Sound Transit is assembling and testing a pair of tunnel-boring machines in Tacoma, to be launched from a deep pit next to Husky Stadium by early summer.

Share story

Sound Transit is assembling and testing a pair of tunnel-boring machines in Tacoma, to be launched from a deep pit next to Husky Stadium by early summer. They each will tunnel to Capitol Hill, while a third machine will dig the shorter segment from Capitol Hill to Westlake Center.

On each rotating head are 48 blocks of teeth, or “rippers,” to churn through soft soil, and 18 disc-shaped tools to break rocks. A screw conveyor, belts and rails stretch about 300 feet behind the head to remove the soil.

The machines are nicknamed Balto and Togo, after heroic sled dogs who helped deliver diphtheria antitoxin to Nome, Alaska, in 1925.

At 21 feet in diameter, the faces are not quite one-seventh as large as the 58-foot cutting heads proposed for the Highway 99 tunnel.