Analysis of Ground-Transducer Coupling in Monitoring Vibration from Railways: A Case Study

Monitoring railway-induced ground vibrations represents a fundamental step in assessing the environmental impact of train passage on buildings and surrounding receptors. For this purpose, accurate measurements of seismic waves are often required. Nevertheless, depending on soil structure, its characteristics or surface, the placement of transducer may affect the shape and phase of the seismic wave determining signal distortion and finally affecting the measurement itself. In particular, the most critical aspect of the sampling phase is represented by the coupling between the seismic transducer and the soil. Slipping or friction of the transducer may generate unacceptable results. For this purpose, different methods of coupling were tested in different conditions with regard to high speed train passages near a location in Rome. To start the test, some steel spikes of different shapes and lengths were tested in order to investigate their response to ground characteristics, surface and resonance frequency. The sampling campaign was carried out by fastening the transducers on the spikes fixed vertically in the soil at a certain distance from the railway track. Sampled data consisting of each passage in Peak Particle Velocity and acceleration were compared with those obtained by coupling transducers as defined in UNI 9916 norms. According to this norm, sensors are buried into the soil at a fixed depth depending on their dimensions. The final part of the sampling is aimed at comparing the sets of obtained values. This analysis is focused on the identification of those parameters to be taken into account in order to select the best coupling method for different conditions.