Finite Element Modeling of New Marine Terminal at the Port of Tacoma

Two dimensional (2D) seismic stability and deformation analyses completed for the design of a new terminal in the Blair Waterway at the Port of Tacoma are presented in this paper. The new terminal or wharf will be constructed on piles driven along a 65-foot high dredged compound slope, with inclinations of 2H:1V and 1.75H:1V for the upper and lower portions of the slope respectively. The primary geotechnical issues for this project relate to slope stability and ground movement under seismic loading in liquefiable loose to medium dense silt and sand. Ground and wharf movements are evaluated under two earthquake conditions (OLE = 75-year return period and CLE = 475-year return period) using the finite element (FE) program, PLAXIS 2D. Time histories used as input at the base of the PLAXIS 2D model were developed from the site specific response spectra analyses using SHAKE. The effect of liquefaction and soil-structure interactions are also considered in the 2D FE deformation and time histories analysis. The results indicate that ground improvement (stone columns) is required to reduce ground deformation and bending moment in the wharf piles to meet the performance criteria recommended in Marine Oil Terminal Engineering and Maintenance Standard 2003 (MOTEMS). PLAXIS 2D is also used to optimize the spacing, depth and diameter of stone columns at the site.