GeoEngineers harnessed cutting-edge modeling to assist with the deepest excavation in New Orleans history, and protect the city from future disasters. Constructors for the Permanent Canal Closure and Pumps project recently completed excavation for the 17th Street, London Avenue, and Orleans Avenue Canal Pump Stations, the heart of New Orleans’ ambitious storm-resiliency program. GeoEngineers provided modeling and analysis that confirmed the cofferdam shoring systems for the 40- to 54-foot excavations could withstand the enormous earth and water pressures, despite being anchored in the region’s soft soils.
“It was a challenge and an honor to work with PND Engineers, Stantec, the New Orleans District US Army Corps of Engineers and the whole Kiewit Louisiana Co.-led team to provide this critical structure for the resiliency and safety of the community,” said GeoEngineers Chairman of the Board David Sauls.
GeoEngineers had to overcome unique challenges to safely shore and unwater the excavation site. New Orleans sits below sea level and the native soil is extremely soft, causing stability concerns for any shoring system. The project required cofferdams to be installed in the water at the mouth of each of the three canals. To protect excavation, the shoring system needed to withstand significant hydrostatic pressures from the surrounding water and soil while limiting cofferdam movement. Any slippage in the weak clay could have led to a progressive failure due to soil deformations, a critical soil-structure interaction.
To meet these site challenges the project team selected a unique OPEN CELL® Cofferdam system designed and patented by GeoEngineers’ client, PND Engineers.
“The OPEN CELL shoring system had never been used before in such challenging conditions, and since this was the largest excavation to use the system, this was a challenging and cutting-edge design validation process,” said Zack Simpson, one of the project’s geotechnical engineers.
Traditional force-based design methods couldn’t provide a realistic evaluation of the soil-structure interaction effects and how much the cofferdam might move under real-world conditions. PND Engineers hired GeoEngineers to perform 3D finite element modeling to better understand this critical issue for use in the design of the OPEN CELL Cofferdam system. This modeling approach predicted the effects of soil deformations to the overall system stability throughout the excavations. The results gave the designer and the project team higher confidence that the OPEN CELL system would safely withstand the enormous soil and water pressures and control ground deformations to maintain the overall system stability.
During excavation the cofferdam performed well and observed wall responses aligned with GeoEngineers’ predictions. Excavation is now complete, but construction on the three canal pump stations is expected to continue through 2017. Once finished, the pumping stations will be able to pump a combined 24,300 cubic feet of water per second out of the city and into Lake Ponchartrain, protecting the city from flooding in the event of a large hurricane or storm event.
For more on the pump stations and the entire Permanent Canal Closures and Pumps project, see this feature article from Engineering News-Record.