Balancing Post SLE Performance and Efficient Construction Detailing for Berth Replacement in Alaska

Abstract: The Port of Alaska Cargo Terminal 1 in Anchorage has a unique terminal layout with STS cranes operating on a deepwater pile-supported wharf connected upland via pile-supported access trestles. As part of the Port of Alaska Modernization Program the terminal is being replaced with a new offshore wharf connected to land by trestles. The new berth is classified as a “seismic” berth, ready for operation shortly after the Safety Level Earthquake (SLE), performance criteria typical for the Contingency Level Earthquake (CLE). As evidenced from the 1964 Alaska earthquake, the region is subjected to high seismicity which is further compounded by the presence of liquefaction induced soil movements within the footprint of the berth. This paper presents the overarching design principles adopted for satisfying the seismic performance objective and associated construction detailing for the replacement berth.

Due to permit considerations and anticipated schedule impacts, the project team adopted a berth layout that implemented an optimized quantity of large-sized, high capacity, open-ended driven steel piles supporting a high-strength concrete superstructure. This performance-based structural design addresses the following: provides structural and geotechnical capacity to resist the 1000 psf live load and 3600 kips of STS crane loading; provides a resilient design through the application of unusually stringent performance requirements; accommodates stiffness requirements across spectrum of conditions varying from the “stiff” silted-in condition and “soft” liquefied condition reflected by a 50 foot of drop in mudline elevations post liquefaction; and satisfies capacity protection requirements despite the large piles and pile-to-cap connections. To address all the above, the team adopted some innovative design solutions such as providing precast/prestressed elements with flared ends to accommodate over-strength capacity requirements, decoupling load transfer yet providing physical connectivity of the interface between wharf and trestles and detailing softer pile-to-cap connections at critical locations. The final design provided a seismically resilient structure, meeting the immediate operation requirement post-SLE seismic event while also being constructible under challenging environmental and operational constraints. The project will begin construction in 2025 with 850-feet of the wharf anticipated to be completed by 2028.

How does this project / topic contribute to the advancement of the industry and profession?: The project applies OLE (operating level earthquake) and CLE design criteria to the CLE and SLE events, respectively. The application of the higher performance criteria to a complex project and the reliance of precast bi-directional pilecaps presented a number of practical challenges including global framing of the wharf and trestle as well as connection detailing. The project provides an example of the application of ASCE-61 to an offshore wharf with long access trestles and extensive use of precast concrete to acceleration construction.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The project implements a combination of pile-to-cap connections varying from fixed to fully pinned to address a suite of geotechnical and seismic conditions. In addition, the connection between the wharf and trestle segments is achieved via a pinned connection that eliminates large chord forces while still transferring shear and axial load and minimizing relative displacement between the wharf and trestles to approximately one inch. Finally, the construction is configured for execution via both marine and/or top-down methods.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect of the project was to provide a design with constructible and repeatable details that meets the stringent post-event seismic performance criteria for a suite of extreme geotechnical and seismic conditions and is also constructible within the challenging environmental constraints of the site and short project schedule.

Who is the target audience for this paper?: The target audience is port owners/operators in high seismic regions, port engineers with interest and/or experience in seismic design of pile supported piers, contractors, and seismic code writers.