TUNNELLING CONSULTANT Geoconsult's explanation for the 1994 Heathrow Express tunnel collapse was 'wrong in almost every particular', the Health & Safety Executive's prosecuting barrister Hugh Carlisle QC claimed this week.
Carlisle said that Geoconsult expert witness Professor Reinhard Rokahr's theory that the cave-in was caused by an unforeseeable and unstoppable landslide was pure conjecture. He added that the professor had admitted NATM was his 'life' and was not therefore an independent witness.
After nearly two days of cross-examination Carlisle rounded on Rokahr. 'You are clutching at straws to support [NATM],' he said. 'You lack objectivity.'
The HSE claimed that Geoconsult's monitoring showed that the tunnel began to fail months before the final collapse. It blamed poor construction of the central concourse tunnel's shotcrete lining and said Geoconsult failed to act.
The Austrian company denied this and claimed that the HSE's expert witness, Sir Alan Muir Wood, had not looked at all the evidence. It said the former ICE President had completely failed to understand the time dependent nature of shotcrete.
Muir Wood claimed the poorly constructed invert joint led to the concourse tunnel 'folding up'. This softened the clay around the tunnel and effectively removed lateral support to the adjacent down-line tunnel. Increased stresses on the tunnel side then caused the shotcrete lining close to the advancing face to fail in bending (see diagram bottom left).
Rokahr said this explanation was not supported by the evidence from sketches drawn on the night of the collapse by site engineers.
The sketches show horizontal cracks and buckled reinforcement mesh in the side walls of the down-line platform tunnel (see diagram bottom right). Rokahr claimed these were shear cracks and could only be explained by a large vertical load on the tunnel.
'It is one thing to make a hunch after one week. It is another to make a judgement after months,' said defence solicitor Arthur Marriott QC, referring to Muir Wood's admission that he had only spent a week preparing his report, while Rokahr spent 10 months on his study.