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Seattle’s biggest bore

Author Jacqueline Detwiler Illustration James Provost

Digging a great big hole sounds easy. Most of us did it at least a dozen times before the age of 5. But what if you had to dig that hole under a swath of the Seattle metropolitan area thick with 100-year-old structures and 30-story apartment buildings? What if the air pressure at the deepest part of the hole were lethal to a human being? And what if, before you even started, you learned you had to use a brand-new 7,000-ton tunnel boring machine—that can’t back up if you make a mistake? That was the task the Washington State Department of Transportation faced when it decided to replace Seattle’s 1950s-era Alaskan Way Viaduct with a double-decker tunnel meant to protect Emerald City drivers from earthquakes. Here’s how they’ll do it.

1. The tunnel boring machine, which is being built in Japan, is too big to traverse the Pacific whole, so it will be shipped in 41 pieces and then assembled on-site. Transporting the largest chunks—which can weigh up to 900 tons apiece— to the assembly area requires a weight- spreading mover to avoid crushing underground pipes and power lines.

2. The 600 cutting tools on the borer’s face will need to be maintained and possibly replaced as it cuts, which can be a problem at such intense air pressure. To accomplish this feat, technicians will either crawl through the machine’s one-of-a-kind hollow, pressurized arms or reach the cutting face through a series of air locks, like deep-sea divers.

3. To make sure Seattle’s buildings remain secure while the machine works beneath them, multiple agencies will monitor the ground and local structures using state-of-the-art systems mounted on buildings or carried by satellites. “We have belts and suspenders and more belts on the suspenders,” says Matt Preedy, the program’s deputy administrator. “We only have one chance to get this right.”

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