A practical method to incorporate scour hole shape effects in Winkler-type pile foundation models for bridges

Scour erosion continues to cause significant issues for the stability and lifespan of bridges worldwide. In Slovenia, extreme flooding in 2023 caused the collapse or failure of many bridges, primarily due to scour, which was exacerbated by debris accumulation. Despite advances in predicting and monitoring scour, it remains among the top reasons for the failure of bridges during flooding. Recent advances in vibration-based health monitoring suggest that scour erosion can be detected using methods such as changes in natural frequency, mode shapes, flexibility-based deflection, and other approaches using offline sensors such as passing vehicle responses. Many of these methods have been trialled numerically where scour is implemented as a reduction in the soil level (or stiffness) around a given bridge foundation. The most common way to model scour is to lower the soil level around a foundation, however, this ignores any contribution that the scour hole shape makes to the stiffness and strength of the soil beneath the scour hole. This paper investigates how the shape of scour holes influences the stiffness and strength of the remaining soil to understand the impact on the modal behaviour of a bridge. A numerical model of a bridge is developed where scour is implemented by removing Winkler springs from the model, and different scour hole shapes are considered in terms of how the remaining overburden influences the stiffness and strength of the soil springs. Scour hole shape properties are considered by means of varying the depth, width, and slope angle of the hole around a given foundation element. For the analyses in this paper, different scour hole shapes are implemented on an example bridge corresponding to local scour holes with narrow width; wider local scour holes; and general scour, where the full soil layer is removed (infinite width). The changes in the modal periods and mode shapes of the bridge in the traffic and river flow directions are assessed to understand the impact of the different scour types on the vibration characteristics.