Hacker News Reader

Comment by ghshephard | Hacker News Reader

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.

View on HN

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?

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.

Sorry.

You girls are giving away all of our secrets!

Are you saying that drilling tunnels is really boring?

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.

It's a little hard to picture so jump to 5m on this video. http://www.youtube.com/watch?v=Dk-8Q42NCSQ

"Earth pressure balance" is the term to google here. This is one of the better explanations I found: http://www.therobbinscompany.com/our-products/tunnel-boring-...

And an animation of the machine that is being used in Seattle: http://www.youtube.com/watch?v=Dk-8Q42NCSQ

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.

>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?

http://www.wsdot.wa.gov/Projects/Viaduct/Contents/Item/Displ...

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.

http://www.hitachizosen.co.jp/english/pickup/pickup008.html

http://www.ncus.ntu.edu.sg/Events/Documents/SRMEG%20Seminar%...

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.

The tunnel is not filled with water though. See abduhl's answer for the correct response.

The finished tunnel isn't filled with water because it's lined with concrete.

The unfinished tunnel might be filled with water if it wasn't for the pressure that the TBM maintains at the cutting face.

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.

It seems they are under water!

They probably have an airlock behind the machine since I doubt the drill head itself is air tight against the walls.

Any bore holes would have to be water proof, or at least pressurized to keep them from filling up.

Reply on news.ycombinator.com