Why The Levees Broke
Investigators are now focusing on engineering mistakes
:Investigators in recent days have assembled evidence implicating design flaws in the failures of two floodwalls near Lake Pontchartrain that collapsed when weakened soils beneath them became saturated and began to slide. They also have confirmed that a little-used navigation canal helped amplify and intensify Katrina's initial surge, contributing to a third floodwall collapse on the east side of town. The walls and navigation canal were built by the U.S. Army Corps of Engineers, the agency responsible for defending the city against hurricane-related flooding.Katrina's storm surge put the floodwalls to the ultimate test. Hours after the storm hit, water poured into the canals from Lake Pontchartrain and added enormous strain to the walls and levees. According to a scenario developed by Bea and other investigators, the already-saturated peat was the path of least resistance, allowing the water to burst through the wall from underneath. At the 17th Street Canal, truck-size chunks of the old earthen levee were heaved 35 feet on a carpet of sliding soil.
Corps officials are not yet convinced. "It is important not to jump to conclusions," said John Grieshaber, chief of the engineering division in the Corps' New Orleans district office. "It's hard to look at the aftereffects and say with a high level of certainty, 'This is what happened.' "
The Corps' actions since the storm, however, suggest that at least some officials are worried about weaknesses in the floodwalls' design. A proposal for rebuilding the floodwalls has set far tougher standards than existed 15 years ago. And the steel pilings, which formerly reached a depth of 20 feet, must now be driven through the peat layer to 40 feet, twice as deep as before.