Identifying the influence of a large alluvial valley on train-induced vibration propagation in Rome by an integrated approach

In recent decades, the number of tunnels has increased within city centres worldwide due to the construction of new underground train lines, thus instigating subsequent issues related to an increase in the amount of vibrations affecting pre-existing buildings and infrastructures; therefore, more studies have been devoted to investigating the interactions between underground train lines and buildings within the framework of a site-city interaction (SCI) scheme. The already designed layout of line C of the Rome Metro was chosen as the research object of a case study focusing on underground train-induced vibration propagation in the city centre of Rome (Italy). Numerical modelling was carried out to analyse the propagation of future vibrations within the large buried alluvial valley and to assess the spatial extension of the induced vibration resentment. Two different subsoil models were considered for this study area: i) a homogeneous model (HoM), in which a homogeneous filling of Tiber alluvia was considered, and ii) a heterogeneous model (HeM), where the Tiber alluvia was distinguished within various lithotechnical units. Specific geophysical measurements were performed to i) record vibrations induced by accelerating, braking and regularly transiting trains to provide input for numerical modelling and ii) define the typical ambient vibration noise of the investigated area. Numerical modelling was then performed using the CESAR-LCPC code, which adopts a finite element method (FEM) solution in the time domain, to simulate the propagation of train-induced vibrations within the Tiber River valley in both the HoM and HeM simulations. The simulation outputs reveal a negligible deamplification effect up to approximately 2 Hz with respect to the considered solicitation in the area located above the tunnels and a maximum effect with respect to the reference ambient vibration noise within 5 and 10 Hz along the valley. The vibrations induced by the accelerating trains can be associated with the highest resentment at the free surface of the valley. A more intense effect generally results in the HeM simulation, which highlights the nonnegligible role of the heterogeneities present within the Tiber alluvia in propagating train-induced vibrations. The numerical outputs highlight that the train-induced vibrations can propagate hundreds of metres away from the underground train location, i.e., up to 400 m astride the axis of the designed tunnels in the case study considered here, and can be distinguished with respect to the regular ambient vibration level.