I think he's gone one step too far on this project. Concrete is something that's necessarily expensive to produce, there's inescapable costs in the production cycle, so while he could probably make it 80% cheaper, he can't make it 99.9% cheaper.
But he might succeed at something somewhere in between the tunnel he is boring in his parking lot and the video.
Context: I lived in Boston during much of the Big Dig. Large tunnel projects scare the heck out of me as a tax payer.
If he retreats from this and builds some kind of pneumatic tube system to transport goods, people, or whatever, he still might have a chance of a win. Those systems have been proposed, and in some cases actually built, and in many cases they've been great ideas.
If he stubbornly insists on pursuing this batshit insane system of tubes he will fail, and he'll fail hard. There is not enough concrete in the world to make that many tunnels.
A rocket can go to the moon and come back safely, that takes an abundance of caution and significant attention to detail, but it's a fixed-length trip. Civil infrastructure has to exist for decades, or in the case of many American undertakings, well over a century. That requires extreme caution.
If you invented a concrete alternative today it'd take at least twenty years for it to be considered a viable alternative to concrete because long-term studies of the mechanical characteristics of it under a wide variety of conditions will have to be taken out.
Concrete is an extremely complicated material, I know people that have gotten a Ph.D. in aspects of it, and it's very well understood. This hypothetical alternative you're talking about has a lot to measure up to.
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...