Posted on 12/26/2013 5:02:00 PM PST by Windflier
Igneous rock on the Mohs scale of mineral hardness runs between 6 and 8. I don’t know what the cutting teeth on Bertha are designed to cut through.
More info here.
http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
I only know what I've read tonight about this project, but reports say that the operator has pressurized the tunnel interior to keep water from leaking in. That would have to be some pretty high pressure, so maybe the claim about workers having to decompress is true.
Air is a fluid. The word you're looking for is liquid.
If, say 1/3 of the face is quartzite and the remainder is sandstone the miner would be pulling continually right or left of its intended direction. It would likely shutdown once it’s alignment is altered.
"Built in Japan the tunnel boring machine, referred to as a TBM, is the largest of its type in the world. You might think okay largest in the world means what? To give you a sense of how massive this machine is the cutting head is 57 feet in diameter. Orient the head to its vertical position and it’s almost the height of a six story building.
393 individual cutting “tools” on the massive head can chew through almost anything it will encounter. Rocks up to four feet in diameter are no problem for Bertha. I wanted to know what the cutting force that Bertha is capable of but the answer is more complicated than I thought. KaDeena Yerkan, manager of Communications and Public Involvement gave me this answer,
“It is difficult to relate the installed thrust force to the force being applied to each cutter. The cutter head has a designed total propulsion force of 392,000kN which is an 88,125,104 pound force! It is not as simple as dividing the cutter force by the cutter number to yield the answer that you requested. The issue is complicated and is affected by geometry and also the way in which the machine is intended to be operated. Bertha only needs to operate at a small fraction of its installed total thrust capability to effectively excavate the soil”.
To put this in layman's terms it’s extremely powerful."
http://www.westseattleherald.com/2013/05/15/features/bertha-not-so-boring-machine
I've done a fair bit of reading about these TBMs tonight, and from what I'm finding, the technology involved is somewhere on the order of NASA's. The power, accuracy, and sophistication of these machines is not to be underestimated.
“We can engineer it to be safe.”
I was reading (or perhaps it was a talk?) about tunnels in EQ zones. It was amazing. Had photos of tunnels where the tunnel was damaged where the fault had occurred - a big crack in the tunnel wall and a few feet of offset due to the fault movement. But on either side of the fault everything was intact!
Took me a minute, but I finally saw what you slipped into that photo. Now we know the problem!
The “caisson disease” while working on the Brooklyn Bridge was where the term “the Bends” first was coined.
It’s not an accident this happened at the start of the fill-in area. It’s probably hard granite mixed with slush that the cutter can’t get purchase on. And it might happen again and again through this area.
While the tunnel is under construction the ground water around the bore hole is kept out by introducing compressed air to keep leakage to a minimum. The rough hole cut by "Bertha" is immediately lined with interlocking ring segments which support the tunnel walls and prevent them from collapsing. The rings are assembled directly behind the machine as it moves forward.
The segmented rings are not watertight and so the contractor uses air pressure to keep the tunnel free from major flooding. Once the boring is complete, the bore hole is lined with concrete which permanently caulks the leaks in the ring structure and the air pressure is allowed to return to normal atmospheric pressure.
During the drilling phase when the bore is pressurized, workers pass through an air lock to enter and leave the work site. I would guess that they use the air lock for a decompression chamber. It's function would be prevent the workers from getting the bends (nitrogen dissolved in the blood stream while working under elevated pressure from forming bobbles if they were returned too rapidly to normal atmospheric pressure).
Once the tunnel is lined, the roadway finished, and ventilation is added to remove exhaust gasses, vehicle traffic can pass through from end to end with no change in pressure to worry about.
Regards,
GtG
An Occam's Razor explanation, which makes sense, except for the fact that the operator claims there's an 'object' stopping them.
I like a good mystery, but it'll be nice to find out what they've really run into.
It is a water drill. There would be no pressure and they could open the door and look.
Exactly... I guess the air is ‘heavy’.
Maybe someone mistakenly flipped the reverse button...
I think the thing can only go in reverse a few feet. It is not only digging the tunnel, but BUILDING the tunnel walls behind it. They can’t just back it out and try something else. They would need to dismantle the boring machine and take it out piece by piece.
I would imagine that they would have had a plan in place when encountering a large boulder in the way. Such a thing would not be unexpected in this geologic setting. I’m guessing it will just take time to dewater and explore to see what it is, where exactly it is located on the face of the cutting head, and how big it is and then take care of it.
Perhaps they can take apart portions of the cutting head to do small drilling of a boulder or something. I would think that the cutting head would get dull over time, and they would have to swap out the head once in awhile like they do oil-well drill bits. But perhaps not.
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