Mammoth foundation

Harvey Canal gate project puts contractor to the test

By Angelle Bergeron

A cofferdam constructed for the concrete foundation, support columns and attachments for the Harvey Canal Sector gate project in the New Orleans area was the largest and most complicated cofferdam ever constructed by Boh Bros. Construction.

"The only thing similar to this was when we built the footings for the Mississippi River Bridge in 1980, which had a huge cofferdam, similar pile system and a large foundation slab," said Vincent Saladino, project superintendent for the New Orleans general contractor. "Instead of a bridge superstructure, this will have flood gates." The project will be completed next summer

The U.S. Army Corps of Engineers project is the centerpiece of the West Bank system of federal hurricane protection levees, floodwalls and gates. The floodgates will remain open most of the time, providing a 125-ft. horizontal clearance for vessel traffic, but will be closed in the event of storm surge.

The 26-ft.-tall, 175-ton steel gates are nestled within the coffer dam in the center of the canal and protrude 11.5 ft. above water level. In the event of storm surge coming north from the wetlands, lakes and canals toward metro New Orleans, the gates will pivot out of their bays to meet and form a barrier.

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Boh Bros.' contract includes the foundation, construction and installation of the gates, as well as some tie-in walls and levee work at the location. To build the 310 ft. by 100 ft. gate foundations, Boh Bros. constructed a huge cofferdam, excavated to minus 26 ft., drove 24-in. steel pipe piles 142 ft. deep, poured a seal slab, de-watered the dam and then constructed the monstrous, concrete foundation that tops out at minus 16 ft., Saladino said.

Boh divided the task into three separate cells, one for either side of the structure and a third for a tie-in wall for a discharge channel that is part of the system.

"We built a fender system to protect the coffer dams and the people inside them from the barge traffic that moved in between," Saladino said. Boh poured nearly 4,000 cu. yds. of concrete to form the seal slab at the base of each cell. "We had to pour the concrete in here before we pumped the water out, or the water pressure from the outside would blow out the sheet pile when we dewatered the coffer dam," Saladino said.

To create the seal slab, the contractor used a pump truck with a boom that rests on the mud bottom. A rich, seven sack mix is pumped underwater into the mud and it balls up until it pushes out into a layer.

"We tied crab trap floats to the hose and used it and buoys to check the elevation as we were pouring," Saladino said. The depth was also marked by a steel plate affixed to a chain. "When the plate hit, we knew the concrete was high enough," he said. "After we poured the seal, we let it cure for about three days and then we started pumping the water out of the coffer dam," Saladino said. "Then you clean it up and you start doing form work and rebar."

The complete project required a number of concrete pours, totaling >> almost 6,000 yards, but the base slab of the structure was the most challenging. The huge slab - 8 ft. thick, 100 ft. wide and 80 ft. long - required a slow reaction fly ash that would retard the cooling.

"The Corps was concerned about the temperature, about differential heat and cracks in the slab, because it was such a mass of concrete," said Mark Cheek, professional engineer and vice president with Beta Testing and Inspection, LLC of Gretna, La.

Cheek installed sacrificial temperature loggers at three different depths in the slab and monitored the internal temperatures over a 14-day period.

"The loggers have two wires coming out that look like speaker wire," Cheek said. "I have a hand-held reader that I plug into the wire sensor to download the data."

Using the data from each logger, Cheek was able to determine the temperature differential between different elevations at different phases of curing.

"The temperature differential wasn't great at all, which means the mix performed better than they expected," he said.

To ensure the quality of the mix and meet Corps specifications, concrete supplier Carlo Ditta Inc. of Harvey, La., had to designate a large portion of the company's ready mix operation to the project.

"We had to dedicate silos for the fly ash and aggregate, which we couldn't find locally, so we could supply the concrete on an as-needed basis," said Sammy Ditta, sales manager. The class F ash was brought in from Shreveport and about 650 tons of the Kentucky limestone (size 460) occupied silo space for a year, Ditta said.

"We had to dedicate the aggregate and F ash in the silos where we would normally store C ash, which is more economical and used more in New Orleans, he said. The storage arrangement caused Carlo Ditta to run out of gravel last October when local companies experienced a trucking and materials shortage because of Hurricane Katrina.

But Katrina wasn't the first thing to slow down the project. The gates were fabricated and ready for installation months in advance, but numerous change orders for the receiving structure resulted in delays at the outset of the project.

The Corps let the original, $40 million contract in November 2003, "but there have been so many design changes that I'd say about 75% of the structural part was delayed."

Initially, the Corps design specifications called for a float-in structure, which was changed to a cast structure. The Corps changed the concrete mix design specifications.

Boh was delayed six months by another contractor performing work in the area, Saladino said.

And as a result of Hurricane Katrina and the new 100-year level standards, the Corps is currently working on more design changes.

As the gates were being placed in the cofferdam Aug. 3-4, the Corps began exploring options for increasing protection at the Harvey Canal to provide the current standard for protection.

"When the project was designed, the authorized height was 9.5 ft., so this was more than sufficient," said Jackie Purrington, Corps project manager. According to the new standards, the gate is 4 ft. short, she said.

"We're looking at what we can do to remedy that."