Amazing that they managed to fill two whole pages of speculation about what might be under that bit of Seattle without once mentioning the giant underground city that actually is under that bit of Seattle. Or at most a few blocks away:
They talked to the local city librarian, so clearly she must have mentioned the tourist attraction where you can go down and see all the buildings that sank into the mud before they decided to raise the street level, oh, around 45 feet.
So now when they're digging a new tunnel 45 feet under that spot, it's tough to understand why they're in any way surprised that they hit something building-ish.
I laughed when I saw the map. Compare [1] from the article with [2], which mostly corresponds to the location of the Seattle Underground.
I went on the Underground tour earlier this year. It's really interesting and pretty funny too; after a large earthquake at one point they used the tunnels to hide a lot of the debris. I'd definitely recommend the tour; that said, I would definitely not want to be down there when something like this hits the wall!
We (my family, i.e.) personally found that tour rather short and significantly overpriced at $17 for about 75 minutes... in spite of that it was packed with tourists who paid extra for priority seating at the start of the tour (which isn't worth anything in practice). A real money spinner.
Overpriced? Sure, but having been on that tour, I would regret having not taken it. Really interesting history, and where else do you get to see an honest-to-goodness sunken city? I'd pay similar money (or more) to go on a (legal) tour of, say, disused NYC subway tunnels, a deactivated nuclear plant, an abandoned mine, a shipwreck, or Kowloon Walled City (when it existed).
> Some residents said they believe, or want to believe, that a piece of old Seattle, buried in the pell-mell rush of city-building in the 1800s, when a mucky waterfront wetland was filled in to make room for commerce, could be Bertha’s big trouble. That theory is bolstered by the fact that the blocked tunnel section is also in the shallowest portion of the route, with the top of the machine only around 45 feet below street grade.
When I read this yesterday, knowing nothing of Seattle history I felt a lot like what the parent is expressing because of the quoted bit.
The article spends 1200 words basically forcing speculation on "the object" (So ominous!) and casting a load of preemptive ("want to believe") doubt on an obvious explanation.
It's the scripted "reality" TV analog for news - manufacturing suspense from the mundane.
That said, I do hope "the object" is in fact an alien spacecraft.
I find it fascinating that the pressure is so high in their (relatively shallow) drilling area that the people doing the inspection have to go through decompression chambers. I'm trying to visualize how that could be, but I'm not sure what's going on - is there basically a "perfect seal" in the front of the drill head they don't want to break? And if so, won't the bore holes they are drilling do just that?
That's the most interesting part of the article for me.
Large TBMs like this generally allow access to what is called the face (the area being excavated) via a double bulkhead system. The ground conditions in Seattle require that the machine be able to control the rate of excavation at all times such that a large sink hole does not form due to uncontrolled loss of ground. This is done by keeping pressure on the face.
The pressures required to keep the face stable are a function of the soil properties, lithology, and groundwater. The typical way for someone to access the face is to pressurize ahead of the machine head with gas and have workers enter under pressure (known as a dive or an intervention). Workers generally are limited in how long they can be under pressure and must go through decompression.
Edit: real quick calculation on my phone puts them at about 70 psi or 4.5 atm of pressure to do a manned intervention, possibly higher for safety
Disclosure: underground tunneling is my profession.
I've always wondered how they get the kind of seal necessary to maintain that kind of pressure ahead of a tunneling machine. Is the drilled surface smooth enough to provide that kind of seal?
At the TBM face, there is a pressure chamber that uses bentonite slurry as the working fluid. The slurry pressure in this chamber is adjusted carefully to equal the local hydrostatic head. This seals against the axial forces on the machine's face.
Also, the TBM's tail shield (armored cover for the ring building arm+trailing gear, aka the TBM's skin) is completely surrounded by a set of stiff steel brushes that are impregnated with heavy grease. These brushes are called "tail seal grease brushes" and have dimensions ~1ft x 1 inch x 1 ft. They protrude radially from the surface of the machine to bear against the walls of the tunnel (perpendicular to tunnel wall at every point). This is probably the seal implementation that you're wondering about.
These things are only true for slurry-face TBM's. There are other sorts of TBM's out there, but the Seattle one is a SFTBM.
I typed up a rant about boring vs drilling and the correct terminology but it sounded super nerdy after reading it.
I'm not really qualified to answer your question as I'm not as intimate with the machines as I am with the ground/structure side. Most likely the seal is maintained by the machine being too big for the hole being cut or by inflatable packers. The walls of the bore can range from extremely smooth to not smooth at all depending on ground conditions.
You don't need to worry about the drilling surface as the TBM's building a concrete tunnel inside of it's self. However, distance from the drill head and friction means your not dealing with full pressure back there.
>Disclosure: underground tunneling is my profession.
That is the best thing I've read all week :) That's an awesome job! How common is that? Do you get hired for individual contracts, or are there entire companies that tunnelling is what they do 24/7?
Top of the boring machine is 60 ft from the surface, but it's 60 ft high so bottom of it is 120 ft. That's 120/33 = 3.6 atmospheres in the ocean, and my reading of the article above is that it may even be higher here due to groundwater pressure. Anyway, you need decompression at anything over 1 atmosphere given enough time as I recall?
Edit: fascinating machine. So yes, it maintains pressure in front of the drill head to resist ground and water pressure.
The water table in Seattle is very high, about 4 feet in that area. So at the point of the blockage they are under 40 feet of water. Every ~33 feet of water is an extra atmosphere of pressure.
Not "might." Absent face pressure, the tunnel would definitely be filled with water. This is precisely why they've gone to the trouble of inventing slurry face TBM's and building a recirculating slurry plant on the topside. Hard rock TBM's are much cheaper because they can get away without doing any of that.
"Chris Dixon, the project manager at Seattle Tunnel Partners [...] said he felt pretty confident that the blockage will turn out to be nothing more or less romantic than a giant boulder, perhaps left over from the Ice Age glaciers that scoured and crushed this corner of the continent 17,000 years ago. " ~ The article.
"Mystery Object Found to be Benign, Everyday Rock" does not make for a great headline.
Edit: I may be underestimating NYT's ingenuity a bit. I suppose "Big Bertha Resumes Drilling After Penetrating Ancient Glacial Super Rock" would serve the purpose.
Print news is now web-based which is all about ad impressions via alarmist or overly dramatized articles. Journalism somehow keeps finding a way to get worse.
This isn't new -- journalism has always been about selling a headline. But even then, it's just about them meeting a demand we have for the curious/ridiculous/obscene.
The area that Bertha is digging in right now is glacial till, so it wouldn't be even a little surprising if it ran into a big glacial erratic. It should be able to chew through those even so, but if it ends up loose and just bounces around in front of the cutting head then it would be a problem, which is probably what's happening.
> A secret subterranean heart, tinged with mystery and myth, beats beneath the streets in many of the world’s great cities.... Now Seattle, at least for now, has joined that exclusive club.
> Tourists seek out the catacombs of Rome, the sewers of Paris
I think it's the opposite: the catacombs of Paris are quite famous (at least in France), and the ancient sewers of Rome are likely to be visited, since sewers are a Roman invention IIRC.
A friend had never seen _2001_, so we watched it just last night. As I read I thought I heard Haywood Floyd's saying, "Its origin and purpose are still a total mystery."
> In October, workers walked through the first
> rings of the highway tunnel being built under
> Seattle’s waterfront toward the _boring_ machine
> called Bertha.
What's so boring about a huge, self-propelled drill making a gigantic hole under a city? … oh (facepalm)
I haven't been able to tell from any reports yet. What is it about this object that they weren't able to discern existing through sonar/ultrasound/core samples/whatever pre-engineering I imagine these sorts of projects do? Is it that it's just so far down? Or do we not even know?
1. We generally do a boring every couple hundred feet along an alignment for tunnels. The bore hole is about 2" in diameter. You can imagine that we miss white a bit of subsurface detail. We rely on experience and local geological history for fleshing out the details and putting things in context. Sometimes we just flat out miss things.
2. Geophysics methods (sonar essentially) is not very good at telling you what something is past "hard", "soft", or "void". In a glacial till you get a lot of all three.
I don't understand your question. A problem for what? For the geophysics methods or for the TBM?
Geophysics methods give a measure of density based on wave reflection/refraction and are therefore at the mercy of energy absorption of a given material. Resolution is a function of energy and so the ground smears in your results. The variability of the ground may or may not pose a problem to the geophysics methods used, it depends on how they do it.
For the TBM, inconsistency in the ground can obviously pose problems. Based on what I know about this project (which is not much), Bertha has mixed face tooling on its cutter head but is primarily a soil machine. This means that large boulders can pose a problem as the cutter head isn't designed to deal with massive, hard objects most likely. The way most of these soil machines work is that they have a cutter head with tooling that essentially scrapes the soil and there are windows in the head that open to an area behind the cutter head which the soil moves through before being spit out as muck from a conveyor or screw jack. This area behind the cutter head generally has a system to crush large rocks that enter this area into sizes that are fit for the muck system. The problem arises when you have a rock too large to fit through the window as the cutter head isn't really equipped to bust up rocks (for multiple reasons).
So what is the typical procedure when encountering a large boulder? Get people there and just drill through it manually? Or would they have to completely remove the entire boulder? (how?)
As a former Seattleite I think there is a decent chance this is just another one of restaurateur Ivar Haglund's futuristic adds like the Billboards for submarines [1]. Perhaps a literal acer of clams.
>> "I’m going to believe it’s a piece of Seattle history until proven otherwise," said Ann Ferguson, the curator of the Seattle Collection at the Seattle Public Library
It is exactly what my biases would lead me to believe it is, without evidence, unless you go to the effort to prove otherwise.
Oh, I had never tried the "privacy mode" of Firefox! Thanks for the heads up!
However why is this working? I already had disabled JS, so the only thing I can think of is my location (Europe), but "privacy mode" will not prevent anyone from seeing my IP, so... wtf?
EDIT: cookies, of course! I had forgotten I already read the NYT recently. Thanks!
EDIT': confirmed by disabling "privacy mode" again, deleting cookies did it.
I'm pretty sure that it's based on cookies. They let you read some number of articles per month for free and once you hit that number, they show the paywall.
http://en.wikipedia.org/wiki/Seattle_Underground
They talked to the local city librarian, so clearly she must have mentioned the tourist attraction where you can go down and see all the buildings that sank into the mud before they decided to raise the street level, oh, around 45 feet.
So now when they're digging a new tunnel 45 feet under that spot, it's tough to understand why they're in any way surprised that they hit something building-ish.