Countermeasures to Protect Bridge Piers from Scour

Abstract

This report is in fulfillment of NCHRP Project 24-7, Countermeasures to Protect Bridge Piers from Scour. The focus of the report concerns alternatives to standard riprap installations as pier scour countermeasures. Two kinds of countermeasures were examined: flow altering countermeasures such as sacrificial piles and armoring countermeasures such as mattresses of cable tied blocks. None of the flow altering countermeasures were found to be overly effective. Under flood conditions in sand bed streams, riprap placed in the absence of a geotextile or granular filter layer was found to gradually settle and lose effectiveness over time even under conditions for which the riprap is never directly mobilized by the flow. This settling is due to deformation and leaching of sand associated with the passage of bedforms. Riprap performance can be considerably improved with the use of a geotextile, especially if the geotextile is sealed to the pier. Another countermeasure that provides excellent protection is a mattress of cable tied blocks underlain by a geotextile tied to the pier. Design recommendations are provided for a number of armoring countermeasures.