Misure mitigatorie dei fenomeni erosivi localizzati attorno a pile dei ponti esistenti

The functionality of a river crossing must be ensured not only under normal conditions but especially during hazardous events, e.g. floods, when the efficiency of the infrastructure becomes fundamental to ensure the mobility of the rescue teams and the evacuation of the inhabitants. However, this is not currently accomplished, so that hydraulic phenomena account for over $50\%$ of bridge failures. Among the hydraulic causes, the localized erosion at bridge piers that mainly develops during flood events plays a relevant role, also because it cannot be recognized or detected through a simple visual inspection. In the present work, the localized erosive phenomena around in-river structures of bridges have been extensively investigated in an effort to clarify the unclear aspects of the phenomena coming from literature analysis. The focus has been mainly devoted on the scour around piers of different geometry, considering circular piers, elongated piers also supported by the foundation, and twin circular pier supported by the foundation. Attention has been posed on the time evolution of maximum depth and volume (or area) of the scouring, as well as on the influence of the flow depth respect to the width of the pier. Additionally, the other concurrent parameters influencing the scour depth in laboratory experiments have been specifically considered. The knowledge acquired in this initial phase was instrumental in preliminarily defining the geometric characteristics required for an anti-erosion mattress. Both past and current rip-rap protection methods have been considered, including launchable stones (developed in the past) and rip-rap mattresses (currently adopted). As an empirical maintenance technique, old bridges are still working thanks to the ancient custom of filling any recognized scour around piers with launchable stones after each relevant flood event. Currently, rip-rap mattresses of appropriate thickness are often suggested as scour countermeasures for existing bridges, as an alternative to structural reinforcements such as foundation deepening. Both the techniques (launchable stones and rip-rap mattress) have been studied through laboratory experiments considering piers of different geometries, suggesting for the mattresses some improvements respect to the technical solution reported in literature. At the end, the performance of monitoring techniques has been investigated, exploring the ability of sonar sensors to continuously monitor the scour evolution, and investigating sensor measurements in several locations around piers comparing them with maximum scour depths. If effective, these monitoring techniques can provide valuable insights to prevent dangerous conditions, even during the progression of flood events. The experimental activities were primarily conducted under clear water conditions, although some rip-rap linings were tested with flow velocities exceeding the critical threshold. The following are the main findings derived from the conducted tests and their critical analysis in the context of existing literature: - new insights into the temporal evolution of maximum scour around shallow and narrow piers have been gained; - enhancements to current rip-rap mattress configurations have been developed, proposed, and subjected to experimental testing; - radar monitoring techniques have been evaluated on complex pier geometries under clear water conditions, demonstrating how the effectiveness in critical scour recognition varies depending on sensor placement.