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Tube tunnels flex to let CTRL drive below

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

The works will enable Central Line trains to keep running during tunnelling.

The operation was originally due to take place last weekend but was aborted by Contract 240 joint venture contractor Skanska/ Costain/Bachy Soletanche after problems with tunnel boring machine (TBM) 'Hudson'.

There were also teething problems with the ground monitoring system near Angel Lane, east of the Stratford station box in east London.

Exhaustive planning has been carried out by project engineers Rail Link Engineering, LUL and the contractor 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.

LUL chief engineer Keith Beattie told NCE that 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 so we are anticipating very little movement. But we will have a temporary speed restriction in place, ' said Beattie.

A sister CTRL tunnel will be bored in January.

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

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