"Unforeseen" events predicted in 1986 study
By Bob Marshall Staff writer
Findings by an Army Corps of Engineers-sponsored panel that the collapse of the 17th Street canal floodwall during Hurricane Katrina was the result of an “unforeseeable” combination of events is contradicted by a 1986 research project done by the corps itself, National Science Foundation investigators said Monday.
The Interagency Performance Evaluation Task Force, working for the corps to investigate the levee breaches, said in its second interim report Friday that the 17th Street failure was caused by rising water in the canal that forced the floodwall to flex away from the canal, causing a separation between the wall and the levee inside the canal. Water pressure building inside the opening then exerted force on a weak layer of soil under the wall and land-side toe of the levee, causing the layer to slip, bringing the levee down and the wall with it.
Spokesmen for the 50-member task force, composed of researchers from academia, industry, state and federal agencies as well as the corps, said a review of engineering literature revealed this specific “failure mechanism” had not been noted prior to design and construction of the project from the late 1980s until 1994.
But in a sharply worded response issued Monday, Ray Seed and Bob Bea, University of California-Berkeley professors heading a 34-person National Science Foundation investigation into the levee failures, said the 1986 corps research make those claims “unfortunate” and “inaccurate.”
Seed and Bea said the 20-year-old test, which included constructing floodwalls on existing levees and raising water levels to determine what pressures the walls could withstand, resulted in the same kind of collapse that toppled the 17th Street structures and flooded much of the city.
“In simple terms this was exactly the ‘unforeseen’ mode of failure” reported by the task force, the statement said.
A spokesman for the corps’ New Orleans District said the task force would be reviewing the National Science Foundation statements and might have comments later. Calls to task force directors were not immediately returned.
The 1986 corps test, done in the Atchafalaya Basin on soils purposely meant to simulate those in the New Orleans area, resulted in a series of events that closely mirror those that would occur on the 17th Street canal during Hurricane Katrina, the science foundation statement said. As water levels rose against floodwalls constructed for the test there was a “dramatic increase” in deflections of the sheet piles, followed by a “gap developing between the sheet piles and the soils, allowing water to flow between the sheet piles and the soils exerting additional hydrostatic pressures on the piles,” the foundation engineers said.
The results of those experiments were widely circulated among corps officials, the science foundation engineers said. Further, the researchers involved in the test alerted the New Orleans District, which was then overseeing design of the area’s hurricane floodwalls, that its findings pointed to the need to find new methods to “analyze both the soils supporting the sheet piling and concrete floodwalls, and the sheetpile/floodwalls themselves,” the foundation statement said.
That never happened, Bea said in an interview. “There was no change in the methods used to do the analysis,” he said. “I doubt this report ever made it to Modjeski and Masters (the general design firm for the 17th Street project) or Eustis (the company doing the soil tests).”
The task force’s statement that the professional literature contains no examples of the 17th Street canal failure scenario, was also challenged by the foundation team, which pointed to a 1997 edition of the Electronic Journal of Geotechnical Engineering, which contains two papers citing the 1986 study.
“The first of these papers,” the foundation statement said, “succinctly observed (page 10): ‘As water level rises, the increased loading may produce separation of the soil from the pile on the flooded side. .¤.¤. Intrusion of free water into the tension crack produced additional hydrostatic pressures on the wall side of the crack and equal and opposite pressures on the soil side of the crack.”
Bob Marshall can be reached at firstname.lastname@example.org or (504) 826-3539.