Geotechnical Aspects Guiding the Deepening of a Previously Deepened Terminal at the Port of Seattle

This paper presents a geotechnical design case history for rehabilitation and deepening of Terminal 5 at the Port of Seattle. Terminal 5 features a berth with an existing toe wall from a previous deepening effort. Because liquefaction may induce instability at Terminal 5, and for economic practicality, the Port of Seattle has a memorandum of understanding (MoU) with the City of Seattle Department of Planning and Development allowing certain limited improvements at piers and wharves not accessible to the public provided that seismic performance is maintained or improved. To satisfy the conditions of the MoU, we used standard of practice limit equilibrium approaches to develop a slope stabilization design for the terminal that includes a deep and large king-pile HZ beam wall system with timber pinch piles. We created time-history dynamic finite element models to compare the current and proposed conditions, verify the limit equilibrium methods, and check deformations. We explored the impact of liquefaction timing on design methods and assumptions. The standard of practice when analyzing slope stability under pseudostatic loading in the Puget Sound region is to assume liquefaction occurs after strong shaking. We investigated the validity of this assumption using the UBCSAND soil constitutive model. The model was calibrated to standard of practice empirical triggering methods and site-specific dynamic laboratory testing. Both liquefaction triggering approaches suggest at least a partial coupling of liquefaction and dynamic shaking should be considered in slope stability analyses.