'Balfour Beatty did not have enough staff experienced in New Austrian Tunnelling Method techniques available for the project, ' says the report. 'Quality was a problem from the beginning.'
The report highlights the fact that Balfour Beatty employed four junior NATM engineers to work under the control of Geoconsult's site engineer, who in turn reported to Balfour Beatty's engineering manager.
Although called 'NATM engineers', the report says they had limited experience of tunnelling and 'no experience of NATM in soft ground.
'Some limited briefing on NATM was arranged, with Geoconsult acting as trainer, ' says the report.
'However this proved insufficient.'
Heathrow Express was the first time a NATM-based tunnelling method had been used in London Clay other than for the trial tunnel built at Heathrow in 1993. Balfour Beatty did have experience of using NATM in rock, most recently on the Channel Tunnel BB's original tender price of £57.5M was the lowest submitted - some £9M less than its nearest technically sound rival. But during the tender stage, BAA's technical review team was concerned about its ability to do the work.
However, ratings were refined, bids tweaked and somehow Balfour Beatty's overall technical performance was ranked first with a revised bid price of £60.7M.
Problems also arose because of flaws in contractor's approach to risk assessment and control.
'Formalised risk assessment was mostly new to Balfour Beatty and others in the industry, ' says the report.
It points out that the contractor was geared to worker safety, rather than identifying engineering and management problems.
Crucially no risk assessments were prepared for repairs when defects were spotted.
'This was almost certainly because repairs were unplanned rather than programmed activities.
Work creating major safety risks went ahead without identification of hazards, ' says the report.
The self-certification system repeatedly failed to deliver a quality product and failed to put in place corrective actions for sub-standard work.
System defect notices were not issued or acted on in a timely fashion and 'the decision was generally taken to leave the defect unremedied and strive for better build quality in future'.
Balfour Beatty pleaded guilty in February 1999 to charges brought by the HSE under the Health & Safety at Work Act.
Fined £1.2M plus £100,000 costs.