A stacked sand-filled geotextile tube structure was specified to form the perimeter of Tern Island with the interior filled with sand. From this island, groins constructed of sand-filled geotextile tubes as a core for the structures project outward. These tubes were installed on top of stone-filled marine mattresses that served as a foundation layer, as well as offering scour protection. Over the geotextile tubes, thicker stone-filled polymeric mattresses were installed as an intermediate armor layer to minimize the overall stone thickness required. Ultimately, a final layer of large armor stone was installed over these mattresses to provide additional protection from a storm surge. The peninsula that projects from the mainland was constructed in a similar manner.
Since the tubes were filled with unwashed quarry sand, a geotextile composite fabric was initially specified to contain the sand fines and reduce potential turbidity, as well as to provide high UV resistance. The geotextile fabric was sand color to blend in with the environment should it become exposed.
In 2014, hurricanes Fran and Jeanne inflicted substantial damage to the Ft. Pierce Marina’s inner docks and caused the complete loss of its outer docks. The client desired a design to provide hazard mitigation against future storm events as well as environmentally enhancing the Indian River Lagoon.
Rather than use a composite geotextile at a significantly higher cost for constructing the geotextile tubes, an innovative woven geotextile was approved as an alternative since it met the specified turbidity criteria, had significantly higher UV resistance than that of the composite fabric, and had a high angle of friction permitting the tubes to be stacked while minimizing the possibility of slippage.
Because the tubes were an integral structural component of the island system, extensive consideration was given to the integrity of the geotextile fabric and seams. This design improved on the design of similar, but much larger, projects previously constructed in the Middle East.The innovative fabric used to construct the geotextile tubes proved to be an ideal alternative to the geotextile composite originally specified at significantly less cost to the owner. Only minimal turbidity from the tubes was observed and no tube slippage or construction damage to the tubes was encountered.Although on a smaller scale, this artificial island system is the first project in Florida to replicate the natural barrier islands along the southern Atlantic coastline of the U.S. A waterfront mosaic was created with the structures having varying heights and widths, all while protecting the marina—its primary purpose. Additionally, this innovative green project permits habitat and recreational activity to coexist in the environment. It provides hazard mitigation against future storm events, as well as enhancing the Indian River Lagoon. Mangroves and coastal dune vegetation were planted to stabilize the islands and provide additional habitat. Oyster shells and lime rock were used at lower elevations to promote the establishment of oyster beds, bottom communities and other essential fish habitats.This is an innovative project that will enhance tourism, fishing, birding, as well as promoting interest in downtown development. Because it provides excellent storm protection as well as being habitat driven, it is a win/win for all parties involved.
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