Correlation of Rock Quality Designation and Rock Scour Around Bridge Piers and211 Abutments Founded on Rock

摘要

Local scour around the base of a bridge pier or abutment occurs as the result of211u001eflow acceleration around the obstruction, formation of a vertical pressure 211u001egradient along the upstream end, and generation of vortices at the base of the 211u001estructure. The flow patterns around piers and abutments are complex in detail, 211u001eand the complexity increases as a scour hole forms in the bed around the 211u001estructures. The potential for local scour at a bridge pier or abutment needs to 211u001ebe estimated so that foundations can be designed to resist failure during large 211u001efloods. While procedures have been suggested for estimating local scour depths at 211u001ebridge piers and abutments located on, or in unconsolidated alluvial material, 211u001eonly interim FHWA (Federal Highway Administration) guidelines (July 1991) in the 211u001eform of memorandum are available for evaluating the scour potential at footings 211u001eplaced on rock. The empirical guides relate quantifiable geotechnical indices to 211u001ea qualitative measure of the ability of the foundation rock to resist erosion. To 211u001etest the validity of some of the more significant FHWA guidelines for assessing 211u001ethe rock scour, on-site inspections of some 400 bridges with footings located on 211u001eexposed rock were performed. A large number of bridge abutments and piers in 211u001eKentucky were found to have been placed on rock foundations that are visible 211u001eduring low flow. These inspections show that scour at bridge piers and abutments 211u001eplaced on rock does occur.