How much were poor quality workmanship and failure to heed warnings to blame for the collapse?
As main contractor for the NATM station tunnels under the central terminal area of Heathrow airport, Balfour Beatty pleaded guilty of failing to ensure the safety of its employees and others in the events leading up to the collapse of the station tunnels. On Monday it was fined £1.2M for its part in the disaster.
Evidence from the Heathrow Express trial shows that a badly constructed invert joint in the central station concourse tunnel was a major contributor to the collapse of the three station tunnels. Even Professor Rokahr, the expert witness called to give evidence for co-defendent Geoconsult, said the badly constructed invert contributed to the surprising speed of the cave in.
It was the contractor's failure fully to investigate and repair this joint after cracks in the sprayed concrete lining were discovered in July and August 1994, two months before the collapse, that led to the disaster.
Compensation grouting of the ground above the tunnels had been carried out in August when higher than predicted settlement was measured in Camborne House, a three storey office block directly above the concourse tunnel. The grouting resulted in a 60mm depression in the concourse tunnel's crown and also led to cracking and deformations in the tunnel walls. After removing the clay running surface to investigate the condition of the invert the contractor discovered the invert had cracks along 46m of its length.
Repair work revealed voids in the sprayed concrete, lack of reinforcement and sections of lining which were only 50mm thick where it should have been 300mm. But despite these discoveries the invert was not investigated from the 54m point back to the tunnel entrance - the section that failed in the October collapse.
The court heard that a letter sent to Balfour Beatty on 5 August from BAA' s Heathrow Central Terminal Area manager Jonathan Allen said: 'We have reservations about structural integrity of the concourse tunnel from the tunnel eye to chainage 54. Please provide evidence of structural integrity.'
'I was concerned that there was no investigation back to the tunnel shaft,' BAA's HEX tunnel superintendent Paul Campbell told the court. 'And I believe that area should have been investigated.'
The court was told that Balfour Beatty responded to the client's concerns in correspondence to BAA during September 1994 when Balfour Beatty said the NATM construction was performing as predicted.
Prosecuting counsel Hugh Carlisle QC said the contractor was responsible for its own quality control and that it had been pushing so hard to meet the demands of an unrealistic tender and a badly behind programme that it had compromised on engineering supervision and, ultimately, safety.
Balfour Beatty tunnelling superintendent Brian Staley told the court that production was stepped up towards the end of September. 'We got instructions to open up all the faces because the job was falling behind,' said Staley. The prosecution and defence agreed that the existence of two new tunnel faces either side of the main concourse tunnel contributed to the collapse.
Balfour Beatty strongly denied prosecution suggestions that they were chasing an impossible tender. 'They bid for this important task. They were not at the bottom of the pile of bids. They were in the middle of the pack,' said the contractor's barrister Geoffrey Nice QC in mitigation.
Balfour Beatty's tunnelling consultant Geoconsult said during its defence that it did not have a quality control role. It also revealed that the contractor had limited the number of staff it could have on site and would not let it use the monitoring systems it wanted.
In mitigation, Balfour Beatty said it was working in a completely alien environment. It had been made responsible for checking its own work under BAA's own Contractor Self Certification Scheme. There was no client's resident engineer on site and this was the first time the controversial NATM technique had been used in London Clay.
Balfour Beatty tunnelling superintendent Brian Staley told the court: 'This was the first job we've all been on where the contractor self certifies quality. I think everyone was inexperienced in doing that. '
'Self certification was imposed as 'this or nothing',' said Balfour Beatty's barrister Geoffrey Nice QC. 'There is no doubt that Balfour Beatty was operating in an environment structurally that they were not familiar with.'
But surely this can only be a partial defence. Balfour Beatty agreed to carry out the £60M tunnelling work under the conditions of the ICE's New Engineering Contract. The evidence at the trial was that it had been challenged by the client over the quality of workmanship and supervision.
If that was the case then Balfour Beatty's behaviour reflects badly on the whole of the British construction industry.