All the complexity has been in geotechnical issues, pockets of mud, sinkholes, then ultra hard rock, then loose shale, damage in blasting operations, needing to pump liquid nitrogen into the rock to deal with subterranean water flow, soluble limestone etc etc.

One of the scariest industrial deaths you can imagine: cutting into a pocket of shale and the TBM and crew just vanishing into the maw of the Midgard serpent and the whole tunnel filling.

https://www.abc.net.au/news/2024-05-22/tunnel-boring-machine...