From: John Cuccia 

On Sat, 04 Mar 2006 09:52:30 -0500, "Gary Wiltshire"
 wrote:

>And the levies having been built to even less than their rated capacity

There were several different failure modes happening during and after the
storm, and you are confusing them.

The levees protecting the east part of the area were built to their
capacity, they were just overwhelmed by storm surge.  Many people are
claiming that the Industrial Canal and Mississippi River Gulf Outlet (MRGO)
levees, which form a large vee-shape with the vertex pointing directly to
eastern New Orleans,  acted as a nozzle to focus and amplify that surge,
leading to overtopping and erosion but I don't know about that.  MRGO did
contribute to considerable coastal erosion which certainly exacerbated the
situation.

The floodwall failures are what you allude to above, and it has been
determined that they were built exactly as per the Corps of
Engineers'-approved design.  The design was faulty, not the construction. 
Here's the latest I saw on the subject.  Sorry for posting the whole
article rather than a link, but the T-P only keeps a couple of weeks of
archived articles online.

=====================
Clerical error may have doomed levee Map maker confuses soil descriptions
Saturday, February 04, 2006
By Bob Marshall
Staff writer

A difference in soil-boring data transferred from one chart to another may
have played a key role in engineering decisions that led to the breach on
the 17th Street Canal floodwall during Hurricane Katrina, National Science
Foundation investigators say.

A cross-section drawing in the project design documents shows a weak layer
of peaty soils between 11 feet and 16 feet below sea level in the area that
failed during the storm. But information in the individual soil borings
that were used to draw the cross section show the peaty layer extending as
deep as 30 feet below sea level.

Investigators said their own borings taken at the site this week confirm
the 30-foot depth, leading them to believe that designers used the flawed
cross-section drawing to set the sheet pilings beneath the floodwalls at
17.5 feet below sea level -- a choice that allowed water to migrate to the
land side of the wall, causing the breach.

"It's pretty obvious the depth of the organic deposit shown on that
cross section is what determined the sheet-piling depth . . . for the whole
canal," said J. David Rogers, a member of the National Science
Foundation team, and a leading authority on levee and dam failures.
"They saw this marshy deposition; they recognized it as potentially
dangerous, so they specified sheet piles that went just beyond the bottom
of that line.

"So it's easy to deduce that if they saw that peat layer going to 30
feet, they would have placed the piling at least that deep. And it appears
pretty clear, at this point, that the transfer of the boring data to the
cross section is where the ball might have gotten dropped."

Shallow sheet pilings

The length of the steel sheet pilings has been a focal point of
investigators since they concluded the wall collapsed before it was
overtopped by storm surge. Sheet pilings provide two critical functions in
the floodwall design: They support the concrete cap that rises to 14 feet
above sea level, and they are supposed to block water inside the canal from
seeping through the levee. Once saturated, the soils supporting a wall can
quickly begin sliding, causing a collapse. With the canal bottom at 18.5
feet below sea level, engineers say the depth of the sheet piles was too
shallow for the notoriously porous soils in what was mostly swamp and marsh
just 100 years ago.

Investigators also have been puzzled by what they say are obvious
engineering errors because the firms involved -- Eustis Engineering for the
soil profiles, Modjeski and Masters for the general design -- were
experienced and reputable in their fields. Work by the National Science
Foundation team, however, may have uncovered a small mistake that had a
huge impact.

The companies have declined to comment.

Rogers, who teaches at the University of Missouri-Rolla, said soil work
always involves a lot of judgment calls and assumptions, because
geotechnical engineers never have a complete picture of what's below the
ground.

"If the data you have to work with is accurate, you're usually OK, but
if it isn't, you're in big trouble," he said. "It looks like the
guys working on the (floodwall design) thought they were anchoring those
sheet pilings in clays, and they were really right in the middle of this
highly organic, highly permeable soils that ran another 10 feet beneath the
tips of those pilings."


Soil profile


The official record of the Army Corps of Engineers project shows Eustis did
the soil borings. The cross-section drawing carries the logo of the corps'
New Orleans district, but Rogers said it is common practice for the corps
to sign documents compiled by consulting firms. "Normally, the firm
doing the soil investigation also does the profile, and the corps puts its
name on the profile in the design document," he said.

A corps spokesperson was unable to respond to a request for information
late Friday afternoon.

Documents show the cross section was constructed using the standard
technique of drilling soil borings -- sinking hollow-core drilling pipe to
about 40 feet. When the pipe is withdrawn, the soils captured inside the
pipe are removed and inspected. As engineers note the composition of the
soils at each depth, a cross-sectional slice of what is below the surface
begins to emerge. On this project, borings were made about every 500 feet
from Hammond Highway to Pump Station No. 6.

The interpretations of each boring were transferred to a written report,
which includes a drawing that uses different symbols to indicate the types
of soils at each depth, documents show. Those boring logs are used to
develop a larger chart that provides a cross-section view of the soils
along entire length of the project.

The logs and drawings for borings in the breach area show a layer of weak,
mostly organic soils -- peat, humus, shells and wood fragments
-- from about 12 feet below sea level down to from 28 to 40 feet
below. Some of those layers also carry the symbol for "fat clay"
but have the words "wood" or "shells" written between
the lines, indicating a mixture, although the written description of the
layers on the log indicates these layers were composed of mostly weak
material.

But on the project cross section, that same area shows the symbols for such
soils ending at about 15 feet below sea level. Below that depth, the
symbols show soils of "fat clay" or "lean clay" --
sticky, impervious soils considered very good for resisting water, Rogers
said.


'Significant finding'


After doing its own soil borings at the breach this week, the National
Science Foundation team found the marshy layers going as deep as reported
in the original boring logs. The team said other borings showed the layer
of weak "marsh" soils continued down the entire length of the
canal, but had its deepest penetration in the area of the breach.

"It's a pretty significant finding," said Rogers, adding that the
marshy layer that extended below the sheet-piling tips was so porous it was
almost completely saturated in the core samples retrieved. Without the
sheet piling blocking that layer, he said, "it's like having a big
pipe that can convey water from one side of the canal to the other."

Rogers said the weak layer may have been strong enough to hold back weaker
storm tides until the canal's capacity was increased by raising the walls
to 14 feet above sea level in 1994, because the extra water pressure on the
weak soils would immediately and dramatically increase their permeability.

"It was like increasing the pressure in the water system in your house
by storing water in a tank above the house," he said. "When you
turn that spigot open, the water pours through. That's what happened when
the (storm surge) rose in the canal. The water just poured through that
layer well under the sheet pile to the other side of the wall -- and the
thing collapsed. It was a sudden and dramatic failure."

Rogers said his team could see no clear reason for the change in soil
symbols when the data was transferred from the boring logs to the project
cross section. While a lot depends on the interpretation of the person
doing the drawings, he said an engineer reading the boring logs should have
drawn the cross section differently.

"The guy doing the soil-stability analysis (used to determine the
sheet-piling depth), probably would be looking more at (the cross section)
unfortunately," Rogers said. "Ideally, they should look at both.
You always should look at the boring logs -- always, always.

"That's like a doctor looking at your file. It's the basic fundamental
information that gets passed on. And the earth doesn't change much. You can
get good data from boring done 60, 70 or 80 years ago.

"And these look like pretty good borings. It just seems like the
interpretations (to the cross section) don't seem to be as good as what
we're doing."

. . . . . . .


Bob Marshall can be reached at rmarshall{at}timespicayune.com or (504) 826-3539.

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