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Exotic TBMs reveal shape of things to come

TUNNEL BORING machines that can produce square, figure of eight shaped or elliptical cross sections will soon be available in Europe.

Kawasaki tunnelling department general manager Yasunori Kondo described recent advances in TBM technology by the manufacturer at a recent British Tunnelling Society meeting.

Most of the new machines are designed to cut down the number of access shafts needed to get the TBM in and out of the tunnelling zone, typically where tunnel diameter changes, at branches or where two TBMs working from each end of the tunnel meet.

Developments included 'Mother-and-child' TBMs that can be used for tunnels with branches. Normally, a shaft would be needed to launch a second TBM to bore the branch tunnel. Instead, the TBM carries up to three child TBM kits which are assembled and launched at junctions to carry on independently.

The new DPLEX series tackles the problem associated with using rotating cutters - the fact that cutters near the outside of the shield travel much further and faster than those closer to the centre. Uneven wear is inevitable and high torque is needed to drive the cutters.

Kawasaki's solution is an oscillating shield driven by parallel links. All cutters travel equivalent distances, so wear is equalised and intervals between cutter replacement is much longer. Drive torque is much lower. Best of all, almost any cross section of tunnel can be produced - 'even triangular, if you want it', Kondo added.

The first DPLEX prototype is actually circular, but Kawasaki hopes designers will take advantage of the new concept's potential and choose more economical cross sections, such as ellipses.

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