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TBMs to take your breath away


EXOTIC TUNNEL boring machines were promised to those attending the British Tunnelling Society meeting at Great George Street on 20 May, and they got them with a vengeance.

Acclaimed as 'truly amazing pieces of equipment' by meeting chairman Eric Snowden, the Kawasaki machines presented to the packed audience certainly lived up to their advance billing.

TBMs that can bore tunnels with non-circular cross sections (News May 27) were the stars of the show, but other developments had major impact on the audience as well - not least the TBM which can execute tight U- turns at will.

Yasunori Kondo, general manager of Kawasaki Heavy Industries tunnelling department, opened his presentation with descriptions of unusual machines in use in Japan and the Far East but unknown in Europe. Most of these were designed to cut down the number of access shafts on a tunnelling project, usually needed to get the TBM in and out of the tunnelling zone.

Typically these occur where a tunnel changes diameter, where it branches, or where two TBMs working from each end of the tunnel meet at the centre. Often these points coincide with obstructions on the ground above. Kawasaki has developed a range of options to cope with these circumstances, starting with the 'double stage' TBM.

On this the shield is made up of two concentric sections. At the point at which the tunnel diameter reduces, the outer annulus is discarded and the centre section carries on to bore the smaller diameter.

Breeding machines

Dealing with a 4.24m diameter branch from a 7.26m diameter main tunnel required the development of a 'mother and child' earth pressure balance machine. As the larger diameter 'mother' TBM approaches the junction operatives begin to assemble a smaller diameter shield within its body. At the junction point a port opens in the side of the mother and the 'child' is launched, to carry on independently up the branch. Kondo said that mother TBMs carrying up to three children were already on the drawing board.

'Docking' TBMs that met at the centre of a tunnel were another option described. After docking and sealing together the shields could be left in place and the main components of the TBMs could then be dismantled and removed back through the original access shafts. The docked shields would eventually disappear behind the lining.

Constructing a tunnel from one valley through a mountain range to another valley is usually a relatively straightforward operation, provided enough is known about the geology of the strata on the route. A pilot bore is often advisable. But when Kawasaki was asked to produce a small diameter TBM to carry out two pilot bores along the projected route of parallel drill and blast road tunnels in Japan, it was faced with a special challenge.

Kondo explained that access to one valley was relatively uncomplicated, and launching a TBM into the mountain was a routine operation. The exit valley, however, was very steep sided. Recovering the TBM after the first drive and relaunching it back into the parallel bore was totally impractical. Kawasaki's solution was radical.

A three-section fully articulated TBM was produced. As it neared the end of the first drive it began a tight 30m radius turn through 180degrees, eventually returning back along the line of the second tunnel to the launch valley.

On the square

'Hybrid' TBMs which can cope with ground conditions varying from hard rock to soft, fissured water-bearing strata on a single drive are another recent addition to Kawasaki's range. But it was the description of machines which could produce non-circular cross sections that really set the meeting alight.

First up was a range of machines the manufacturer has dubbed 'H&V'. These are capable of boring two circular parallel bores at the same time, arranged at any angle to each other from the horizontal to the vertical. Kondo said that two overlapping bores involved a complicated lining which took longer than usual to assemble, but it was possible to bore them far enough apart for conventional segments to be used.

Very different diameters for the two bores are possible, as are 'platform bores', where auxiliary overlapping small diameter shields each side of the main shield bore out the areas for station platforms.

Very large box-section tunnels - to house major stations, for example - can be constructed with the 'multimicroshield' TBM. This effectively excavates vertical and horizontal rectangular slots which are then backfilled with concrete to form the main tunnel structure. But it was Kawasaki's prototype machine for boring square section tunnels that first drew gasps from the audience.

Kondo showed a brief video that clearly illustrated how a conventional rotating shield fitted with auxiliary planetary action cutters was capable of excavating a square face. Hardly had the audience absorbed the impact of this unconventional concept when Kondo introduced something even more revolutionary - the DPLEX series.

This tackles head on the main practical problem associated with using rotating cutters to bore the tunnel - the fact that the 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 the interval between cutter replacements 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, to the astonished the audience.

Kondo raised the biggest laugh of the evening responding to a suggestion from the floor that DPLEX machines must be more expensive than the standard TBMs. With mock sadness he replied: 'Unfortunately not.'

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