Tunneling toward a new SR 99 corridor

The SR 99 Alaskan Way Viaduct is being replaced with an approximately two-mile-long tunnel underneath downtown Seattle. Replacing the viaduct with a tunnel allows the highway to remain open for much of construction, thus minimizing closures and impacts to traffic. Once the tunnel opens, the viaduct will be taken down to clear the way for new public space along Seattle’s downtown waterfront.

The massive tunneling machine specifically built for the project is nicknamed Bertha. 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 building the tunnel, is responsible for ensuring she functions properly.

How a tunneling machine works

The SR 99 tunneling machine was designed specifically for the ground conditions beneath Seattle. The graphic to the right illustrates some notable parts of a tunneling machine, and the short video below shows how Bertha works. A motorized, 10-foot-long model of Bertha is on display at Milepost 31, our information center in Seattle's Pioneer Square neighborhood. 

Performing hyperbaric maintenance

Performing regular maintenance on Bertha, the SR 99 tunneling machine, is critical to the success of the tunnel drive. Most maintenance tasks can be completed under normal atmospheric pressure, but some tasks at the front end of the machine can only be completed in hyperbaric conditions  those in which the air pressure is greater than the atmosphere we live and breathe in every day. 

To safely perform these tasks, crews must stabilize the ground in front of the machine. They do this by injecting a type of clay, known as bentonite, into the front end of the machine. This creates a seal that prevents water and soil from entering – and air from escaping – their work space. 

Next, crews over-pressurize the space by introducing compressed air, which pushes against the bentonite to counteract the ground and water pressure at the front end of the machine. This newly created "hyperbaric" work space has pressure levels that are higher than regular atmospheric pressure, similar to conditions found in an underwater dive. The graphic below illustrates the process.
This video from July 2016 shows crews working in hyperbaric conditions. Additional videos of hyperbaric work and other construction can be found at our video page.

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 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 and manufacturer Hitachi Zosen completed repairs to the machine in December 2015.



The SR 99 tunnel route. Click to enlarge.

The SR 99 tunnel route. Click to enlarge.