two separate stretches about 50 m long, were excavated from the main shaft using the Artificial Ground Freezing technique to guarantee excavation stability and hydraulic seal. The paper presents the main results of the monitoring carried out along the whole complex construction process for a total duration of about seven years; the presented data consists of: measurements of pore water pressures inside and outside the excavations; measurements of displacement of retaining structures and ground anchors forces; temperature measurements in the vicinity of the freezing probes around the tunnels excavation to check the frozen ring formation; measurements of subsidence of surrounding buildings. The case history is a very complete one and valuable lessons are learnt on the effectiveness of the construction techniques further than on the design methodologies adopted. Qualitative and quantitative relationships among the various sets of measurements are also outlined, such an aspect representing a valuable feature of the presented case history.