WASHINGTON - Soil tests indicate that a soft, spongy layer of swamp peat underneath the 17th Street canal floodwall was the weak point that caused soil to move and the wall to breach during Hurricane Katrina, an engineer who has studied the data says.
"The thing that is remarkable here is the very low strength of the soils around the bottom of the sheet pile" base of the floodwall, said Robert Bea, a geotechnical engineer at the University of California, Berkeley, who examined the test results. Bea is a member of the National Science Foundation team that is studying the levee system's performance during Katrina.
Bea said other data shows the same peat layer also runs under the London Avenue canal breaches and probably was instrumental in the collapses there as well.
Investigators are focusing on the 17th Street and London Avenue canal levee walls because, unlike other parts of the system, they were apparently not topped by Katrina's storm surge. That could mean a design or construction flaw is to blame for the collapses - and for the flooding of much of central New Orleans.
Army Corps of Engineers officials say they must determine whether human error played a role in the breaches. If it did, they say, they may have to rebuild the canal walls immediately so they don't pose an additional risk during next year's hurricane season.
Investigators from the Corps, the American Society of Civil Engineers and the National Science Foundation have spent the past two weeks examining the floodwalls and other parts of the levee system. At the two drainage canal breaches, they say the culprit appears to be the layer of peat around or not far beneath the base of the walls.
The Corps of Engineers has not yet released the results of soil borings in the breached areas to the outside investigators. But the Corps included some soil boring data for the 17th Street canal breach this week in a contract for temporary repairs posted on its Web site. The Times-Picayune asked Bea to examine the results.
The canal walls consist of a concrete cap on a steel sheet pile base, driven 17.5 feet deep at 17th Street and 16 feet deep at London Avenue, Corps design documents show. Bea said that the soil boring data shows the peat layer starts about 15 feet to 30 feet beneath the surface and ranges from about 5 feet to 20 feet thick.
Signs of trouble appear in graphs in the Corps' soil data showing the "shear strength" of the soil, its ability to resist deformation and lateral motion. In one boring, at 27 feet, the soil strength is near the bottom of the scale, about 0.02 tons per square foot. Eight feet deeper, the strength is 0.25 tons per square foot - more than 10 times greater. At 70 feet, the strength even greater: 0.6 tons per square foot.
The data also show the soil at the peat level has a high water concentration. Put together, those data indicate it would be very vulnerable to the stresses of a large flood, Bea said.
At 17th Street, the soil moved laterally, pushing entire wall sections with it. Bea and other engineers say that as Katrina's storm surge water filled the canal, water pressure rose in the soil underneath the wall and in the peat layer. Water moved through the soil underneath the base of the wall. When the rising pressure and moving water overcame the soil's strength, it suddenly shifted, taking surrounding material - and the wall - with it.
"Think of a layer cake - in the middle I've got my icing. All of a sudden, I push on the top of my piece of cake, and what it's moving on is this weak, slick icing. The whole thing moves, " said Thomas Zimmie, a civil engineer at Rensselaer Polytechnic Institute who is on the National Science Foundation team and surveyed the levees this week.
The Lakefront area used to be a swamp, and was filled over the decades as development advanced northward toward Lake Pontchartrain. But the soft swamp and river soils - layers of mud, peat, sand and silt - remain under the surface and can pose problems for those trying to anchor structures in them. The normal solution, engineers say, is to drill piles as deep as possible so that they go all the way through weak layers and are more firmly anchored.
The contractor who built the 17th Street Canal reported problems drilling the sheet pilings in the soil. Segments of the wall leaned slightly when the concrete was poured, according to a legal ruling in a contract dispute over the matter. An administrative law judge ruled that was due to the unusual bracing system used to build the structure rather than unexpected soil conditions.
Bea said that while the investigators have theorized the Corps missed the peat layer in soil tests before the wall was built, that data they now have shows the peat would be hard to miss.
"The soil profile that we've got in front of us is showing that peat layer is large in extent, not narrow. They are mapping it between multiple borings. My suspicion, or fear, that they had missed it between two borings is not justifiable. It looks like it's about a thousand feet wide. That used to be a swamp. We built levees and cut canals in it, but never went in there and took out the peat."
John McQuaid can be reached at 202-383-7889 or at email@example.com.