Army Corps Report Details 17th Street Levee’s Failure

Weak clay soils combined with tilting concrete storm walls caused the failure of the 17th Street levee during Hurricane Katrina, according to an Army Corps of Engineers investigation released Friday.

The corps’ first official findings about the levee failures in New Orleans do not indicate whether they resulted from defects in design or construction, although the wall apparently withstood less force than it should have.

A 465-foot section of the 17th Street levee was breached Aug. 29, flooding the Lake View section of New Orleans. Breaches occurred on two other major canals in New Orleans; the analysis of those failures is incomplete.

As the 17th Street Canal filled up with water from the surges that inundated Lake Pontchartrain, the top of the wall on the canal began to deflect or tilt toward the neighborhood and away from the canal, according to the report.

The tilting of the concrete wall opened a gap between the soil and steel-sheet pilings that had been sunk 17 feet into the ground. The water raced into the gap and to the bottom of the sheet piling, exerting heavy pressure along the entire height, the investigation found.

Despite those intense pressures, the levee should have remained standing, said Lewis “Ed” Link, a University of Maryland professor who is leading the investigation. Indeed, other storm walls in New Orleans suffered similar deflection during Katrina and did not fail.

But the soil was weak in ways the corps did not anticipate. The soil directly under the levee was compacted and strong, the result of the massive weight of the levee and storm wall sitting over the foundation for decades. However, at the toe of the levee in the backyards of residents living near the storm wall, it was comparatively weak.

As the water pushed against the steel sheet piling, it caused the entire foundation of the levee to slide backward toward the homes, Link said.

David Daniel, head of an external review committee organized by the American Society of Civil Engineers, said the gap also compromised half the strength of the levee. The gap negated any strength that would have come from the canal side of the levee.

At the 17th Street levee, the failure did not occur in a layer of organic peat, as earlier surmised.