From: John Cuccia 

On Mon, 19 Dec 2005 14:31:37 -0500, "Frank Haber"
 wrote:

>Any civil engineers here?  That article is fabulous, but doesn't give a
>picture of the standard calculation of safety factors.  Is it an RMS thing,
>with outlying data ignored?

Well, according to the article, designers are supposed to find the weak
spot(s) and apply a safety factor of 1.3 to the design at those locations. 
Doing that should yield higher safety factors at most other places, but in
this case did not.  That's why I think this is the money quote from the
article:

===========
"If I can do a hundred cross sections of a levee and a few are 1.3,
that's OK. But here all the factors are around 1.3," Rogers said.
"From the outside, that's suspicious. If you come across something
like this, you say maybe the answers they want to get are driving the
analysis, not the analysis driving the answers."
===========

The Times-Picayune has been really good a covering this story from the
beginning. If you do a search on their site for either "levee
design" or "floodwall" you get the whole list of articles
from their post-Katrina archives.

>I've no problem with the implied ignoring of surveys and improper basic
>assumptions.  That soil looks like really slippy-slidey stuff, and it seems it
>wasn't properly surveyed.

They knew the soil was weak long before they designed the floodwalls, as
indicated in the first paragraph of the article.  Note that the floodwalls
weren't designed and built until the 90s.

===========
After a 1980 flood caused a stretch of the city's London Avenue canal levee
to collapse, the Army Corps of Engineers proposed replacing it with a
fortified design called a T-wall, with sheet pile foundations driven 26
feet deep. And in 1981, a study by Metairie design firm Modjeski &
Masters found that proposed higher levees along New Orleans' 17th Street
Canal likely would fail in high water because they were built on "very
soft clays with minimal cohesion."
===========

>And then we come to the political and costs/bidding pressures.  I wonder
>whether there is a "crisis in morality" in the civil trades,
mirroring the
>almost total abdication of probity by the accountants over the last thirty
>years?  Bet that last one gets a rise out of someone here (g).

It's entirely possible that the Corps, who have a long history of NIH
syndrome (try John Barry's "Rising Tide: The Great Mississippi Flood
of 1927 and How It Changed America" for a taste) , and who initially
recommended floodgates at the canal/lake interface, were pissed that locals
did not accept that proposal and proceeded to do sloppy engineering because
their noses were out of joint.

There were two reasons for the lack of acceptance by locals.  First, the
people responsible for pumping water out of the city (the Sewerage and
Water Board, I believe) would not be able to pump rainwater out during a
hurricane if the floodgates were closed.  Second, maintenance costs are
local and the local pols didn't want the expense of maintaining floodgates.

It's also possible that there were budgetary constraints, but in that case
it would have been better to ask for more money than to build something
that had little chance doing its job?

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