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Cofferdam to repair damaged DLR tunnel

TUNNELLING CONTRACTOR Nishimatsu has confirmed that it will carry out a Heathrow Express-style cofferdam repair to the Docklands Light Railway extension tunnel wrecked by a compressed air explosion two weeks ago.

Work on the repair will start this week. Engineers will first have to construct a 14m deep cofferdam around the damaged tunnel using continuous flight auger concrete piles.

The decision to excavate and replace the damaged section from the surface followed last weeks abandonment of proposals to replace the tunnel lining from inside (NCE last week). High groundwater flows and uncertainty over the condition of the tunnel meant that repairs from inside were considered too dangerous.

Preliminary investigations from the surface have confirmed that up to eight of the tunnels 1.2m long precast concrete rings were blown apart by a compressed air blast. The tunnel is also thought to contain a significant amount of debris from the blast and is being flooded by groundwater at around 5 litre/s.

To achieve a waterproof seal between the piles and the undamaged rings, engineers will flood the tunnel until groundwater flows become static. Once the cofferdam is in place the damaged section will be excavated and the tunnel dewatered. A reinforced concrete box will then complete the tunnel. Nishimatsu project manager Peter Fisher claimed the repairs will take three months.

Construction joint venture for the 181M private finance project Mowlem/Mitsui- Nishimatsu is likely to bear the cost of repairs as part of its contractual risk. It is also likely to face prosecution by the Health and Safety Executive. An HSE source said: I would be very surprised if we didnt prosecute. It was very fortunate that no-one was killed.

Matthew Jones

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