Correlation of Rock Quality Designation and Rock Scour Around Bridge Piers and Abutments Founded on Rock

Abstract

Local scour around the base of a bridge pier or abutment occurs as the result of flow acceleration around the obstruction, formation of a vertical pressure gradient along the upstream end, and generation of vortices at the base of the structure. The flow patterns around piers and abutments are complex in detail, and the complexity increases as a scour hole forms in the bed around the structures. The potential for local scour at a bridge pier or abutment needs to be estimated so that foundations can be designed to resist failure during large floods. While procedures have been suggested for estimating local scour depths at bridge piers and abutments located on or in unconsolidated alluvial material, only interim Federal Highway Administration (FHWA) guidelines (July 1991) in the form of a memorandum are available for evaluating the scour potential at footings placed on rock. The empirical guides relate quantifiable geotechnical indices to a qualitative measure of the ability of the foundation rock to resist erosion. However, the procedure lacks documented proof or verification by means of either experiment or observation. To test the validity of some of the more significant FHWA guidelines for assessing the rock scour, on-site inspections of some 400 bridges with footings located on exposed rock were performed. A large number of bridge abutments and piers in Kentucky were found to have been placed on rock foundations that are visible during low flow. These inspections show that scour at bridge piers and abutments placed on rock does occur. As a means of evaluating rock scour at existing sites, a rock scour hazard rating system was proposed. Based on the inspections, rock scour is not a significant problem in Kentucky. Statistically, only about 0.5% of the observed bridges had significant vertical scour. Generally, when scour was observed the depth of vertical scour was less than about 10 in. In the few cases observed, the scour holes could be easily repaired by filling with concrete. An approximate relationship between vertical rock scour depth next to the abutment, or pier, and Rock Quality Designation (RQD) was developed. Also, an approximate relationship between horizontal rock scour penetration beneath a pier, or abutment, footer and RQD is presented. Generally, the geology at sites where scour was observed consisted of interbedded layers of limestone and shale layers, or very thin shale partings. Freezing and thawing apparently caused the blocks to break up and flooding velocities of the streams tended to wash out the blocks around footers.