Scour and foundation damage at theVespucci Bridge on the Arno River in Florence (Italy)

The Amerigo Vespucci Bridge on the Arno River in Florence (Italy) was built in the 50’s of the last century. The bridge has three spans and a total length of 155.5 m. The deck is in pre-stressed reinforced concrete, the piers, the abutments and the foundations are in reinforced concrete. The foundations of the piers consist of caissons supported by bored piles, with diameter 0.40 m and length 15 m. The piles are weakly reinforced only in the first 3 m from its head. At the time of the construction of the bridge, the subsoil stratigraphy consisted of a surficial alluvial layer of sand and gravel with a thickness of 10-12 m overlying a deep layer of overconsolidated clay. The bridge is located immediately downstream of a hydraulic weir which is heavily inclinated with respect to the water flow direction. Such inclination causes a deviation of the water flow and situations that can be very different from each other even within a few tens of meters of distance. In particular, the right pier of the bridge is affected by a conspicuous deposition phenomenon while close to the left pier the riverbed has lowered causing a deep scour around and below the caisson foundations. At present the depth of the erosion is variable and has a maximum value of about 7.85 meters. The lowering of the riverbed has brought to light the foundation piles, exposing them to the erosive action of the water and the transported debris. Therefore, many piles appear to be damaged, broken or even absent. The phenomenon described was highlighted by surveying the riverbed and the foundation structures by multi-beam technology. In this paper the bearing capacity reduction of the pier foundation is estimated.