THE HEALTH & Safety Executive has appointed test house AEA Technology to investigate the high pressure air blow-out on Docklands Light Railway extension earlier this year.
However, an HSE source described the cause of the explosion which wrecked the southbound tunnel near Island Gardens as 'obvious'. The source claimed there was little doubt that the overburden at the point where the tunnel failed was insufficient to balance the internal air pressure.
AEA is concentrating its investigation on the adequacy of the overburden. It is carrying out a finite element analysis of the failure method, to check whether calculations by Japanese contractor Nishimatsu Construction were correct.
The blast, in February, happened while Nishimatsu was preparing to excavate a cross passage and sump underneath the Thames to the northbound tunnel of the extension (NCE 26 February). Operatives were carrying out compressed air testing and had built up the pressure to more than 2.2 bar when the massive blow-out occurred below a school playground.
No-one was hurt, but buildings up to 100m away were damaged by debris thrown into the air. It was 'very fortunate' that no-one was killed said the HSE source.
Nishimatsu finished the repairs to the tunnel last week after constructing a 22m long cofferdam using 900mm diameter secant and contiguous piles around the damaged section. Shattered precast concrete segmental bolted tunnel linings were replaced on a like for like basis rather than using an alternative proposal of casting an insitu concrete box around the damaged section.
Project manager Peter Fisher claimed that the accident had not caused a delay to the overall programme of the project, which is due to open in early 2000. 'We moved the cross passage construction into the northbound tunnel, which took it off the critical path,' he said.
Nishimatsu had originally installed the northern air-retaining bulkhead just 60m from the tunnel entrance at a depth of 8m below ground level for its first attempt at the cross passage. But for the recovery Fisher said it reduced air pressure in the tunnel to 1 bar by dewatering, and moved the bulkheads to positions just 30m either side of the cross passage where the depth is around 20m for the recovery.
The HSE inquiry is expected to continue for another six months.