Building resilience: Terracon helps deliver safer Alligator River crossing

Client

Skanska

Location

Columbia, NC

Challenge

Eastern North Carolina faced a major transportation obstacle: a 55-year-old swing-span bridge over the Alligator River that caused frequent traffic delays because of maintenance and mechanical failures. As a vital hurricane evacuation route to the Outer Banks, the bridge needed a reliable, resilient replacement.

However, the site’s geotechnical complexity — soft marine clays beneath dense sands, variable soil layers, and water depths up to 14 feet — presented significant engineering challenges.

The replacement bridge will be a new, modern fixed-span bridge, casually referred to as the “Alligator River Bridge.” It is one of North Carolina’s most complex infrastructure projects, designed and built via collaboration with Terracon, Skanska, and the North Carolina Department of Transportation (NCDOT).

Solution

Terracon’s innovative geotechnical approach was key to addressing complex soil conditions and optimizing foundation design. Fieldwork involved eight over-water borings from barge platforms, depths exceeding 150 feet, and adaptive protocols to overcome unpredictable weather and logistical constraints. All testing adhered to AASHTO standards in an NCDOT-certified Tier II laboratory.

What set this project apart was not only the technical rigor but the field complexity. Mobilizing heavy and awkward equipment from docks and crew boats to barge platforms required precision and coordination. The team faced unpredictable weather — high winds, sudden spring storms, and intense sun glare off the water — while operating in cramped barge conditions.

Safety was paramount, with daily tailgate meetings and constant communication with boat captains to support safe extraction when conditions worsened. As one team member put it, “There are no heroes here, just guys that go home every night,” underscoring the harrowing nature of the work.

Results

Terracon’s data-driven insights enabled precise foundation design for pile-supported bents, providing structural integrity and long-term resilience. Importantly, our work was delivered on schedule and within budget. The new fixed-span bridge, scheduled to be completed in 2029, will:

• Eliminate traffic delays and mechanical failure risks.
• Enhance safety as a hurricane evacuation route.
• Support multimodal transportation with dedicated bike lanes.
• Integrate broadband infrastructure for digital equity.

Terracon was honored with the Grand Award in Transportation and the People’s Choice Award at ACEC North Carolina’s 2026 Engineering Excellence Awards for our work on the Alligator River bridge replacement project.