Protecting Bridges from Scour

Scour is the primary cause of bridge failures in the United States. Unlike most forms of deterioration that occur gradually, scour can destroy bridges quickly, especially during floods. After several bridge scour failures in the 1980s, the Federal Highway Administration (FHWA) and the engineering community changed the way bridges are evaluated for vulnerability. Bridges now undergo regular underwater inspections, evaluations to predict performance during floods and monitoring of streambed elevations. Corrective Plans of Action (POAs) are developed for vulnerable bridges. Today, National Bridge Inspection Program Metric No. 18 (Scour Critical Bridges) and Inspection Item 113 (Scour Rating) quantify scour conditions, inspection requirements, and designate compliance. To comply with FHWA scour directives, bridge owners must prioritize evaluations and implement POAs for scour-critical bridges. In April 2012, the FHWA provided another in a series of formal scour memoranda to all 50 states that clarified the expectations for implementing POAs, provided strategies for prioritizing various categories of bridges, and reinforced a 2008 directive that scour-critical ratings must be assigned to bridges with unknown foundations by 2010. If a bridge is rated scour critical, a POA must be developed and corrective action implemented. The Maine Department of Transportation’s scour management plan is highlighted as an example of how a state can meet the FHWA expectations. Although rating bridges for scour is part of the challenge, the ultimate goal is to protect them. One countermeasure that is commonly used in Europe and shows promise in the United States is partially grouted riprap (PGR). PGR construction involves placing riprap on top of a gravel filter base and/or geotextile fabric. The voids between stones are partially filled by a flowable cement-based grout, interlocking the armoring stones. This increases both the mass of the repair and its hydraulic stability. Compared to traditional countermeasures, PGR requires less material, reduces the need for channel excavation, and improves performance during floods.