After Hurricane Sandy, New Jersey plans to erect a wall to protect the shore
February 2014 - Hurricane Sandy hit the U.S. Eastern seaboard in 2012 and left in its wake devastation that spanned from the coast of Maine to Florida, its damage extending as far south as the Caribbean and Haiti. In efforts to protect itself from future storms, the state of New Jersey is currently in the process of erecting a steel wall spanning four miles of 45-foot-long 1/2-inch-thick sheeting.
The calculations made by a design engineer determine what dimensions are necessary to prepare the wall to withstand hurricane-strength winds, as well as the wear-and-tear Mother Nature will inflict on the barricade on a daily basis. The wall will serve as an unwavering buffer between the elements and the lives it protects. Made from ASTM A 572 Grade 50 steel, the project is expected to take approximately 180 days to complete.
“The design engineer performs a load calculation to accurately reflect the expected wave/tidal surge loading on the bulkhead,” says Bob Considine, press officer at the New Jersey Department of Environmental Protection. “Then, using these calculations, they pick a sheeting section that provides that amount of strength with an additional safety factor built into the design.”
Reinforcing the wall requires precise engineering and planning. “The contractor will first survey the alignment of the bulkhead. They will then set up their driving template, which is typically called ‘falsework,’” Considine says. Falsework are temporary platforms for laborers to work from as well as give a template to ensure the sheets are driven plumb, square and are properly aligned.
Sheets are then driven in pairs sequentially close to grade before going back to drive to finished grade, Considine says. “At that point, either another crew or the laborers of the sheeting crew will come and attach the cap and scour apron to the sheeting.” The wall’s scour apron will be 10 feet wide and is directly attached to the sheeting. It is a single sheet of geotextile fabric. At the landward side, 10 feet away from the bulkhead is a sewn-in loop that is to be filled with concrete to serve as a weight in order to keep the geofabric pinned down.
The sheeting won’t be tethered to anything but will have a cantilevered design to be driven to elevation, -30 ft. NAVD88, deep into the sand.
Engineers have two options for coating the steel, which needs to stand up to weather conditions. According to Considine, one option is to use coal tar epoxy to coat the sheets to prevent them from rusting. The other approved method is to use A690 marine steel. “That’s an uncoated steel sheeting specifically designed to produce a surface coating of rust which then protects the remainder of the sheet,” he says.
The sheeting crew needed to complete the job includes an excavator/crane operator, two laborers, who handle the fastening of the sheeting and alignment during driving, and a laborer to operate the generator for the vibrator hammer. “To this end, the contractor may employ a separate crew to install the cap, scour apron and ladders,” he says. “This crew could also range in size from two to six laborers to handle the fastening of the cap to the sheeting and the attachment of the geotextile fabric to the cap as well as a welder or two for the ladder attachment for one site. There would also be a site supervisor.”
Due to the relatively quick turnaround for the project, Considine says several crews will be needed to complete the construction within the contract time. “Due to the quantity of sheeting, the delivery schedule will be key to ensuring the contractor has the proper supplies to provide constant construction.”
Hatch Mott MacDonald, an engineering firm with multiple locations throughout the U.S., performed the design and created the plans with layout and dimensions. “They were chosen since they were the Borough of Mantoloking’s municipal engineer and have expertise in coastal structure engineering,” Considine says.
Challenges facing engineers and workers include driving strata. “While the driving should be through sand and mud, the contractor may encounter unforeseen obstructions such as rocks—so that’s one thing to keep an eye out for,” he says.
Funded by a combination of state and federal funding, the Federal Highway Administration will pay 80 percent of the cost, which is estimated around $36 million, provided from Sandy Emergency Relief funds. According to Considine the state will fund the remaining 20 percent of project costs. MM