It came and went without fanfare. As of October 5, the Washington State Department of Transportation has had possession of the new State Route 99 tunnel under Seattle.

The state took possession from contractor Seattle Tunnel Partners.

It’s known as “substantial completion."

It doesn’t mean work is done, but it can operate as designed, according to WSDOT. That means if traffic had to go through it now, it could, although the roads leading to the tunnel aren't completed.

That’s what the upcoming three-week closure of the viaduct entails, as those roads, off- and on-ramps, and other infrastructure is switched over to the tunnel. That work involves paving and concrete work.

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The plan is to close the viaduct permanently after the evening commute on Jan. 11.

“It’s a very important milestone,” WSDOT spokesperson Laura Newborn said.

Newborn said the tunnel systems are all tested and signed off,. What remains are so called “punchlist” items such as small painting jobs and work to complete repairs to the North Operations Building after a functional test in September led to non-structural damage to an elevator door and wall board after air pressure was directed into the wrong part of the building.

The state says “final completion” will come when the last of the work, along with the last of the paperwork, is signed off.

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“The tunnel will not be idle for the next couple of months,” Newborn said, adding WSDOT can now bring its own operations and maintenance people into the tunnel to begin training for their jobs.

The turnover is considered a contractual technicality, the actual date only becoming clear after the paperwork has worked its way through.

Why didn’t the tunnel open before Thanksgiving as WSDOT suggested it might earlier this summer?

The agency backed off after it couldn’t guarantee it could make that date and have the major north-south route handling some 90,000 vehicles a day out of service during the height of the holiday shopping season.

Also see | Seattle tunnel test damaged building more than first reported