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Comment by nickff | Hacker News Reader

The tunnels have been structurally stable for a long time, sealing them seems much easier and more reliable than boring a new tunnel, sealing it, and hoping it remains stable.

What kind of control do you think you'd have that would make the 'artificial' tunnel better?

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It'd have several desirable properties:

1. You'd have a flat smooth surface that you could apply a liner to, instead of a surface that undulates and (probably) has fissures.

2. The diameter would also be constant, so that you could design your liner to fit based on known dimensions. Curves would also be of a known radius.

3. Also, our tunnel would go exactly where we want, and not wander around based on some ancient flow pattern. Or form oxbows where it loops back on itself.

4. So far as a structural liner, if you look at some of the tunnels under rivers where there is the danger of water intrusion, to my knowledge none of them have ever used a natural formation. Brunel used a brick structural liner under the Thames, and modern TBMs use interlocking concrete panels with rubber gaskets.

You seem to be assuming that the whole tunnel should be one continuous structure, and that the pressure vessel must occupy the whole volume. A segmented structure, possibly with pressure vessels that are either flexible or of a smaller diameter than the outer walls may be more practical. I'm also not sure whether luncrete is really a practical option, and you certainly won't have clay for bricks.

I think you're making a lot of assumptions, and don't have a comprehensive view of how the structure(s) would be designed and built.

A space station in a bottle. Except the bottle is a lava tube.

They've been structurally stable under a specific, unchanging load for a very long time. If suddenly they are pressurized to 1 atm, that may very well change.

It may, but usually it would mean they are even more durable, since the air pressure works against the gravital pressure from above. But lots of experiments are definitely needed.

Preventing it from collapsing by increasing structural durability seems wise.

The grandparent didn't say anything about reinforcing the TBM-made tunnel, it just asserted that it would be controlled. What techniques would be used for "increasing structural durability" that are only applicable to TBM-made tunnels?

I would expect a lava tube from a drained lava flow to vary greatly. We see many have collapsed over the ages. It would provide an insulated underground tube miles in potential extent. That said, the danger of a roof fall would be low in some spots and ready to fall anytime in others. This is still rock, about 6 tonnes earth mass per cubic meter (one tonne on the moon). That means a very strong tunnel liner is needed. It also needs to be air tight. Research into the use of regolith as an ingredient of a concrete needs to be done. One traditional concreted mass is baking bricks of regolith. More research into brick baking from various lunar materials is also needed. A blown up fabric habitat would work inside this sealed brick tunnel.

Can you do that without water? That would be the ultimate so the little that is available doesn't have to be used in construction materials

With enough energy, one can sinter the regolith together. Using water would be ridiciously expensive, unless some meaningful water reservoirs on the moon are found or an easy way to extract the very little existing water.

Yes, ground regolith and other local minerals might be available to mine - grind to powder, press into a shape(s) that can be Lego'd into the tunnel you want. Sintered blocks are very strong in compression and 1-2 layers will allow long buildings to be made in the lava tubes or on the surface. crushed regolith will serve as insulating berms that embed structures.

There are hints there will be water in parts of the regolith. I am not sure how a water based cement would endure heat/vacuum, or only vacuum cycling - would it crumble? Inner places = less heat, but vacuum is all around. I tend to think that a low temperature melting mineral mix might work to bond blocks - a ceramic solder?

I imagine these bricks will be modeled so they stack easily i.e. legoregolith bricks.

Yes, pressed and shaped they could be baked until they were a strong ceramic load bearing shape that suited wall, roofd, door, ceilings Made with precision molds they would be a tight fit and they could be cemented together with a fusing cement that melted at a lower temperature than the blocks, as water would be hard to find and might fume off into the vacuum. In truth there will be 20-30 years of step by step progress and R&D before we reach the stage of doing this on the moon

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