THE HEALTH & Safety Executive alleges that Geoconsult, which was employed by Balfour Beatty to provide technical supervision and monitoring of tunnelling work using the New Austrian Tunnelling Method, ignored its own monitoring instruments which showed a collapse was imminent up to two weeks before the cave-in.
Appearing as an expert witness, former Institution of Civil Engineers President Sir Alan Muir Wood claimed that a badly formed tunnel lining, particularly at a joint in the invert which was difficult to construct, ruined the integrity of the main concourse tunnel structure.
'Whatever the contribution of other factors, the poor invert caused the failure,' he said.
Muir Wood told the court that in design and build tunnelling such as NATM, an effective system for monitoring tunnel and ground movements was essential. He claimed it was clear from engineering records, monitoring data and eye witness accounts that there were ongoing quality problems in the invert joint to which Geoconsult should have reacted.
Throughout August 1994 a programme of remedial work led to more than 72m of the invert being broken out and replaced, in one place revealing lining only 50mm thick which should have been 300mm.
But the repairs did not include the first 44m of the tunnel. The HSE says when this section started to buckle in October 1994 it was too late to do anything to prevent the collapse.