Urban Dark Fiber Distributed Acoustic Sensing for Bridge Monitoring under Road Traffic Sollicitation

The vulnerability of urban assets, encompassing soils, buildings, and infrastructure, is intricately linked to human activities, environmental exposure, and societal vulnerabilities. By employing Distributed Acoustic Sensing (DAS) technology on telecom fiber networks, we have developed an information model that facilitates data extraction, exchange, and networking to enhance decision-making regarding civil engineering assets. In collaboration with the APRR-AREA French company, our focus lies on two long-span bridges situated along a 25 km stretch of telecom optic fiber on the A40 motorway concession — known as the "autoroute des Titans" — in eastern France. Our objective is to characterize the deformation of these structures under heavy vehicle traffic, aiming at discerning behavioral disparities between spans and identifying potential plasticization zones. Employing a standard beamforming approach, we track vehicles using DAS data records. A statistical analysis of signal amplitudes helps to establish standard deformation thresholds and to detect anomalous deformation events. We conduct diagnostics to identify the origins of these events through DAS signals identification and classification. Simultaneously, we continuously monitor the vibrational characteristics of the structures, including frequencies, damping, and modal shapes, to ascertain if traffic-induced deformations enter the plastic regime. Throughout the 68-hour acquisition campaign, we tracked 5 423 vehicles with weight ranging from 2 568 to 73 123 kilograms. On one of the bridges, our analysis revealed 402 events exceeding the 5-sigma threshold, 58 surpassing the 10-sigma threshold, 33 exceeding the 12-sigma threshold, and 4 surpassing the 15-sigma threshold. We will analyse if these events coincide temporally and spatially with those detected with conventional long-base extensometers deployed all along the bridge deck. There is no indication of plastic deformation. This study highlights the potential of utilizing DAS technology applied to telecom optic fibers to complement specialized instrumentation for monitoring the behavior of long-span bridges.