Tunneling toward a new SR 99 corridor

Bertha, the massive machine responsible for digging the SR 99 tunnel beneath Seattle, is the largest machine of its kind in the world. Tunneling beneath Seattle allows us to replace the SR 99 Alaskan Way Viaduct while minimizing closures of the highway during construction. When the tunnel opens, a two-mile stretch of SR 99 will move underground, allowing us to remove the viaduct and clear the way for new public space along Seattle’s downtown waterfront.

Bertha's background

Bertha was manufactured by Hitachi Zosen Corp., a Japanese firm that has successfully built more than 1,300 tunneling machines, a number of them for large-diameter tunnel projects. As Bertha’s owner, Seattle Tunnel Partners, the contracting team that’s building the tunnel, is responsible for ensuring she functions properly.

How a tunneling machine works

How does a tunneling machine work? The answer, as you might expect, is complicated. The SR 99 tunneling machine is designed specifically for the ground conditions beneath Seattle. The graphic to the right illustrates a few of its more notable parts.

Check out this short video (or view on YouTube) to see the inner workings of a tunneling machine, or visit Milepost 31 to see a motorized, 10-foot-long model of Bertha. Our Flickr site has photos of Bertha and her progress to date. 

Bertha's journey 

Bertha was shipped from her manufacturing facility in Japan to Seattle in spring 2013. She was then reassembled in an 80-foot-deep pit to the west of Seattle’s stadiums. After a series of thorough tests, Bertha was launched into the soils beneath Seattle on July 30, 2013.

In December 2013, STP stopped tunneling approximately 1,000 feet into the tunnel drive after measuring increased temperatures in the tunneling machine. While investigating the cause of the high temperatures, STP discovered damage to the machine’s seal system and contamination within the main bearing. STP is working to repair the seal system and replace the main bearing so that crews can resume tunneling.

  • You can watch crews repair Bertha on our time-lapse camera.
  • These graphics (pdf 2.5 Mb) illustrate the steps STP is taking to resume tunneling.
  • View a narrated video (links to YouTube) explaining STP's repair plan in detail.

Click to view the SR 99 Tunnel Project interactive construction map.

Click to view the SR 99 Tunnel Project interactive construction map.


Click to enlarge and learn about the anatomy of a tunnel boring machine.

Click to enlarge and learn about the anatomy of a tunneling machine.


The SR 99 tunnel route. Click to enlarge.

The SR 99 tunnel route. Click to enlarge.