Effect of the relative bridge-landslide orientation on the seismic behaviour of a bridge

In the analysis of landslide-infrastructure interaction, parameters such as the orientation of the bridge relative to the landslide movement and the foundation characteristics play a crucial role, significantly influencing the structural response even under identical seismic excitation. This study presents a three-dimensional numerical analysis of an idealised seismic-induced landslide scenario, aimed at systematically assessing the influence of bridge-landslide orientation and foundation systems. Two configurations are considered, with the bridge axis aligned and orthogonal to the landslide direction, each analysed with three deep foundation solutions: pile groups with different diameters and numbers, and large-diameter caisson foundations. The results show that although the landslide is activated only by seismic excitation in both cases, the structural response varies substantially depending on the relative orientation between the infrastructure and the landslide. In the parallel configuration, the behaviour is governed by a longitudinal compression mechanism of the deck, transmitting horizontal forces even to elements outside the unstable volume. Conversely, in the orthogonal configuration, internal piers are dragged along the landslide direction, with deformations concentrated on the directly affected elements and limited stress propagation across the structure. The effectiveness of the foundation system also strongly depends on the bridge-landslide orientation: increasing stiffness significantly improves the response in the parallel case, while its benefits are more limited in the orthogonal case. Therefore, the interplay between structural geometry and landslide movement direction emerges as a key factor, substantially affecting the performance of the adopted foundation solutions.