3 - A TBM has a bunch of cutters on the front; big blades that cut through rock and whatever else they run into. Since they're cutting through rock, they wear o

3 - A TBM has a bunch of cutters on the front; big blades that cut through rock and whatever else they run into. Since they're cutting through rock, they wear out pretty quickly, so they routinely have to stop the machine and send workers up to the front to replace those tools. The site is frequently pressurized to prevent the tunnel from flooding, so the workers have to go through a pressurization chamber before they can get to work, slowing the process down. That maintenance is needed because of how much material has to be cut, not by how long it's sitting there, so going faster means going through more material and wearing down the tools faster. You'd have the same total amount of time spent maintaining it (assuming going faster doesn't wear the tools down even faster, which is iffy). So just digging faster runs into Amdahl's law; you speed up a portion of the process but the total gains are less because the other parts are fixed. (And, as others have said, yes there's a faster way; cut and cover.)