Looking into the future the Pelican feeding its young from a self-induced wound in its own breast (as depicted, mysteriously, on the state flag of Louisiana) is accepted as an appropriate symbol of both self-sacrifice and rebirth. Through his selfless efforts, man is raised from the slavery of ignorance to the condition of freedom conferred by wisdom. Given the current state of affairs in Louisiana, one hopes that the understanding of the Pelican as a symbol shall point the way towards a new consciousness of ourselves as a whole, and lead us to face our futures with strength, grace, wisdom and faith, to learn from our mistakes and carry our successes and zest for living to future generations.

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Officials Knew About Weak Soil Under Levee

By Staff Writers John McQuaid & Bob Marshall
Reprinted from: http://www.timespicayune.com
Oct. 22, 2005
New Orleans

17th Street Canal breach was just above soft peat

Original soil tests and design documents for the 17th Street Canal flood wall show layers of weak soil under the wall's steel base that investigators believe gave way during Hurricane Katrina, likely causing the breach that flooded large portions of the city.

Army Corps of Engineers officials and the contractors who designed and built the wall were aware of the soft soil and took it into account in their design calculations, the documents show.

That only deepens the mystery about whether the wall breached because of a design or construction flaw, some factor unaccounted for in the calculations -- or was hit by forces beyond its design capabilities.

"According to the analysis, they've got the soil strength test. It doesn't show exactly the input for the analysis, but assuming they used it and came out with factors of safety, it's showing the numbers are safe. So it leaves an open-ended question," said Peter Nicholson, a geotechnical engineer from the University of Hawaii who is heading up an American Society of Civil Engineers team looking at the levees.

Before the Corps of Engineers builds a levee or flood wall, it typically bores into the soil to get information to be used in design calculations. The stronger the soil, the better it is able to resist forces pushing against it and remain in place.

The borings for the 17th Street Canal were done in 1981 by the Eustis Engineering, a Metairie firm. Those results and other documents were contained in case files from a dispute between the corps and the contractor that built the wall, A.E. Pittman Co. Those files were obtained by The Times-Picayune.

A 400-foot section of the flood wall breached, starting about 400 feet south of the Old Hammond Highway bridge over the canal. The boring data show weak soils throughout that area, extending almost a mile south of the bridge, engineers who examined them said, and a particular concentration of peat in the area of the breach.

Unusual pressures

One boring taken in the breach area shows a layer of "soft black humus w/wood" from 15 to 21 feet below sea level. Humus, or peat, is a soft, spongy soil made of decayed organic material that investigators say may have played a role in the breaches.

The flood walls are built on steel sheet piling driven to a depth of 17 feet below sea level, corps documents show. Original design documents showed a proposed depth of 10 feet, and the design documents show calculations were made with the wall base at 12.8 feet. The depth was apparently changed later, according to corps New Orleans district engineer Walter Baumy, who said the 17-foot depth is correct.

Based on surveys of the 17th Street and London Avenue canal walls, investigating engineers from the National Science Foundation and the American Society of Civil Engineers say that Katrina's high water likely created unusual subterranean pressures under the walls.

Those pressures appear to have made soil under the sheet pile give way, moving the steel sheet-pile-and-concrete walls along with it. That opened the way for water to pour through, flooding much of central New Orleans.

Engineers studying the levees also say that other, unknown factors, including structural problems in the walls, could also have contributed to the breaches.

Comparing the soils

Investigators are focusing on the 17th Street and London Avenue canals because evidence shows they were breached even though water didn't flow over their tops. That could indicate a design or construction flaw. Eyewitness accounts and other evidence shows that levees and flood walls in other parts of the city, such as along the Industrial Canal, were topped by floodwaters first, then breached or eroded.

Investigators now will likely compare the data from the original soil samples to post-Katrina soil borings taken Sept. 29 and 30. The older tests show that the soil layers are somewhat stronger than they were post-Katrina.

For example, several of the newer borings show the soil about 20 feet down has shear strengths ranging from about 100 to 200 pounds per square foot, much weaker than deeper layers. An old boring from that area shows a shear strength of 282 pounds per square foot.

Robert Bea, a geotechnical engineer from the University of California, Berkeley, said the additional weakness may be an effect of Katrina's storm surge, which would have weakened the soil as it pushed water through subterranean levels under the wall.

Bea theorizes that the design firm, New Orleans-based Modjeski and Masters, could have followed correct procedures in calculating the safety factors for the flood walls. But he added that corps procedures may not account for changes in soil strength caused by the changes in water flow and pressure during a hurricane flood.

Such changes can occur when variable soils are subjected to unusual pressures, engineers say.

"Certain soils can lose strength by remolding," Nicholson said. "When you remold certain soils, they will have less tested strength. Generally, what happens with soft soils like this is that they gain strength over time. But you can have strained softening." Nicholson said it was too early to tell if that had happened in this case.

Sudden sliding effect

The boring data show generally weak soil layers of alternating peat and soft clay, with decaying trees and other organic matter mixed in throughout the nearly mile-long area along the canal surveyed in 1981.

Bea speculated on why the wall may have given way in one place and not others. If a particularly weak layer of soil was exposed at one section of the canal bottom, it could have acted as a conduit for water to move through the soil, something that could have set off or contributed to the sudden sliding effect that occurred at the 17th Street Canal, he said.

"Things that we don't know come into play," he said. "If the peat layer was exposed in the canal at that point due to dredging, water can find its way into the peat where it's delayed in other areas. That may be why we found some walls in other areas that were on the bloody edge of dropping, but didn't."

John McQuaid can be contacted at john.mcquaid@newhouse.com or (202) 383-7889. Bob Marshall can be contacted at rmarshall@timespicayune.com or (504) 826-3539.