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Comment by timr | Hacker News Reader

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?

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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.

Sorry.

You girls are giving away all of our secrets!

Are you saying that drilling tunnels is really boring?

"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

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

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