For a waterproof fit, the gates have Teflon-coated rubber seals with a 1½-in. radial on the bottom and J-bulbs on the vertical sides. "This is very similar to seals used on other Corps projects, so the Corps is familiar with maintenance," Zahid says.
Also, Bio-ARCADIS proposed using a hydraulic-ram cylinder system to lift and lower the gates because it requires less maintenance than the commonly used hoist-and-drum system, Zahid says. "It is like a piston-and-shock-absorber design, with an electrical linear positioning system embedded in the tower track to control the stop height."
The track consists of two stainless-steel plates on either side of the slot. The plates are one foot wide and 3/8 in. thick and embedded in the towers with grouting from -18 ft to +37 ft elevation.
"There is only a ½-in. clearance on each side of each roller to the rails, which is why we brought in the surveyor," Calcaterra says. "He surveyed both the gates and wheel assemblies at our St. Louis Humboldt facility, where the gates were manufactured before we barged them down. He surveyed the rails and slots in New Orleans. That gave us a great deal of comfort going into the VLG installation."
The Leica GeoSystems Scan Station sends out a laser pulse beam at a certain intensity and measures 4,000 X, Y and Z coordinates per second in a 900-ft range, Collins says. "It's a stand-alone station, a piece of hardware that will measure any surface without the aid of a receiver," he says. That means it requires no secondary device and only one operator.
To ensure the rails are plumb, Collins set up a 1/8-in. by 1/8-in. grid and shot a laser from the top of the tower down the length of the rail. Thus, he was able to see along the 55 ft of rail any encroachments undetectable to the eye, such as flaws in steel fabrication mill tolerance or in the concrete installation. "By the human eye or conventional methods of measuring, you wouldn't be able to see every 1/8 inch," Collins says. "We would have had to slowly move the gate up and measure incrementally."
It took Alberici about two and a half hours on Jan. 22 to install the first VLG at Seabrook. The fit was so snug, the crew had to take the seal off one side to ease it into place, Calcaterra says. On the second VLG, the crew removed the seal first, allowing the contractor to install the gate in less than 90 minutes on Jan. 31. After installation, the seals were bolted in place, and the rubber connections were vulcanized.
To make the lift, Alberici used a Manitowac 4100 ringer crane set up for 600 tons. The ringer has a 36-ft-dia ring and was rigged with 220 ft of boom in the mast. "The wind was the major thing we were concerned about because we needed 10 mph or less to make the lifts," Calcaterra says.
Although use of the laser scan system isn't expected to shave as much time off installation of the sector gate in March, Calcaterra says, "it assures us the steel gate and as-built concrete will fit together within the tight tolerances. It also provides the client and us [with] a permanent record—a model—of all actual surfaces and elevations within the structure. This is valuable information to have due to lack of access under normal, submerged conditions."
The contractor expects to save field time on the sector-gate installation by doing most of the welding on the gate components at its St. Louis facility. However, welding the lower section to the kingpin assembly will have to be done on-site. "The final welding-in-place is required due to the 26-week lead time for delivery of the base casting components, which will ship directly from our supplier to the project site," Calcaterra says.
Because the project was awarded as an early-contractor-involvement contract, the contractor was engaged at 35% of design. "[That early involvement] helped our team better work with the contractor and figure out options that helped with the fabrication schedule, without sacrificing engineering requirements," Zahid says.
Bandy says the fast-paced schedule —planning, design and construction all done in a year and a half—would have been impossible "without such a good contractor." With its innovative cofferdam design, Alberici managed to meet the Corps' goal for flood risk reduction by June 2011. The project is scheduled for completion in July.