What caused the canal walls to collapse in New Orleans?

The Times-Picayune makes an interesting story of a technical report. This is the key, though.

The canal walls consist of a concrete cap on a steel sheet pile base, driven 17 ½ feet deep at 17th Street and 16 feet deep at London Avenue, corps design documents show. Bea said 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 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.[...]

Bea said that while the investigators have theorized the corps missed the peat layer in soil tests before the wall was built, the 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."

Someone tried to skimp on the cost of the walls lf the canal in my opinion.