- The floodwall on the 17th Street Canal levee was destined
to fail long before it reached its maximum design load of 14 feet of water
because the Army Corps of Engineers underestimated the weak soil layers
10 to 25 feet below the levee, the state's forensic levee investigation
team concluded in a report to be released this week.
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- That miscalculation was so obvious and fundamental,
investigators said, they "could not fathom" how the design team
of engineers from the corps, local firm Eustis Engineering and the national
firm Modjeski and Masters could have missed what is being termed the costliest
engineering mistake in American history.
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- The failure of the wall and other breaches in the city's
levee system flooded much of New Orleans when Hurricane Katrina slammed
ashore Aug. 29, prompting investigations that have raised questions about
the basic design and construction of the floodwalls.
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- "It's simply beyond me," said Billy Prochaska,
a consulting engineer in the forensic group known as Team Louisiana. "This
wasn't a complicated problem. This is something the corps, Eustis, and
Modjeski and Masters do all the time. Yet everyone missed it -- everyone
from the local offices all the way up to Washington."
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- Team Louisiana, which consists of six LSU professors
and three independent engineers, reached its conclusions by plugging soil
strength data available to the corps into the engineering equations used
to determine whether a wall is strong enough to withstand the force of
rising water caused by a hurricane.
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- "Using the data we have available from the corps,
we did our own calculations on how much water that design could take in
these soils before failure," said LSU professor Ivor van Heerden,
a team member. "Our research shows it would fail at water levels between
11 and 12 feet -- which is just what happened" in Katrina.
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- Not deep enough
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- Several high-level academic and professional investigations
have found that the sheet piling used in the design to support the floodwalls
was too short for the 18.5-foot depth of the canal. In addition to holding
up the concrete "cap" on the walls, the sheet piling is supposed
to serve as a barrier preventing the migration of water from the canal
through the porous soils to the land side of the levee, an event that rapidly
weakens the soils supporting a wall and can cause it to shift substantially.
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- The corps has long claimed the sheet piling was driven
to 17.5 feet deep, but Team Louisiana recently used sophisticated ground
sonar to prove it was only 10 feet deep.
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- Van Heerden said Team Louisiana's latest calculations
prove investigators' claims that a depth of 17 feet would have made little
difference. He said the team ran the calculations for sheet piles at 17
feet and 16 feet deep, and the wall still would have failed at a load of
11 to 12 feet of water.
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- Investigators have been puzzled by the corps' design
since it was made public in news reports. They said it was obvious the
weak soils in the former swampland upon which the canal and levee were
built clearly called for sheet piles driven much deeper than the canal
bottom. It was not a challenging engineering problem, investigators said.
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- Prochaska said a rule of thumb is that the length of
sheet piling below a canal bottom should be two to three times longer than
the length extending above the canal bottom.
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- "That's if you have uniform soils, and we certainly
don't have that in the New Orleans area," he said. "It kind of
boggles the mind that they missed this, because it's so basic, and there
were so many qualified engineers working on this."
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- Corps approved design
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- According to records, Eustis Engineering provided the
detailed analyses of the ability of soils along the path of the levee to
withstand water pressure once the wall was built on top. The information
was provided to Modjeski and Masters, the contractor that designed the
wall for the corps. If the project followed normal procedures, the engineers
with those firms were using design criteria spelled out in various corps
handbooks. "You use the corps cookbook, and you usually have to work
it out using corps (computer) programs," Prochaska said.
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- Private-sector engineering work must be reviewed by
corps personnel in relevant sections. In this case, legal documents show,
the work was reviewed by engineers in the corps' geotechnical and structural
engineering branches, as well as the flood control structures section.
It was approved and accepted by the district's chief engineer at the time,
Chester Ashley, according to the documents.
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- Robert Bea, a University of California, Berkeley professor
who led a National Science Foundation investigation of the levee failures,
said the mistakes made by the engineers on the project were hard to accept
because the project was so "straightforward."
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- "It's hard to understand, because it seemed so
simple, and because the failure has become so large," Bea said.
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- "This is the largest civil engineering disaster
in the history of the United States. Nothing has come close to the $300
billion in damages and half-million people out of their homes and the lives
lost," he said. "Nothing this big has ever happened before in
civil engineering."
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