Riprap Protection of Bridge Abutments under Clear Water Conditions

Criteria for selecting riprap to protect bridge abutments against scour were investigated. experiments were conducted with a spill-through abutment under clear water conditions for various riprap sizes, apron sizes and apron extents. The abutments were constructed from the bed sediment and riprap was laid on top, to a thickness of two stone diameters. Tests were run for a period of 24 hours, after which the abutment was assessed for failure, either total, partial or not at all. An abutment was adjudged to have failed if the side slopes of the abutment (including the riprap blanket) had shifted in any way. Initially, the riprap apron was constructed following the HEC-23 recommendations. This recommendation was found to be conservative in all cases, and was reduced by increments in subsequent tests until an abutment failure occurred. This process was carried out for two abutment lengths and three riprap sizes. A useful relationship was found between the dimensionless riprap size and apron extent, showing a clear zone of partial failure.