Structural-Geotechnical Procedures for New Wharf Design

This paper presents the collaborative design procedures utilized by structural and geotechnical engineers to design a pile-supported, two-berth container terminal wharf located at the Port of Tacoma in Tacoma, Washington. The wharf structure design complies with the performance and displacement based design criteria of the California State Land Commission, Marine Oil Terminal Engineering and Maintenance Standards (MOTEMS). Combined use of finite difference and static pushover models optimized the performance-based design to achieve significant reductions in both construction costs and construction schedule. Geotechnical investigation, testing and analysis indicated that potentially liquefiable soils present a risk of unacceptable slope and wharf structure displacement during design-level seismic events. Initial geotechnical and structural analyses indicated that extensive upland and in-water ground improvement was needed to limit the displacement to acceptable limits. Further analyses and modeling, performed in close collaboration between the geotechnical and structural engineers, were completed to evaluate the nonlinear soil-structure interaction, resulting in a more efficient design. The extent of soil liquefaction and resulting slope displacement was evaluated by completing finite difference modeling using Fast Lagrangian Analysis of Continua (FLAC) software. A two-dimensional, nonlinear static pushover analysis was conducted to evaluate the wharf structure response using the computer program SAP2000. Wharf displacement, variable structure damping and pile hinging obtained from the SAP2000 pushover analysis were incorporated into the FLAC time history analysis to evaluate the effect of nonlinear soil-structure interaction on the seismic performance of the wharf. The design optimization revealed that less ground improvement was needed than initially thought and that the number and embedment depth of foundation piles could be reduced, resulting in a significant reduction in estimated costs. The reduction in ground improvement and number of piles also has a significant impact in reducing the duration of in-water work; hence, the design achieved a more suitable and achievable construction schedule, as desired by the owner.