GNGTS 2022 - Atti del 40° Convegno Nazionale

368 GNGTS 2022 Sessione 2.2 Geophysical investigations and bridge experiments. The bridge has a total length of about 378 m and consists of 10 spans. The first and last span have a length of 33.55 m, the remaining have a length of 34 m except for the span between columns 6 and 7 which has a double length of 73 m. The bridge is supported by 9 columns and the monitored span is between the columns 4 and 5 (Fig. 1). Geophysical investigations have been carried out in order to characterize the soil foundation below the bridge. These investigations would have also suggested the depth of the seismic bedrock, and therefore the depth of the perforation in which install a sensor with the objective to record the ground motion without being altered by the interaction with the bridge. The seismic tomography was realized under the western half of the bridge (columns 1-6). The resulting 2D profile shows a difference in the thickness of the surface layer between the two ends of the investigated line. It also clearly highlights the foundations in correspondence of the pillars (Fig. 2a). This difference in thickness at the ends of the investigated line was also observed by two 1D profiles obtained from MASW (Fig. 2b, c). Finally, from the microtremor measurements it is possible to observe the presence of two frequency peaks: one of about 1.6 Hz and one of about 4.3 Hz (Fig. 2d). Fig. 2 - Results of geophysical surveys: seismic tomography (a), MASW (b and c), microtremor (d), and their location (e). The results of the preliminary geophysical investigation suggested to drill the perforation at least 40 m deep. The perforation has been finally drilled down to 51 m and posthole 3-axial seismometer (model Nanometrics Trillium PH 120 s) has been placed its bottom (Fig. 1c) and set at 100 Hz sampling rate. A 3-axial accelerometer (Nanometrics Titan) has been placed at surface, next to the perforation top (Fig. 1c), at sample rate of 200 Hz. This couple of sensors, installed in January 2022, will be maintained installed for a long-term monitoring. In order to calibrate the velocimeter in the well, a velocimeter was installed at ground surface. Both sensors acquired a seismic event that occurred in January 2022. The difference between the surface and down-hole sensor is notable (Fig. 3). While the former is constantly affected by the noise of the morning traffic over the bridge (Fig. 3a), in the latter the waveforms of the event are clearly recognizable (Fig. 3b) even though a notable amount of noise is still present. In order to characterize the vibrating modes of the bridge, a devoted experiment had been carried out in April 2022. It was a temporary experiment lasting some hours; because the