Comment Re:Doesn't sound very stable... (Score 5, Informative) 339
The machine puts up tunnel walls as it goes.
http://gizmodo.com/big-bertha-is-digging-seattles-massive-underground-fre-662469199
Concrete panels go in right behind the bore head. Infront of the maw is ground below the water table. The bore head forms a seal and the tunnel behind the bore head is pumped dry of water that leaks through.