New US 21 Bridge is Vital Link to South Carolina Barrier Islands

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, and a GeoEngineers team led by King Chin and Zack Simpson helped the South Carolina Department of Transportation (SCDOT) get it done.

In 2017, SCDOT began planning to replace the aging swing-span structure with a higher fixed bridge, eventually awarding the project to a design-build team which included GeoEngineers as geotechnical subconsultant. 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 that satisfied the client and earned our team the contract. To learn more about the technical details of this project, see the full project profile.

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.

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