Veterans Drive in St. Thomas, U.S. Virgin Islands, is a 2-mile-long seaside road that offers breathtaking views of the harbor and sea and serves as an anchor point for the city of St. Thomas.
In 2017, American Bridge Company contacted WSP USA to provide value-engineering design services to improve a half-mile seawall portion of the road, which was badly in need of repair. The project included road widening, pavement reconstruction, seawall construction, and related work as part of a larger revitalization of the St. Thomas Waterfront and Downtown Charlotte Amalie.
The owner originally proposed a stacked, precast concrete block design, but the team determined that such a structure would be too risky and too expensive. Also, the supply of materials at the time was undependable. After considering several alternative options, the team selected a precast concrete counterfort wall design that could be produced and transported from Coastal Precast Systems in Virginia.
“The precast concrete counterfort gravity wall design incorporated all of the benefits of the design-build process by tailoring a unique design to meet the owner’s requirements, the contractor’s abilities, and the budget,” says Justin Berglund, project manager for American Bridge Company.
GPS replaces divers
The design features individual, standard precast concrete wall units composed of a tapered base slab, a wall stem, and single or double counterforts based on height of soil retained; some counterforts were as tall as 16 ft. The precast concrete producer developed the piece-wise layout of the units in the plant to ensure they would fit the exact alignment of the project site.
“The overall design and fabrication of the pieces allowed some flexibility in trimming portions of top of the precast concrete modules to fit the site conditions,” Berglund says.
WSP worked with the precast concrete producer to define element shapes and sizes and provide input for fabrication, shipping, handling, and installation. To meet aesthetic goals, the seawall pieces feature a pigmented face and a troweled finish that was achieved through the inclusion of a special precast concrete formliner. To meet the stringent design life and durability requirements, the precast concrete producer used a high-quality marine concrete mixture and stainless steel reinforcement.
Once cast, the pieces were shipped via ocean barge from Virginia to St. Thomas and placed for storage on the seabed in the vicinity of the final installed location.
The use of full-height units counterfort wall pieces instead of quay wall blocks reduced the number of crane picks from 6600 to 251. “This approach allowed us to reduce the wall installation time frame by nine months,” Berglund says. It also eliminated the need for divers to set the blocks, which was a deemed a significant safety issue. Instead, the team used underwater GPS equipment attached to an excavator to guide the precast concrete pieces to the correct height and alignment.
The first half of the wall required excavation of the existing foundation soils, which were replaced with crushed gravel prior to placing the seawall footing. “This excavation would have proved very difficult if we had used the original wall design, as the risk of the wall footing washing out prior to backfilling would have been a real concern,” Berglund says. By using the precast concrete counterfort wall option, the crews were able to place the wall segments to full height with one crane pick followed immediately by backfilling.
“The precast concrete module concept was an effective and elegant solution to the seawall design and construction for Veterans Drive widening project,” Berglund says. “The concept can be considered to be a viable option for shallow piers, wharves, and other shore-protection projects where durability, resiliency, and aesthetics are critical requirements.”