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Crossrail tunnels: Delivery driver

Crossrail’s first tunnel boring machine is now being built on site ready to start work in March. Claire Symes reports from west London.

Crossrail’s tunnels will start to become a reality later this year, with four tunnel boring machines (TBMs) set to start work on the underground sections of the long-awaited new rail network. Parts for the first Crossrail TBM have now arrived on site near Paddington from German manufacturer Herrenknecht’s factory.The machine will be assembled ahead of its scheduled March launch date.

The first 1,100t earth pressure balance (EPB) machine will be used to drive the 6.16km from Royal Oak east to Farringdon, and will shortly be followed by a second machine which will bore the parallel twin tunnel. In total six EPB and two mixshield TBMs – all from Herrenknecht – will be used for the 21km of twin bore tunnels on the new 118km rail link from Maidenhead to Shenfield in Essex and Abbey Wood in Kent.

£10M machine

The first machine to arrive costs £10M and will be 140m long. It will use a 7.1m diameter cutter head and 7.08m shield to drive the bore, which will be lined with precast concrete segments, grouted into position, to form the tunnel’s 6.2m internal diameter. As each concrete ring is grouted into position, the TBM will use 58,000kN capacity hydraulic jacks to drive itself forward. Crossrail anticipates a tunnelling rate of about 100m per week.

In total six EPB and two mixshield TBMs – all from Herrenknecht – will be used

Crossrail’s EPB machines will operate at pressures up to 300kPa, and will mainly be tunnelling through London clay, although the eastern end will move into the sands and clays of the Lambeth group and Thanet sand, before the mixshield TBMs take over to drive the north Kent tunnels through chalk.

Herrenknecht division manager for traffic tunnels Gerhard Wehrmeyer says Crossrail set a very clear specification for the TBM design. “All the EPB machines are very similar, although there are a few minor differences between what each contractor has ordered,” he says. “Crossrail agreed a common specification in order to ease the parts and maintenance issues.”

Common route

The common procurement route has helped speed up the delivery of the machines, cutting delivery time from the normal 12 months to 10. “Each machine has 2,000 drawings and 500 assembly drawings, so being able to duplicate these between the machines has helped reduce the design work needed,” says Wehrmeyer.

“Crossrail agreed a common specification to ease the parts and maintenance issues”

Gerhard Wehrmayer, Herrenknecht

One area where Crossrail’s specification went above and beyond the normal specification was in terms of the belt scales fitted to the conveyors. “Only one manufacturer – Australia’s CST – could match the high specification,” says Wehrmeyer.

Crossrail chief engineer Chris Dulake explains that this demand was placed on Herrenknecht to help avoid a collapse similar to that in Lavender Street in east London when TBMs for High Speed 1 passed through previously unrecorded well shafts. “The high accuracy of the belt scales will help ensure that the material being extracted at the face as the TBM advances exactly matches the expected volume,” he says. “If there is any difference between the two, then we know there is a problem.”

Launch of this first TBM is planned for March, and it is expect to reach Farringdon in summer 2013. The second EPB machine will soon arrive at Royal Oak for assembly, while work on the remaining four EPB machines and the two mixshield TBMs continues in Germany.

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