Solution
The USACE selected Duffield Associates to design a replacement jetty. Our engineers and scientists performed an as-built survey of critical features, bathymetric survey, and geotechnical and geophysical investigations of the submerged conditions as well as comprehensive laboratory testing.

The key elements in the existing jetty which enable it to resist forces on it are the presence of a soil berm on the jetty’s channel side (currently subject to sloughing, scour, prop-wash) and the jetty cells. In discussions with their technical staff, we were able to demonstrate that the berm could be protected and, therefore, it would provide needed resistance if the existing jetty were rehabilitated.

The advantages of rehabilitation included: a long-term solution to the original problem; an approximately $13 million savings – a $7 million rehabilitation versus an estimated $20 million replacement; shortened construction schedule benefiting navigational safety and port operations; and reduced environmental impacts associated with subaqueous land disturbance.

As a result, our proposed solution won acceptance for rehabilitating in lieu of completely replacing the jetty.

Innovative design solutions for the rehabilitation included:

• Innovative modeling of the global stability to capture benefits of soil/structure interaction.
• A unique protective layer of precast concrete “blankets” to keep the “berm” of unconsolidated sediments in place between the jetty and berth/navigation channel while enabling reestablishment of the subaqueous environment.
• Design of a rigid concrete cap over each cell to replace the strength lost from corrosion and partial collapse of the cells.
• Creative cell reinforcement and force transfer mechanisms to achieve safety factors consistent with contemporary design practices.
• Phasing of the project to enable continued operation of the berth during construction. A phasing sequence and a comprehensive set of work procedures was developed from a review of historical usage records and discussions with project stakeholders including the USACE, the Port of Wilmington and the berth’s principal user.

The construction was completed in approximately 11 months, well within the original estimate of 15 months.