Structural Reliability of Bridge Caisson Foundations under Scour: A Framework Linking Experimental Data to Vulnerability Assessment

Scour around bridge foundations is a leading cause of bridge failure, and it can progressively degrade a foundation's stiffness well before it threatens bearing capacity outright. Most of the impedance-based and vibration-monitoring literature linking scour to a measurable structural response has been developed for shallow or slender foundations, where the coupling between horizontal and rocking response is small and often neglected; how a deeply embedded caisson, whose foundation impedance is dominated by extensive sidewall contact rather than base contact, responds to progressive and directionally asymmetric scour has received comparatively little quantitative attention. This thesis addresses that gap with a three-dimensional finite element model, built in PLAXIS 3D with the Hardening Soil model with small-strain stiffness, of a deeply embedded caisson foundation. Its scour geometry is driven directly by a physical flume experiment, scanned at several progressive durations, rather than approximated from an idealized scour profile, so the model tracks an irregular, asymmetric excavation as it actually develops around the foundation. The intact model is validated against established analytical solutions before any scour geometry is introduced, and the same probing procedures used for that validation are then applied at every scour stage to extract the full impedance matrix, including its off-diagonal coupling term, which links horizontal and rocking response and grows in relative importance with embedment depth. Beyond the linear stiffness degradation itself, the thesis compares that degradation against the caisson's nonlinear pushover capacity and its coupled dynamic frequency response, to establish whether the three measures move together or reveal different aspects of the same underlying process. It also situates its results against the existing caisson- and monopile-scale literature, most of which addresses shallower or more slender foundations, to explain why a deeply embedded caisson's directional response to scour might diverge from that literature's expectations. Finally, the thesis discusses the feasibility of extending the same point-cloud-to-finite-element pipeline and frequencymonitoring approach to a real, field-scale bridge, where a bathymetric survey and a vibration-based monitoring system could together supply the scour geometry and the measured frequency response needed to apply this diagnostic approach outside the controlled conditions of a flume experiment.