Clerical error may have doomed levee
By Bob Marshall Staff writer
A difference in soil boring data transferred from one chart to another may have played a key role in engineering decisions that led to the breach on the 17th Street Canal floodwall that flooded much of New Orleans during Hurricane Katrina, National Science Foundation investigators say.
A cross-section drawing in the project design documents shows a weak layer of peaty soils running between 11 feet and 16 feet below sea level in the area that failed during the storm. But information in the individual soil borings that were used to draw the cross section show the peaty layer extending as deep as 30 feet below sea level.
Investigators said their own borings taken at the site this week confirm the 30-foot depth, leading them to believe that designers used the flawed cross section drawing to set the sheet pilings beneath the floodwalls at 17.5 feet below sea level - a choice that allowed water to migrate to the land side of the wall, causing the breach.
"It's pretty obvious the depth of the organic deposit shown on that cross section is what determined the sheet piling depth . . . for the whole canal," said J. David Rogers, a member of the National Science Foundation team, and a leading authority on levee and dam failures. "They saw this marshy deposition, they recognized it as potentially dangerous, so they specified sheet piles that went just beyond the bottom of that line.
"So it's easy to deduce that if they saw that peat layer going to 30 feet, they would have placed the piling at least that deep. And it appears pretty clear, at this point, that the transfer of the boring data to the cross section is where the ball might have gotten dropped."
The length of the steel sheet pilings has been a focal point of investigators since they concluded the wall collapsed before it was overtopped by storm surge. Sheet piles provide two critical functions in the floodwall design. They support the concrete cap that rises to 14 feet above sea level, and they are supposed to block water inside the canal from seeping through the levee. Once saturated, the soils supporting a wall can quickly lose their strength and begin sliding laterally, causing a collapse.
With the canal bottom at 18.5 feet below sea level, engineers say the depth of the sheet piles was too shallow for the notoriously porous soils in what was mostly swamp and marsh just 100 years ago.
Investigators also have been puzzled by what they say are obvious engineering errors because the firms involved - Eustis Engineering for the soil profiles, Modjeski and Masters for the general design - were experienced and reputable in their fields. Work by the National Science Foundation team, however, may have uncovered a small mistake that had a huge impact.
The companies have declined to comment.
Rogers, who teaches at the University of Missouri-Rolla, said soil work always involves a lot of judgment calls and assumptions, because geotechnical engineers never have a complete picture of what's below the ground.
"If the data you have to work with is accurate, you're usually OK, but if it isn't, you're in big trouble," he said. "It looks like the guys working on the (floodwall design) thought they were anchoring those sheet pilings in clays, and they were really right in the middle of this highly organic, highly permeable soils that ran another 10 feet beneath the tips of those pilings."
The official record of the Army Corps of Engineers project shows Eustis did the soil borings. The cross section drawing carries the logo of the corps' New Orleans district, but Rogers said it is common practicepratcie for the corps to sign documents compiled by consulting firms.
"Normally the firm doing the soil investigation also does the profile, and the corps puts its name on the profile in the design document," he said.
A corps spokesperson was unable to respond to a request for information late Friday afternoon.
Documents show the cross section was constructed using the standard technique of drilling soil borings - sinking hollow-core drilling pipe to about 40 feet. When the pipe is withdrawn, the soils captured inside the pipe are removed and inspected. As engineers note the composition of the soils at each depth, a cross-sectional slice of what is below the surface begins to emerge. On this project borings were made about every 500 feet from Hammond Highway to Pump Station No. 6.
The interpretations of each boring were transferred to a written report, which includes a drawing that uses different symbols to indicate the types of soils at each depth, documents show. Those boring logs are used to develop a larger chart that provides a cross-section view of the soils along entire length of the project.
The logs and drawings for borings in the breach area show a layer of weak, mostly organic soils - peat, humus, shells and woody fragments - from about 12 feet below sea level to 28 to 40 feet below sea level. Some of those layers also carry the symbol for "fat clay" but have the words "wood" or "shells" written in between the lines, indicating a mixture, although the written description of the layers on the log indicate these layers were composed of mostly weak material.
But on the project cross section, that same area shows the symbols for such soils ending at around 15 feet below sea level. Below that depth, the symbols show soils of "fat clay" or "lean clay" - sticky, impervious soils considered very good for resisting water, Rogers said.
After doing its own soil borings at the breach this week, the National Science Foundation team found the marshy layers going as deep as reported in the original boring logs. The team said other borings showed the layer of weak "marsh" soils continued down the entire length of the canal, but had its deepest penetration in the area of the breach.
"It's a pretty significant finding," said Rogers, adding the marshy layer that extended below the sheet piling tips was so porous it was almost complete saturated in the core samples retrieved. Without the sheet piling blocking that layer, he said, "it's like having a big pipe the can convey water form one side of the canal to the other."
Rogers said the weak layer may have been strong enough to hold back weaker storm tides until the canal's capacity was increased by raising the walls to 14 feet above sea level in 1994, because the extra water pressure on the weak soils would immediately and dramatically increase their permeability.
"It was like increasing the pressure in the water system in your house by storing water in a tank above the house," he said. "When you turn that spigot open, the water pours through. That's what happened when the (storm surge) rose in the canal. The water just poured through that layer well under the sheet pile to the other side of the wall - and the thing collapsed. It was a sudden and dramatic failure."
Rogers said his team could see no clear reason for the change in soil symbols when the data was transferred from the boring logs to the project cross section. While a lot depends on the interpretation of the person doing the drawings, he said an engineer reading the boring logs should have drawn the cross section differently.
"The guy doing the soil stability analysis (used to determine the sheet piling depth), probably would be looking more at (the cross section) unfortunately," Rogers said. "Ideally, they should look at both. You always should look at the boring logs - always, always.
"That's like a doctor looking at your file. It's the basic fundamental information that gets passed on. And the earth doesn't change much. You can get good data from boring done 60, 70 or 80 years ago.
"And these look like pretty good borings. It just seems like the interpretations (to the cross section) don't seem to be as good as what we' re doing."
Bob Marshall can be reached at firstname.lastname@example.org or (504) 826-3539