The Prince William County Board of Supervisors awarded Nature Bridges the contract for Neabsco Creek Boardwalk in September of 2017. This boardwalk, located in Woodbridge, Virginia, is the newest segment of the 700-mile Potomac Heritage Scenic Trail, which runs along the Potomac River form the Allegheny Plateau to the Chesapeake Bay. Thirty-eight miles of the trail run in Prince William County alone. Locally, the trail links the town of Occoquan to Belmont Bay, the Occoquan Bay Life Refuge, Veterans Memorial Park, Featherstone Wildlife Refuge and Rippon Lodge and Rippon Landing before crossing the new boardwalk into the Metz Wetlands. From the Metz Wetlands, the trail connects to Leesylvania State Park, Powell’s Landing County Park, and to U.S. 1 near Dumfries, Virginia.
The boardwalk took ten years to envision, gather funding, design, permit, and construct. The landscape architect, Lardner/Klein Landscape Architects, P.C., headed up the project’s multi-disciplined team that included civil, structural, and geotechnical engineering, natural and cultural resource experts, archaeologists, and interpretive designers. The design team researched, inventoried, and evaluated the dynamic processes associated with the creek’s hydrology, aquatic vegetation, surrounding surface features, and the rapidly urbanizing watershed. Plant and animal communities provided the primary path layout for the boardwalk so that it fits into the natural setting of Neabsco Creek and its wetlands.
Prince William County and Lardner/Klein worked with a group of dedicated stakeholders with the goal of enabling the public to enjoy nature’s beauty while preserving a delicate ecosystem. The group included trail enthusiasts, supporting organizations, and government agencies. Neabsco Creek serves as a regional destination for environmental education, birding, hiking, and access to natural areas in eastern Prince William County, Virginia.
The collaborative design approach addressed the complex issues associated with flooding, storm surge, and related environmental permitting and established a high-quality user experience. After a preliminary layout was developed, archaeologists used probes to determine the depth of the tidal deposits, locating each of the historic channels and helping the geotechnical crew to better determine the pile depths required to achieve a stable boardwalk. The path and changing elevations of the boardwalk provide opportunities to experience the wetland from different perspectives. The higher elevations, which include five longer-span bridge sections, accommodate flood flows, while the lower elevation sections are designed to accommodate large storm events by allowing periodic over-flow of the boardwalk.
The boardwalk winds through a series of naturally established “outdoor rooms,” each with a different story to tell and places to get close to nature and appreciate the beauty of the wetlands. Long and graceful curves helped provide striking views of each of the distinct ecological niches found throughout Neabsco Creek while the boardwalk drops in elevation periodically to immerse visitors in the wetland environment. Steel cable railings met the project’s safety requirements while maintaining the transparent views, and most importantly, allowing water to flow over the boardwalk during flood events.
Neabsco Creek Boardwalk is 10’ wide and 3,300’ long, with a two-level observation deck, a single-level observation deck, and three overlooks. The decks and overlooks provide opportunities for outdoor classroom use. The boardwalk encourages visitors to become invested in preserving this natural treasure … “by engaging visitors with its aquatic life and the ways humans can both harm and heal the environment. By encouraging an appreciation of nature, we hope to inspire its protection.”-Board of Supervisors.
The key design and construction approach to preserve the delicate wetlands was the “top-down” construction method, which is Nature Bridges’ preferred method. The top-down construction method is a “green” construction method that protects ecologically sensitive vegetation and wildlife habitats. We use an excavator that is modified with a hydraulic vibratory hammer that drives two sets of piles up to ten feet apart, and to the required depth. We then install pile caps, stringers, and decking on the ten-foot section. Once the ten-foot section of boardwalk is completed, the excavator is moved to the end of the completed section to drive the next set of piles.
All boardwalk materials were staged at the beginning of the boardwalk, near the Rippon Road. Materials were fed to the construction area by a forklift that is modified to carry materials parallel to its body, so that long materials, such as piles and stringers, can be carried down a very narrow path. This process is repeated until the boardwalk is completed. This method has minimal impact on sensitive environments, as all heavy construction equipment stays on top of the structure, keeping it from entering the fragile aquatic ecosystems.
Neabsco Creek Boardwalk was engineered to be built using the top-down construction method, but needed a change in one detail. When we submitted our engineered shop-drawings to the project engineer, they were initially rejected because we had modified the header-to-pile connection detail. The project engineer’s connection entailed notching one side of the top of the pile and bolting the header horizontally through the portion of the pile that was left from the notching. We modified this so that the pile was not notched, with a wider header that covered the pile end with vertical pinning of the header to the pile.
The notched pile would meet the end use design loads, but could have impacted the safety of our employees and the longevity of the structure. The horizontal connections through the small part left of the pile didn’t provide a moment resistance connection, which could allow the header to roll off the pile. We could not allow our heavy