Geotechnical and Structural Challenges: New Seattle Multimodal Terminal at Colman Dock Project

Washington State Department of Transportation (WSDOT) and Washington State Ferries (WSF) are replacing the aging and seismically vulnerable parts of the Seattle Ferry Terminal at Colman Dock in Seattle to maintain its critical role as a regional multimodal transportation hub. The project includes new concrete trestles, terminal building, and other structures—all to be supported on driven pipe piles and drilled shafts. The project site is located in an active seismic zone. Coupled with high seismic demands, the offshore site is underlain by non-liquefiable soft silt, overlying liquefiable silt, and overlying glacial soils. The main geotechnical and structural challenge is lateral instability of the soils overlying the glacial soils and the consequential impact on the driven pipe piles and drilled shafts. Advanced soil-structure interaction (SSI) analyses using numerical modeling—rather than the conservative empirical approaches—were performed and peer-reviewed to optimize the design. The analyses overall resulted in shorter pile embedment into the glacial soils.