Bridge abutment scour countermeasures
The use of riprap and cable-tied blocks as scour countermeasures at bridge abutments is investigated. Riprap is the most common armouring scour protection method used at bridge abutments and approach embankments. Despite the widespread use of riprap protection, the guidelines for its use at bridge abutments are based on limited research. The aim of the experimental study was to determine the requirements of riprap and cable-tied block apron countermeasures to protect bridge abutments from scour damage, and to produce design guidelines for their use.
The two types of bridge abutments used in the experimental study were a spill-through abutment situated on the floodplain of a compound channel, and a wing-wall abutment sited at the edge of the main channel. The spill-through abutment experiments were run under clear-water conditions, and the variations in the scour hole geometry were measured for different abutment and compound channel geometries, apron widths and apron types. The wing-wall abutment experiments were run under live-bed conditions, and the settled apron geometries were measured for different flow depths, flow velocities, apron widths, apron types and apron placement levels. The flow fields around the abutments were also measured for both abutment types.
The clear-water spill-through abutment results show that the protection aprons do not significantly reduce the scour depth at abutments, but instead deflect the scour hole further away from the abutment, protecting it from scour failure. The experiments also show that cable-tied block aprons allow the scour hole to form much closer to the abutment compared to equivalent riprap aprons. Equations were developed to predict the scour hole position and size, and the minimum apron extent required to prevent the scour hole from undermining the abutment. For the live-bed wing-wall abutment experiments, the troughs of the propagating bed-forms undermined the outer edges of the aprons, causing them to settle. Equations were developed to predict the settled apron geometry at the equilibrium scour conditions.
The predicted scour hole depth and position for clear-water scour conditions, or the predicted apron settlement geometry for live-bed scour conditions can be used in a geotechnical stability analysis of the abutment. The geotechnical stability analysis forms the basis of the abutment scour countermeasure design procedure, which was developed from the experimental study. Further experimental work is required to increase the robustness of the bridge abutment scour countermeasure design procedure and make it applicable to a wider range of situations.