Pier Design Using a Mixed Orthogonal Lateral System for Liquefaction and Seismic Loading

The subject of this paper is a pier designed for the US Navy on Hood Canal in Washington State. The pier is located on a slope that is expected to experience deep liquefaction and was designed to be capable of resisting downslope flow of the top 60-ft of in situ soil while also resisting significant inertial seismic loads. The design process included significant collaboration with the geotechnical engineer, who performed soil-structure interaction analysis using FLAC software. Because the direction of kinematic loading was so well defined, the pier was designed with different types of seismic-load-resisting systems each in orthogonal direction (i.e., a mixed system). The resulting design utilizes a stiff batter-pile system in the direction of the kinematic loading, paired with a flexible plumb-pile system perpendicular to the direction of kinematic loading. Because the same piles function in both systems, the two are coupled, with the results in one direction affecting the results in the other. This required a sound, first-principles-based analysis, which resulted in unique pile connection details to accommodate the coupling behavior.