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Take a 360-Degree Dive With Seattle's Massive Tunnel Borer

Tag along as Bertha nears the end of its painstaking, four-year journey.

WSDOT

A five-story-tall, 6,700-ton mechanical centipede that chews through clay and rock like it’s marshmallow—that could easily be a butt-kicking robotic “Jaeger” in the next installment of Pacific Rim. In reality, though, it is Bertha, the planet’s largest tunnel-boring machine that, after four years, is about to break from the earth in Seattle.

If you’re unfamiliar with this beast of many moving parts, the Washington State Department of Transportation’s got you covered with a fascinating, 360-degree video of Bertha’s operations. With less than two blocks to go, the borer has almost completed the roughly 2-mile-long SR 99 highway, which will replace the antiquated and quake-vulnerable Alaskan Way Viaduct when it opens around 2019.

Your Virgil in this journey 200 feet below Seattle is the WSDOT program administrator Joe Hedges, who begins by pointing out a massive “ring erector.” This device slams multiple 18-ton slabs into the walls to make loops that reinforce the tunnel, sort of like an industrial arterial stent; so far it’s built about 1,300 such rings.

After a glance at some hyperbaric chambers, the camera shifts to what Hedges calls a sight that “few people ever get to see”—Bertha’s pilot house. Here the machine’s various operators steer it like a great, blind earthworm toward the eventual exit. The technology is sophisticated, but not invulnerable to unforeseen hitches. Bertha was sidelined for two years for repairs starting in 2013, after it hit a buried pipe and damage was subsequently discovered. (The cause of the damage is under litigation between the state and the tunnel contractors, with the state arguing the 8-inch pipe couldn’t have caused such a mangling.*) It also recently ground to a halt to correct for a minor error in direction.

There are also shots of a swollen yellow vein carrying life-giving air to the tunnel, a conveyor belt moving out Bertha’s ground-up leavings, and much more to excite fans of large, complicated thingamabobs like Bertha—which, if all goes according to schedule, will finally see the light of day this spring.

*Clarification: This post has been updated with additional information.

About the Author

  • John Metcalfe
    John Metcalfe is CityLab’s Bay Area bureau chief, based in Oakland. His coverage focuses on climate change and the science of cities.