Project Description

The super flood entrance structure is a portion of an overall modernization project that aims to create a navigation lock system to reduce tide dependency for submarine movements in and out of the Portsmouth Naval Shipyard’s Dry Dock 1 in Kittery, Maine.

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Per the original contract documents, the engineer of record, Stantec, and project owner, the Naval Facilities Engineering Systems Command (NAVFAC), envisioned the structure being cast on land in a dry setting. The completed structure would then be launched and floated into place via waterway and towed behind a barge. The Cianbro team recognized that site conditions presented challenges to constructing the entrance per contract details.

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Cianbro’s plan called for casting the structure on a barge at their Ricker’s Wharf facility in Portland, Maine. Due to the project scale, the structure’s sides would hang off the edges of the barge and over the water. The team used almost 2,700 cu yd of self-consolidating concrete—the most Cianbro had ever undertaken on one project. Although massive in scale, the team reduced the weight of the structure’s base by building it with 30 void spaces between an upper and lower slab. As a result, the area from the base of the caisson seat to the top of the structure was a hollow structure with tapered walls.

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The strategy enabled the team to successfully transport the entrance on the Fore River and put it in place at the shipyard, where it arrived on June 22, 2021.

Once the structure was set in place with strand jacks, the team filled the 5,000 cu yd of void space of the entrance structure—which had an extensive amount of rebar already installed—with marine concrete batched on site. The project completed on time and on budget in May 2022.

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