Seismic Behaviour of a Viaduct Affected by Foundation Scour: A Numerical Study

Foundation scour is one of the major causes of failure for bridges over waterways. Its detection is challenging, and limited financial resources often delay necessary repairs and retrofitting. Thus, scoured bridges remain more exposed to future extreme events, such as earthquakes, to which they may be more vulnerable due to the weakened support conditions. Therefore, predicting the behavior under the combined presence of scour and seismic events accounting for soil-structure interaction becomes necessary to understand the long-term performance of a road bridge. This paper presents the results of a numerical study carried out on a continuous deck viaduct located in northwestern Italy. The viaduct is a seismically isolated multi-span steel-concrete composite deck, supported on short piers. The foundation system for each pier is a mat on large-diameter piles. Some piers are subject to lateral scour, due to the erosive action of a river flowing parallel to the viaduct. A finite element model was developed using ABAQUS to investigate the influence of foundation scour on the dynamic response of the viaduct under seismic motions. This study compares the results of variants of the numerical model: one representative of the original configuration, i.e. without scour, and two corresponding to alternative scour scenarios. The superstructure and the seismic isolators were modeled through elastic elements and viscoelastic springs, respectively. Instead, the soil-pile interaction was introduced through the beam-on-nonlinear-Winkler foundation method. Foundation scour induces a significant increase in the peak horizontal displacements at the scoured piers, up to 15-20%. A significant change in both the amplitude and the frequency content of the deck displacements is also observed. These preliminary results contribute to a deeper understanding of the response of structural systems under the combined presence of pre-existing foundation scour and seismic loading, which is crucial for an adequate assessment of bridge vulnerability in seismically active regions