'Balfour Beatty did not have enough staff experienced in NATM techniques available for the project, 'says the report.'Quality was a problem from the beginning.'
The report highlights the fact that Balfour Beatty employed 'three junior engineers'to work under the control of Geoconsult's site engineer and, although they were called 'NATM engineers', they had limited experience of tunnelling and 'no experience of NATM in soft g round' .
'Some limited briefing on NATM was arranged, with Geoconsult acting as trainer, 'it says.'This proved insufficient.'
Heathrow Express was the first time that a NATM-based tunnelling method had been used in London Clay, other than the trial tunnel carried out in 1993.However HSE believes enough was known about the method and the behaviour of London Clay for 'safe economic construction to be achieved'.
The trial tunnel had proved the method could be used but similar tests were not carried out before use of compensation grouting, which, says the HSE, 'did not have a well-defined theoretical basis'.
Balfour Beatty was aware of the potential risks and the need to maintain high quality workmanship throughout tunnel construction.But the HSE severely criticises the contractor's whole approach to risk assessment and control from day one.
'Formalised risk assessment was mostly new to Balfour Beatty and others in the industry, 'the report says, pointing out that a system of proformas was introduced which did not really address the issues.
'The proforma approach kept the focus on routine worker safety.It did not encourage the strategic identification of high-level engineering and management issues essential to the success of NATM.'
Crucially, when production problems were identified - such as the critical errors in the construction of the invert - no risk assessments were prepared for the repairs.
'This was almost certainly because the repairs were unplanned rather than programmed activities.Work creating major safety risks went ahead without identification of hazards.'
The self-certification system repeatedly failed to deliver a quality product and failed to put in place corrective actions for substandard work.System defect notices were not issued or acted upon in a timely fashion and 'the decision was generally taken to leave the defect unremedied and strive for better build quality in future.'
Throughout construction of the concourse tunnel, HSE says, 'Balfour Beatty and Geoconsult were aware of defective workmanship although the full significance was not appreciated.'
Balfour Beatty pleaded guilty in February 1999 to charges brought by the HSE under the Health and Safety at Work Act.The firm was fined £1.2M plus £100,000 costs.
The HSE believes Geoconsult's approach to the design of temporary linings contributed to the collapse.
'The numerical model was not correctly calibrated against the results available from the trial tunnel, 'it says.
'Compensation grouting was not formally included in the calculations to predict the loads in the lining.The failure to adequately predict surface settlement from multitunnelling activities was to have significant consequences.'
HSE also criticised Geoconsult's approach to risk control, especially the buildability of its designs.Minor variations in the construction of the lining, particularly at the invert, were also found to have major implications on structural performance.
'The margin for error was small and rigorous inspection and monitoring during construction was necessary, 'the report says.
HSE highlighted inappropriate staffing levels as a continual problem for Geoconsult.Its onsite presence was limited to one engineer who was less experienced than originally anticipated.
Geoconsult had little input in development of work quality systems and voiced no objections when Balfour Beatty's production teams were handed responsibility for certifying their own work.Despite being technical advisers, Geoconsult played no part in the worksite risk assessments.
Limited site resources were compounded by Balfour Beatty's choice of computer software for monitoring the tunnelling data.
Different software, says the HSE, may have given the Geoconsult engineer more time to focus on the more important issues.
'Geoconsult's engineer was hard-pressed, ' says the report.'There were extended periods, particularly at weekends, when no expert assistance was available. It should have been apparent that Geoconsult's site engineer was overloaded.'
As cracking in the tunnel became more ev ident, invest igat ions were ordered to check the condition of the invert joint. In places this was found to have failed and had been poorly built, often only 50mm thick rather than the 300mm specified.
'It was unacceptable that people only became aware of the severe distortion by the appearance of cracks, 'says the report.'The timely analysis of the monitoring data and an engineering review should have identified incipient damage.'
Geoconsult pleaded not guilty in February 1999 to charges brought by the HSE under the Health and Safety at Work Act.Its subsequent appeal was rejected.The firm was fined £500,000 plus £100,000 costs.
British Airports Authority
BAA chose NATM early on in the project, procured on a design and construct basis.This meant that the contractor's design team took sole responsibility for the primary linings with secondary linings designed separately by Mott MacDonald.
'This reduced Mott MacDonald's interest in the impact the permanent works had on temporary works, 'HSE says.'Likewise it limited the ability of the temporary works designer to influence the permanent design.'
It also gave BAA distance from responsibility, says HSE, and was compounded by the adoption of a performance specification which passed all settlement and technical risks to the contractor.'This meant that BAA's supervisory team was less able to intervene during design and construction of the NATM works.'
Use of the New Engineering Contract complicated matters further, as parties were unfamiliar with the contract and specifically with the requirements of self-certification.
This again limited BAA's ability to closely supervise the work and 'placed greater onus on the contractor to complete the work satisfactorily'.
'BAA chose to oversee the construction work indirectly through a system of infrequent audits.These proved ineffectual, 'HSE says.
'BAA's decision that they required staff competent in auditing rather than knowledgeable in tunnelling further reduced their ability to influence the work.'
Taylor Woodrow was originally to act as project manager on BAA's behalf but this was later transferred, along with some TW staff, to BAA.
However, insufficient resources were available for BAA to carry out its planned audit programme.Of the 18 tasks assigned to the planning supervisor under the CDM regulations, only one was ever carried out when BAA assumed control of the project.
The report adds: 'BAA's management systems failed to challenge the contractor sufficiently robustly on safety critical issues, in particular the quality and integrity of the CTA concourse tunnel.'
No charges were brought against BAA by the HSE in connection with the HEX collapse.