Alaskan Way Viaduct / SR 99 Tunnel Replacement

Tetra Tech’s High Performance Buildings Group, under contract to JH Kelly, provided comprehensive commissioning services for the four-lane, two-mile Replacement Tunnel project on the Alaskan Way Viaduct.


  • Size: 4-lane, 2 miles long
  • Construction Cost: $4.25 billion
  • Client/Owner: Washington State Department of Transportation
  • Architect: HNTB Architecture (part of Seattle Tunnel Partners joint venture team)
  • Contractor: Seattle Tunnel Partners (Dragados USA and Tutor Perini Corp. joint venture)
  • Completion Date: 2019

Tetra Tech provided commissioning services for the systems serving both the tunnel and the operations buildings at each end. The data for each of these facility process systems travel through the central supervisory control and data acquisition (SCADA) system, which centralized all functional testing on the project.

The replacement tunnel carries approximately 110,000 vehicles per day to their destinations and allowed for demolition of the previous viaduct structure, which was part of the transformative Seattle Central Waterfront redevelopment process.

The replacement tunnel was bored with the world’s largest tunnel borer—nicknamed “Bertha,” after Seattle’s only female mayor, Bertha Knight Landes—that featured a 57.5-foot diameter, weighed 900 tons, and cost $80 million.

Commissioned systems:

  • Mechanical, electrical, and plumbing
  • Building automation
  • Illumination
  • Ventilation
  • Drainage
  • Fire suppression and smoke control
  • Toll readers
  • Signage
  • Carbon monoxide

Images courtesy of WSDOT