The idea behind TBC is fine, which is to figure out how to tunnel as quickly as possible, with the assumption that the current methods are less than fully effic

The idea behind TBC is fine, which is to figure out how to tunnel as quickly as possible, with the assumption that the current methods are less than fully efficient. The original TBC at the tunnel across the street from SpaceX was a proof of concept that used essentially off-the-shelf tunneling tech so that the team could learn how the current state of the practice was accomplished, and that they could improve on it. I don't know if you're familiar with tunnel boring (and if you are, sorry I'm explaining something that you probably know better than I do), but essentially the phases of digging, mucking, and reinforcing are not done concurrently, and I think that was the first things they were trying to do, and then move on from there. But yes, smaller tunnels were part of it.