The first parallel suspension bridge built in such close proximity to an existing span, this design-build project also presented complex issues with stratigraphy, diverse topography, fault-zones, landslide-prone slopes, contaminated soils and endangered species. Shannon & Wilson provided geotechnical and earthquake engineering studies for preliminary and final design of the bridge and approach structures.

Shannon & Wilson performed subsurface explorations, including drilling marine borings in water deeper than 150 feet, extending to approximately 200 feet below mudline.  We used this information develop foundation recommendations for the bridge's two support towers.  Tower foundation alternatives included large-diameter driven piles, drilled shafts, and sunken caisson foundations. Shannon & Wilson developed preliminary recommendations for all three foundation types, and delivered detailed recommendations for the sunken caisson alternative-the foundation type ultimately selected.  Our studies included 2-D and 3-D finite-difference studies to evaluate lateral pressures on the caisson walls under seismic-loading conditions.  In addition, we used the finite-difference studies to evaluate the sinking of the caisson.

The new bridge anchorages required 12- to 75-foot-deep excavations located along the steep, marginally stable hillsides adjacent to the Tacoma Narrows.  We evaluated hillside stability and provided recommendations for design of the anchorages as well as the excavation support system.  We performed soil-structure interaction analyses using 2-D and 3-D finite studies to model the resistance and movement of the anchorages under static and seismic loading conditions.