Hereford Inlet seawall repair project
Marine mattresses on the seawall in New Jersey.
Geosynthetics | August 2006
North Wildwood, N.J.
Location: Hereford Inlet is located on the New Jersey coastline, near the southeasternmost tip of the state, near Cape May.
Application: This project involved reconstructing the inlet’s shoreline seawall (Photo 1). Portions of the structure were undermined by tidal scour and storm erosion. Failure of the seawall would pose a long-term threat to nearby buildings and other shoreline infrastructure.
Challenges: Strong tidal currents, steep underwater slopes, and deep-water sections meant that traditional construction methods would have been difficult to implement.
Conditions: The seawall is located on New Jersey’s Atlantic Ocean shoreline. The site has maximum water depths of 60 ft. Underwater visibility is typically less than 3 ft. Strong tidal currents also affect the construction zone.
Alternatives: A conventional solution would have involved installing an unsupported geotextile fabric to contain eroding soil. That fabric would then have been capped with bedding and armor stones to prevent the material from shifting and settling. The U.S. Army Corps of Engineers concluded that this approach was impractical because of the challenging conditions at the site.
Solution: A conventional solution would have involved installing an unsupported geotextile fabric to contain eroding soil. That fabric would then have been capped with bedding and armor stones to prevent the material from shifting and settling. The U.S. Army Corps of Engineers concluded that this approach was impractical because of the challenging conditions at the site.
Each compartment was lined with nonwoven geotextile fabric and filled with 1.5- to 3-in. stones. Installation of a top panel completed the mattress assembly.
“This is our first experience with the Triton Marine Mattresses,” said Rich Marzucco, project manager for Agate Construction Co. “They are easy to work with and follow the contours that they are laid on.”
Small stones in the mattress compartments helped anchor the geotextile until it could be lifted into place and secured with layers of armor stone. Construction began in January 2005, with expected completion in late 2006.
Up to 5 mattresses were tied together to form a 20-ft. x 32-ft. unit. Ganging mattresses in this way enabled the installer to place more units with each lift (see Photo 4). The project crew used a 4-point lifting frame and a combination of shore- and barge-mounted cranes to lift and place each unit. Because of murky conditions on the underwater sections, the lift operator used sighting aids to facilitate mattress placement.
By the time Agate Construction completes the work, its crews will have placed more than 2,900 units. Each unit is anchored with armor stones weighing 500 to 800 lbs. each.
The system is ideal for the rough waters along the New Jersey coastline,” said Jeff Fiske, Triton Coastal and Waterway Manager. “Traditional methods are much more challenging in these conditions.”
The Army Corps is using the system on several other coastal projects, including the repair of the Townsend Inlet seawall in Avalon, N.J., 10 miles northeast of Hereford Inlet.