The Evolving “Factor of Safety.”
Reed Mosher – US Army Corps of Engineers researcher
Bob Bea – engineering professor, University of California-Berkeley
J. David Rogers – engineering professor, University of Missouri-Rolla
A major city in the United States is flooded when poorly engineered levees fail after a major hurricane passes close to, but not directly hitting, the city. The agency that designed and built these levees, the USACE, tries foolishly to absolve itself from blame by claiming that the levees were built properly but the hurricane was just too strong. They say the failure was unforeseeable.
But the JLE, Justice League of Engineers, comes to the defense of the city’s residents to say, “This is not so! The levees should have held. The people of this fare city have been wronged!” This is the story of their continuing struggle to get the USACE (a.k.a. the Legion of Doomed Levees) to admit they were wrong and pay for their transgressions.
In last week’s episode, the JLE pointed out that the west side, or Metairie side, of the 17th Street Canal floodwall was in danger of “incipient failure” had it not been for the *actual failure* of the east side flooding New Orleans and parts of Jefferson Parish.
In this week’s episode, the USACE defends itself:
Mosher doesn’t agree with Bea and Rogers that a breach in the west floodwall was imminent, but he does affirm the inadequacy of the floodwalls’ “factor of safety,” which is a numerical value used to dictate how much over-building is needed to offset risky situations like poor soils.So says the Legion of Doomed Levees. They admit that the factor of safety *was not as high as it should have been,* but set the factor *barely above* where failure occurs.
Design standards for both floodwalls south of Hammond Highway Bridge was supposed to be 1.3. Bea and Rogers say their calculations indicate that the factor of safety at the west floodwall at the time the east wall collapsed was 1. Mosher, who said failure occurs at 1 or less, sets it at between 1.1 and 1.2.
See? It wasn’t the USACE’s fault! They may build crappy levees, but they don’t build crappy levees that fail.
Take that, residents of Southeastern Louisiana! [Evil laugh]