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Wirth a fortune

Technical feature: Wirth

A massive tunnel borer built at Erkelenz in Germany is one of eight huge machines about to start work on Section Two of the Channel Tunnel Rail Link.

Mike Winney reports.

Brute force meets high technology in the mighty Wirth earth pressure balance tunnel boring machine that has just been put through its final tests at Erkelenz, Germany.

Now the 8.165m diameter, £7.2M monster is being carefully dismantled and packed up for shipping to London. It is destined for the Channel Tunnel Rail Link (CTRL) Contract 240, where it will drive one of the 4.7km running tunnels from Stratford to Barrington Road.

Wirth has manufactured two identical machines for this contract's joint venture of Costain, Skanska and Bachy Soletanche.

The second machine has been assembled in Lyon, France, at Wirth's sister company NFM because there was not enough factory floor space at Erkelenz for the 950t of shield and backup equipment.

In all, four pairs of TBMs are being built for the CTRL's London and Thames tunnels. Each pair is being produced by a different company and uniquely matched to the ground conditions expected along its section of the route.

Wirth's forte is hard rock tunnelling machines and this robust heritage is very apparent in the massive, heavily armoured, 100t cutter head of the machine at Erkelenz. The only thing differentiating the head from that of a hard rock tunneller is the absence of roller disk cutters.

But these can be fitted as an option. The body of the head and all the back up machinery such as the hydraulic rams which apply crowd force to the face are designed to carry the dynamic loads of the machine smashing and jarring away at a solid rock face.

Nothing is being left to chance because, although the main work of the TBM will be excavating densely packed Thanet Sand, the tunnel line dips into Chalk and its capping layer of Bullhead Beds, very hard flints. Tungsten carbide tipped picks and teeth on the head are designed to rip into this material, break it up and feed it down into the toughened screw conveyor that will pull spoil out from the face as the shield is jacked forwards.

Constant jacking forwards of the huge perforated head to match the volume of ground excavated from the sealed chamber behind the head disk is the essential characteristic of an EPB tunnelling machine, minimising inward movement of soil at the face and hence surface settlement.

'We plan to use only picks, ' says Wirth's chief designer Werner Hensgens. But if the picks prove to be not up to the job they can be changed for roller disk cutters by the crew, who will have access to the face via an airlock pressurised at up to 3 bar to hold back groundwater.

Regional dewatering of the Thanet aquifer in the area of the tunnels should mean that such extreme pressures are never required. But, should there be problems with the dewatering operations, equipping the TBM with this high pressure facility will insure against delays.

Another special design feature of the Contract 240 machines is double articulation of the main shields. This was a requirement from CTRL project manager Rail Link Engineering which was closely involved with specifying all TBMs. The front section of the 10.9m long, three part shield at Erkelenz is actively articulated from the centre body by 29 hydraulic thrust cylinders. Where hard material such a flint is met in only one sector of the face the shield can be steered sharply to compensate the tendency for the whole machine to be thrown off line.

Passive articulation of the tail section is intended to reduce the overbreak beyond the bolted precast concrete lining that will be assembled inside it, and also help the tail follow through the ground with minimum resistance. This double articulation makes the shield section considerably more complex than more conventional TBMs.

Probing ahead to test the ground can be done using a large drill on the axis of the machine and from an array of kit set around the perimeter of the central body section. These drills can push probes out through slanting, reinforced holes formed in the double skin of the shield. Eight ports around the rim of the rotating cutter head itself will allow a polymer foam mix to be injected into the ground to fluidise excavated material and give some lubrication to the shield.

Statistics of the big tunneller are hard to comprehend compared with the performance of more humdrum machinery. The five water cooled electric motors turning the head absorb 1.5GW of power - enough to run a fair sized town. On start up they generate a torque of 15,500,000Nm on the main shaft - roughly equivalent to the largest diesel railway loco available hung on the end of a crank handle 100m long.

Contrasting with this brute force is the air conditioned control cabin. It is packed with display screens that would not look out of place in a substantial permanent installation such a petrochemical process works. Almost every facet of the machine's operation can be interrogated through the screens' graphic interfaces - right down to the actual volumes of lubrication grease being fed to vital parts such as the water cooled main bearing. Alarms indicate when something is overheating or where there has been excessive wear of the specially wired picks.

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