Permanent Canal Closures and Pumps Project

Since Hurricane Katrina flooded much of New Orleans, local and federal authorities have been working to prevent a similar disaster. The Permanent Canal Closures and Pumps Project (PCCP) was an effort by the United States Army Corps of Engineers (USACE) to protect the city by building three new canal closure and pumping station structures at the 17th Street, Orleans and London Avenue outfall canals. These stations will protect the city from storm surges and alleviate pressure on New Orleans’ canal infrastructure by moving water out of the city and into Lake Pontchartrain to the north in the case of a large hurricane.

Excavation for these structures wasn’t easy. Because New Orleans sits below sea level and the native soil is extremely soft and unstable, GeoEngineers had to overcome unique challenges to safely shore and dewater all three project sites. The massive excavation for the 17th Street Pump Station was the deepest in the city’s history, extending up to 54 feet below grade. Adding to the difficulty, the Pump Station had to be built in the water at the mouth of the 17th Street Canal. For the project to move forward, the team needed to design a cofferdam that could withstand significant hydrostatic pressures from the surrounding water and soil. The cofferdam walls had to be anchored in the weak surrounding clay and soil, and substantial movement could have led to a cascading failure due to the loss in soil strength.

GeoEngineers’ client, PND Engineers, Inc., planned to use an innovative OPEN CELL^® Cofferdam system of their own design. The USACE had strict standards for how much the OPEN CELL system could deform inward under the pressure of the soil and water during construction, and the team was unsure how large the deformations would be. Traditional force-based methods of predicting deformation weren’t able to give deformation estimates, so GeoEngineers took a more novel approach to validate the OPEN CELL shoring approach.

Our GeoEngineers team used advanced 3D finite element modeling to evaluate the shoring system’s performance for each of the canal structures before construction even began. We also performed a series of lab tests to refine the soil properties used in our models. GeoEngineers found that their models supported the use of the OPEN CELL system, and construction moved forward.

GeoEngineers also had to adjust to changing conditions during construction. When the team discovered damage to the OPEN CELL system that could have compromised cofferdam integrity, GeoEngineers reacted quickly to model and develop repair scenarios to safely keep construction on track.

Approach

• Using data from earlier drilling explorations, we went back to the lab to improve the accuracy of the soil properties for the soft soils on site using advanced analysis. This provided an accurate data set on which to base our models.
• We used state-of-the-art finite element modeling to estimate OPEN CELL^® Cofferdam deflections and sheetpile stresses in incredibly challenging soil conditions for an unusual shoring system.
• Developed a PLAXIS 3D mesh model based on boring data from critical sections. This visual model showed cross sections of soil and water at the site with predicted stress and pressure data.

Results

The project faced significant challenges due to the soft soils and high water table. Our accurate 3D numerical models confirmed that the OPEN CELL® Cofferdam system would withstand the enormous pressures from both the canal and surrounding soft soils. With this information in hand, the project team was able to satisfy the client and regulators that the project could be built safely. Together, these three pump stations will provide protection and dewatering for the city of New Orleans during large storm events, potentially saving both property and lives.