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I'm not up to speed on tunnel boring. Do they really grind away the whole cross-sectional area, instead of doing something like a hole-saw? Where you only machine away some of the circumference?

https://en.wikipedia.org/wiki/Hole_saw

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So you cut a one meter-deep circle in a chunk of rock. How do you get that circle removed? The back is still attached to the rock behind it.

Bore deep enough that the weight of the cantilevered piece breaks off. Or use additional leverage to break that chunk off.

And then falls on the drill, making it stuck and bends the attachment. This is a solved problem: https://en.wikipedia.org/wiki/Tunnel_boring_machine

No matter what, the amount of material removed is the volume of the tunnel.

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