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 tunnel boring machine. We call it our ride to a new SR 99 corridor.

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 late 2015, a two-mile stretch of SR 99 will move underground, allowing us to remove the viaduct and open up nine acres of 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 – our tunnel boring machine is the world’s largest. Crews finished assembling the machine in Japan in late 2012, and are now putting it through a series of tests. When testing is completed, the machine will be shipped across the Pacific Ocean in more than 40 pieces that will be reassembled in an 80-foot-deep pit  (pdf 589 kb) to the west of Seattle’s stadiums. The machine will arrive in spring 2013, and tunneling is scheduled to start this summer.

Japanese firm Hitachi Zosen Corp. is manufacturing the machine for Seattle Tunnel Partners (STP), the contracting team that’s building the tunnel. Hitachi has successfully built more than 1,300 tunnel boring machines, a number of them for large-diameter tunnel projects. The machine will not officially become the property of STP until it has tunneled approximately 1,000 feet without any issues. As owner of the machine, STP is responsible for ensuring it functions properly before and during tunneling.

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 our 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.

Our machine can handle almost anything, but that doesn’t mean our crews won’t use extreme caution. They will constantly monitor ground conditions as they drive the machine forward. Safety measures begin before tunneling even starts, when some 160 buildings above the tunnel alignment are examined and fitted with monitoring equipment (pdf 755 kb) that allows crews to detect even the slightest movement. Buildings and other structures that are thought to be sensitive are 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.