SEATTLE – Both directions of the State Route 99 tunnel in Seattle will close the first two weekends in October for its first six-year inspection. The tunnel opened to travelers in February 2019 to replace the Alaskan Way Viaduct and provide safety and mobility improvements along Seattle's central waterfront.
Closure details
From 10 p.m. Friday, Oct. 4, until 6 a.m. Saturday, Oct. 5, and from 10 p.m. Friday, Oct. 11, until 6 a.m. Sunday, Oct. 13, both directions of the SR 99 tunnel will close.
To conduct part of these inspections, Washington State Department of Transportation crews will begin to reduce lanes at 9 p.m. with the tunnel fully closed at 10 p.m. Southbound traffic will detour to the Harrison Street off-ramp and northbound traffic will detour to the Alaskan Way off-ramp. Additionally, the Sixth Avenue and Royal Brougham Way on-ramps will close at 9 p.m.
Travelers should seek alternate routes during these inspections and expect delays because several other WSDOT and Seattle Department of Transportation closures are scheduled the same weekends.
Other work
People traveling in the Puget Sound region should be aware of several other closures at the same time as the tunnel inspections, including:
- The Ballard Bridge in Seattle
- 10 p.m. Friday, Oct. 11, to 5 a.m. Monday, Oct. 14
- All northbound lanes of Interstate 5 from 54th Avenue East in Fife to SR 18 in Federal Way
- 10:30 p.m. Friday, Oct. 4, to 7 a.m. Saturday, Oct. 5
- 10:30 p.m. Friday, Oct. 11, to 7 a.m. Saturday, Oct. 12
- 10:30 p.m. Saturday, Oct. 12, to 8 a.m. Sunday, Oct. 13
- All southbound lanes of I-5 from SR 18 in Federal Way to 54th Avenue East in Fife
- 11 p.m. Friday, Oct. 4, to 7:30 a.m. Saturday, Oct. 5
- 11 p.m. Friday, Oct. 11, to 7:30 a.m. Saturday, Oct. 12
- 11 p.m. Saturday, Oct. 12, to 8:30 Sunday, Oct. 13
- Both directions of the SR 520 bridge, between I-5 in Seattle and 92nd Avenue Northeast in Clyde Hill
- 11 p.m. Friday, Oct. 11, to 5 a.m. Monday, Oct. 14
Inspection details
In February 2001, a 6.8 magnitude earthquake struck near Olympia, causing several foundations of Alaskan Way Viaduct columns to shift as much as 5 inches. Engineers believed the viaduct would have collapsed had the earthquake lasted a few moments longer.
During the next decade, state and local agencies studied more than 90 alternatives to replace the aging viaduct, which carried SR 99 through Seattle. In 2009, a deep-bored tunnel was chosen. Construction finished in February 2019. The unique structure of the tunnel and federal regulations require specialized inspections every six years.
The inspections take two weeks to complete, but crews only need to close the tunnels during the two weekends.
The inspections will include an overall condition assessment of the tunnel’s electrical systems and checks of the fire detection system, communication systems, lighting, jet fans, cameras and variable message signs.
The inspections also will include an overall condition assessment of the tunnel’s
mechanical system, including operational testing of the centrifuge fans and other aspects of the ventilation system, drainage system and generator.
To perform these specialized inspections, the WSDOT Bridge Preservation team brings in inspectors and vendors from out of state.
People can get real-time traffic information on mobile phones with the WSDOT traffic app and the WSDOT real-time travel map.