Four-day SR 99 Closure Shines Spotlight on Tunnel’s North Portal

You may have heard that SR 99 will be closed for four straight days by the state starting Friday night, August 22. It’s the longest full closure of SR 99 in Seattle since crews demolished the southern mile of the Alaskan Way Viaduct in 2011. While this is probably not welcome news for travelers, it is a sign of progress at the tunnel’s north portal.

The 2011 demolition of the southern portion of the viaduct cleared the way for construction of the tunnel launch pit and the commencement of tunneling. During this month’s closure crews will demolish the SR 99 bridge above Broad Street, which will clear the way for continued construction of ramp and roadway connections at the tunnel’s north portal.

North Portal of the Tunnel

The shape of those connections is a little hard to imagine now, but this visualization from the Washington State Department of Transportation helps show how all the pieces of the portal will function. As you can see, the city’s two-way Mercer Street is a key component to making this portal function. You might be surprised to learn that a big piece of the north portal, the tunnel entrance, is mostly complete. In the next year you’ll start to see the outlines of the lanes and ramps that connect to the tunnel entrance. Since much of this change can’t be seen on the ground, the construction camera views on the Alaskan Way Viaduct Replacement Program website are the best way to keep track of progress toward the future north portal of the SR 99 tunnel.