The Permanent Canal Closures and Pumps (PCCP) project is the capstone piece of New Orleans' extensive hurricane risk-reduction system constructed following Hurricane Katrina in 2005. The project encompasses three separate pump stations and floodgate structures located at the mouths of three outfall canals on the south shore of Lake Pontchartrain. These three canals, running from the south to north, act as the primary conduits for discharging stormwater from New Orleans.
Prior to Hurricane Katrina, these canals were operated with pump stations located in a series two to three miles upstream of the mouths at Lake Pontchartrain. This left long stretches of levee on each side of the canals exposed to storm surges entering from the lake. Three levee failures occurred along these canals during Katrina two at London Canal and one at 17th Street Canal. The failures resulted in major flooding of the city, causing catastrophic damage and loss of life.
Following initial canal levee repairs after Hurricane Katrina, the US Army Corps of Engineers (USACE), in partnership with the Louisiana Coastal Protection and Restoration Authority (CPRA), took action to move the line of flood protection to the mouths of these three canals at the edge of the lake. By cutting off the storm surges from Lake Pontchartrain at the entrance to the canal, more than 10 miles of canal levee would no longer be exposed to storm-surge water elevations, significantly decreasing the risk of failure.
Initially, temporary pump station and gate systems were installed (the interim closure structures) at each canal to provide this increased protection. The goal of the PCCP project was to replace these temporary structures with permanent pump stations designed to protect against a 100-year storm surge and evacuate the stormwater from the associated design event. This closure system would also allow non-storm canal flows to bypass the pump station and flow directly to Lake Pontchartrain. During specified storm conditions, this bypass canal is closed off by lift gates to complete the floodwall protection at the mouth of each canal. Stormwater in the canals, collected from the New Orleans area, is then pumped over this flood barrier and into the lake.
The project was completed by the design-builder PCCP Constructors JV (Kiewit Louisiana South Co., Traylor Bros. Inc. and the M.R. Pittman Group, LLC) with Stantec, Inc. as the lead designer, PND Engineers, Inc. as the cofferdam designer, and GeoEngineers, Inc. as the cofferdam numerical modeler.
The PCCP project incorporated some of the deepest and largest excavations ever accomplished in this region and required a significant structure, along with careful construction sequencing, in order to successfully complete the project. Two cofferdams and two substantial permanent retaining walls were required for each site. For the small cofferdam enclosing the by-pass structures, a classical, braced-frame cofferdam was deployed with an excavated depth of 18 feet. The second, larger cofferdam enclosed the pump station footprint. The largest of these pump station cofferdams at the 17th Street Canal was 265 feet long and 165 feet wide. The deeper pump station cofferdams posed compelling challenges, given the 50-foot excavation depth and the necessity to limit interferences and penetrations from the cofferdam and the permanent structure.
The New Orleans area presents extreme challenges to a deep cofferdam. Common to most of the southern Louisiana region is a high water table. Underlying soil deposits are primarily composed of unconsolidated clays with occasional sand layers to a substantial depth below the surface. The history backdrop of the PCCP project included the notorious geotechnical failures near the site; the difficult, soft ground conditions; large magnitude of hydrostatic loading; the uniqueness of the cofferdam support system; and the need for unobstructed access to the subsurface permanent work.
After evaluating preliminary designs and overall costs of different cofferdam systems, the design-builder selected the OPEN CELL SHEET PILE™ system. An OPEN CELL system with a pile-supported tremie is an innovative and unique design application for a deep cofferdam.
The design was made complex by the project criteria, soft ground conditions and complex site layout. In addition, the pump station cofferdams needed to be fully un-watered to EL -50 feet without excessive ground improvement. The project footprint at each site was limited for by-pass structure and pump station due to the existing canals, levees and floodwalls where the design-builder needed to maintain canal flow throughout construction. This meant that each step of design required careful analysis and operations planning to protect the site and the city from storm and flooding events. This required detailed planning in the site layouts, operations and sequence of construction.
The OPEN CELL cofferdam design was a unique application of the OPEN CELL SHEET PILE system. Rather than using the system to support an engineering fill structure, the system supported the in-situ soft soils of the site and retained and sealed the cofferdam on all four sides from water intrusion into the large field of construction. The design-build team advanced the OPEN CELL system design under an intense review process by the USACE, given that no guidelines existed for such a structure within the USACE's design practice. The team presented and defended this shoring method, which had never before been used for a project of this magnitude and historical legacy.
Meticulous force-based design, soil structure interaction numerical modeling and physical testing were all techniques used to present and gain USACE acceptance of the shoring design. The cofferdam in which the impressive permanent pump stations would be constructed was a short-lived but vital feature to enable the design-build team to bid, win and successfully deliver a major infrastructure project to USACE for the civil defense of New Orleans from storms and flooding.
For the pump station designer, the OPEN CELL cofferdam offered flexibility in design development of the permanent structure without any need to contemplate or coordinate the locations of obstructions or interferences from temporary cofferdam struts or braces.
For the construction team, the resulting cofferdam system enabled planning of work operations completely free of obstruction or interference, with optimal production rates and the ability to place large equipment near the face of the cofferdam and hoist large equipment and materials into the field of construction.
Unique application of piles
Unlike a braced cofferdam and closed cell structure, the OPEN CELL structure mobilizes the available soil resistance via the large exposed diaphragm area of the tailwall system. The tailwall is simply a long series of flat web sheet pile that extend into the retained earth at junctions between each curved face arc. Adhesion and friction of soil to the flat web sheet pile creates a portion of the net resistance. The extensive series of interlocks in each tailwall, acting similar to the deformation on rebar, mobilizes the balance via bearing against the soil fabric. The two mechanisms and the selected spacing of tailwall elements create a block of mechanical stabilized soil, with the face arc retaining the forces arising from earth and hydrostatic pressure, as well as from equipment. This configuration is also highly efficient, given that steel sections are almost exclusively working in tension. A simple analogy is that the face arc of the OPEN CELL system is like the canopy of a parachute, and the tailwall mimics the effect of shroud lines anchoring the face arc to the surrounding soil. In soft ground, the mechanism is most effective, since even with weak soil the available capacity can be mobilized across a large area.
Given the extreme hydrostatic pressure in the unwatered condition, the OPEN CELL cofferdam design for PCCP captured and utilized the driven pipe piles to provide stability at the base of the excavation and to augment the lateral capacity of the tailwall system. While OPEN CELL walls were able to safely support the excavation work prior to unwatering the cofferdam, the extreme earth, equipment and hydrostatic loading on the cofferdam system after unwatering required additional support. For this support, the design-build team incorporated the lateral and vertical support of a tremie-placed concrete seal slab and permanent piles in the bottom of the cofferdam to support the base of the OPEN CELL cofferdam walls. This component was common to all three project sites.
The cofferdam and tremie slab acted as an integrated system to enable the design-builder to perform the deep excavation required to build the pump station foundation and superstructure in a dry work environment. The tremie slab was sealed, which meant the slab resisted full hydrostatic 1.6 tons per square foot of uplift and soil heave pressure under the slab without venting or water pressure relief pumping from under the slab. The challenge of the sealed slab was to develop a system strong enough to safely support the uplift pressure, yet efficient in order to limit the depth of excavation to place the seal slab. A conventional gravity-supported seal slab would have needed to be over 30 feet deep to resist the 51 feet of water head pressure at the base of the slab. This slab depth would have resulted in additional excavation and cofferdam wall undermining.
In order to optimize the concrete seal slab, the design-build team incorporated the tension capacity of the permanent pipe piles designed to support the permanent pump station structure. By "nailing" the temporary concrete seal slab to the underlying soils, the slab achieved additional uplift resistance against the large hydrostatic forces. At the 17th Street pump station cofferdam, 465 separate 24-inch- and 30-inch-diameter pipe piles and adhesion to perimeter cofferdam sheet piles helped to provide uplift resistance hold down the concrete seal slab.
A single shear ring welded to head pipe pile provided reliable mobilization of the concrete seal tributary to the pile section. Concrete adhesion to the temporary cofferdam sheet piles provide vertical resistance to uplift around the perimeter of the cofferdam. The design-builder was able to achieve high production rates and tight tolerances while driving the permanent piles through highly turbid waters within the cofferdam prior to unwatering. The final top elevation of the permanent piles were 50 feet below the water surface. This underwater pile driving was made possible by the use of hydraulic impact hammers.
Cost saving measures
The OPEN CELL cofferdam represented a material savings on steel versus a traditional braced frame cofferdam alternative. However, the primary cost saving associated with the cofferdams was the efficiency of construction after the sheet piles were installed. The OPEN CELL cofferdams provided a free field of construction within the cofferdam excavation across a large area. Cost-saving efficiencies were realized in the excavation of the cofferdam and the construction of the permanent structures within the cofferdam confines. This free field of construction allowed for the placement of large permanent equipment, formwork panels and material quickly and safely without having to adjust or allow for internal bracing and the sequential removal or re-strutting of cofferdam walls and walers.
The robust strength capacity of the OPEN CELL cofferdam system also permitted the construction crews to place heavy equipment loads near the cofferdam face without additional pile supports to distribute the load away from the walls. This free field of operation outside of the cofferdam also resulted in efficiencies and cost savings in construction.
Innovative project management
The PCCP project was developed as a design-build project the first major project of its kind for the USACE, MVN District. Under this context, the design of the pump station, pump station cofferdam and permanent retaining walls were developed concurrently in order to compress schedule and deliver the project on time. In order to execute the project successfully, the cofferdam design and subsequent start of construction needed to occur before completion of the permanent pump station design. Simultaneously, both the pump station and pump station cofferdam designs were subject to independent design review by USACE and their engineering and agency partners.
The cofferdam design was developed based on the initial design development and general project criteria of the pump station and surrounding support structures. Modifications to the cofferdam occurred in real-time as design progressed on the permanent pump station and surrounding structures. The open field of the OPEN CELL cofferdam allowed for additional flexibility and optimization for the permanent structure design because internal bracing or re-strutting of the cofferdam walls was not required.
Management or mitigation of environmental considerations
The OPEN CELL cofferdam system proved to be an optimal and robust solution to achieve the stated USACE design objectives for the PCCP project. These objectives were consistent with the project mission to provide for public safety and property protection within New Orleans.
The city and surrounding areas are now better protected as a result of the project. The threat of adverse environmental ramifications from wide-spread flooding of the surrounding neighborhoods and city are immense as evidenced by the results of Hurricane Katrina. The application of this innovative support of excavation method using driven pile technology will continue to grow and evolve to address new environmental issues that arise from society's demands and new pressures arise from the effects of climate change.