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.
Berger/ABAM designed a 28-inch precast octagonal pile made of 8,000-psi high-performance concrete. Three gallons of corrosion inhibitor were added per cubic yard of concrete. The piles have a 50-year design life, says Bill Wieners, vice president of marketing at Northeast Concrete Products LLC, the piles' precaster. The 28-inch octagonal pile is a departure from conventional East Coast designs for such components, which typically range up to 24 inches and are square in shape. "The octagonal design is structurally more efficient than a square pile," says Kumar.
The piles were transported to a marshalling yard on the river approximately two miles north of the construction site. About 150 piles were stored at the marshalling site, where they were loaded onto barges for transport to the LLTF. Atkinson Construction, which is owned by Clark Construction, built the entire open-wharf structure.
The piles were installed into 2-foot-deep, 36-inch-diameter rock sockets. Because the rock depths varied - from 25 feet under water to 190 feet - pile lengths had to vary accordingly. So as platform construction proceeded, the contractor loaded onto the barges exactly the lengths of pile needed for the construction at hand. "With precast piles, we could get within 5 feet of the actual length needed," says Wieners. "They could tell us the lengths of piles needed within two weeks of actual delivery, and that's what we would cast. So we minimized wasted concrete materials by casting as close to the pile's full bearing depth that the field required."
For piles longer than 110 feet, Berger/ABAM specified a composite pile of concrete and steel. The precaster embedded a large steel "tip plate" into one end of a typical 90-foot precast pile, after which the contractor would weld, for instance, a 60-foot steel-pipe pile onto the tip plate at the lower end of the precast pile. About 25 percent of all piles had the composite design. After the pile was lowered into place, the contractor cut it precisely to the correct height at the top.
To connect the piles to the pile caps, the precaster embedded eight 2-inch-diameter sleeves, each several feet long, into the tops of the piles. The length of sleeves allowed some margin for error, so that when the pile top was cut off, enough sleeve length would remain in which to place an epoxy-coated rebar. The rebars were epoxied into place and extended up into the pile caps.
The platform structure used single-, double-, and triple-pile caps. A single-pile cap covered one pile, a double covered two, and a triple covered three. When the pile caps were in place, the contractor did a closure pour. "That produced a monolithic system of pile and pile cap fully bearing on bedrock, with an actual capacity of 300 tons per pile," explains Wieners.
Control Of Tolerances
A central issue for the precast system, Kumar says, was control of tolerances both for pile location and for the erection of the beams and deck slabs. To account for tolerances, the project team adopted the practice that all caps and beams must be placed on the gridlines and that any pile tolerance issues must be handled within the cap itself. The project team avoided shifting a cap to accommodate a misplaced pile, which would have required shifting a beam and could have created interference of the projecting steel reinforcement. "Designing sufficiently large voids in the caps for the piles to target accommodated this approach," says Kumar.
Precast concrete beams dropped on top of the pile caps and precast panels were fitted into place on top of the beams. Closure pours connected the beams to pile caps and tied the deck slabs to the beams. The precast panels were typically 14 by 6 feet and 8 inches thick. To carry electrical conduit in the deck, a 12-inch thickness of cast-in-place concrete was placed on top of the precast slabs. Bayshore Concrete Products was the precaster for the pile caps and deck slabs.
Specialized precast sections - vaults and tunnels - had to be developed to accommodate the extensive utilities within the shipbuilding platform. These utilities included water, sewer, construction gases, steam and compressed air. Oldcastle Precast's Rotondo Precast unit handled the precasting of the vaults and tunnels as well as the hundreds of beams used in the project. The vaults and tunnels were designed to be modular with the deck system, to be erected on the transverse beams instead of deck panels, Kumar notes.
The decision to employ three precasters "was driven by the schedule and cost," Kumar says. The combination of three precasters - Northeast Concrete Products, Bayshore Concrete Products and Oldcastle's Rotondo Precast - helped the project team meet an aggressive schedule and solved logistics problems with component storage.
"The cost of forming the deck, heating concrete and pouring the decks over water is very expensive," says Wieners. "By casting the concrete off-site in a PCI-certified plant, you're certain to get the design strength and long-term performance that the owners intended."
Tapping Piles Into Place
The Kennebec River presented Atkinson Construction with a real challenge. All 1,350 piles had to be socketed into bedrock - and the top-of-bedrock elevation varied greatly. In some places, bedrock was 160 feet deep; in others rock protruded from the water. In addition, the depth of overburden varied from zero to 100 feet.
To drill the pile holes, Atkinson used a special barge fitted with a series of 36-inch-diameter frames, which created a template for precisely locating the piles. A global positioning satellite (GPS) was used to fix the template location before drilling. Next the contractor inserted a drill into the casing and drilled a hole through the overburden and into the rock. Rock sockets were typically 2 feet deep by 36 inches in diameter.
When the drill was withdrawn, Atkinson used a barge-mounted crane fitted with a special sling to lift the piles up. Each pile was lowered into the 36-inch casing and dropped into its rock socket. "They would just tap them into place," says Bill Wieners, vice president of marketing for Northeast Concrete Products LLC. "Putting those piles in the tubes was just like putting a shotgun shell into a shotgun."
With a series of piles in place, the contractor placed collars around them and installed steel bracing between the piles. An electronic leveling system adjusted the bracing to make the piles exactly plumb, said Wieners, who witnessed the process. Next the contractor used a machine placed atop the pile to cut each one to its specified height.
Precast piling is ideal for such variable-depth conditions, Wieners says. "With other systems, you would manufacture the piles to one theoretical depth everywhere to provide the total resistance that you needed," says Wieners. "But with prestressed piles, you only need to use the amount of pile material required to support the load in an end-bearing condition."
Building An Underwater Landing Site
The open wharf is not the facility's only structure built with precast concrete. Located just downstream of the facility, in the river, are three sets of six landing grids that can support the drydock exactly at LLTF deck elevation during vessel-transfer operations. Each grid resembles a bridge bent; it is topped with a precast concrete beam 104 feet long. The grids' tops are 15 feet below mean low-water level. One set of six landing grids serves each of the three shipways on the transfer facility.
The beams were fabricated by a local highway contractor, Reed & Reed Construction, which had a casting yard on the river bank up-river from the project. These "precast bent sections," weighing about 400 tons each and measuring 81/2 feet wide by 6 feet deep, then were barged directly to the site to sit on top of the five 72-inch-diameter drilled shafts socketed into bedrock. In some cases, bedrock was high enough that Atkinson placed the beams to bear directly on the rock, says Tom Shafer Jr., project manager with Moffatt & Nichol, the Baltimore-based engineering firm of record for the LLTF.
To assemble the landing grids, the precast beams were rolled down to the water's edge on a system of rails. The beams then were transferred onto platforms, from which Weeks Marine lifted them onto a barge. The barge, fitted with a 500-ton crane, floated the beams onto the water and placed them over the drilled shafts.
Vertical steel H-piles key the precast beams into the drilled shafts. Each beam was fabricated with a 12-inch hole located over each drilled shaft. That allowed Atkinson to pass a concrete drill through the hole and drill about 6 feet deep into the shaft. The contractor then dropped a steel H-pile into the hole and grouted it into place, says Coy Butler, an associate with Moffatt & Nichol.
To form a bearing pad for the drydock, the contractor bolted a mat of 8- by 8-inch timbers to the tops of the precast beams. "That provided a relatively soft spot for the drydock to rest," says Butler.