HURRICANE STORM surges in New Orleans pushed a sheet piled levee wall out of position before it failed, flooding a part of the city, engineers said this week.
The surge exerted huge lateral forces on a whole section of sheet piled levee on the city's 17th Street canal, causing a 139m long breach on its the east side.
It occurred as Hurricane Katrina's storm surges pushed water from Lake Ponchartrain into the canal last August (NCE 8 September 2005).
Initial results of centrifuge modelling of the failure were released last week by the US government-appointed Interagency Performance Evaluation Task Force (IPET).
These show that as canal water levels rose, the flood wall moved landward. The movement created an underwater gap between the canal side of the floodwall and the clay embankment.
Water was then able to push down against the toe of the sheet pile, causing it to shift landwards.
Weak shear strengths in the clay layer below the sheet pile and hydrostatic forces on the pile toe pushed the sheet pile vertically along a slip plane in the clay.
Centrifuge testing of models of the 17th Street levee were led by ICE vice president Scott Steedman, who is heading up one of 10 IPET teams.
'If you have clay next to a sheet pile wall and water flooding on top it's very easy for the two to separate, ' said Steedman.
INFOPLUS View a video of the centrifuge and storm surge models at www. nceplus. co. uk