Mechanics of Riprap Failure at Bridge Piers

TLDR

Pier scour is addressed by placing coarse riprap stones around bridge piers, yet riprap can fail through shear, winnowing, or edge failure modes. The study investigates riprap stability around cylindrical bridge piers and proposes a semi‑empirical stone‑sizing method. Laboratory flume experiments identified shear, winnowing, and edge failure modes, and the authors developed a semi‑empirical sizing procedure illustrated with field data from three bridge sites, supplemented by two empirical relationships linking riprap thickness and cover to stability. The experiments showed that riprap can fail alone or in combination, but a sufficiently thick layer prevents winnowing, can tolerate partial breakup while rearmoring the scour hole, and two empirical relationships link thickness and cover to stability.

Abstract

Engineering methods to deal with pier-scour problems include placing coarse riprap stones around the bridge pier. This study aims to investigate the stability of a riprap layer around a cylindrical bridge pier. Experiments conducted in a laboratory flume identified three different modes of riprap failure. They can act singly or combine to render a complete failure of the riprap layer. These modes of failure are riprap shear failure, winnowing failure, and edge failure. The study proposes a semiempirical method to size stones for riprap protection. The procedure for this method was illustrated with field data from three different bridge sites. The experimental data show that a thick riprap layer can prevent winnowing in the absence of a filter layer. The thick riprap layer can also sustain a partial breakup of the layer with the capability of rearmoring the scour hole, preventing a total disintegration of the riprap layer. Finally, the study proposes two empirical relationships, which describe the effect of riprap thickness and cover on the stability of the riprap layer.