Tunneling will close the SR 99 Alaskan Way Viaduct for one to two weeks, likely in early 2014

More than 1,000 feet of soil separates Bertha from the spot where she’ll pass beneath the Alaskan Way Viaduct a few months from now. That gives drivers plenty of time to prepare for Bertha’s big crossing, which will require an extended around-the-clock closure of the viaduct.

Crews will close the viaduct for one to two weeks to allow Bertha to tunnel beneath the structure (pdf 913 kb). The machine could reach the viaduct in early 2014, but a specific date for the closure won’t be determined until later this fall, after crews have a better understanding of how quickly Bertha is moving.

Temporarily closing the viaduct is one of several precautions crews are taking to reduce the likelihood of ground settlement during tunneling. Like most structures, the viaduct was built to withstand some settlement, but we want to do everything we can to minimize potential damage to the structure. Keeping traffic off the viaduct while we tunnel beneath it will allow us to respond more quickly if one of the hundreds of sensors we installed on the structure detects movement that concerns us.

A number of factors make this section of the tunnel route unique, including the viaduct’s structural condition, soil conditions in the area and the close proximity of the tunneling machine to the foundation of the viaduct – approximately 15 feet.

Fortunately, we have people who’ve done this kind of thing before. Many members of our team helped successfully build a 40-foot-diameter tunnel about six feet beneath La Sagrada Familia, one of Barcelona, Spain’s most famous churches. The viaduct may not be a cathedral, but protecting it is our top priority. Closing the structure temporarily will help us keep SR 99 open to traffic until the tunnel opens in late 2015.

Stay tuned for closure details. We’ll pass them along as soon as we know more.