Posted on 11/10/2005 9:28:36 AM PST by Incorrigible
BY JOHN McQUAID
When the Texas construction firm AquaTerra Contracting began work on an Army Corps of Engineers hurricane protection project on New Orleans' West Bank, it encountered a serious problem: Its floodwalls wouldn't stand up straight in the mushy soil.
AquaTerra workers tried driving steel sheet piling down to the 55-foot depth the design required for the walls' foundation, company CEO Clay Zollars said. But the piling began to lean inward.
Zollars said the corps decided to nearly double the depth of the steel foundation to 105 feet. That didn't work either.
"Before we completed the wall, it began to lean and sink also," Zollars said. "The pilings were inadequate. The corps corrected that by installing some additional reinforcing steel in the concrete, but the wall still is leaning."
The top of one section of the 10-foot wall is more than a foot off the vertical, he said. AquaTerra is seeking $5 million it says the corps owes it for the extra work on the $11.1 million contract. Corps officials won't comment on the case because of the dispute.
The problems illustrate one of the basic obstacles to building reliable levees -- or any heavy structure -- in south Louisiana: It's a swamp.
Questions about soil are at the heart of investigations into why some of New Orleans' levees breached during Hurricane Katrina. Investigators say poor foundation conditions almost certainly led to the breaching of floodwalls along the 17th Street and London Avenue canals in New Orleans, flooding large parts of the city. The walls broke without being topped, suggesting a design or construction flaw. Layers of soft soils and organic matter under the wall foundations were not able to withstand the underground pressures generated by high waters in the canal, engineers say.
"They were struck with a bad situation, and they made a poor choice with those floodwalls, trying to put a structural wall on plastic soils. It's like putting bricks on Jell-O. There isn't a lot of support," said J. David Rogers, a veteran forensic engineer at the University of Missouri-Rolla who specializes in dam and floodwall failures.
What is unclear is how the corps and its contractors went forward with designs that some engineers now say appear fundamentally flawed. A team of engineers at the University of California at Berkeley studying the levee failures said that the corps' design standards do not seem to have accounted for all the soil uncertainties, raising questions about the design of the entire levee system.
The challenges of building floodwalls in weak, wet soils are well-known to engineers. A corps design manual warns that "by their very nature, floodwalls are usually built in a flood plain which may have poor foundation conditions."
Unexpected problems with weak soil have cropped up before. The AquaTerra case resembles a 1990s dispute concerning the 17th Street Canal floodwall. Segments of that wall also tipped off-center when the concrete wall sections were poured, requiring additional work and sparking a legal tangle. As with AquaTerra, the corps left the leaning walls in place.
A slightly tilted wall wouldn't necessarily be a safety hazard, engineers say, and it's not clear if the 17th Street Canal floodwall's early problems are directly linked to its ultimate failure.
Mississippi delta soil is notoriously unpredictable, both in composition and the ways it responds to stress. It's squishy and wet, with alternating layers of sand, silt, soft clays and peat, imbedded with the odd shells and decaying organic matter such as cypress trees.
Engineers must figure out how to imbed stable structures in this gumbo that will remain upright and withstand occasional extreme pressures from hurricane storm surges, winds and waves.
To do that, they depend on a delicate balancing of the forces of friction and gravity.
Floodwalls, skyscrapers, homes and other structures are typically built on steel or concrete piles imbedded in the earth. They get some support from the bottom tip of the structure, the way legs hold up a table. But most of the work is done by friction. Pile foundations are held immobile by friction between the soil and the surface of the pile. Long piles offer more security because they have more surface area and generate more total frictional force. Multistory buildings in downtown New Orleans are anchored by concrete friction piles extending hundreds of feet below the surface.
But a foundation is only as strong as the soil it's built in, and in engineering terms, strength is the soil's ability to resist forces acting on it and remain in place.
The area around the breached canals was swamp before it was drained or filled to make way for the city's residential expansion in the early decades of the 20th century and after World War II. Before Katrina inundated it, it looked like any neighborhood. But a completely different landscape lurks just under the surface.
"If you fly over the LaBranche Wetlands (upriver from the city), you will see wet and dry areas, areas with vegetation and areas with none," said David Lourie of Lourie Construction, a New Orleans-based soil engineering firm. "If you imagine some of that occurring at depths of 50 or 100 feet underground, that's what we've got in New Orleans residential areas."
Forces acting on the swamp for hundreds of years before humans decided to make it livable deformed it in peculiar ways, Lourie said, creating an unpredictable underground terrain.
"Through the passage of time, changes in Gulf water levels, changes in river flows, some of those (soil) surfaces were eroded or cut away," he said. "There were natural variations in the surfaces. They weren't all flat like a tabletop. You can have variations block to block. ... On one block you are over the center of a channel, and you could be only a block away and not over the same channel."
That means the requirements to anchor a foundation can also vary block by block. That's why detailed soil testing is essential before building a levee, or any big structure, to identify exactly what's below ground.
Documents show that in the 17th Street and London Avenue floodwalls, original soil borings were done about every 300 feet. It's not clear if there were later surveys that collected more data, but investigators say the soil surveys could have missed spots of soil weakness, and that could have created unidentified weak points in the walls.
In designing a wall, engineers weigh not only structural questions, but also issues of expense versus the high cost of failure.
Floodwalls "must be designed for the most economical cross section per unit length of wall, because they often extend for great distances," a corps design manual says. "Added to this need for an economical cross section is the requirement for safety. The consequences of failure for a floodwall are normally very great since it protects valuable property and human life."
Engineers say that the corps standards required an unusually low safety factor for the floodwalls, perhaps a remnant of a time when most levees protected sparsely populated rural areas, not cities and suburbs. A higher safety factor would require stronger walls -- and cost more.
Nov. 10, 2005
(John McQuaid can be contacted at john.mcquaid@newhouse.com. Bob Marshall contributed to this story.)
Not for commercial use. For educational and discussion purposes only.
I thought it was only a quarter French. :-)
Fund the entire project with fair market disposal fees.
The new city - Tel New Orleans - would be up above the mosquitoes and would be the South's new 'Shining City on a Hill'.
Touche.
The solution is to drive twice as many piles and angle them towards each other for mutual support. In cross section the levee would be pyramidal in shape, just like an earthen dike. If weight is an issue, you could make it hollow in the middle.
Expensive? Definitely. That's why engineers are lousy politicians, they don't like compromising or cutting corners (or at least good engineers don't).
Maybe they could sell the swamp soil to farmers up north. Watch the tabasco spring up in MN.
That makes it a compliment of questionable merit. LOL
I was raised by an engineer. I owe much of the decency and order of my life through having been brought up under the orderliness and structure and logic with which he lived, moved, and had his being. Whatever his politics were, I never knew. But you surely knew where you stood with him. You folks nailed it. If you want truth, and to plot a successful course, talk to the engineers.
Merriam-Webster online:
tou·ché
Pronunciation: tü-'shA
Function: interjection
Etymology: French, from past participle of toucher to touch, from Old French tuchier
-- used to acknowledge a hit in fencing or the success or appropriateness of an argument, an accusation, or a witty point
"...dead man anchors on the river/sea side."
Is this what Jimmy Hoffa is doing now?
Bite your tongue! Jimmy Carter was an engineer and look how that turned out.
I say this after thirty five years as an engineer; engineers need direction. They are great at detail planning once a goal has been set by visionaries, however, neither you (nor I) would want to live in a world ruled by engineers. Total logic devoid of compassion, passion trumped by facts, no thank you! I prefer my politics with a generous dose of human failings.
Regards,
GtG
Forgot about Jimmah Carter. Perhaps you have a point. I still prefer the logic of Engineers to the lying and back stabbing of Politicians.
Herbert Hoover, too...
Thats true today only because the Corp of Engineers has been moving heaven and earth (lots of that) to prevent it's moving west. The formation process of the delta used to push sediment out into the Gulf which deposited new acreage. The newly forming delta slowed the river's flow causing it to seek a new path to the sea. Over a course of years the Mississippi river has entered the Gulf anywhere from Texas to the Mississippi line.
Their (Corp's) recent tinkering with flood control on 'ol man river has caused a large increase in flow along a route intended to take "overflow" only. The result will be the Mississippi will change course and move toward Texas, leaving NO in a swampy backwater.
By channeling the river (straightening it's course and confining it to a concrete lined bed) the flow velocity is increased and the delta building process is stopped. The higher velocity flow carries the silt and sediment with it into the deeper water of the Gulf. The net result is the Mississippi Delta is shrinking and sinking into the abyss.
If you still think NO is viable real estate consider this: the Delta is also sliding south as it sinks. It is a large gob of semi-plastic mud and goo that is slumping and sliding off the North American plate.
Thought you might like to know before we blow $250,000,000,000 (a quarter trillion) trying to stop the inevitable.
Regards,
GtG
You can clearly see this by looking at Google Earth around the area. It's all old filled in oxbows and bends.
Not to be a naysayer, but this doesn't at all address the other concern that I heard mentioned--the seepage pressure through the berm underneath.
When asked if it was true Jimmy was buried below the goalpost at the Meadowlands, Rudy Guliani said "I think there's a little piece of Jimmy everywhere"
Thanks for the ping.
In order to keep a complex subject organized, I've posted an answer and some new data here:
http://www.freerepublic.com/focus/news/1517817/posts?page=31#31
My blame the nutrias theory is still valid, but it just ain't sexy to blame a creature slightly lower than a politician.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.