>> "then relaying the lining." Former tunnel boring engineer here. Modern TBM's usually line the tunnel with prefab concrete sectional rings, which are built

>> "then relaying the lining." Former tunnel boring engineer here. Modern TBM's usually line the tunnel with prefab concrete sectional rings, which are built by a human-controlled hydraulic arm at the front of the TBM's trailing gear, immediately behind the cutterhead chamber. As far as I know, cutting heads that could bore soils/rock could not also bore through metal lining. This is limited by the material properties of the cutting surfaces, typically hard metals or coated ceramics. Heads with acceptable wear rates in hard rock or soils will likely have unacceptably high wear rates against metals. And 3-5 years is probably a drastic underestimate of project duration.