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Cast box likely for DLR blast repair

REPAIRS TO the Docklands Light Railway Lewisham extension's south tunnel are likely to follow the model set following the Heathrow Express collapse in 1995.

Engineers are currently assessing damage to the tunnel, blown apart in a violent compressed air explosion in the early hours of 23 Feburary. Insufficient overburden due to a wrongly positioned bulkhead is being blamed for the incident during a compressed air test on the 5.2m tunnel.

The most likely method will be to expose the tunnel from the surface and replace the damaged section with a cast insitu concrete box.

Access to the tunnel is still too dangerous to confirm damage but it is thought that at least two of the 1.2m long rings had been completely destroyed.

Work is set to be hampered by groundwater flowing into the tunnel from the highly perm-eable Thames Gravels, estim- ated at 5litre/s. Access from the surface will require a sealed access shaft. Keller Ground Engineering has been pressure grouting to stem the flow of water to the crater and stabilise ground around the damaged tunnel. Probing has also been carried out to establish the extent of damage.

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