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Army Corps Megaprojects: The Benefits of Innovative Foundations

The IHNC Floodwall Structure

In April 2008, Shaw Environmental & Infrastructure (now CB&I) was awarded the contract to build the Inner Harbor Navigation Canal (IHNC) Lake Borgne Surge, Barrier near New Orleans. The surge barrier was part of·the largest civil works design-build project awarded by the USACE. The surge barrier was designed and constructed as part of the greater Hurricane and Storm Damage Risk Reduction System (HSDRRS).

The Inca/Gerwick Joint Venture was the overall Engineer of Record for the project and a subcontractor to Shaw. Ben C. Gerwick (now COWI North America) was the Engineer of Record for the flood wall component of the surge barrier as well as the sector gate monolith foundation. Eustis Engineering was the Geotechnical Engineer of Record for the flood wall surge barrier.

The design and construction of the project was highly successful as evidenced by many awards and citations including the ASCE OPAL award naming the project as the Outstanding Civil Engineering Achievement of 2014.

Lessons Learned - Construction access in the soft soil conditions of the marsh posed a significant mobility issue. At the project outset, Shaw dredged a 150 ft (46 m) wide access channel so that floating marine equipment could be used for access to construct the floodwall in-the-wet.

The basic floodwall design and detailing by Gerwick allowed procurement of structural components to begin before the design was complete. Effective use was made of multiple precast components including cylinder piles, closure piles, battered piles, etc., so that fabrication and construction could proceed along multiple paths simultaneously. An overall contraction of the construction schedule was realized.

As the floodwall structural design evolved, design details were developed so different soil profiles along the wall alignment could be addressed by small adjustments in the basic design. At the project start, it was decided to use vertical concrete cylinder piles assembled from short segments. Use of segmented cylinder piles allowed fabrication of the segments to start before the design was complete. Additional segments could be added to the standard basic length as the structural need arose. In addition, a reinforced concrete cage, required in the upper portion of the vertical pile to address maximum design bending moments, could be adjusted as needed along the project alignment.

Floodwall Design Details - The design of the braced floodwall incorporating the precast cap allowed tuning the design moment in the cap to vertical pile connection. The tuning allowed less conges­ tion in the connection and easier installation, resulting in accelerated construction. The caps were also detailed to account for the expected, field driving tolerances of the vertical and battered piles.

SSI (soil-structure interaction) analyses were successfully used to verify the capacity to resist deep-seated failure planes in soft soils , while limiting lateral deflections to acceptable limits tc prevent progressive collapse responses. Multiple analysis methods were used including a nonlinear soil spring approach and a global finite element approach. Both methods independently confirmed the magnitude of the controlling design moment and maximum displacements. A primary lesson learned was the robustness of the battered pile floodwall structure and its insensitivity to variations in soft soils.

The decision by the main floodwall subcontractor, TMW (Traylor-Massman-Weeks), to build a construction trestle allowed excellent dimensional control of vertical and battered piles and greatly contributed to accelerated construction. Tight control of the battered pile positioning to obtain the required tolerances was achieved using an adjustable guide system in a three-story tall battered pile template designed by the contractor. The subsequent placement of precast cap units over multiple vertical pile stub outs and associated battered piles was easily accomplished. A primary lesson was that reasonably achievable field tolerances built into the design, combined with a contractor dedicated  to maintaining tight field control of tolerances, will yield exceptionally good results.

USACE embedded its own review team with Shaw and the Inca/Gerwick Designers of Record to expedite communication, familiarity with the design, and final review and approval of released for construction (RFC) documents. This resulted in faster overall approvals as concerns could be vetted and resolved in a team approach, and avoided lost time in back and forth response in addressing issues.

The State of Louisiana, which assumed control of the floodwall and urge barrier system from USACE when completed, also embedded reviewers with Shaw and the design team. The communication fostered team cooperation.

IHNC Design Innovations - The scour stone behind the floodwall at the base of the plunge pool dissipates energy from the waves spilling over the top. In these soft soils, one lesson learned was, that the scour stone contributed to reducing bending in the vertical concrete cylinder piles.

In general, Gerwick engineers and designers spent considerable effort optimizing the structural details as small changes in details translated into significant savings in construction costs because of the length of the wall. At the interface with adjacent structures, a basic expansion joint flap detail was developed that was adjusted to suit differing juncture conditions and installed using a similar construction method at all junctures.

Steel piles with a diameter of 3 ft (0.9 m) were used for the sector gate foundation as installation was deemed easier than a combination of vertical and battered concrete piles. The steel piles provided similar performance and deflection to the battered pile alternative, with a reduction in schedule.

 

One of the major lessons learned was the positive aspect of communication and individual inclusion in a megaproject. There were numerous weekly meetings with workers at all levels involved. The result of the massive inclusion effort was that the entire team stepped up and took ownership, even when decisions to move forward were contrary to a specific individual's recommendation.