SR 99 Bored Tunnel

Washington State Department of Transportation/HNTB Corporation

Seattle, Washington

SR99 Bored Tunnel

Photo: WSDOT

Hart Crowser was the lead geotechnical engineer for Seattle Tunnel Partners, a design-build team responsible for the $1.1 billion bored tunnel alternative for the SR 99 Alaska Way Viaduct replacement in Seattle. The 58-foot-diameter tunnel is the world’s largest diameter soft ground bored tunnel. Along its 1.7-mile length, the tunnel passes below more than 150 buildings and critical infrastructure, including buried utilities, other tunnels, and elevated highway structures.

The American Council of Engineering Companies awarded the Washington State Department of Transportation and WSP USA the 2019 Grand Conceptor Award for being one of the nation’s top engineering achievements. Hart Crowser supported Seattle Tunnel Partners, which built the  tunnel. The program competed with 196 projects worldwide for the ACEC award.

Hart Crowser was responsible for geotechnical engineering design of the bored tunnel liner, as well as excavation and ground support of the cut-and-cover tunnel sections at each end, which added another mile to the total tunnel length. Ground conditions were particularly challenging at the tunnel’s south approach, where the tunnel boring machine launch pit was been excavated. The launch pit extended more than 90 feet below ground surface in an area where poor-quality fill soil overlies glacial sediment. In this area, the groundwater table is within a few feet of the ground surface, and fill soil with a significant amount of historical development debris extends more than 40 feet deep. Discontinuous, artesian groundwater conditions in the underlying glacial soil required excavation dewatering and reinjection of groundwater to prevent settlement damage to nearby structures, roadways, and utilities. Hart Crowser designed a combined temporary and permanent ground support system consisting of secant piles with a combination of tieback anchors and internal bracing. At the north end of the tunnel, Hart Crowser designed a temporary excavation support system consisting of soldier piles and lagging, which was more appropriate for ground conditions in this area than the secant piles used at the south approach.

For the cut-and-cover structures at both ends of the tunnel, Hart Crowser provided the earthquake engineering design including soil liquefaction, lateral spreading, and dynamic lateral earth pressure analyses. We performed site response analysis to provide input ground motions for the dynamic numerical modeling, including 1-D and 2-D linear equivalent and non-linear effective stress analyses. We used FLAC to perform static deformation analysis and develop soil springs and displacement time histories. The results of our FLAC analysis were used as input to the structural engineer’s racking analysis. For the bored tunnel, Hart Crowser used FLAC to develop static loads and soil springs in support of the structural design of the tunnel liner.