Problematiche geotecniche ed ambientali connesse con la realizzazione di stazioni metropolitane

The new C Line of Rome Underground -T4, T5 and T6A lots- mainly runs into volcanic deposits deriving from the Colli Albani apparatus, widely extended in south-eastern areas of Rome. The local hydrogeological framework is very complex due to the large variations of permeability in function of granulometry, cementation processes and secondary fracturing of pyroclastic strata. The bottom of the excavation of the underground stations lies 25/ 30 meters below the groundwater level. In order to allow dry conditions and to prevent bottom heave during excavation, the groundwater level is lowered by deep wells systems. Some dewatering field tests showed how the local stratigraphy strongly affects the dewatering efficiency. Steady state filtration FEM analyses allowed to compare permeability of soil with Lefranc in situ tests results and to verify the efficiency of grouted plugs. Due to the presence of a densely built urban environment, the selected solution for the construction of the stations consists of a “cut and cover” excavation about 35-55 m depth retained by multi-propped diaphragm walls to minimise settlements and prevent damage to the structures. The diaphragm walls were carried out with hydromills and self guided mechanical grab. An intense programme of monitoring was set up to control and prevent the effects induced by the excavation during the entire construction progress. The work presents the in situ measurement of ground surface settlement due to lateral diaphragm walls installation of some stations of the new C Line using different construction techniques. Numerical analyses carried out to interpret situ measurements for two different stations of T4 lot, namely Pigneto and Malatesta, showed the main influence of presence of soft soil on the settlements due to diaphragm wall installation.