The Alaskan Way Viaduct will close for about two weeks beginning April 29, as a precaution while tunnel-boring machine Bertha passes underneath, the state announced.
The shutdown will start before the morning commute, from South Spokane Street to just south of the Battery Street Tunnel.
Normally, as many as 90,000 vehicles and 30,000 transit riders use the viaduct at its busiest point. The Washington State Department of Transportation encourages people to work from home, form carpools and look for updates at www.99closure.org.
“We do need those 90,000 drivers a day to find another way to travel for two weeks starting on April 29,” said spokesman Travis Phelps, who predicted spillover slowdowns as far as Bellevue.
Local officials are making several temporary arrangements to keep people moving.
For instance, traffic police will work several intersections, including in Belltown and South Lake Union.
In a departure from other shutdowns, drivers entering downtown from the north will be allowed through the Battery Street Tunnel southbound to take the little-used Western Avenue offramp. Nearby, the northbound onramp at Western Avenue will be open and without any risk from converging viaduct traffic.
On the other side of downtown, traffic will be detoured to Sodo surface streets.
Some transit buses that use the viaduct, including Route 120 serving Delridge Way and White Center and the RapidRide C line from the West Seattle Junction, will shift to Fourth Avenue South. Burien routes 121, 122 and 123 will move to other Sodo streets.
Near the West Seattle water-taxi dock, shuttles and 360 temporary park-and-ride spaces will be provided. Five trips will be added to the Vashon water-taxi schedule.
Ferry and bicycle access around the viaduct will remain open.
The two-week closure will make it easier for contractors or the state to inspect the viaduct and inject grout to support the foundations, in the event Bertha is suspected of causing the soil to sink. A lack of vibration from traffic will allow more accurate measurements with devices that record changes as slight as a half-millimeter, said tunnel spokeswoman Laura Newborn.
Excavation for the Highway 99 tunnel under the seismically vulnerable viaduct brings a number of challenges, as dramatized by a Jan. 12 sinkhole nearby that did not damage the highway.
Bertha will pass beneath Yesler Way — just 20 to 40 feet below steel rods that help support the viaduct, which in that area has sunk 6 inches since the Nisqually earthquake of 2001. The machine will grind through mostly compact glacial soil. But that dirt is topped by watery fill and wood fragments, so a slight void might quickly spread upward.
Working in its favor, though, the viaduct is wrapped in carbon fiber, braced by steel frames, and supplemented by the buried steel rods. In addition, contractors for Seattle Tunnel Partners planted diagonal steel rods above where Bertha will dig, to form a sort of A-frame to resist the settlement of dirt.
A similar closure known as Viadoom lasted nine days in October 2011, while the south end of the viaduct was demolished to make room for the tunnel project. Rush-hour traffic began as early as 4 a.m. as drivers looked for a head start.
On the two worst weekdays, traffic delays were seen as far north as Bothell. Freight trains sometimes blocked east-west passage through Sodo, adding to the misery. Bicycling increased, but a commuter who jogged to work was hit by a semitruck next to the low West Seattle swing bridge.
Conditions have changed since then, including the arrival of an Amazon office campus in north downtown, and 32,000 new city residents. Travel options have increased, including a new ramp from First Avenue South to the West Seattle Bridge. Light-rail trains that serve Sodo now continue beyond downtown to reach Capitol Hill and the University of Washington.
