Geotechnical and hydraulic processes during scour at spill-through bridge abutments

Most cases of scour-induced abutment failure at spill-through bridge abutments show a geotechnical failure of the spill-slope extension of the abutment's earth-fill approach embankment. Extensive research and numerous publications have examined the flow hydraulics at spill-through abutments, but very few have addressed the geotechnical aspects and failure mechanisms of the abutment itself. This thesis shows how abutment failure is attributable to the combined effects of hydraulic erosion and geotechnical instability and how the geotechnical strength of the spill-slope soil influences the extent and location of abutment scour. The thesis entails experiments with a numerical slope-stability model and the conduct of laboratory flume experiments using strength scale-reduced models of spill-through abutments. These numerical and laboratory experiments revealed that geotechnical and hydraulic processes cause abutment scour to occur in a significantly different manner than previously documented in literature. Scour progresses initially at the toe and waterline of the abutment's upstream corner, doing so by hydraulically eroding the spill-slope soil that leads to under-cutting and ultimately toppling of blocks of exposed spill-slope soil. This sequence continues until the spill-slope is eroded back to the abutment column, and the abutment is breached. The experiments successfully used scale-reduced model spill-slope soil that field case-study information indicates replicated the geotechnical and hydraulic erosion of actual abutments. Results regarding the exact relationship of spill-slope soil strength on abutment scour depth are inconclusive, indicating the need for further experiments using a more refined instrumentation system than available for the present study.