The engineering of the 17th Street Levee

During Hurricane Katrina, one of the major disasters that hit the city of New Orleans was the failure of the Levee at 17th Street. People are going over this with a fine-tooth comb and the initial reports are not good. The Times-Picayune has the story:

17th Street Canal levee was doomed
Report blames corps: Soil could never hold

The floodwall on the 17th Street Canal levee was destined to fail long before it reached its maximum design load of 14 feet of water because the Army Corps of Engineers underestimated the weak soil layers 10 to 25 feet below the levee, the state's forensic levee investigation team concluded in a report to be released this week.

That miscalculation was so obvious and fundamental, investigators said, they "could not fathom" how the design team of engineers from the corps, local firm Eustis Engineering and the national firm Modjeski and Masters could have missed what is being termed the costliest engineering mistake in American history.

The failure of the wall and other breaches in the city's levee system flooded much of New Orleans when Hurricane Katrina slammed ashore Aug. 29, prompting investigations that have raised questions about the basic design and construction of the floodwalls.

"It's simply beyond me," said Billy Prochaska, a consulting engineer in the forensic group known as Team Louisiana. "This wasn't a complicated problem. This is something the corps, Eustis, and Modjeski and Masters do all the time. Yet everyone missed it -- everyone from the local offices all the way up to Washington."

Team Louisiana, which consists of six LSU professors and three independent engineers, reached its conclusions by plugging soil strength data available to the corps into the engineering equations used to determine whether a wall is strong enough to withstand the force of rising water caused by a hurricane.

"Using the data we have available from the corps, we did our own calculations on how much water that design could take in these soils before failure," said LSU professor Ivor van Heerden, a team member. "Our research shows it would fail at water levels between 11 and 12 feet -- which is just what happened" in Katrina.

I would take this with a minor grain of salt though... From the same story:

The corps has long claimed the sheet piling was driven to 17.5 feet deep, but Team Louisiana recently used sophisticated ground sonar to prove it was only 10 feet deep.

It seems that 17.5 feet was still too small but the discrepancy between the engineered depth and actual depth might also point to cuts made in the construction -- seeming to build to spec but shorting materials and labor to make more money for the construction company. It will probably turn out to be a mix of both -- the Engineering companies are not lightweights (Eustis Engineering, Modjeski and Masters) and you can bet that if either of them signed off on a project, they had some very smart Senior Engineers review the design, even if the design was done by a junior partner. I used to work for an Engineering Company in Seattle and design projects of this magnitude are not taken lightly.