Laboratory investigation of geotechnical and hydraulic influences during abutment scour

Scour of spill-through abutments occurs due to the combined influence of geotechnical and hydraulic processes induced erosion acting on an abutment's compacted earthfill approach embankment. This thesis is focused on the geotechnical process associated with the failure of the compacted earth spill-slope region of spill-through abutments, and the effects of the geotechnical strength of spill-slope soil on the extent, depth, and rate of abutment scour. Extensive laboratory methods were conducted in which the strength properties of model spill-soil were controlled, and hydraulic models of compacted model embankment soils were prepared, all in order to determine how soil shear strength affects abutment scour. The experiments were mainly done using sand compacted to varying extents. These experiments let to new and useful insights addressing abutment erosion and failure. Erosion begins at the embankment spill-slope's upstream corner where flow constriction around an abutment exposes spill-slope soil to the highest values of flow velocity and turbulence. This initial erosion continues towards the middle portion of the spill-slope face, and then progresses downstream. Erosion is marked by the formation of undercut, exposed vertical blocks of embankment soil whose failure occurs relatively quickly once the spill-slope face begins eroding, and slows as spill-slope erosion increases the area of flow at the abutment. Increased flow area reduces flow velocities and thereby eases hydraulic erosion of the channel around the abutment and of the abutment itself. This thesis shows that abutments formed of soils with higher shear strength take longer to eroded, have bigger blocks of failed soil, and tend to produce deeper scour of the channel around the abutment. This thesis indicates requirements for promising further research regarding the influence of soil strength on abutment scour.