Collapse Mechanism of a Bridge-Size Reinforced Concrete Box Culvert Following Scour-Induced Loss of Soil Support

A three-cell, cast-in-place reinforced concrete box culvert of bridge size ( 8 × 8 m cells; total length 24 m) experienced catastrophic settlement 9 months after a major flood. Field investigation revealed severe and asymmetric scour at the culvert foundation, and forensic analysis of the observed settlement and cracking established that the resulting loss of support extended over approximately 80% of the culvert length. Finite-element analysis incorporating soil–structure interaction and nonlinear wing wall contact showed that, under this condition, the culvert entered a metastable restrained state sustained by frictional forces at rigid wing walls. Loss of this restraint triggered cantilever action, causing shear and flexural demands at a critical section to exceed capacity and produce sudden collapse. Parametric analysis further indicated that culverts in this compromised configuration may experience 20–30% higher seismic demands than under dead load alone. The study identifies a restraint-controlled delayed collapse mechanism in large box culverts on erodible soils and highlights the need for design and assessment procedures to consider partial support loss and wing wall restraint, which are not addressed in conventional provisions.