BackgroundAfter years of storm erosion along a 1.1-mile stretch of State Route A1A/Ocean Drive on Hutchinson Island, Florida, any additional erosion could undermine the road, affecting access to residences and a Florida Power and Light nuclear power plant. The Florida DOT needed a long-term solution to both stabilize the shoreline and prevent future erosion from storms.
Project ScopeReynolds, Smith and Hills, Inc., engineered a wall to go inbetween the road’s edge and the shore. The entire 17 foot (5.2 m) tall wall would be below grade. The top one foot (0.3m) would be embedded in a 2 x 2 foot (0.6 x 0.6m) concrete cap, with the top of the concrete at grade. To meet “storm hardening” specifications, the wall needed to withstand being exposed by a hurricane to the depth of 6 feet (1.8 m).
The engineers considered walls made from steel sheet pile, concrete, and deep soil mixing (DSM). However, all of these materials are highly vulnerable to the extremely corrosive effects of saltwater. Another option was CMI’s horeGuard® SG-950 vinyl sheet pile: the low-maintenance, cost effective synthetic sheet pile is virtually unaffected by corrosion and comes with a 50-year materials warranty.
However, neither the engineers nor FDOT had used vinyl sheet pile on this scale before. Therefore, they specified that a 20 linear foot (6 m) test wall be installed on-site, and then removed so the sheets could be fully inspected for damage and integrity. Prior to construction the FDOT’s Construction Engineering Inspection (CEI) contractor visited a site using SG-950 to better understand its installation process and material properties.
ConstructionThe site has cemented sand layers and soil blow counts in the high 60’s. Fortunately CMI — who provided on-site consultations for every step of the project — had prior experience to deal with the difficult driving conditions. First, with two weeks’ notice, it provided a CMI PileClaw® Mandrel especially designed for the large SG-950 profile. Second, CMI arranged to have two back-up PC-950 mandrels ready as needed. With the use of CMI’s patent Mandrel and prior experience, the contractor was able to achieve and install an average of 240 lf per day.
PerformanceAlthough the new wall has not yet taken a direct hit from a hurricane, it has withstood record-setting local rain events, and FDOT is evaluating ShoreGuard sheet pile for their future projects. The contractor, Zep Construction — who had also not used PVC sheet pile before — had originally advocated for steel sheet pile to counter the difficult driving conditions. However, by the end of the project, Zep was very comfortable working with the ShoreGuard product.
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