Bridges over rivers and streams are under constant attack by the powerful forces of moving water, particularly during spring snowmelt or after significant rains. Unlike gradual structural deterioration, powerful floods can quickly destroy bridges. According to the Federal Highway Administration (FHWA), erosion around bridge foundations (piers or abutments) – scour –is the leading cause of bridge failures nationally. After several high-profile bridge collapses in the mid 1980s that were attributed to scour, FHWA required bridge owners to rate structures for scour risk, predict their performances during floods, and develop plans of action (POAs) to protect the traveling public before, during, and after severe weather events.
Rating bridges for scour is only part of the challenge. Protecting them is the ultimate goal to ensure public safety and to avoid the potential loss of life from scour-related bridge failures. Also, repairing a bridge is much less costly than replacing one. The New Hampshire Department of Transportation (NHDOT) developed a statewide program to evaluate and monitor scour-susceptible bridges and recommend needed countermeasure repair and protection strategies on a priority basis. This provided NHDOT an opportunity to consider a new type of countermeasure: partially grouted riprap (PGR).
PGR is new in the United States, but has been used widely in Europe with great success. PGR repair offers several advantages compared with traditional countermeasures by generally using less material, requiring minimal channel excavation, and performing better during floods – providing more effective scour protection. Presently, the FHWA lists PGR as a recommended protection alternative in its latest scour design manual, HEC 23 (www.fhwa.dot.gov/engineering/hydraulics/pubs/09111).
PGR construction involves placing angular stone on top of a gravel filter base and/or geotextile fabric. A flowable cement-based concrete grout is used to partially fill the voids between stones, thereby interlocking the armoring stone and concrete grout. This increases the mass of the protective stone and concrete layer, and significantly increases its hydraulic stability.
New Hampshire's first PGR installation was performed on a 25-foot, single-span bridge in the town of Holderness that was damaged by severe spring floods and ice jams in 2011. This flood event resulted in one abutment being undermined, a washed-out roadway approach, and minimal remaining embedment of the substructure footings. PGR repair was recommended by NHDOT's consultant, CHA, based on several factors, including the minimal excavation required to place the recommended 18-inch-thick stone layer. This reduced excavation depth would not threaten or undermine the abutment footings, as compared with the excavation depth required for placement of typical stone protection. Further, it was determined that the PGR repair will withstand twice the expected peak flow velocities.
Installation occurred during the fall of 2011, and the resulting PGR scour protection is expected to keep the bridge in service for many years. This successful repair is believed to be the first PGR installed by a state DOT, and one of the first installations in the United States. NHDOT and CHA are developing construction specifications for PGR and expect to introduce it to the contracting community throughout 2012.
Bill Horne, P.E., is a senior transportation engineer and project manager at CHA. He has 20 years of structural, geotechnical, and hydraulic engineering experience evaluating scour at thousands of bridges. He can be contacted at email@example.com.
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