Another sluggish traffic weekend awaits Eastside drivers, when four days of lane closures block the westbound Interstate 90 floating bridge starting Thursday afternoon.

Contractors for the Washington State Department of Transportation will replace a giant expansion joint installed in 1989, that officials say might break someday and force an emergency closure if it isn’t replaced.

Traffic was funneled into a single westbound lane by 2 p.m. Thursday. All westbound lanes will close starting 10 a.m. Friday until 5 a.m. Monday, a WSDOT update says.

However, the walk-bike lane and the West Mercer Way onramp to westbound I-90, which both closed last weekend for joint construction, will stay open this weekend, allowing a trickle of traffic onto the floating part of the bridge. That’s possible because this entire I-90 joint is above the island, and engineered as two halves. Those meet in the “gore point” between the onramp and the mainline, which allows a two-phase replacement project.

Sound Transit bus routes 550 and 554 westbound will miss Mercer Island Transit Center and detour to the Highway 520 bridge instead of I-90 starting Friday morning, a rider alert says. Returning eastbound, they’ll cross the lake on I-90 as usual.

The westbound closure conflicts with a Seahawks football game Sunday afternoon and a Huskies football game Saturday night. (Sound Transit will operate Sounder trains to and from the Seahawks game.)

“We have been trying to schedule this work for several months, but supply chain issues have pushed us back into September, and we need to finish the project while there is still good weather to allow the concrete to cure,” said a statement by Shawn Wendt, engineer for the $1.8 million maintenance project.

In 2009, WSDOT replaced a similar expansion joint that’s over water, on the westbound Homer Hadley Bridge, one of the two I-90 bridges crossing Lake Washington. Expansion joints are gaps between concrete road decks that allow for normal thermal expansion. On I-90, some joints also accommodate varying water levels that affect the bridge slopes.

Eastside drivers should detour over the Highway 520 toll bridge, or drive around the lake using Interstate 405, project officials said. Drivers going west across Mercer Island must exit I-90 at Island Crest Way.

The disruptions follow last Saturday’s full closure of I-405, to replace a broken stream culvert below the freeway south of I-90. That triggered Eastside congestion on I-405, plus westbound traffic jams on the floating bridge and Mercer Island.

Traffic snarls should ease on westbound I-90 entering Issaquah, where crews will finish a re-decking project that closed most lanes, by 5 a.m. Friday. Concrete had been damaged over several years because of water runoff from an abandoned coal mine. Repairs are on schedule, and an asphalt truck is expected to arrive early Thursday, said WSDOT spokesperson Amy Moreno. Stop-and-go traffic during some busy periods stretched as far back as 3 miles this week, she said.

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