BAA elected to use NATM to construct station tunnels and procured on a design and construct basis. This meant the contractor's design team had to take sole responsibility for the primary linings with secondary linings designed separately by Mott MacDonald.
'The separation reduced the permanent works designer's interest in the impact that the permanent works had on the temporary works, ' it says.
'Likewise it limited the temporary works designer's ability to influence the permanent design.'
The result also gave BAA distance from responsibility, says the HSE. This was compounded by the adoption of performance specifications which passed all settlement and technical risks to the contractor.
This, it says, '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, it says, as parties were unfamiliar with the contract and specifically with the requirements of self certification.
This further limited BAA's ability to supervise the work closely 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, ' says the HSE, pointing out that NATM excavation was never audited in the Central Terminal Area.
'BAA's decision that they required staff competent in auditing rather than knowledgeable in tunnelling further reduced their ability to influence the work.'
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.