> How competitive is the manufacturing/design market for TBMs? There's a bunch. > Or are there good physics reasons why TBMs can't be much more efficient than

> How competitive is the manufacturing/design market for TBMs? There's a bunch. > Or are there good physics reasons why TBMs can't be much more efficient than they already are? Mostly that tunnelling through unknown ground is risky (even if you're taken samples, you don't know what every meter is like!) so you don't want to move too fast, you're having to move vast quantities of spoil out, you need to keep the cutting blades cool (they're under immense pressure and friction so they get _hot_, and cooling is hard when you're in a confined tunnel) and we only have materials so hard (so you can't speed up the cutting head)…