Keeping the Alaskan Way Viaduct safely open to traffic during SR 99 tunnel construction is a top priority. It’s one of the main reasons we’re boring a tunnel – to minimize disruption at the surface as we replace the viaduct.

Recent media reports have stated that ground settlement due to tunnel construction could cause us to close the viaduct. These reports are inaccurate.

The viaduct is still vulnerable to earthquakes, but it remains safe for everyday use. The viaduct settled as a result of the Nisqually earthquake and we strengthened the viaduct in those areas so it remains safe for drivers.

We have no plans to close the viaduct until after the tunnel opens to traffic. Crews inspect the structure four times per year – the next inspection is scheduled for this weekend – and have taken a number of steps to protect it during construction.

We anticipated some ground settlement would occur during construction of the tunnel, which is taking place near the viaduct. That is why the viaduct has been strengthened in this area and more than 100 monitors have been installed on the viaduct to measure how much the ground beneath the structure moves.

Those monitors told us that the viaduct settled up to four-tenths of an inch at one location along the viaduct near where tunnel construction is underway. This settlement is well within the limits established by WSDOT’s structural engineers.

We will continue to carefully monitor the viaduct during construction and will take additional steps to reinforce the viaduct if needed.

We’ll be posting additional details soon about the work we’ve done to protect the viaduct. In the meantime, please feel free to contact us if you have questions or concerns.

Learn more about the Alaskan Way Viaduct Replacement Program.

 by guest blogger Natalie Graves

The SR 99 Tunneling Machine (Bertha) in the launch pit.

The power is on, the cutterhead is hooked up and two miles of Seattle soil await the teeth of Bertha, the world’s largest tunneling machine. Aside from running final tests, there’s only one thing left to do before the massive machine’s launch beneath downtown Seattle later this month: say goodbye.

On July 20, Gov. Jay Inslee, WSDOT and Seattle Tunnel Partners will host a public celebration at Bertha’s launch site, west of CenturyLink Field, from 11:30 a.m. to 3 p.m. Following the brief dedication ceremony at 11:30 a.m., attendees can walk the construction site, talk with project staff, learn about the tunneling operation and view Bertha from one of the walkways that span her 80-foot-deep launch pit. This is your chance to meet Bertha face-to-cutterhead-face.

It isn’t the first time WSDOT has hosted a “come one, come all” event along Seattle’s waterfront. In 2011, WSDOT closed the Alaskan Way Viaduct to demolish the southern mile, and hosted a final walk on the viaduct that attracted more than 3,000 attendees. WSDOT and the SR 99 tunnel contractor, Seattle Tunnel Partners are prepping the construction site for visitors and Bertha is ready to meet her fans. The event includes activities designed to teach kids about science and engineering and provides the opportunity for attendees to sign their names on one of the concrete segments that will form the tunnel’s walls. To add a little flavor to the day, food trucks will also be on-site.

The celebration is one for the history books, so don’t miss out. Tunneling starts at the end of July after final testing is complete and we won’t see Bertha again until she emerges near South Lake Union in approximately 14 months. For more important information, such as directions to the event site and restrictions, visit the program’s website.