Permanent Canal Closures and Pumps Cofferdams - Support of Deep Excavation in Soft Ground Using the Open Cell Sheet Pile System-Paper

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The Permanent Canal Closures and Pumps (PCCP) project added permanent flood gates and pump stations at the mouths of New Orleans’ 17th Street, Orleans, and London Avenue canals. The structures help protect the city from Lake Pontchartrain storm surges while pumping rainfall from the canals. Their deep excavations—up to elevation -53 feet—were challenging because of soft, low-strength soils, high groundwater, limited construction space, and the need to keep canals operating.<br /><br />The design-build team selected OPEN CELL sheet-pile cofferdams. Unlike conventional braced systems, these structures use the strength of surrounding soil: curved sheet-pile faces transfer lateral loads through tension to buried tailwalls. This created an unobstructed work area without perimeter ground improvement or internal bracing. A pile-supported tremie seal was also used to address the high water pressure.<br /><br />Because the system was unusual and the ground conditions difficult, the team combined first-principles design, laboratory testing, three-dimensional soil-structure modeling, and destructive sheet-pile testing. Models incorporated soil strain-softening and estimated potentially large movements. Field monitoring tracked wall deflections, structural stresses, and curvature, with predefined response thresholds up to evacuation and flooding if needed.<br /><br />During construction, a change to partial “in-the-dry” excavation caused greater-than-originally-predicted movement. Updated modeling showed the cofferdams were behaving as expected under the revised loading sequence, and thresholds were adjusted using field data. All three cofferdams were successfully completed, flooded, and removed in 2017.<br /><br />The project demonstrated that innovative cofferdam systems can be used safely in soft ground when design assumptions are clearly communicated, analytical limitations are understood, monitoring is suited to field conditions, and the design team remains engaged throughout construction.

Keywords

PCCP project

New Orleans canals

flood gates

pump stations

OPEN CELL cofferdams

sheet-pile cofferdams

soft soil excavation

3D soil-structure modeling

field monitoring

storm surge protection