Predicted and observed settlements induced by the mechanized tunnel excavation of metro line C near S. Giovanni station in Rome

Abstract

This paper deals with the effects induced by the mechanized excavation of Rome metro line C in the area of an old masonry building, the Carducci school. Class A settlements predictions are obtained performing full 3D soil-tunnel-structure interaction numerical analyses, using a simple elastic perfectly plastic soil constitutive model. The developed model realistically simulates the main excavation and construction features influencing the induced settlements, such as tunnel advancement, front pressure, TBM-EPB design (shield’s weight, overcut and conicity), tail void grouting and grout hardening over time. The measured settlements are reported and compared with the results of numerical analyses performed before (class A prediction) and after tunnelling; the latter carried out to implement in the model the front pressure and TBM conicity actual values, both higher than assumed in the design. Since before the excavation the foundations were reinforced with micropiles, and these were not modelled, the comparison between monitoring data and numerical predictions is limited to the settlements outside the building. Monitoring data are also compared with further analyses conducted using small-strain soil stiffness and using a constitutive model able to reproduce the non-linearity of soil behavior (Hardening Soil). The different predictions of the two models are investigated analyzing the vertical strains distributions and the stress paths around the tunnel. Finally, a reasonable interpretation for the remaining differences between numerical results and field data is proposed and used to back-analyze the settlements, obtaining a satisfactory agreement. The results confirm the effectiveness of the proposed 3D numerical approach, associated with relatively simple soil constitutive models, as a tool to predict tunnelling-induced settlements both in the design and the construction phase, independently of the geotechnical context.