Geotechnical Components of a Seismic Soil-Structure Interaction Model for a Pier

A major pier realignment at the Port of Tacoma was designed to the recently published ASCE 61-14 standard for seismic design of piers and wharves. Standard requirements led the design team to choose a performance-based seismic design approach. Nonlinear deformation modeling was used to analyze dynamic soil-structure interaction (SSI) to quantify the demands on the piles and the wharf structure from seismically induced soil/slope movements. Because of the poor soil conditions at the site and the relatively high seismic hazard of the Pacific Northwest, the design considered coupling of soil liquefaction and ground shaking. The impacts of modeling liquefaction are discussed in terms of the amount of slope deformation and the ground improvement required to sufficiently stabilize the slope. A dynamic time-history analysis model was developed using the finite element method (FEM) software PLAXIS to reasonably capture the soil-structure interaction. As specified in ASCE 61-14, pile performance was evaluated under three seismic hazard levels. Demands on the piles were tracked throughout ground shaking to determine the maximum demands from simultaneous inertial and kinematic loading. This paper describes the geotechnical and seismic details of the SSI model, including: Subsurface explorations and lab tests required to develop soil parameters for dynamic analysis. Assessment of three seismic hazard levels, and selection and scaling of ground motions for each hazard level. Modeling parameters such as boundary conditions, FEM mesh, and dynamic time step. Development and calibration of soil constitutive models, including the UBCSAND liquefaction model. Modeling of structural elements and connections in PLAXIS. Ground improvement used to mitigate liquefaction and stabilize the slope.