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Alongside the maze of barricades, flaggers and cranes that make driving through South Lake Union a brain teaser, construction crews will soon block Highway 99 to remove a bridge.

State officials gave early warning Wednesday about a four-day project that will close the highway from Valley Street to the Battery Street Tunnel.

And for two of those days, the Alaskan Way Viaduct will close, too.

The work starts late Friday, Aug. 22, and wraps up at 5 a.m. Wednesday, Aug. 27.

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A bridge on Aurora Avenue North, where it crosses a now-defunct segment of Broad Street, is being removed and replaced with pavement on top of a berm — to provide space and better connections for future Highway 99 tunnel ramps.

Motorists coped easily with recent lane shutdowns on the Interstate 90 East Channel Bridge, but viaduct closures have a history of fouling up Interstate 5 and surface routes.

“If folks think this won’t be a big deal, they’re wrong,” said Matt Preedy, a West Seattle resident and deputy Highway 99 administrator for the Washington State Department of Transportation.

The closure plan goes like this:

• An overpass where Aurora Avenue North crosses the now-defunct Broad Street segment will be removed, an unusual operation that will take the entire four days. The old overpass can’t be kept because the elevation is too low to match the future tunnel entry-exit ramps, Preedy said.

Workers are already filling the space beneath Aurora with soil and concrete rubble, so a new roadbed can be paved over fill.

• On Friday night, Aug. 22, southbound Alaskan Way Viaduct traffic will cease at 10 p.m., but northbound traffic from Royal Brougham Way South through the Battery Street Tunnel and beyond will stay open until midnight for football fans leaving the Seahawks game.

A damaged expansion joint on the viaduct, at the Seneca Street offramp, will be repaired, a two-day operation. The viaduct reopens at 5 a.m. Monday, Aug. 25, but Highway 99 will remain closed two more days from the Battery Street Tunnel to Valley Street. This means northbound traffic must exit at Western Avenue all day that Monday and Tuesday.

• Ivy will be removed from sections of the viaduct.

• Workers will replace 81 concrete panels in Sodo, also a two-day operation.

The state chose to perform the Sodo work at the same time as the overpass replacement, to avoid spreading multiple projects over several weekends, Preedy said.

Meanwhile, the north portal of the future Highway 99 tunnel is taking shape near Denny Way.

Despite the prolonged stall of tunnel-boring machine Bertha, pedestrians can now envision where ramps would emerge connecting the double-deck tube to Aurora Avenue North.

The decommissioning of lower Broad Street has long been envisioned as part of the city’s Mercer Corridor Project. Eventually, surface streets will be connected east-west across Aurora, after bypass traffic moves underground.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom

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