Numerical modelling of subway train vibrations propagating in the Tiber River alluvial valley in Rome

In the last decades, a rise of vibrations produced by train transit took place in the city centres worldwide due to the construction of new subway lines, followed by the need of evaluating the interactions of these vibrations with pre-existent buildings and infrastructures. In this regard, the present study investigates the propagation scenario of train-produced vibrations in the historical centre of Rome due, for example, to the C-line subway, which is currently under construction. Numerical simulations were carried out in order to test and to tune the numerical tools able to analyse the propagation of the future vibrations within the buried alluvial valley. The test site area of the Prati neighbourhood was selected, located on the right bank of Tiber River. Both homogeneous and heterogeneous subsoil models were designed, and geophysical measurements were carried out to record vibrations originated by operating trains and to define the typical ambient vibration noise for the investigated area. The finite-element code CESAR-LCPC was used for the numerical simulation, also adopting a simplified Site-City-Interaction scheme. In addition, the surface wave content was quantified in the motion simulated at the ground surface of the valley by adopting an advanced analytical approach using the time-frequency Normalized Inner Product (NIP) filter based on the Stockwell Transform. The numerical modelling results output: i) a negligible deamplification of the vibrational input immediately above the tunnels and ii) a relevant perturbation of the ambient vibration noise due to the underground train transit. In addition, a not negligible role of the Tiber alluvia heterogeneities as well as of the presence of buildings can be observed on propagation and surface wave content of the train-induced vibrations.