The engineering mistakes that led to the canal levee failures that flooded most of New Orleans during Hurricane Katrina were found and then dismissed in the Army Corps of Engineers' design review process in 1990, an investigative team reviewing the failures says.
Documents, obtained by The Times-Picayune and provided to forensic engineers studying the levee breaches, show project engineers made a critical mistake in assessing soil strengths on the 17th Avenue Canal project, said Robert Bea, a University of California-Berkeley professor who is a member of the National Science Foundation team.
Corps documents show the mistake of overly optimistic levee strength was detected by its Vicksburg, Miss., office, which directed local engineers to make changes. But when the chief engineer in New Orleans replied that the results were based on "engineering judgment," his superiors dropped the issue.
Bea said the discussion in the 16-year-old "design memo" points to the key decision that created fatal problems on the 17th Street Canal levee and could reveal a systemic problem that will show up during investigation into the London Avenue and Industrial Canal levees, which also breached during the Aug. 29 storm.
"From all the data we have, from all the documents made available to us, that exchange highlights where the key mistake was made in the design process, and how it was allowed to stand," Bea said this week.
"The design engineers didn't account for the weak layers in that swamp, and the Vicksburg office picks that up in review. But the New Orleans office says it's our professional judgment this is OK. In our business, that's an acceptable answer. But it's an answer Vicksburg can disagree with -- but it didn't.
"And from the documents we have, the issue is never raised again. At least not until Katrina comes along."
Investigators have long suspected engineering mistakes were at the heart of the levee and floodwall breaches. Not only did the structures fail before they reached design capacity, but documents show the designs were not appropriate for the weak soils and the depth of the canals, investigators said. Yet discovering why skilled engineers at reputable firms came up with obviously faulty designs, and how those mistakes were missed in the corps' lengthy review process, has stumped investigators.
Bea said the 1990 documents provide a two-part answer to one of the key questions in the investigation of the engineering behind work on the 17th Street canal and may hold true for failures at the London Avenue and Industrial canals as well.
The first part involves the choices engineers made to measure soil strength. A standard practice in levee design, Bea said, is to model soil strengths on the weakest layers in a project area. That allows engineers to design a structure that will withstand the most severe tests. That didn't happen on this project, Bea said.
The errors began when the crew from Eustis Engineering, the chief soil investigators for the project, decided to use an average of soil strengths taken from samples in an area that stretched over great distances, in some cases as much as 6,000 feet, Bea said. While averaging always involves some risk because it can miss inconsistencies in the subsurface, that practice is considered acceptable in areas featuring uniform layers of strong soils or rock, he said.
'A big no-no'
But Bea said his profession considers the practice "a big no-no" when dealing with levees because those structures "almost always have layers of different strength soils." And it was especially egregious at the 17th Street canal, he said, because, like most of New Orleans, the area is a former swamp with a subterranean soil profile that shows an unpredictable weave of mostly weak soils. A layer of very weak peat might be found near the surface or hidden 30 feet deep anywhere along the length of a canal.
That error was then compounded when the engineers decided to assess the horizontal soil strength at a depth that features more strong soils than weak soils, records show. The result was a rating showing the soils could withstand 380 pounds of pressure per square foot. That led the engineers to use, among other things, shorter sheet pilings and narrower levees.
Had they taken averages from shorter reaches, and at a depth just a few feet higher that featured more weak soils and was clearly visible in the soil profiles used by the engineers, the strength would have averaged just 231 pounds per square foot rather than 380, Bea said.
That result would have required the design team to call for sheet pilings that were at least twice as thick as those used and driven to 50 or 60 feet below sea level, rather than the maximum of 17 feet below sea level, which the corps has said is the deepest depth shown on final design documents.
"The way they did the soil strength analysis was the first big mistake, and it set things in motion," Bea said. "But it wasn't the last."
A second mistake
The second critical mistake was made, Bea said, when the team did the "slope stability analysis" -- a standard engineering exercise for determining if a levee will hold up at design capacity, in this case, 14 feet of storm surge. Once again, records show the engineers chose to accentuate strong soil layers and downplay weaker layers. That violated an essential element in the process, Bea said.
"You're looking for the weakest surfaces, the areas that could cause failure, and they didn't do that," Bea said. "Part of it was they had already assigned soil strengths that were too high. That showed up in this analysis."
The final mistake, Bea said, was made by the corps.
The initial soil investigations were done by Eustis, then reviewed and accepted by lead design team Modjeski and Masters, on a joint dredging project by the Sewerage & Water Board and the Orleans Levee Board. That work was accepted by the corps for the final part of the improvement project, which involved capping the steel sheet wall already embedded in the levee. Eustis did not reply to requests for comment; an attorney for Modjeski and Masters said his client declined to comment.
Engineers at the New Orleans office of the corps reviewed all of the work it received, then incorporated it in its final design plans. Corps officials have said that once they accept outside work, it becomes their own. Further, the federal agency had overall responsibility for the purpose of the project, which was to provide protection against a hurricane storm surge that might reach 14 feet in the canal.
The New Orleans office accepted the plans, but the regional office in Vicksburg had questions.
On a list of 17 problems noted, the Vicksburg office says the soil stability analysis is too optimistic.
Eugene Tickner, then chief engineer of the New Orleans district, responded only that the variables chosen in the New Orleans calculations were based on "engineering judgment." Bea said that should have raised a red flag with the regional office.
"You would prefer to have that judgment backed up with examples and numbers, but (the New Orleans office) doesn't offer very much," Bea said. "At that point, I would have asked for more information. But Vicksburg doesn't."
Instead, the issue was dropped and does not surface again in the trail of documents currently available on the project. The one clear opportunity to correct the engineering mistakes that led to the failures, Bea said, was missed.
"It's pretty clear, looking back on it with the information we have available at this point, that Vicksburg didn't like what had been done," he said. "We'll never know why they didn't pursue it."
Tickner, now living in North Carolina, could not be reached for comment. Fred Bayley, the chief engineer in the Vicksburg office at the time, retired in 1993. Now 73, he said he doesn't remember the issue or much of the details of the project. What he remembers most about New Orleans is the challenge its tortuously weak and layered soils posed for engineers.
"Even if you took borings every 5 feet, you might not get an accurate picture of what you were dealing with down there," he said. "Everything you did down there was a risk, because of those soils."