I recall one speculative engineering idea: Use hot hydrogen to reduce oxides and then extrude resulting silicon into a glass-like tube lining.
On the other hand: Transforming soil and rock into bricks might be less energy-intensive.

This has application on mars too. Clever fella, this Musk guy.

". . .turn the excavated dirt into bricks to construct tunnel structures. This would negate the need to cart the soil off for disposal. . ." I doubt that soil volume can be compacted to any significant degree. So unless you want to pack the tunnel solid with bricks, most soil would still need to be carted off.

"....unlike flying cars, tunnels....won't fall on your head." Famous Last Words.

"TBM, called Godot"
Love the naming. Brilliant.

I'm always amazed when something like this is treated as groundbreaking and clever. That Elon guy is so smart and crafty! No offense but it all been done before...
►Seattle is in the process of tunneling with Becky. ►How about Niagara Falls Ontario tunneling 10 km to a depth of 14km?

Go for it, Elon! Cut the cost of tunneling by 10. Wonderful!
Why aren't we selling/giving this excavated soil to countries like Brazil, to keep their farmers from doing more slash and burn on the rainforests? Slashed farms last a year or two at most. End foreign aid cash to Brazil and give 'em dirt to farm in, fer Crikey's sake.

It would be cheaper to extrude tunnels on top of the ground or above the ground from building to building, city to city, using a known material. Forget about struggling with mud, silt and rubble.

Tunneling and creating structures as it goes? Yeah, more convinced than ever that this is being designed for a Mars mission. It makes all the sense in the world to use this work to set up non-earth colonies. For any rocky planet, the process would likely be the same for the people living there.

The problem of soul-destroying traffic has already been solved in so different ways, but this one is the kinkiest. Given that we have already colonized Mars, this idea is long overdue. No point in reviewing the tunnels idea' fundamentals, let's jump straight to solving issues with the boring, it's more fun. The rats will hate to explore a zillion of new venues if ever built though. I am sure Elon will not use this venture to siphon money to his other endeavours. All his bets are sure to win! Big!