Nola.com: Floodwall failure was foreseen, team says
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Floodwall failure was foreseen, team says
Corps' own 1986 study showed collapse possible,
scientists say
Tuesday, March 14, 2006
By Bob Marshall
Staff writer
Findings by an Army Corps of Engineers-sponsored
panel that the collapse of the 17th Street Canal
floodwall during Hurricane Katrina was the result
of an "unforeseeable" combination of events are
contradicted by a 1986 research project done by
the corps itself, National Science Foundation
investigators said Monday.
The Interagency Performance Evaluation Task Force,
working for the corps to investigate the levee
breaches, said in its second interim report Friday
that the 17th Street failure was caused by rising
water in the canal that forced the floodwall to
flex away from the canal, causing a separation
between the wall and the levee inside the canal.
Water pressure building inside the opening then
exerted force on a weak layer of soil under the
wall and the land-side toe of the levee, causing
the layer to slip and bringing the levee down and
the wall with it.
Spokesmen for the 50-member task force, composed
of researchers from academia, industry, and state
and federal agencies, as well as the corps, said a
review of engineering literature revealed this
specific "failure mechanism" had not been noted
before the design and construction of the project
from the late 1980s until 1994.
But in a sharply worded response issued Monday,
two University of California-Berkeley professors
leading a 34-member National Science Foundation
investigation into the levee failures said the
1986 corps research make those claims
"unfortunate" and "inaccurate."
Ray Seed and Bob Bea said the 20-year-old test,
which included constructing floodwalls on existing
levees and raising water levels to determine what
pressures the walls could withstand, resulted in
the same kind of collapse that toppled the 17th
Street structures and flooded much of the city.
"In simple terms this was exactly the 'unforeseen'
mode of failure" reported by the task force, the
statement said.
A spokesman for the corps' New Orleans District
said the task force would be reviewing the
National Science Foundation statements and might
have comments later. Calls to task force directors
were not immediately returned.
The 1986 corps test, done in the Atchafalaya Basin
on soils purposely meant to simulate those in the
New Orleans area, resulted in a series of events
that closely mirrors those that occurred on the
17th Street Canal during Katrina, the science
foundation statement said. As water levels rose
against floodwalls built for the test, a "dramatic
increase" in deflections of the sheet piles
occurred, followed by a "gap developing between
the sheet piles and the soils, allowing water to
flow between the sheet piles and the soils,
exerting additional hydrostatic pressures on the
piles," the foundation engineers said.
The results of those experiments were widely
circulated among corps officials, the foundation
engineers said. Further, the researchers involved
in the test alerted the New Orleans District,
which was overseeing the design of the area's
hurricane floodwalls, that its study suggested the
need to find new methods to "analyze both the
soils supporting the sheet piling and concrete
floodwalls, and the sheet pile/floodwalls
themselves," the foundation statement said.
That never occurred, Bea said in an interview.
"There was no change in the methods used to do the
analysis," he said. "I doubt this report ever made
it to Modjeski and Masters (the general design
firm for the 17th Street project) or Eustis (the
company doing the soil tests)."
The task force's statement that the professional
literature contains no examples of the 17th Street
Canal failure scenario, also was challenged by the
foundation team, which pointed to a 1997 edition
of the Electronic Journal of Geotechnical
Engineering, which contains two papers citing the
1986 study.
"The first of these papers," the foundation
statement said, "succinctly observed (page 10):
'As water level rises, the increased loading may
produce separation of the soil from the pile on
the flooded side. . . . Intrusion of free water
into the tension crack produced additional
hydrostatic pressures on the wall side of the
crack and equal and opposite pressures on the soil
side of the crack."
. . . . . . .
Bob Marshall can be reached at
rmar...@timespicayune.com or (504) 826-3539.