Confusion about responsibility for supervising construction work on the Heathrow Express tunnels was at the heart of the 1994 collapse, according to witnesses at the trial. The project was to use the New Austrian Tunnelling Method for the first time in London Clay, and tightly controlled quality supervision was vital if disaster was to be avoided.
'It was absolutely essential that the design and construction should be entirely integrated,' former Institution of Civil Engineers president and expert witness for the prosecution Sir Alan Muir Wood told the trial.
'The way in which the whole management of this project was proceeding, there was this great break between engineering and construction and this, to my mind, is the fundamental ill effect which led to all the things happening.'
The trial revealed confusion within the HEX construction team about who was ultimately responsible for quality control. In a written statement read to the court BAA Central Terminal Area manager Jonathan Allen criticised the unique contractor self certification scheme developed by BAA for the project.
Under the scheme there was no client resident engineer on site. Instead, main contractor Balfour Beatty's own engineers were responsible for checking the quality of work on the tunnels. BAA's own HEX project managers were then responsible for auditing the contractor's self certification scheme. BAA's HEX project management team comprised personnel from BAA plus secondees from contractor Taylor Woodrow and consultant Mott MacDonald.
'I was concerned with the lack of understanding in the HEX team and Balfour Beatty over CSCS. The misunderstanding was that Balfour Beatty felt that HEX should not be involved,' Allen said in his statement.
'It was because of inconsistencies within HEX [in its approach to CSCS] that Balfour Beatty was able to exploit chinks within the system.'
Taylor Woodrow engineer Paul Campbell, construction superintendent for the project management team, told the court that the HEX team auditing of the contractors' self certification system 'did not work very well'.
A statement by Kurt Laubbichler, Geoconsult's head of NATM design on the HEX project, also stressed the problems of implementing the scheme. 'Supervision and quality control was based on self certification,' the court heard.
'We found out that miners are a particular breed of people. They're not easy to handle, and you would not expect them to impose any self certification. They're very reluctant to take orders from young engineers.'
Balfour Beatty tunnelling consultant Geoconsult's role was also unclear. The firm was subcontracted by Balfour Beatty to provide design supervision and monitoring for the NATM works. But while its engineers had a role in quality control this was not the consultant's primary function.
The jury heard that this was underlined in a letter from Balfour Beatty project director Alan Myers to BAA a month before the collapse. The letter said: 'The primary function of [the NATM engineers] is not quality control. Quality control is the responsibility of the construction teams carrying out the work. This is in accordance with our contractors' self certification scheme.'
The need for strong engineering supervision was especially important because the project was using NATM. The technique requires engineers to monitor ground and tunnel movements as work progresses so they can alter the tunnel lining design to suit conditions. Good monitoring data and close communications within the construction team were therefore essential.
Permanent works designer Mott MacDonald was against the self certification system from the start. In a statement read out in court Mott MacDonald principal engineer Robert Gee, who was seconded to the HEX team as engineering and design manager, explained that Mott MacDonald felt that allowing the contractor to check off its own work would risk lowering quality. 'Motts' preferred option was for the work to be supervised in a more traditional way,' he said.
The court also heard that Health & Safety Executive inspector Martin Thurgood had raised concerns about management of the NATM work with HEX in October 1992 - 18 months before the tunnelling work in the central terminal area began.