Seattle, WA

SR 99 Alaskan Way Viaduct Replacement

Washington State Department of Transportation (WSDOT)


The SR 99 Alaskan Way Viaduct Replacement design-build project included a single-bore tunnel under downtown Seattle. The tunnel has a stacked configuration with two lanes on each level and operation buildings at the north and south ends. It replaced the remaining section of the SR 99 Alaskan Way Viaduct, a double-decked highway that was damaged in the 2001 6.8‑magnitude earthquake.

Fast Facts
  • Utilized the world’s largest diameter (57.5-foot [17.5 m]) tunnel boring machine (TBM)
  • The tunnel features wall-mounted sensors that sample the air every few seconds and turn on ceiling-mounted jet fans when emissions reach a certain level
  • The TBM utilized earth pressure balance technology to address the complex ground conditions

Scope of Work

Delve Underground led construction management services, including preconstruction activities consisting of review and comment on design submittals, schedule analysis, tunnel systems integration support, and assistance with geotechnical monitoring. During construction, services included on-site resident engineering and inspection services with staff being co-located and working alongside the in-house WSDOT construction management staff. Delve Underground reviewed contractor schedule updates, provided impact assessments, assisted with inspection of building construction, participated in planning for systems integration and commissioning, and assisted WSDOT in resolving contractual issues.

Challenges & Innovations

Project challenges included construction sites constrained by a waterfront location and an urban setting, and issues surrounding the TBM, causing a two-year project delay. Near project completion, Delve Underground facilitated detailed planning for the TBM extraction at the North Portal, with an emphasis on minimizing the impacts and duration of work in the right-of-way and adjacent properties. The TBM was ultimately dismantled within the extraction pit with virtually no impact to the traveling public and no impact to the overall project schedule.