A sudden, unexpected ground movement caused the Heathrow Express tunnels to cave in and there was no way it could have been foreseen.
So said Austrian tunnelling expert Geoconsult, in its defence against Health & Safety Executive allegations that it endangered workers' lives by failing to prevent the October 1994 collapse.
Defence witness Hannover University professor of under- ground construction Reinhard Rokahr claimed piles supporting a previously built structure and escalator shaft above the collapse area 'knitted together' the ground making it behave like a mono- lithic box which transferred high loads to the ground above the tunnels (see figures right).
The collapse was then triggered by unexpected softening of the clay between Cambourne House and the face of the downline platform tunnel, which caused the monolithic block to slide into the excavated tunnels. He believed the softening was in part caused by water soaking into the ground through a collapsed wall in an escalator box.
Rokahr claimed cracking observed during the collapse was not consistent with folding up of the invert. He did however feel the poor quality of workmanship affected how the tunnel behaved during the collapse, but was not its cause.
Rokahr argued that instruments monitoring tunnel and ground movement showed readings in line with predictions until two days before the collapse. He added that the speed of the collapse could not have been predicted.