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London Tube tunnels flex to let CTRL drive below

LONDON UNDERGROUND has carried out special works to its Central Line tunnels near Stratford in east London so they can 'flex' as an 8.15m diameter section of Channel Tunnel Rail Link is bored eastward as little as 4m below it.

Exhaustive planning has been carried out by project engineers Rail Link Engineering, LUL and Contract 240 joint venture contractor Skanska/ Costain/Bachy Soletanche to prevent damage to the 1930s Tube tunnels and avoid costly line closures.

The LUL bores consist of cast-iron rings with a concrete invert on which the timbersleepered track rests.

Enabling works removed concrete between the sleepers, exposing the cast-iron ring joints. LUL engineers then loosened the connecting bolts to allow the tunnel to flex, reducing the risk of overstressing the lining.

According to LUL chief engineer Keith Beattie, a battery of highly complex monitoring equipment has been installed to detect ground movement, expected to be between 4mm and 5mm, while modelling already carried out will enable early prediction of remedial works should they be needed.

'The ground is Woolwich and Reading Beds and very compact Thanet Sands and the TBM is a state-of-the-art earth pressure balance machine.

'The compact sands are standing up extraordinarily well with little pressure needed to hold the face. But we have a temporary speed restriction in place, ' said Beattie.

INFOPLUS www. nceplus. co. uk/magazine/ rail

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