Experimental and numerical analysis of vibrations induced by underground trains in urban environment

This paper deals with the experimental measurements and numerical predictions of vibrations produced by underground railways, which propagate through the subsoil up to the ground surface. Two test sites were chosen along the new underground railways in Turin and Naples, which are representative of light and heavy railway transit systems, respectively. The vibrations generated by the railway operations were experimentally characterized by monitoring train transits across selected sections. Arrays of velocimeters were placed on the railway tracks and on the ground surface, and, for the case of Naples, also along a 30 m deep borehole. Up-to-date techniques for data acquisition and processing allowed identifying significant records in terms of peak amplitudes and frequency content. The vibrations recorded at the source allowed one to calibrate the analytical modelling of the dynamic interaction between the rolling stock and the railway, taking into account the inertia of vehicles, the track roughness and the mechanical properties of the superstructure. Surface and borehole geophysical tests made it possible to define horizontally layered visco-elastic models of the subsoil at both the selected test sites. The numerical simulation of the ground propagation of vibrations was carried out using a 2D finite difference code (FLAC 5.0), with the experimental record at the track level as kinematic input motion at the inner boundary of the tunnel. The results of the analyses were processed in terms of peak amplitudes at variable depths and on the surface; these latter values were compared with experimental records and threshold levels suggested in international standards.