U.S. 21 Harbor River Bridge Replacement

For more than 80 years, the US 21 Harbor River Bridge was the only way to access a string of three barrier islands on the South Carolina coast. It was also the only hurricane evacuation route available to the several thousand people who call Fripp and Harbor Island home. Over the years the low swing-span bridge had become vulnerable to washouts and storm damage. A modern replacement was necessary.

In 2017, the South Carolina Department of Transportation (SCDOT) began planning to replace the aging swing-span structure with a higher fixed bridge, eventually awarding the project to the United Infrastructure Group (UIG) design-build team which included Infrastructure Consulting & Engineering (ICE) and geotechnical subconsultant GeoEngineers. A national GeoEngineers team headed by our performance-based engineering team provided comprehensive geotechnical services including site exploration, soil characterization, geotechnical design, project management, scheduling and construction oversight.

Building a bridge in a tidal marsh brings its share of geotechnical challenges. Subsurface investigations revealed thick shallow layers of soft, highly compressible silt and clay as well as underlying layers of potentially liquefiable sand. In such complex conditions the owner must carefully balance cost and risk to find a foundation design that is both safe and efficient. During the design-build process, GeoEngineers collaborated closely with the project team and SCDOT to understand their goals, ultimately delivering a series of alternative technical concepts (ATCs) that satisfied the client and earned our team the contract.

The completed bridge is more than 3,000 feet long and 65 feet high, giving plenty of clearance for boat traffic. Supporting the bridge are two 18-foot earthen embankments and a total of 21 bridge bents founded on either driven piles or drilled shafts. The bridge embankments are supported by ground improvement consisting of closely spaced driven timber piles and earthquake drains to mitigate the long-term soil compression and liquefaction risks. A preload program with wick drains was also implemented along the roadway embankments to further mitigate these risks.

Approach

• Geotechnical Design: GeoEngineers analyzed the performance of the bridge which included long term consolidation, liquefaction susceptibility, and global stability for unimproved and improved conditions both during and after construction.
• Foundation Design: GeoEngineers provided recommendations for the bridge foundations which consist of 8-foot-diameter drilled shafts over the middle half of the bridge and driven piles (HP 14×117 steel and 24-inch prestressed concrete) for remaining bridge bents.
• Ground Improvement Program: To meet the project performance criteria, GeoEngineers designed a ground improvement program which included closely spaced driven timber piles, earthquake drains, preload program with wick drains, and reinforced soil slopes at the bridge embankments.
• Construction Observation: GeoEngineers was heavily involved during the construction of the bridge and provided recommendations related to pile driving criteria and surcharge placement/removal timing among other things. Our work helped the contractor remove the surcharge earlier than originally planned and shortened the construction schedule by several months.

Results

Thanks to the hard work and ingenuity of the project team, the new Harbor River Bridge was completed on budget and approximately two months ahead of schedule. A potentially hazardous bridge has been replaced with a modern fixed-span structure that will reliably link Harbor, Fripp and Hunter Islands to the mainland while allowing uninterrupted boat traffic.