Seismic Design of South Park Bascule Bridge Supported on Sunken Caisson Foundations

The South Park Bascule Bridge crosses the Duwamish Waterway near downtown Seattle in Washington State. The new bridge will be a double-leaf trunnion bascule bridge and will be constructed on an alignment approximately parallel to the existing bridge. The existing bridge has suffered significant deterioration over the past 79 years and damage from the Nisqually Earthquake in February of 2001. The project corridor is generally underlain by soft or loose soils extending to a considerable depth. Drilled shafts and sunken caissons were both considered for the foundations of the bascule piers. Based on the site condition, seismic requirements, constructability, environmental impact, and construction cost, caisson foundations were selected for the bascule piers of the South Park Bridge. This paper will focus on the merits of caisson foundations for the bridge under seismic loading. The results of non-linear time history analysis incorporating soil-structure interaction will be presented. Other measures that were incorporated in the design of the bascule bridge will also be discussed.