Investigators Link Levee Failures to Design Flaws

A 1988 document reveals that Corps officials took careful measurements of the peat layer and tested the soil in a laboratory to calculate its relative strength, according to Robert Bea, a professor of engineering at the University of California at Berkeley and a member of the NSF investigating team. Based on those calculations, the Corps designed a concrete-and-steel floodwall anchored to the earth by steel pilings driven to a depth of 20 feet.

"The depth of the pilings becomes important," said Bea, because the "tips of the sheet piles may not have penetrated the peat." Meanwhile, because the canal was dredged to an even greater depth, water was able to penetrate the peat layer from the inside, investigators said.

"There was a gap where water could get through," said Ivor Van Heerden, the deputy director of the LSU Hurricane Center and the leader of the Louisiana forensic investigation. "Water was able to get around or through those pilings to the other side and start weakening the structure."

Reports of problems with the soft underlayer began to surface even before the floodwalls were finished. In 1994, the now-defunct Pittman Construction Co., a New Orleans firm involved in levee construction, claimed in court documents that floodwall sections were failing to line up properly because of unstable soils. An administrative law judge dismissed the complaint on technical grounds in 1998, without specifically addressing the allegations about weak soils.

Katrina's storm surge put the floodwalls to the ultimate test. Hours after the storm hit, water poured into the canals from Lake Pontchartrain and added enormous strain to the walls and levees. According to a scenario developed by Bea and other investigators, the already-saturated peat was the path of least resistance, allowing the water to burst through the wall from underneath. At the 17th Street Canal, truck-size chunks of the old earthen levee were heaved 35 feet on a carpet of sliding soil.

Corps officials are not yet convinced. "It is important not to jump to conclusions," said John Grieshaber, chief of the engineering division in the Corps' New Orleans district office. "It's hard to look at the aftereffects and say with a high level of certainty, 'This is what happened.' "

The Corps' actions since the storm, however, suggest that at least some officials are worried about weaknesses in the floodwalls' design. A proposal for rebuilding the floodwalls has set far tougher standards than existed 15 years ago. And the steel pilings, which formerly reached a depth of 20 feet, must now be driven through the peat layer to 40 feet, twice as deep as before.

If design flaws are confirmed, the task of preparing the 200-mile levee system for the next hurricane may be far more complex -- and more expensive -- than first believed, said Gordon Boutwell, president of Soil Testing Engineers Inc. and a member of the ASCE investigative team.

"Nothing beats a full-scale field test, and this was a full-scale field test," Boutwell said. "Some structures did the job they were supposed to, but some were total failures -- and those you can't just leave alone. And you can't expect to just stack them higher and walk away."

Designer David Murray contributed to this report.