Balancing Post-SLE Performance Objectives and Efficient Construction Detailing for Offshore Pile-Supported Berth Replacement in Alaska

The Port of Alaska Cargo Terminal 1 in Anchorage has a unique layout with Ship-to-Shore (STS) cranes operating on a deepwater pile-supported berth connected to the upland by pile-supported access trestles. As part of the Port of Alaska Modernization Program (PAMP), the terminal is being replaced with a new offshore berth connected to land by two new trestles. As evidenced by the 1964 Alaska earthquake, the region is subject to high seismicity, which is further compounded by the presence of liquefaction-induced soil movements along the shoreline. In recognition of the overall seismicity and accounting for the critical nature of the terminal to the region, the new berth is classified as a “seismic” berth, requiring the berth to be ready for operation within a week of the 975-year return period Safety Level Earthquake (SLE). This paper presents the overarching design principles adopted for satisfying the high seismic performance objective and associated construction detailing for the replacement berth. As a result of environmental considerations and the short construction season in Anchorage, the project team adopted a berth layout that implemented an optimized quantity of large-sized, high-capacity, open-ended, driven steel piles supporting a precast concrete superstructure. The performance-based structural design addresses the following requirements: it provides structural and geotechnical capacity to support the 1,000 psf live load and 3,600 kips STS crane loads; it provides a resilient design required by the performance objectives of a seismic berth; it provides necessary lateral stiffness across a spectrum of geotechnical conditions varying from the “stiff” silted-in condition to the “soft” liquefied condition, including future berth deepening; and it satisfies capacity protection requirements, despite the large piles and pile-to-cap connections providing elastic behavior for the SLE conditions. The design team adopted innovative design solutions, including precast/prestressed elements with flared ends to accommodate over-strength seismic capacity protection requirements, decoupling load transfer yet providing physical connectivity of the interface between wharf and trestles, and detailing specialized pinned pile-to-cap connections at critical locations. The final design provides a seismically resilient structure that satisfies the post-SLE seismic event requirement of near-immediate operation while also being constructable under challenging environmental and operational constraints. The project is scheduled to begin construction in 2026 with 870 ft of the wharf anticipated to be completed by 2029.