Comment by schiffern

>Concrete is something that's necessarily expensive to produce

That's true. But techniques like compressed stabilized earth blocks can reduce the required percentage of concrete from ~30% to 4%.[1] For example, recently a 4% concrete and 8% ash CSEB was shown to be as strong as class 30 concrete.[2] Fly ash can also be used. Obviously considerations like longevity and manufacturability need to be validated, but the strength is there.

Essentially you're mechanically squeezing all the air out of the concrete, thus reducing the cement requirement. Since TBM tunnels are made from precast formed concrete currently, it seems like a natural cost (and CO2) saving improvement.

[1] https://www.researchgate.net/publication/267637559_Effect_Of...

[2] https://www.hindawi.com/journals/je/2016/7940239/