The World's Largest Tunnel-Boring Machine Must Be Saved
Bertha has stalled deep beneath Seattle, leaving the city's planned waterfront renaissance hanging in the balance.
What do you do if you're operating the world's biggest tunneling machine and something goes wrong? You're digging along, everything fine, the machine's five-story maw about to chew beneath the skyscrapers of one of the great American cities. Then suddenly one day things are not so fine. Bertha—that's her name, in honor of Seattle's first woman mayor, Bertha Knight Landes—hits something. A few days later her temperature starts rising. Not good. Then her cutting head stops spinning.
Now what? What do you do when the world's largest tunneling machine is, essentially, stuck in the mud? Bertha is 60 feet under the earth, and you're on the surface watching a squirmy public swap rumors of cost and delay on the $1.35 billion tunnel component of an even larger transportation project, and the naysayers are howling: Just you watch, Bertha will be abandoned like an overheated mole, boondoggle to end all boondoggles. Because, don't forget, when you're boring the world's largest tunnel, everything is bigger—not just the machine and the hole and the outsize hopes but the worries too. The cynicism.
What do you do?
Here's what you do: You try to tune out the media. You shrug off the peanut gallery's spitballs. You put off the finger-pointing and the lawsuits for now; that's what the lawyers are paid for afterward. You do the only thing you can do. You put your head down and you think big, one more time. You figure out how to reach Bertha and get her moving again.
This is a rescue story.
Ask a Seattleite why he likes it here and he'll invoke the things we Seattleites always say: good fish. Better coffee. White sails on the blue water of Puget Sound. Never high on anybody's list is the Alaskan Way Viaduct. For 61 years the elevated double-decker freeway that slices along the waterfront has been the city's grim, gray mule, carrying roughly one-third of Seattle's north–south traffic while effectively divorcing the city from its waterfront, as so many other highways have done around the nation—from New York City's FDR Drive to Boston's Interstate 93 before the Big Dig buried it.
In 2001 a magnitude 6.8 earthquake rattled Seattle, cracking the aging viaduct. As years passed and the road deteriorated, the city argued about what to do. Finally, in 2009 local and state leaders decided: the viaduct would fall. In its place a waterfront renaissance would bloom as 26 blocks along Elliott Bay rejoined the city. James Corner Field Operations, visionary of the acclaimed High Line project in Manhattan, was hired to imagine a string of walkways, parks, public piers, bike paths, beaches—even a swimming pool on a barge—that would knit the city's core and its shoreline together and transform the place into an urban waterfront to rival those of Sydney, Copenhagen, Vancouver.
The costliest and most complicated puzzle piece—the one that would make all of this possible—would also be one of the least visible. A 2-mile tunnel would replace the hulking viaduct. The tunnel would whisk traffic underground from the Seattle Seahawks' stadium, just south of downtown's high-rises, north to the Space Needle and South Lake Union.
Seattle's tunnel wouldn't be very long—just 1.7 miles of it bored through the earth—but it couldn't be just any tunnel. It needed to be big enough to hold four lanes of traffic across two decks, with cars traveling at highway speeds. It would have to dive deep, more than 200 feet below downtown's heart, to avoid disturbing the city's skyscrapers and old buildings. The machine would have to be wily enough to dig through Seattle's funky soils, everything from glacial till to pudding, the latter a legacy of early city fathers, who flattened the lumpy pioneer town into the salt marshes to create the modern city by the sound.
The requirements emerged: Bertha's cutterhead—her face—would be 57½ feet across, as tall as the viaduct she was replacing. She would have hundreds of teeth to chew with. She'd digest the muck she chewed and then build the tunnel behind her as she worked, so she would be 326 feet long, as long as a home run over the right-field fence at nearby Safeco Field. She would weigh as much as the Eiffel Tower and would use enough power to light a town of 30,000 people. She'd be able to generate so much thrust—44,000 tons—she could send 13 space shuttles into orbit. And, of course, she'd be burly, because by the time she burrowed through the subterranean darkness and emerged on the other side she would have shed 9 tons of solid steel.
Bertha would be all of these things. She would be the biggest tunnel-boring machine ever built.
A look at what it will take to snatch Bertha from the depths.
1. This summer Seattle Tunnel Partners sank 73 concrete pillars in a huge ring in front of Bertha's face. Workers then excavated the hole until it was deep enough to swallow an 11-story building.
2. Though she's running hot, Bertha can still move. Soon she will chew through the front of the protective wall until her head rests on a concrete cradle in the rescue pit.
3. Finally, a custom crane called a modular lift tower will raise Bertha's 2,000-ton face, tilt it, and set it down. Workers will replace Bertha's bearing assembly and add 86 more tons of steel ribs and plates.
To understand how a tunnel-boring machine works, think of an earthworm. The worm eats. It pushes forward. It expels. This is Bertha in a nutshell. As her 886-ton cutterhead revolves at about one rotation per minute, 260 spinning and stationary teeth chew the soil before her. Nozzles on her cutterhead spray saliva-like conditioners that transform the soil into the consistency of toothpaste. The soil is pushed through the large, mouthlike holes in her face. Then this chewed-up earth—it's officially called tunnel muck by engineers—enters a chamber where it's stirred and conditioned still more.
Next comes digestion: High pressure deep in a tunnel forces the muck up Bertha's gullet, a massive ribbon screw that works like an Archimedes' screw, but whose shape ("It's like a Slinky," Dixon said) allows it to swallow boulders up to 3 feet wide. The screw carries the muck farther back into Bertha, until it's deposited in her "intestines": a giant conveyor belt that reaches out of the tunnel and dumps its waste onto a barge on Puget Sound. The conveyor belt will grow as Bertha digs until it eventually stretches some 9,000 feet to the tunnel's end. Without this system the project would need an average of 200 dump trucks per day rumbling through downtown to carry away all the muck that Bertha digests.
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