Earthquake Mitigation at Washington United Terminal Tacoma, WA

A 600-foot marginal wharf extension at the Washington United Terminal at the Port of Tacoma is supported on precast concrete piles installed through a relatively steeply inclined, riprap-protected dredge slope. The dredge slope beneath the wharf is potentially unstable during severe seismic events. The geologic and seismic setting at the Port of Tacoma and procedures for establishing operating level and contingency level earthquake design spectra in accordance with MOTEMS are described. Partial or complete liquefaction of portions of the soil column under anticipated earthquake loads results in unacceptable lateral spreading. Stone column ground improvement is used at the Port of Tacoma to improve stability and performance of the dredge slope during earthquakes. Wharf pile foundation and soil interaction is assessed using the Newmark displacement method and Plaxis finite element analysis. Optimizing stone column spacing and location relative to the dredge slope and the foundation piles also is discussed. Estimated lateral spreading at the crest of the unimproved dredge slope ranges from a few inches to several feet. A zone of ground improvement is necessary beneath the dredge slope to reduce the lateral spreading and keep load transfer to the wharf piles below acceptable levels. The construction sequence was carefully specified and detailed stone column and pile foundation installation specifications were developed for the project to manage the potential for pile installation difficulties. Separate bid items were requested for regular pile driving and pile driving in stone column improved ground.