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Soil failure caused floodwalls to collapse, investigators believe

NEW ORLEANS—Hurricane Katrina's storm surges were never high enough to go over the walls of two canals whose failure caused massive flooding in western and central New Orleans, investigators have determined.

The finding appears to eliminate an early theory about why walls failed along the 17th Street and London Avenue canals—that the concrete and steel walls were overtopped by floodwaters, which then scoured out the soil in back of the walls, leading to their collapse.

A team of engineers instead found evidence of a massive movement of the embankment underneath sections of the floodwalls. The water pushed one section of the 17th Street levee back 35 feet.

Why the soil failed is still "a mystery story" awaiting further investigation, said Raymond Seed, a civil engineering professor from the University of California at Berkeley.

Adding to the mystery, the floodwalls along the nearby Orleans Canal made it through the storm intact, even though they were subjected to similar surges of water, and the floodwalls on the other two canals only broke in three places, he noted.

Investigators were not willing to say whether construction or design flaws contributed to the collapse, noting they needed more time to study documents and blueprints. Investigators have not been able to examine the "as-built" drawings that would show the depth of the steel piling inside the walls and other construction details, Seed said.

The collapse of the floodwalls on the 17th Street and London Avenue canals is of particular interest because most of New Orleans' flooding was caused by water flowing through those breaches.

The investigators also said they had eliminated one widely reported theory that a drifting barge was responsible for a breach in the floodwall along the Industrial Canal, flooding New Orleans' Ninth Ward.

Investigators said they had confirmed that a barge did strike the canal's floodwall. But they said the point of impact occurred at the southern end of the breach and could not have been responsible for it.

Seed leads a team funded by the National Science Foundation. They were joined by a team from the American Society of Civil Engineers. The U.S. Army Corps of Engineers is assisting in the investigation.

The concrete and steel floodwalls that line the London Avenue, 17th Street and Orleans canals leading into Lake Pontchartrain were built atop levees in the mid-1980s to add more flood protection to the canals.

Some engineers have suggested that their 11-foot height was a possible contributor to the failure, but Seed said he thinks this issue is "a red herring." He said there is nothing intrinsically wrong with a wall that high as long as it is structurally sound and well anchored.

In a formal statement, the team said that while some levees in the New Orleans area had been overtopped by floodwaters, that was not the case along the 17th Street and London Avenue canals.

At the 17th Street canal breach investigators found "evidence that a section of the levee embankment that supported the floodwall moved approximately 35 feet laterally ... We also saw evidence of soil mass movement" along the London Avenue canal, the team said.

Scientists at the Louisiana State University hurricane center said recently that they found no evidence of overtopping based on computer modeling of the storm and on-site investigation. The hurricane center is helping with the NSF-ASCE investigation.

The team will make additional soil borings and expects to issue an interim report in a month, with a final report to come later.

Seed said the investigation should enable the Corps of Engineers to rebuild the levees stronger than they were, as well as design a future system to protect the city from storms stronger than Katrina.

The soil failure is probably only part of the mechanism that caused the breaches. Levees and floodwalls are typically built in difficult soil conditions, on marshy land composed of sand, silts, clay deposits and peat.

But structures can be designed to cope with these conditions, Seed said.

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(c) 2005, Knight Ridder/Tribune Information Services.

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