Corps: Floodwall break caused by unforeseen stresses
3/10/2006, 4:11 p.m. CT
By JANET McCONNAUGHEY The Associated Press
NEW ORLEANS (AP) — A foundation problem — although not the one targeted by earlier studies — caused the 450-foot-long break in a floodwall and levee on New Orleans' western edge when Hurricane Katrina hit on Aug. 29, the U.S. Army Corps of Engineers said Friday.
Part of the levee was pushed 40 feet backward, and can be seen above the water — along with that section of floodwall — in an aerial photograph published as part of the second report by the task force set up to find out why the levees broke.
Unprecedented high water pushed back the floodwall, which is set into the center of the earthen levee. Once water got between the floodwall and the front half of the levee, it effectively cut the levee in half lengthwise.
The floods then pushed the floodwall, and the half of the levee behind it, backward on a layer of soft clay below the surface, the Interagency Performance Evaluation Task Force report said.
The floodwall's design didn't include either the possibility that water could get between it and the levee or that the clay might be unstable, corps officials and others said in a news conference. And, they said, it took both to create the break: neither alone would have done it.
"We are incorporating the information into our current repairs, and incorporating it into our assessments for the future," said Col. Lewis Setliff, who is in charge of the levee repairs that the Corps wants completed by the beginning of next hurricane season on June 1. "We are also evaluating the repairs we are making ... to see if we need a change of course."
The combination appears to be something new, said Ed Link, project director for the study.
"I would not say right now that it's never happened or it's never been documented. But we've never seen it yet," he said. "We're still looking. We would very much like to learn whatever anyone else has learned."
Setliff said the corps is looking at all of the "I-walls" — vertical concrete barriers anchored by vertical sheet steel pile — in New Orleans-area levees, said Setliff.
The 17th Street Canal and two others that broke during and after Hurricane Katrina will be cut off at the mouth by new floodgates if a hurricane approaches this year, to keep high water in Lake Pontchartrain from stressing their levees and floodwalls.
And, wherever it can, the Corps is replacing I-walls with "T-walls," which have a horizontal concrete base and are anchored by steel beams driven diagonally through the levee.
Those should be "significantly more resistant" to such failures, Ed Link, project director for the study, said in a news conference which began 15 minutes after the 332-page report and more than 400 pages of appendices were released Friday on the Internet.
The clay that failed was not brought in to build the levees. Rather, it is the naturally occurring 20-foot thick layer beneath the layer of sand and peat that analyses by other groups had targeted as a likely culprit for the break.
"The failure plane was not in the peat. It was in the clay below the peat. That became the weakest part of the system," Link said.
The original soil samples probably did not find the "zone of weakness" at the "toe" of the levee, the rear edge, where it slopes to level ground and private property, Link said.
He said the task force is also analyzing soil along the Orleans Canal, which has similar walls, to learn why they withstood the storm surge while others failed.
The report released Friday is preliminary. The final report is due June 1.
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