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Test house asked to report on Docklands blowout

THE HEALTH & Safety Executive has appointed test house AEA Technology to investigate February's high pressure blowout on the Docklands Light Railway extension, under construction in east London.

AEA will concentrate on the adequacy of the overburden at the point where the accident occurred. It is carrying out finite element analysis of the mode of failure to ascertain whether calculations by Japanese tunnelling contractor Nishimatsu Construction were correct. It has been claimed that the overburden at the point where the tunnel failed was insufficient to balance the internal air pressure.

The blast wrecked the southbound tunnel near Island Gardens while Nishimatsu was preparing to excavate a cross passage and sump underneath the Thames to the northbound tunnel of the extension. Operatives were carrying out compressed air testing and had built up pressure to more than 2.2 bar when the explosion occurred below a school playground.

Delays to the project, due to open in early 2000, have been reduced by constructing the cross passage from the northbound tunnel.

Nishimatsu has now changed the construction method, with air pressure in the tunnel reduced to 1 bar by dewatering and the bulkheads positioned 30m either side of the passage. The first attempt saw the company place the northern bulkhead just 60m from the tunnel entrance with just 8m of overburden. The new position is at 20m depth.

Nishimatsu finished repairs to the southbound tunnel last month after constructing a 22m long cofferdam using 900mm diameter secant and contiguous piles around the damaged section.

The HSE inquiry is expected to last for another six months.

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