Tunneling toward a new SR 99
It’s not rocket science, but it might be just as impressive: a custom-built, tube-shaped machine that bores its way through the earth, building a tunnel behind it as it goes. Industry folks call it a tunneling machine. We call her Bertha, the world's largest tunnel digger.
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 in 2016, 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.
Better than a shovel
At 57.5 feet in diameter – roughly as tall as a five-story building – the SR 99 tunneling machine is the world’s largest. The machine was shipped to Seattle on the Jumbo Fairpartner, after successfully undergoing testing in Osaka, Japan, where it was built. It arrived in Elliott Bay on April 2, 2013 and was reassembled in an 80-foot-deep pit to the west of Seattle’s stadiums.
Japanese firm Hitachi Zosen Corp. manufactured the machine for Seattle Tunnel Partners (STP), the contracting team that’s building the tunnel. Hitachi has successfully built more than 1,300 tunneling machines, a number of them for large-diameter tunnel projects. As owner of the machine, STP is responsible for ensuring it functions properly.
There’s a lot to keep track of when you’re tunneling beneath a bustling city. Steering, for instance. And of course the soil in front of the machine. The important thing to know about soil is that not all of it is the same. In fact, there are eight different types of soils along the SR 99 tunnel route. In general, the looser the soil, the more likely it is to move as you tunnel through it. Sand, for example, is harder to control than clay. Other things workers might encounter underground: boulders, gravel, logs and various man-made objects.
Tunneling crews constantly monitor ground conditions as they drive the machine forward. Safety measures began before tunneling started, when some 200 buildings above the tunnel route were examined and fitted with monitoring equipment (pdf 912 kb) that allows crews to detect even the slightest movement. Buildings and other structures that were thought to be sensitive were stabilized prior to tunneling. There are a number of ways to do this, including ground improvements and construction of angled walls below the ground that hold the earth in place above the tunnel. More information about our building monitoring program is available in the library.