I asked Don, the Contractor, who sits in the same coffee shop in which I edit papers and code, to read the comments from this blog re the failure of New Orleans floodwalls.
I had thought some of them quite good, so I was shocked when Don finally finished and said "Bullshit," which was the strongest language I had ever heard him use.
He did not like the attempts to pin blame on someone. He skipped over the contractors who built the walls and said that the mistake was to put the walls there in the first place. They were built on silt. Water can leak through silt and, as it flows, widen the pathway. In effect, he predicted that the dirt base of the floodwalls had been eaten out from below.
Don has only a high school degree, but knows how to understand how things work. He makes a good living running his own small business. He grew up in New Orleans and has been down there to look around. But, he is no civil engineer.
The next day, a team of Civil Engineers agreed with Don.
The dirt base of the walls seemed to have been undercut. The entire structure, dirt and wall slid sideways. There still seems to exist, as someone here has pointed out, the possibility that the design was not followed - that it demanded a deeper structure than was actually built. Don seems to wonder how deep into silt would be deep enough.
Don said that the barge had no signs upon it that it had banged forcefully against a wall. Even had it been pressed against the wall by the rather mild (under 120 mph) winds, that would not cause 400 feet or so of dirt, steel and concrete to slide sideways?
Thanks for the ideas. Unlike Don, I found them interesting, teaching me more than I had known.
4 comments:
I'm no engineer, but the original "narrative" created by the MSM was that the storm surge caused the levee breach. Because this was the first meme put into people's minds, it is likely to be the only thing remembered by anyone.
On the other hand, we had this story last week which indicated that LSU experts have said that there was some problem in the construction which caused the problem.
Whether it was corruption, as previously suggested, or poor placement, as Don suggests, I have no idea. I am glad, however, to see the discussion moved into the appropriate direction and away from the "if Bush hadn't fought in Iraq this wouldn't have happened" line of thought.
david:
I live in an area that has floods sometimes.
In areas like this there tends to be a lot of sand in the soil.
I have seen this happen. The force of the water simply erodes the earth beneath.
But it takes a lot of water.
I don't know whether they should rebuild or not, In some was it would seem at least part of the city would be more suitable to a big city park or something than residential areas.
But this kind of storm is so rare that this particular set of circumstnces might not come about again in our lifetimes.
CP,
If water undercut the wall then it was ipso facto a design error. I suppose that it is possible that the top of the sheet piling was not attached to the base of the wall footing but if that is in fact what happened then the problem reverts to the type of soil and level of its compaction in the area between the bottom of the footing and the top of the sheet piling. The sheet piling itself is impermimiable to water flow. It is typically driven to a depth greater than the depth of the water adjacent to it. For eample, the maximum depth of the water adjacent to my house is 25' - sheet pile have to be driven to 28'+ in order to be effective. The sheet pile at the 17th Street canal were not undercut, if the water did undercut the wall then there had to be a gap between the wall foundation bottom and the sheet pile cap. I'd love to see the design details for that wall.
Don's point of "stuff happens" is what engineering is all about. Levees are not exactly a new subject to engineers and building on top of them and behind them is a practice that predates writing. Don is not making a very convincing argument.
David,
Corps procurement procedures do not lend themselves to corruption at all. If subsidence was the primary factor - the ground sinking beneath the wall and opening a gap between the bottom of the wall footing and the top of the sheet pile then a comparison of core samples from the pre-constrution analysis with core samples taken today will show it. It is possible that the very act of driving the sheet piles affected the underlying stability of the soil into which they were driven (similiar to the effect of an earthquake on soils in the Bay Area). Shooting grades for the sheet piling tops is very easy to do and will provide a fairly conclusive answer.
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