In this forum of informed minds, is there anyone who can find the latest theory on the failure of flood-wall on the 17th Street Canal in New Orleans? This failure seems to have been responsible for most of the flooding in the city.
The theory favored by the Corps of Engineers, which had the virtue of both exonerating it and providing it a chance at more funding, was that the storm-surge topped the wall, and, falling behind it, washed away supporting dirt covering the base, or the steel supports of the base, of the wall. One problem faced by this theory seems to be lack of evidence that the canal's water got within two feet of the top of the wall. The solution to this detail has been that a short-duration surge might not leave any direct evidence. I do not know anyone personally who actually believes such a thing, but the Corps seemed to be sticking to it as recently as a few days ago.
My own favorite theory of the moment is that a large Boh Brothers barge which was parked in the canal (?!) was forced by water or wind against the wall. The mooring of the barge could have allowed one end of the barge, and not the other, to press against the wall. This could have concentrated the force on a small part of the wall. A 100 mile per hour wind pressing on a large, empty barge sitting high, might create enough additional force to exceed design specks? Or, a heavily loaded barge (getting facts about the barge seems nigh impossible ... and this might itself have an interesting reason) propelled by the wave, if there was such a thing, of the storm surge, and swung laterally by a mooring, might have struck the wall with enough force to crack it.
Additionally, given that we are in Louisiana, I have wondered about sloppy construction. The design of the wall might have been good, but the contractor might have been a nephew of the governor, for example, and willing to cut corners. My theory is that the barge cracked the wall, water came through, washing away supporting the dirt berm which had not been built to proper specs, and the wall slid sideways (as photos show that it did). By the way, the barge is sitting among ruins of houses in the Ninth Ward.
6 comments:
Wizbang has been tracking this to some extent.
PC,
Examination of the barge should be determinant concerning whether did, in fact, make contact with the wall. If it was loaded, it should still be loaded and I would lean toward only a loaded barge swinging from one anchor being able to cause a breach. A loaded hundred ton (if that is its size) barge swinging from a single anchor or even with an improperly set second anchor could batter a wall to the point of a point failure occuring.
I tend to lean toward a failure on the part of the testing lab charged with some very critical steps in the design and construction process. There are four areas where the testing lab may have erred. The first is in the initial soils analysis from which the design was derived. The engineering calcs for a retaining wall are dependent upon the soil typology and analysis. Soil typology would have been determined through test bores on the dry side of the wall prior to the design and reconstruction beginning. The depth to which the sheet piles are to be driven is determined by the soil type and density. The second area of potential failure by the testing lab regards the actual logging of the piledriving operation. Piles are typically driven either to a specific depth or to a specific resistance and if done properly will perform to the criteria established by the soils report. The third area of potential failure would be improper inspection of the rebar web that provides the flexural component of the design. Concrete is able to flex and return to its original position due to the steel web within. If shortcuts in lapping distances and tieing procedures occurr, the potential for failure is enormous. The fourth area of testing lab responsibility is in the testing of the concrete used.
If I were investigating this I would be looking at the barge and looking at the testing lab even more closely than I looked at the barge. Any bad construction practice that occured would have to have had the collusion of the testing lab as cover. If such collusion occured it's a relatively easy matter to review and test the concrete and steel debris, determine the actual depth of the sheet piling and and review the testing lab's reports. The section that failed was reconstructed rather recently and the records are easily obtainable. Unless the testing lab's offices flooded out.
Btw - I'm sitting in my house on a levee as I type and I purchased the sheet piling that keeps the house from sliding into the bay in the back yard. Watched 'em drive it and watched the testing lab engineer log each pile.
Knuck,
The Corps may have hired the soils lab (which can be different from the TL). They may also have relied upon soils data previously collected - and that's where their big exposure to liability lies. If there is a peat component to the soil a new soils report would be indicated.
The Owner typically contracts with the TL separately from the General Contractor. The lines of responsibility are kept clean by doing so. I'm unsure as to whether common practice if for the Corps to assume hiring responsibility regarding the TL or whether the Levee Board would retain that task. A very interesting question where collusion and corruption may be involved.
In at least one place, the weight of the water seems to have actually pushed the earth down at the base and heaved it outward, distorting the walls enough to pull them apart at the rubber brackets.
I saw photos. The water would have been spilling through the gaps and that's why people thought they overtopped.
This is one heck of a great blog.
Regarding the soil characteristics, wouldn't they change over time?
CP (sorry for the previous inversion),
I've been looking around a bit more concerning the failure and I think you might find this blog useful for the reports contained. I'm a bit wary of ascribing the failure to the expansion joint design because such a joint is generally designed in a "tongue and groove" manner which prevents the hingeing moment described. Interesting stuff though.
CP,
Here is a NYT Report that goes back to soil conditions.
This seems to put the liability on the Corps.
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