Wasn't there a big tunnel collapse some years back, during work by Balfour
Beatty? Fortunately it was section not yet in use, or a disater might have
occurred. I seem to remember tunneling methods used by Austrian engineers were
tried, but these had worked in granite, not London Clay.
Perhaps better informed readers can recall details of the event?
I do remember a minor incident blocking the lines to Heathrow one Sunday
morning - a tower crane had come down across it - fortunately it came down in
the night, though one family living nearby had a narrow escape when the top of
the crane crashed through their roof.
Next morning I witnessed some passengers clamouring at the Acton Town ticket
office, apparently unable to comprehend that the besieged clerk could not get
the obstruction cleared at once and trains running again just for them.