Bath Iron Works Land Level Transfer Facility
Design goals: Create a shipbuilding facility with nine acres of deck over land and six acres over water.
Precast solutions: A precast system of piles, pile caps, beams and deck slabs cut a year or more from the original design with steel pipe piles.
Components: 1,350 high performance 28-inch octagonal precast concrete piles, which ranged in length from 25 to 110 feet for a total of 111,000 feet of piling. Pile concrete was an 8,000-psi mix. The pile-supported, six-acre portion of the deck was built with 669 pile caps, 1,012 precast beams and 1,700 deck panels typically 14 by 6 feet (total 3,381 pieces). Other precast components included 141 utility tunnels and laterals, 226 tunnel lids and 39 utility vaults for a total of 406 pieces. Drydock landing grids were built with 18 precast beams, each 104 feet long and 8.5 feet wide by 6 feet deep in cross section.
It's a gargantuan structure. Located on the Kennebec River in Bath, Maine, the facility is known as a Land Level Transfer Facility (LLTF). But behind the big name is a 15-acre, high-performance concrete platform upon which its owner, General Dynamics, will build ships for the U.S. Navy using the latest innovations in ship-building technology.
From the LLTF, ships can be moved to a floating dry dock for launch. Nine acres of the gigantic deck are built on a retained fill structure, while the remaining six acres extend over water and were constructed primarily with a precast concrete system supported by 1,350 precast concrete piles. Land and waterside portions support four 300-ton gantry craneways that serve three shipways and one outfitting pier.
Originally, the over-water platform was designed "with a lot of cast-in-place concrete and steel-pipe piles," says V.K. Kumar, vice president with Berger/ABAM, the project's engineering consultant. "Some project managers with Guy F. Atkinson [owned by Clark Group, the project's design-builder] came to us before the job was bid, and they wanted us to value-engineer the design. Our first thought was, why don't we use precast concrete piles?"
Precast construction solved two difficulties, Kumar points out. First, Bath's severe winters make cast-in-place construction difficult, if not impossible, for four months of the year. The second problem was that the design called for the deck to be built just six feet above mean high water. With cast-in-place construction, "some of the falsework and forms would have to be below water," says Kumar. "The tides would affect the formwork." Those factors led Kumar to propose a precast option. "It made sense to the Atkinson people and to us to make it a precast deck," he says. "You can build your precast components throughout the year, and erect the deck even in the severe winter weather."
Moreover, the precast system produced a fast-track construction schedule. The combination of precast piles, pile caps, beams and deck slabs took just two years to build, saving as much as one full year over a cast-in-place system, he says. Precast pile construction started in 1999 and the entire over-water deck was complete by the summer of 2001.
In addition, redesigning to a precast system meant that longer spans could be used. As a result, about one-third fewer piles were needed - 1,350 instead of the 2,200 piles called for in the original design. Plus, precast concrete piles cost about 50 percent less than steel-pipe piles, Kumar says. Pipe piles filled with concrete would have required a coating of painting, as well as a cathodic protection system, for corrosion protection. Kumar estimates that his firm's work saved millions of dollars in project costs.
"Steel piles were chosen in the early design because these piles are installed in rock," says Kumar. "The mindset was to use steel and a drilled hole. But we decided that as long as we had to drill a hole anyway, we should look at concrete and see what it would take to design a concrete pile with a 300-ton capacity." They found it worked just fine.