Viaduct to close in the fall? ‘Viadoom II’ looms

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 57 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.

Officials don’t know when a closure might occur.

Talk of a traffic closure comes as a surprise, after proponents’ earlier claims that a deep tunnel would keep cars flowing during construction.