Posted on 10/10/2005 1:19:41 PM PDT by Between the Lines
NEW ORLEANS — The storm surge from Hurricane Katrina rose with such force that water flowed underneath levees near Lake Pontchartrain, pushing them in and cracking massive concrete flood walls, according to preliminary evidence gathered by investigators.
Sinkholes and tipped concrete walls could shed new light on how the earthen structures broke during the flooding of New Orleans.
Why they broke is not resolved. The next step for the investigation is to determine whether the levees were adequately designed and properly built, or if the ferocity of the storm caused them to fail.
The Army Corps of Engineers initially suspected the levees here that protected central New Orleans were topped by floodwaters. That clearly happened in other areas, such as the Lower 9th Ward.
But Raymond Seed, a University of California at Berkeley engineering professor who is leading an investigative team sponsored by the National Science Foundation, said Friday that waters from Katrina did not flow over the levees here.
Instead, the flood protection failed when the soils that make up the levees gave way, Seed said.
The levee that failed at the 17th Street Canal slid 35 feet, he said. One of the two breaches at the nearby London Avenue Canal failed in a similar fashion.
"Levees tend to be built in very difficult situations on poor site conditions because you're essentially turning marshy land into land you can stabilize and do things on," Seed said.
Understanding why the levees broke after Katrina hit Aug. 29, flooding 80% of the city and several nearby communities, will be the key to rebuilding the hundreds of miles of flood barriers in ways that are less vulnerable.
Investigators from Berkeley, the American Society of Civil Engineers (ASCE) and the Army Corps of Engineers are studying a previously undisclosed site on the London Avenue Canal that nearly failed. The levee walls at this location stopped just short of collapsing, preserving the evidence of what happened.
Water at the London Avenue location made its way under the levee and began to erode it from within, creating several sinkholes, said Peter Nicholson, a University of Hawaii engineering professor who heads ASCE's team. The water also appears to have lifted up adjacent backyards by several inches and moved sections of the levee.
One of the concrete walls in this section cracked as the walls began to give way.
The investigators took pains not to assign blame for the failures. They said they will review construction records at the areas that failed and take soil samples to determine why the levees moved.
But none of the telltale evidence of erosion from water flowing over the levees exists, suggesting that whatever water came over the top was minimal. Investigators now believe the water rose to within about 2 ½ feet of the top of the walls, which are 14 feet above sea level.
By contrast, the Lower 9th Ward in New Orleans and nearby St. Bernard Parish were flooded when the hurricane surge rose over the top of flood walls.
Bush's fault!
Thats because Bush had them blown up!
They should have cut a slurry trench down to a competent layer, and used a cemnet/bentonte plastic concrete backfill. Duh, and decent geotech knows that.
1. Most of the pumping stations, that everyone was screaming about the first few days.. were shut down onpurpose, becuase they pumped the water into the drainage canals...so until the levee breaks wer fixed..they were just pumping the water around in circles..back into the city..
2. They showed pics of the barge that caused the biggest break..It was huge..and no way a levee can withstand that impact..
3. There was a large all brick house across the street, about 100 feet from the levee on one of the canals.directly opposite the break...You could see a hole punched clear through the house...and out the other side..like if you apply a high pressure hose line to a big block of ice..That gives you some ides of the force of the water..the pressure, that built up...
Ever heard of a phenomenon called "sand boils" (I think)? I believe I read about them in "The Rising Tide" about the 1927 flood.
The American River levees have had sand boil problems, so the do the slurry trench thing. There was a story a week or two about a kids playhouse lifted up from a sand boil. They didn't call it that, but that's what it was. Piping is another term for something similar.
You are well read on floods, eh?
Also referred to as a "heave" -- when the underlying soil becomes so saturated that it approaches a fluid state -- and the weight of the water above pushes it out...and up.
Just curious about loads of stuff. No chance of a flood where I live.
If the levees broke, water would still be pouring into the city, no?
Has it been definitely proven that the barge broke the levee or did it go thru the hole when the levee broke.
They sandbagged the breaches.
As I understandit, Heave is a little different. Heave is spread over a larger area, whereas piping, which causes sand boils is more localized through "pipelines" of more granular or weaker material.
We experience heave when excavating large foundation holes without properly drawing down the water in the area. Within the heave over the whole ecavation, there are often several sand boils.
http://earthquake.usgs.gov/image_glossary/sandboil.html
Just the name "Sand Boil" I had to do a search.
I think it's pretty conclusive..
</Sarcasm>
The pics indicate a slab failure, anecdotal evidence says the weak layer was 40 feet down. The sheet piles weren't driven to refusal, they assumed it'd never happen, so the curtain only went down about 32 feet IIRC.
What's the shear strength on the cement mixture after it sets up?
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