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The Alaskan Way Viaduct will probably close for several days this fall as a safety precaution when the mammoth Highway 99 tunnel-boring machine passes underneath.

You might call this “Viadoom II.” A closure in 2011 caused major backups.

Such a closure is probable, though officials hope to avoid it, Dave Sowers, an engineering manager for the state Department of Transportation, said Friday.

The world’s biggest tunnel drill, at 57½ feet in diameter, will be passing 80 feet below the street, but only 14 feet beyond the lowest buried pilings that reinforce the old viaduct, said Sowers. The half-century-old structure already was wrapped in carbon through this zone, so that vibrations from tunnel machine “Bertha” won’t damage its crossbeams.

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Officials don’t know when a closure might occur.

Chris Dixon, project manager for Seattle Tunnel Partners construction group, said drilling will begin in early July at Sodo. The machine will advance 1,500 feet, taking two to three months, he said, before it dives diagonally under the old viaduct from southwest to northeast, at Yesler Way.

Talk of a traffic closure comes as a surprise,

after proponents’ earlier claims that a deep tunnel would keep cars flowing during construction — and assurances last year from DOT exec Matt Preedy that carbon-wrapping would

“keep the viaduct safely open to traffic while we build the tunnel.” The West Seattle Blog
first reported
on closure plans

Thursday, after hearing of them at an unrelated bicycling forum.

But Sowers said Friday a potential closure “has always been in the works,” and as teams learn and discuss the project more, they’re leaning toward extreme caution. He said the contract calls for more temporary closures late in the project, to connect the new Sodo and South Lake Union tunnel portals to the highway.

Commuters in October 2011 endured a seven-day shutdown known as Viadoom, while the south end of the Viaduct was demolished.

On two days, severe congestion reached as far away as Bothell. Buses and cars in Sodo waited behind crossing trains at railroad crossings to reach the two-lane lower West Seattle Bridge.

Driving options should be somewhat better this time, because of the new First Avenue South ramp from Sodo to the upper West Seattle Bridge.

Sowers said the duration of any closure this year depends partly on how Bertha performs during its early weeks, churning through soft soil along the waterfront.

Meanwhile at the pit, Dixon said the job is proceeding well. Workers Friday were setting rebar and pouring concrete to build the front end of a huge cradle — the starting line where Bertha will chew through a hard wall into wet soil, at an initial 3-degree descent.

Mike Lindblom: 206-515-5631 or

On Twitter @mikelindblom

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