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Soft tunnel vision


THE THAMES Tunnel has become the first UK beneficiary of a powerful new tool in the launch and capture of TBMs in difficult water-logged ground conditions.

Amec Spie JV (ASJV), working as a partner to principal contractor Hochtief Murphy JV, has used glass fibre-steel composite diaphragm wall reinforcement cages to allow construction of a 'soft tunnel eye' through the TBM reception chamber diaphragm wall on the north side of the river.

The soft eye will allow the TBM to cut through the headwall without having to stop to allow the T50 steel reinforcement, typical of the diaphragm wall, to be broken out.

Physical constraints at the northern approach meant that a more conventional slurry block could not be built on the external face of the reception chamber.

This method is being used on the southern side where a slurry block will act as a seal to enable the TBM to leave the launching chamber without causing water ingress into the excavation.

Working from design concepts provided by Hochtief Murphy JV, Dr Marco Arduini of Hughes supplied glass fibre reinforcement instead of steel for the tunnel eye.

ASJV carried out detailed design of the composite steel and glass fibre reinforcement bar cages.

One of the main challenges was to maintain cage integrity during lifting and handling. The three composite cages for the receiving panel had to be rotated through 90°, which meant over half of the 20t cage weight would be supported by the glass fibre section.

A series of tensile tests was carried out to model the interaction of the glass fibre bars with the steel bars, and to establish the slip capacity of the lap joints under different fixing arrangements.

'Creating a tunnel eye in diaphragm walls is difficult, ' says Amec specialist businesses director Richard Wright. 'By taking advantage of the piling and tunnelling expertise within the unit, we have delivered an innovative engineered solution.'

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