A SUDDEN, unexpected landslide caused the Heathrow Express tunnels to cave in and there was no way it could have been forseen.
So says Austrian tunnelling expert Geoconsult, which this week started its defence against Health & Safety Executive allegations that it endangered workers' lives by failing to prevent the October 1994 collapse.
Geoconsult claims a large clay slide directly above the centre of the three tunnels caused a massive 'monolithic block' of concrete and clay to fall on to the tunnels, setting off an unstoppable chain of events. This theory was put forward by defence witness Hanover University professor of underground construction Rheinhold Rokahr.
Rokahr claimed the faulty tunnel invert pointed out by the prosecution was nothing to do with the cause of the collapse and that subsequent independent studies showed that even with part of the invert removed the tunnel should have stood up for up to 80 days.
Instead, Rokahr blamed the the unexpected softening of clay between Cambourne House and the face of the down line platform tunnel. He believes the softening was caused by tunnelling work beginning on the two platform tunnels and by water soaking into the ground through a collapsed wall in an escalator box constructed under an earlier contract.
Geoconsult argues that instruments monitoring tunnel and ground movement showed readings in line with predictions until two days before the collapse. It also claims the speed of the collapse could not be predicted.