Research Pays Off: Evaluating Bridges with Unknown Foundations for Vulnerability to Scour: North Carolina Applies Risk-Based Guidelines

This Research Pays Off article describes how the North Carolina Department of Transportation (NC DOT) saved more than $7 million by implementing National Cooperative Highway Research Program risk-based management guidelines for scour at bridges with unknown foundations.

For approximately 10 percent of the estimated 600,000 bridges that span waterways in the United States, the “as built” information—that is, the details of the final structure—is not available or is missing. The National Bridge Inventory (NBI) of the Federal Highway Administration (FHWA) classifies these as bridges with unknown foundations. Scour is the removal of material such as sand and rock around a bridge foundation—the abutment and piers—by flowing water. Scour affects the stability of the foundations of bridges over water and contributes to an estimated 60 percent of all U.S. bridge failures.

In 2004, the NC DOT established a plan of action for bridges with unknown foundations.  It did so by performing pile integrity testing (PIT), which is labor intensive and time consuming. From 2004 to 2011, NC DOT evaluated 1,398 bridges at a total cost of $2.7 million, but 4,602 bridges with unknown foundations still required evaluation.

A quicker, less expensive, but reliable method was needed. NC DOT found such a method in the National Cooperative Highway Research Program (NCHRP) Web-Only Document 107: Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations, which details a risk-based approach to managing bridges in the absence of foundation information.

To carry out its action plan for bridges with unknown foundation, in 2010, the NC DOT’s Geotechnical Engineering Unit used bridge inventory reports to select 3,752 bridges from the 4,602 still to be evaluated. NC DOT officials then used the NCHRP risk-based management guidelines to evaluate the bridges with unknown foundations for vulnerability to scour. The evaluation of these bridges was completed in 2012 at a total cost of $21,000, or $5.60 per bridge.

The average cost to North Carolina DOT for evaluating a bridge with the conventional method was $1,900—which would have meant a total of more than $7 million to evaluate all 3,752 bridges. Furthermore, the maximum number of bridges that NC DOT could evaluate with the conventional method was 200 in a year, which would mean that processing the 3,752 bridges would have taken more than 12 years. In contrast, using the risk-based approach enabled NC DOT to complete the evaluations in three months. In addition, the scour vulnerability evaluations have ensured the safety of the traveling public, with no traffic disruptions.

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