Tunnel boring machines (TBMs) bore a gross diameter hole that's then lined (often with concrete segments) for support. The net diameter of the hole behind the T

Tunnel boring machines (TBMs) bore a gross diameter hole that's then lined (often with concrete segments) for support. The net diameter of the hole behind the TBM is less than the diameter of the TBM, particularly the shield, which is the business end. Maintenance is done in place. Fun Chunnel fact: when the TBMs driving from England and France met, they drove one below the other and abandoned it in place. The other was dismantled and removed in pieces.