Eagle Island

Wilmington, North Carolina, is a city of 80,000 people and is located on the stat’s southeastern coast. A network of sandy barrier islands rest just offshore and protect the mainland from oceanic forces, which can be tricky here due to the proximity of the continental shelf and Gulf Stream currents. Tucked between the Brunswick and Cape Fear Rivers and opposite of Wilmington’s port is Eagle Island, a historic site that hosts a number of military and cultural stopping points, such as a retired battleship and old plantations.

Also, it’s the site of an important dike network, much of which is used to contain dredge spoils from the surrounding rivers. This creates an excellent habitat for sea birds – which is especially important given the region’s expanding population and land redevelopment needs.

Projects have been put in place to increase Eagle Island’s spoils capacity by raising and reinforcing the dike beams. This summer, the United States Army Corps of Engineers (USACE) sought to improve two of the dikes cells (Numbers 1 and 3). Bids were solicited and a contract was awarded to a state-based contractor, Aldridge Brothers Inc., Robbinsville, N.C.

Installation

Working from USACE’s Wilmington office design, the contractor acquired and installed reinforcement materials from Synteen Technical Fabrics. Two types of reinforcement geosynthetics were used: SF 11 biaxial geogrids and SF 500 high-strength polyester fabric. Flint Industries of Georgia performed some factory fabrication to reduce the need for field seaming on some of the rolls. Of the geogrids, 136,000 yd. (113,713 m) were shipped in 17ft. (5.1 m) wide rolls. A complementary 115,000 yd. of the polyester fabric was shipped as well in 36-ft. (11-m) panels that varied in length from 62 to 82 yd. (56 to 75 m).

The 36-ft. panels were seamed in the field by the contractor, who used prayer seams to a 200 lb./in. break strength. The installation process involved a careful balance of earthwork and material handling between layers of sand, geogrid, sand, polyester fabric, and fill.

Vital work

The project called for a wide, flexible geogrid that had the requisite tensile strength and 2% and 5% strain values. The polyester fabric was high strength (>3000lb./in. ultimate tensile strength), and the manufacturer was able to custom manufacture individual panel lengths for the contractor.

Of note, cell Number 1 needed to be completed in August so dredging activity could continue on schedule. The site’s barge, when present, costs $50,000 per day, regardless of the site’s readiness.

Kim Roberts of Synteen, www.stf-inc.com, and Larry Gindstaff and Fred Parker of Aldridge Brothers contributed information and images to this piece.