it can only back up 2 or 3 feet?
I think that is a design flaw, considering its pretty much stuck there for now.
It does more than just dig the tunnel, it builds the tunnel as it goes, putting up panels every 6 feet or something. Not a design flaw - but yes, a very risky thing.
Several years ago just north of Seattle (but same geology) they built the Brightwater Sewer tunnel. One of the machines got stuck. They brought in another one, which was in the area working on another tunnel, to dig from the opposite direction. The one that got stuck had to be dismantled in the hole and taken out piece by piece.
With the current project, no spare 56-foot diameter TBM’s around though.