Prediction of Railway-Induced Ground Vibrations in Tunnels

The authors of this paper present the results of a study concerned with the assessment of the vibrational impact induced by the passage of commuter trains running in a tunnel placed underground the city of Rome. Since the railway line is not yet operational, it was not possible to make a direct measurement of the ground vibrations induced by the railway traffic and the only way to make predictions was by means of numerical simulations. The numerical model developed for the analyses was calibrated using the results of a vibration measurement campaign purposely performed at the site using as a vibration source a sinusoidal vibration exciter operating in a frequency-controlled mode. The problem of modeling the vibrational impact induced by the passage of a train moving in a tunnel is rather complex because it requires the solution of a boundary value problem of three-dimensional elastodynamics in a generally heterogeneous, nonsimply connected continuum with a moving source. The subject is further complicated by the difficulties of modeling the source mechanism, which constitutes itself a challenge even in the case of railway lines running at the surface. At last, the assessment of the vibrational impact at a receiver placed inside a building (e.g., a human individual or a sensitive instrument) requires an evaluation of the role played by the structure in modifying the computed free-field ground motion. So far, few attempts have been made to model the whole vibration chain (from the source to the receiver) of railway-induced ground vibrations, with results that have been only moderately successful. The numerical simulations performed in this study were made by using a simplified numerical model aimed to capture the essence of the physical phenomena involved in the above vibration chain including the influence of the structural response as well as the dependence of the predicted vibration spectra on the train speed.