I’ve had personal acquaintance with half a dozen or more tunnel boring machines (TBMs). Including a 25-foot diameter cutterhead TBM I lived with – or suffered with, more accurately – for nine long, cold, wet, hard rock miles below the floor of Massachusetts Bay. In its time, the 1990s, that machine was one of the largest TBMs the world had yet seen.
On Tuesday, I got a good look inside the new Seattle TBM, the one that will bore the new, Viaduct replacement tunnel. I clambered up and down ladders, inched along catwalks, balanced by holding tight to bronze hydraulic fittings and finally crawled on my knees past a steel bulkhead right to the center core of the monster machine now poised in a pit west of Qwest Field: the green, for now, “cutterhead” that will rotate and scrape and grind its way under Seattle. Jammed into quarters made tight with hydraulics and electronics, I marveled to my guide Greg Hauser, Deputy Project Manager for Seattle Tunnel Partners, “I’ve never seen anything like this.”
“That’s because there’s never been anything like this,” replied Hauser (below), a lifelong project-hardened mole – as tunneling engineers often are called – without batting an eyelash. “This, for a machine, is almost unbelievable. I call it a work of art.”
Hauser is in charge of testing, launching and operating the TBM. No cockiness accompanies his awe at the scale, power and complexity of the machine. He’s grateful for what he calls the “shakedown cruise,” slow and cautious, that will soon begin for the first 1,500 feet underground. He spends his time already anticipating the challenges of tuning the machine’s operation and its delivery of each daily advance.
A lot of tunneling goes on around the world, most of it unheralded by surface dwellers for its importance to modern infrastructure. At 57-feet across, Seattle’s new machine has a handful of recent forerunners that approach its scale. But this TBM is big, really big.
It weighs in at 7000 tons, the equivalent of more than 30 orange and black BNSF Dash 9 diesel locomotives. Its 24 huge electric motors — that's the #18 motor below — generate the 25,000 horsepower that will slowly rotate the cutterhead (at about one revolution per minute) as it chews and grinds the ground – TBMs don’t actually drill into the ground. The TBM’s 56 thrust jacks that push the rotating cutterhead (the business end of the boring machine) against the ground ahead exert 44,000 tons of thrust, or 13 times the thrust of the engine and booster rocket that lifted the space shuttle into orbit. The machine is powered by what is essentially a 26 kilovolt extension cord. Seattle City Light built the dedicated feeder line just for this purpose. An extension cord!
Seattle’s press and public get a close-up look at the behemoth on Saturday before it starts to disappear underground sometime next week. It will re-emerge after its 1.75 mile tunnel drive to Aurora Avenue North near Denny Way. But the machine is already an international rock star, engaging engineers and other experts who are now in Seattle from Spain, Japan, Germany, the United Kingdom, Ireland, Australia, Russia and France, and just about every corner of the United States. The homegrown workforce of building trades men and women, including sandhogs, operating engineers and electricians bring valuable, pertinent experience from Sound Transit and Brightwater tunnels, has never worked on a TBM like this.
Moles everywhere will be keeping tabs on how well Seattle’s TBM performs. And how will performance be measured? There are, of course, a myriad of measures to be tracked every day, indeed every hour by command officers like Greg Hauser and their teams. But the two key parameters are utilization and production.
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