Observed and computed performance of mechanised tunnelling in the historical centre of Rome

The project of the Line C of the Rome underground involved the use of two earth-pressure balance (EPB) tunnelling boring machines (TBMs) to excavate the twin tunnels in the city centre, having the priority of minimising potentially detrimental interferences with existing monuments and historical buildings of invaluable historical value. This gave rise to a massive monitoring program to evaluate the effects of the mechanised tunnelling on the nearby monuments, providing the opportunity for field investigation of ground response. This paper presents and discusses the monitoring results collected from a greenfield test site located close to the Basilica of San Giovanni and instrumented with extensive surface and subsurface instrumentation. Volume losses observed at the test site were low, being about 0.3 % and 0.45 % for the first and second tunnel drives, respectively, the higher volume loss resulting from plastic strains induced by the excavation of the first tunnel. This also induced non-symmetrical settlement troughs and cumulative subsurface horizontal displacements directed towards the second tunnel. The paper also presents a 3D numerical analysis to describe the ground response to tunnels excavation, simulating the progressive advance of their faces. The analysis results were found to satisfactorily reproduce the observed surface and subsurface ground movements induced by tunnelling. Comparing the observed and computed displacement fields helped to identify some uncertainties in the field measurements, demonstrating how numerical analyses can be used to predict the outcome of complex geotechnical problems, provided a suitable constitutive model is carefully calibrated using laboratory and site investigations.