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 last month.
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 Pontchartrain into the canal last August (GE October 2005).
Initial results of centrifuge modelling of the failure were released last month 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 under ater 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 was led by ICE vice president Scott Steedman, the only non-American appointed to head an IPET research team (see 'Race to raise the levees', page 28).
'If you have clay next to sheet pile wall and water flooding on top it's very easy for the two to separate, ' he said. 'The evidence from the centrifuge model fi ts all the facts that we know about the 17th Street breach.'