Floodwalls seem to have been built to ACE specifications

Was the floodwall failure at the 17th Street Canal due to bad design by the Army Corps of Engineers, or bad construction by contractors?
They pulled eight sections of the steel sheets that were driven into the ground, and so far that seems to indicate that they were installed to the designated depth: "Engineers Reverse New Orleans Levee Finding".

...The steel had been sunk into the ground to prevent water from saturating the soil and destabilizing the flood walls. Initial testing by sonar had indicated the sheet pilings were driven to only about 10 feet below sea level, even though the design called for 17.5 feet below sea level...
...Brig. Gen. Robert Crear said the length of the sections pulled all exceeded 23 feet. About six feet of the sheet piling was above sea level, leaving a little more than 17 feet below sea level - in accordance with design specifications.
Engineers also plan to test the concrete and the reinforcing bars in the flood wall to ensure they were made properly.
Also, engineers must try to figure out why the sonar tests yielded bad results on how deep the sheet pilings were driven.