With creative use of laser scanning, Alberici Constructors has shaved four weeks off its schedule to install two 120-ton steel vertical-lift gates that are part of a $165-million complex designed to block storm surge from entering the eastern flank of New Orleans.
Mitch Collins, a surveyor and project adviser for St. Louis-based Alberici, used a $150,000 Leica GeoSystems Scan Station to scan both the gates and the slots in their vertical concrete-tower abutments to make sure everything was aligned and that the gates would slip in within a strict, half-inch tolerance. This assurance— achieved with laser scanning—made it possible for the contractor to install the stainless-steel rails that support the gates' wheels and secondary concrete before the gates were on-site.
That sequence was a significant departure from how the job would have gone without the scans, says Dave Calcaterra, Alberici project manager and vice president. "Otherwise, we would have had to wait until the gates were here, install [them] to make sure they fit, take them out to place the rails, pour secondary concrete and re-install the gates," he says.
Alberici has used laser scans in its work for about five years. "It's important for the company to invest in new technology that either gets the job done quicker or saves the client money by seeing problems they may not see," Collins says.
The system paid for itself on this portion of the project, says Calcaterra.
The vertical-lift gates, or VLGs, are part of the Seabrook Floodgate Complex, which includes a 95-ft-wide navigable sector gate, 1,600 linear ft of concrete T-walls, a roadway ramp and upgrades to an Alabama Great Southern Railroad floodgate. The complex ties into the existing U.S. Army Corps of Engineers hurricane and storm-surge risk-reduction system, designed to protect New Orleans from floods that have a 1% chance of occurring each year—a so-called 100-year event.
The complex is under construction where the north end of the Inner Harbor Navigation Canal meets Lake Pontchartrain in eastern New Orleans. It rests on 414 foundation piles 74 ft to 86 ft long and 32 in. in dia. The pipe piles are driven through consolidated sand on 7-ft centers and built to an elevation of +16 ft. The system will work in conjunction with the IHNC-Lake Borgne Surge Barrier, nearing completion at the other end of the canal on the western edge of Lake Borgne.
Alberici holds the early-contractor-involvement contract for the whole Seabrook project. A joint venture of Bio Engineering Group, Salem, Mass., and ARCADIS-US, Highlands Ranch, Colo., designed it and is providing engineering during construction.
Maurice Bandy, retired project manager for ARCADIS-US, says the complex is unusual in that it has both a sector gate and VLGs. "To my knowledge, there is not another project like Seabrook anywhere in the country," he says.
Initially, the Corps anticipated using only a navigable sector gate at Seabrook, but hydraulic studies indicated that installing only a sector gate would increase water velocity in the canal. "The water going through there is really fast due to tidal change," Bandy says. "We were concerned that by adding only a sector gate, we would increase the scouring and undermine the support structures." Adding the two VLGs remedied the problem.
The 120-ton steel VLGs are 50 ft wide, 34 ft tall and 8 ft thick. They feature a 5/8-in.-thick skin plate on the flood side and W-truss members on the protected side, all coated with a corrosion-resistant coal-tar epoxy. "When designing these gates, we had to look for loads from both sides," says Anwar Zahid, project manager for the Bio-ARCADIS joint venture and designer of record. "We designed with thousands of combinations of load cases," he says. "This gate can resist water height up to elevation +16 ft on the flood side, with the reverse head at elevation +4 ft."
Each gate has five stainless-steel wheels on either side to guide it in the slots, easing movement and transferring lateral load to the concrete-tower abutments, says Eric Stricklin, Corps project manager. The towers are positioned -18 ft to +57 ft elevation and have a 20-ft by 50-ft footprint. The 9–in.-thick wheels have a diameter of 26 inches and a greaseless lubrication system to minimize grease exposure to the water, Bandy says.