So, you want to build a boardwalk...

Port Penn, DE - Boardwalks provide access to a very complex ecosystem for nature enthusiasts, hikers, bicyclists and scientists alike. The marsh ecosystem is fascinating because it is teaming with life, from insects to fish to birds to animals. The challenge is to provide access without destroying the marsh in the process. The Delaware Coastal Heritage Greenway Council recently constructed a boardwalk in Port Penn, as a segment of the Coastal Heritage Greenway Trail.

The village of Port Penn is situated on the Delaware Bay approximately three miles south of the Chesapeake and Delaware Canal. The entire village has been placed on the National Register of Historic Places. The Port Penn Historical Society was formed to protect historic artifacts and to promote the history of the area. A museum was started as a repository of artifacts for the community, which has long relied on the neighboring marshland for its livelihood. The community still has a very strong bond with the marsh and some residents were skeptical of whether a boardwalk could be constructed without destroying the resource.

The marsh, although located within an impoundment, has a very healthy stand of cat tail reeds and is teaming with wildlife, including muskrats, carp, fiddler crabs, dragon flys, and an assortment of marsh birds (including Great Egret, Glossy Ibis, Clapper Rail, Seaside Sparrow, Marsh Wren, and Yellow Throated Warbler). Much of the marshland habitat in Delaware has been invaded by Phragmities, which raised the concern that constructing the boardwalk would allow Phragmities to invade the marsh at Port Penn.

First, Investigate

When the Delaware Division of Parks and Recreation, in conjunction with the Coastal Heritage Greenway Council, decided to consider constructing a boardwalk in Port Penn, they made a concentrated effort to investigate advances in boardwalk construction technology, in an effort to reduce potential negative impacts on the environment. In addition, the boardwalk would have to comply with structural design criteria; be handicap-accessible; meet wetlands and subacqueous land regulations, including an environmental assessment and use recycled materials to the greatest extent possible.

We gathered environmental data to determine the most appropriate alignment and elevation for the boardwalk. We assimilated this data via aerial photographs, a subsurface evaluation, a topographic survey, an environmental assessment, the exploration of regulatory issues and an onsite survey. We chose the shortest distance across the marsh, which avoided existing muskrat houses, to reduce the level of impact. Environmental conditions dictated a deck elevation of four feet above the mean surface water elevation to avoid bisecting the ecosystem and to comply with Us Army Corps of Engineers recommendations. The elevated deck necessitated the use of accessible ramps on each end for an overall boardwalk length of 360 feet.

Issues to be Considered

Selecting the foundation system presented several interesting issues. Test borings revealed very soft sandy silt with varying amount of organics to depths up to 6.5 feet and medium density poorly graded sand with some gravel below. This type of soil is not well suited for shallow foundations. The geotechnical engineer offered three options: driven timber piling, jetted timber piling, or galvanized steel helical soil anchors. We considered characteristics of each, including compressive and uplift capacity, lateral load resistance, environmental impact and installation procedures.

In order to achieve the design capacities, the geotechnical engineer estimated timber pile lengths of 20 feet if driven and 25 feet if installed by jetting (high pressure water is used to open up soils and to push pile into place). In addition, jetted piling would still require driving for the last three to five feet. The pile lengths were somewhat long due to uplift requirements necessary to resist flotation and overturning. Pile uplift, developed through skin friction between the pile and the surrounding strata, is significantly reduced when piles are jetted, thus longer piles are needed.

We chose helical anchors primarily for ease of installation and thus minimal construction impact on the environment. Helical anchors also alleviate the threat of chemicals leaching into the marsh from the treated wood pilings. The anchors, which have a 1 1/2-inch-square galvanized steel shaft, came in seven foot long lead section and extensions of three and one half feet, five feet and ten feet. The lead section includes a helix of varying diameter, depending on soil conditions. The helix used in this project was 14 inches. Although generally used for tension applications and foundation underpinning, this technology was readily adaptable to boardwalks. In place of the large pilings and heavy equipment required for installation, the helical anchors were hand carried across the marsh and set into place with a hand-held drive unit powered by a gas generator. The anchor was then driven until the torque on the anchor reached a specified level.

The resultant disturbance to the marsh was minimal, involving only foot traffic over an eight-foot width. All 125 anchors were installed within the first two weeks of construction. The varying lengths of extensions allowed for pier-by-pier field adjustments of embedment depths based on a minimum torque criteria, thus maximizing use of materials. We placed galvanized steel pipe sleeves with a saddle to accept the timber pile cap over the shaft of the anchor. The soft upper strata dictated the use of short batter anchors for lateral resistance.

We encountered only one problem during anchor installation. The gravel present in the lower soil strata made it difficult to maintain alignment of piers with only the hand-held drive unit to stabilize the anchor. The contractor cut the corner off of the leading edge of the helix, which minimized anchor movement as it encountered gravel. Any additional realignment needed was achieved by careful adjustment of the turnbuckle connection between the vertical and batter anchors while bolting the pile caps to the sleeves. The contractor indicated that on future projects the drive unit would be modified to provide greater control over the alignment of the anchors during installation.

Until recently very little progress has been made in construction technology for boardwalk applications. Wood pilings have been pressure treated with various chemicals to slow the inevitable deterioration of the pilings due to water contact. These chemicals would then leach into the marsh-land environment releasing potentially devastating compounds into a very fragile and productive ecosystem. Standard wooden decking and handrails were replaced with a wood-polymer composite.

COASTAL HERITAGE GREENWAY

To protect and preserve our treasured areas of open space, Delaware has initiated grassroots efforts to create greenways within communities throughout the state. While Delaware's first greenway was established over a century ago in Wilmington, one of Olmstead's legacies, more recently a statewide greenway program was created to connect existing parks, wildlife areas, historic sites, communities, schools and places of work, to our neighborhoods. During the past two years 24 greenway projects were initiated in our small state. More than $4 million from the State Greenway Fund, legislative discretionary funds, local government matching funds and private donations have been used to create trails and pathways that showcase Delaware's diverse natural and cultural resources.

The state's most comprehensive greenway project to date is the Coastal Heritage Greenway, spanning a 90-mile corridor along Delaware's coast, extending from the Delaware River to the Atlantic ocean. The Coastal Heritage Greenway Council, co-chaired by the Lieutenant Governor and the CEO of Delmarva Power and Light, sets objectives for the greenway corridor. The greenway highlights quiet "salty" villages which were early ports, marsh meadows, wildlife refuges, and historic structures. Trails which connect to the greenway traverse villages and natural areas and interpret these features.

Many of the greenways are works-in-progress, with milestones of accomplishment being attained every day. Through the greenway program, small communities are reorienting their thinking about river corridors from considering a river through town as an obstacle to be crossed and a place to dump garbage, to an asset to provide community recreation, relaxation and rejuvenation. Like the Coastal Heritage Greenway, the other greenway efforts are the result of collaborations between Delaware's citizens, their communities and their local and state legislatures.