Fondations de digues sous-dimensionnées à la New Orleans

[JacquesL]

Posté le 13/12/2005 19:33:34

Experts question Jeff floodwall safety
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Experts question Jeff floodwall safety
Problems may be hidden, engineers say
Sunday, December 04, 2005
By Sheila Grissett
East Jefferson bureau

Experts don't yet know why the floodwall on the Jefferson Parish side of the 17th Street Canal didn't collapse during Hurricane Katrina, but investigating forensic engineers say no one should take much comfort until more is known about the sheet piles and soil conditions beneath the wall.

"Should people living west of that wall be concerned? Damned right, they should be concerned," said Bob Bea, a University of California-Berkeley engineering professor helping lead a National Science Foundation investigation of southeast Louisiana's crippled hurricane protection system.

"No one should sleep easy until we know what's down there," said Bea, a former Shell Oil Co. chief engineer who lived in New Orleans during Hurricane Betsy and lost everything in that landmark 1965 storm. "It's guilty until proven innocent. There's too much at stake."

Katrina slammed the area Aug. 29, overtopping many flood-control structures and breaching floodwalls in three canals, including the east side of the 17th Street Canal. Although much is still unknown about the condition of the west levee and wall, Bea and other investigators are troubled by a set of design plans that indicate the sheet piles on the west side may be only eight feet below sea level, which they said is far less than would be required to secure the sheet pile in the super-soft southeast Louisiana soil known to underlie that part of the levee.

If that turns out to be correct -- and experts examining the levee system don't yet know if those were final construction plans -- the west wall sheet piling would be two feet shallower than the east wall that failed and nine feet less than the depth the Army Corps of Engineers has said sheet piles on both sides of the 17th Street Canal are buried.

Team La. pays for tests

Corps officials said they are still searching among several hundred boxes of paper for the final set of construction drawings showing how the wall was actually built.

But, wary of waiting for those documents, Team Louisiana last month paid for sonar testing that showed sheet piles adjacent to failed wall sections were driven to only 10 feet below sea level on the east side of the 17th Street Canal, and were 16 feet less at London Canal, site of another major breach, than was shown in preconstruction documents.

Team Louisiana is a forensic engineering team of six Louisiana State University professors and three independent engineers working under contract for the state Department of Transportation and Development. The corps last week released the results of its own sonar investigation that verified Team Louisiana results on the 17th Street Canal.

Bea and Team Louisiana engineers said the same sonar tests are needed to determine the stability and long-term viability of the 17th Street Canal's west levee and wall.

The corps has announced no immediate plans to do that work, and the independent investigators said they don't have the money to do it.

"Unfortunately, we are limited by financial constraints," said Ivor van Heerden, associate director of the LSU Hurricane Center and a leader of the state team. "There are 350 miles of levees (and structures), and we were hired to focus on the breaches. It's a case of time and money, though my guess is the project will be expanded and, we hope, to check it all."

The corps has said it didn't do the design work for the two projects that added higher levee walls on both sides of the 17th Street Canal. But corps officials did approve the designs and construction contracts for both sides. A special task force within the corps is conducting its own probe into what went wrong, with oversight and input from Bea and others representing the National Academy of Sciences and the American Society of Civil Engineers.

The corps also has hired the local engineering firm of Linfield, Hunter & Junius to help Army engineers analyze the integrity of walls on both sides of the 17th Street Canal, which drains some 10,000 acres in New Orleans and east Jefferson Parish.

A second private contractor, URS Corp., has also been retained to analyze other outfall canals in the system, corps spokesman Jim Taylor said.

Corps still evaluating

The corps has not yet released its own preliminary study into system failures although agency brass have testified before Congress that it agrees that southeast Louisiana's notoriously unstable soils likely contributed to some of the failures, including the 17th Street Canal collapse.

Corps engineers also recently surveyed the west floodwall, and by year's end, Army engineers will have compared those findings with the floodwall's design to document whether the structure shifted during the storm, Taylor said this week.

"An initial inspection indicates that the west wall came through the hurricane in good condition, but if we have any concerns after comparing the survey results and design plans, we'll certainly do a more detailed analysis," he said.

The corps has multiple teams of engineers focusing on different design issues involving the many parts of the area's levee system, including a systemwide review of what worked, what didn't, why and what's next.

As part of that evaluation, corps contractors this week will pull as many as four wall segments and their sheet pilings adjacent to the breach on the New Orleans side of the 17th Street Canal to see exactly how long the sheets are. The removal will be done under the watchful eye of the FBI and other criminal investigators, and those sections will be stored in a secure building for further forensic review, Taylor said.

But Taylor said he's not aware of any immediate plans to measure sheet pilings under the west wall or take new soil borings on that side, and without that evidence, independent investigators said they are left to work with only a partial set of facts and their own observations.

More than just luck

Bea applauds the corps' decision to survey the west wall's post-Katrina alignment, but said it's only a start. "They need to find out what is really down there by determining the depths of the sheet piling and current characteristics of the soils," he said.

Above ground, Bea said, he has seen "seeps, sand boils and depressions in the earthen levee on both sides of the canal" indicating water movement. He has also seen signs of distress in some of the concrete wall joints, which, as the weakest parts of the wall, are where Bea said movement always shows up first.

"I study what others may see as failures or successes, and there's often a very fine line between them," Bea said. "In this case, one side of the canal may not have failed, while the other side did, due to small differences in the demands made on them by the storm surge and wave action."

Bea identified several factors that may have contributed to the failure of the New Orleans side, including the uprooting of a large tree near the toe of the earthen part of the levee.

"This very large oak tree had been felled by the wind before the levee failure, (its) root ball pulled out of the (ground)," he said.

That may have "uncorked the bottle," he said, providing an exit for water from the levee that had moved through soft, peaty soil beneath the shallow sheet piles. Similar unrecognized anomalies may be present all along the canal, which could invite catastrophic results during future storms, he said.

More berm and a road

On the Jefferson side of the 17th Street Canal, one of those "small differences" that might have protected against failure could be the two extra feet of grassy berm and the pavement of Orpheum Avenue, which separates the levee from residential encroachment.

"We've done no analyses of the west wall," said Team Louisiana consulting engineer Billy Prochaska. "We were hired to study the breaches and, at this point that's what we're doing. But we do know about some similarities and differences.

"You do have a different geometry, with a street behind the west wall -- not swimming pools and trees. Plus the land behind the levee on the Jefferson side is about two feet higher, and I suspect that the weight of that pumped-in fill helped matters there," Prochaska said.

But that's the end of any potentially good news for the west wall.

"Plans show that sheet piles over there are two feet shorter, and the soils are very similar," he said. Although there's apparently been no new soil borings at the west wall since Katrina, investigators said those taken over the years indicate very similar soils on both sides of the canals -- peaty soils so unstable that they would have required pilings to be driven 40 to 50 feet below sea level in order to keep water in the canal from leeching through the swampy soil and undermining the walls.

David Rogers, a University of Missouri-Rolla engineering professor who specializes in floodwalls and levees, said that because of the soil's instability, he's still not certain that any amount of sheet piling could render the area's outfall canals capable of handing storm surges.

'A mirror image'

"I can't tell you much about the west wall until we can see more. Right now, everything's covered up," said Rogers, who is investigating for the National Academy of Sciences.

"But I can tell you that everyone has to assume that it's a mirror image unless evidence proves to the contrary because the multiple breaks in the floodwalls there suggest a systemic problem with the foundation," he said.

"It all points to the soil. Everyone's looking at the peat, and I agree," he said, referring to the layer of soft, organic material believed to be the remains of the area's wetland past. "It's the most compressible soil on the planet, especially when combined with drought, which it was there."

Rogers speculates that it may take some sort of massive redesign and retrofitting of all outfall canals, or even their abandonment as drainage conduits in favor of floodgates, to keep Lake Pontchartrain at bay.

One possibility might be to use the 17th Street Canal as a corridor for pipes to move water from pump station No. 6 to the lake.

"It has to be understood that you had a massive failure that will cost a lot of money to address," he said. "And while I think it's a no-brainer that the bulkhead walls weren't deep enough, I don't think deeper ones could have prevented this, only slowed it down."

As for the need to evaluate the west wall, Rogers suggests thinking of the 17th Street Canal as a sort of plane crash.

"This was akin to one wing ripping off an airplane. It was a catastrophic, systemic failure that has left the whole thing tarnished and in need of assessment."

. . . . . . .

Sheila Grissett can be reached at sgrissett@timespicayune.com or (504) 883-7058.

Discussion en français sur fr.sci.geosciences :

A la suite du passage de Katrina a la Nouvelle Orleans, une equipe de
chercheurs (Team Louisiana) a enquete sur les causes des inondations.

Selon les conclusions de l'enquete, qui seront publiees cette semaine,
les digues du canal de la 17eme rue a la Nouvelle Orleans etaient
sous-dimensionnees par rapport a la charge maximale de 4,27m d'eau
atteinte cette fois-ci. Une erreur de calcul s'est glissee dans la
conception des digues. L'agence de developpement locale Eutis
Engineering, Modjeski and Masters au niveau national et l'Army Corps of
Engineers ont successivement approuves les plans de conception sans
remarquer que les caracteristiques du sol sous-jacent n'avaient pas ete
correctement prises en compte dans les equations.

Les elements de support des digues n'etaient simplement pas assez
enfonces suffisamment en profondeur eu egard a la faible tenue du
substratum. Cette erreur grossiere est la plus couteuse de toute
l'histoire de la construction aux Etats-Unis.

Pour en savoir plus, contacts :
http://www.nola.com/news/t-p/frontpage/index.ssf?/base/news-4/113368350467800.xml
Sources :
http://www.nola.com/news/t-p/frontpage/index.ssf?/base/news-4/1133336859287360.xml
http://www.nytimes.com/2005/12/01/national/nationalspecial/01levees.html?adxnnl=1&adxnnlx=1133910046-cvLXZFva9U6/xuNCRXOhKg

Cette information est un extrait du BE Etats-Unis numero 13 du
8/12/2005 redige par l'Ambassade de France aux Etats-Unis. Les
Bulletins Electroniques (BE) sont un service ADIT et sont accessibles
gratuitement sur http://www.bulletins-electroniques.com
Site : http://www.bulletins-electroniques.com/actualites/31039.htm

Redacteur : Emmanuelle Delbecque, deputy-envt.mst@ambafrance-us.org

Arte a consacré une soirée Théma à Katrina il y a 10 jours.
Je résume ce que j'en ai retenu:

Les digues ont bien été défaillantes, mais c'est un peu simple de ne considérer que cet aspect. Comme souvent, c'est multifactoriel.

Les autorités avaient bien vu le danger il y a quelques années. Les 3 plus grandes catastrophes potentielles étaient une attaque terroriste sur New York (réalisée en 2001), un séisme "big one " en Californie et un cyclone dévastateur sur la Nouvelle-Orléans.

En 2004, la FEMA avait bien simulé cette dernière catastrophe et répété les procédures en grandeur réelle. Ce n'est pas franchement rassurant de constater que même avec ce genre de mesures, on puisse aboutir à un tel fiasco  (je pense notamment au cas d'un accident nucléaire en France s'il arrivait réellement).

Pire: le scénario du pire envisagé lors de cette simulation mettait en scène un cyclone passant un peu à l'ouest de la Nouvelle-Orléans. Le cyclone tournant dans le sens inverse des aiguilles d'une montre, la ville prenait alors le maximum d'intensité des vents.

En fait, Katrina est passée légèrement à l'est de la ville pour aller ravager les côtes du Mississipi. Ce n'est pas le cyclone en lui-même qui a abouti à la catastrophe.

Ce qui a été dévastateur, ce sont les vagues venues du golfe du Mexique et accompagnant le cyclone: autrefois, les marécages du delta auraient bien davantage amorti ces inondations, mais les digues installées sur le Mississipi n'ont fait que réduire leur étendue. Les eaux ont donc facilement gagné la Nouvelle-Orléans par un canal. Elles ont d'abord rompu une digue via ce canal. Puis dans un deuxième temps, c'est le lac Pontchartrain qui a fait monter le niveau d'eau dans 2 autres canaux: les eaux de ce lac n'ont pas débordées, mais ont exercé une telle pression latérale sur une digue que celle-ci a glissé par rapport à ses fondations et s'est alors affaissée.

Je résume ici seulement l'aspect technique de la catastrophe. Arte a également montré les insuffisances de l'organisation, de la logistique après l'inondation, des responsabilités politiques...

Les cyclones (de cette partie du monde au moins) semblent suivre des cycles de 30 à 40 ans: leur intensité était très forte dans les années 40 à 60. Puis ils ont globalement baissés des années 70 à fin des années 90. Depuis quelques années, cela repart à la hausse, avec des années 2004 et 2005 exceptionnellement violentes.

Ce n'est pas la fréquence des cyclones qui varie, mais l'intensité des cyclones: il y a moins de cyclones de classes 1 à 3, mais plus de cyclones de classes 4 et 5.

Indépendamment de ce caractète cyclique, il y aurait aussi une hausse globale de ces violence des cyclones à cause du réchauffement climatique:  0,5°C de température supplémentaire observée dans les eaux de l'Atlantique suffirait à expliquer cette tendance.

--
Ronano