SR-99 Alaskan Way Viaduct Tunnel Project, WSDOT & City of Seattle
The earthquake-damaged Alaskan Way Viaduct is currently in the process of being replaced with an underground tunnel as part of WSDOT’s multi-phase relocation of SR99 and redevelopment of the waterfront in downtown Seattle. CivilTech is providing engineering services for both designers and contractors on this design-build project. Our tasks have included final design details for the gravity sewer system replacement work on Alaska Way between South King Street and South Main Street, structural engineering for the North Approach Temporary Support of Excavation, multiple shoring designs for electrical and utility line relocations along the corridor, and tunnel equipment support designs. Final design for the gravity sewer system replacement included analysis of pipes spanning the tunneling zone, pipe design including pipe lining and coating system, pipe support and connection details, and analysis of manhole structures, manhole design, and detailing for non-standard precast and cast-in-place manholes. The North Approach Support of Excavation included design of the perimeter soldier pile wall with a combination of tiebacks and internal corner braces. The perimeter shoring wall is approximately 1200’ long and up to 93’ deep. Design coordination includes maintaining clearance between tiebacks and existing utilities and multiple buildings’ pile foundations. CivilTech has also designed other shoring systems, including soldier pile, soldier pile and sheet pile walls with internal bracing, trench box, and plate-and-jack shoring systems to support a variety of contractor activities. Other services have included the design of concrete formwork for micropile and cast-in-place pile grade beams, heavy equipment drilling platforms, concrete vault repair, micropile support systems for existing utilities, and RFI and submittal review assistance. Challenges for this project include designing within a high density urban environment, reclaimed land, and unidentified utilities and obstructions, coordinating with multiple agencies and contractors, and expedited timeframes and material substitutions to maintain the construction schedule.