What would you call them? Crossrail's tunnel-boring machines (TBMs), which will start boring the 21km twin-bore tunnels under London in March, are looking for a name.
Annie, Bertha, Hudson, Brunel, Maysam and Judy were the names of the TBMs on the capital's last major rail tunnelling project, the 18km of the Channel Tunnel Rail Link which crossed east London.
The challenge for the eight Crossrail TBMs – whatever their names – will be much greater though, because of the complexities of boring across central London, and the different ground conditions that will be encountered on the southern spur which runs through Canary Wharf.
Manufacture
The TBMs are being manufactured in Germany by Herrenknecht AG, with the first two already completed and shipped to London. The TBM components of these two machines are being assembled in Westbourne Park for the start of the first tunnel drive, from the Royal Oak Portal.
Each TBM weighs approximately 1000 tonnes and will be up to 140m in length with an external diameter of 7.1m. This allows for an inside tunnel diameter of 6.2m once the concrete tunnel segments are in place.
At the front of the TBM is a full face cutter head which rotates at 1 to 3 rpm. As the TBM advances forward the cutter head carves into the tunnel face, removing the spoil, and the machine slowly advances.
Of the other TBMs, two will be delivered to Limmo Peninsula in Docklands, two will be transported to Stepney Green in east London, and another will go to the Plumstead Portal site for the Thames Tunnel, the latter having two cutter heads.
Types of TBM
There will be two different types of TBM to reflect the differing ground conditions along the Crossrail route. Six will be Earth Pressure Balance (EPB) machines, which will be used for the main running tunnels between Royal Oak and Pudding Mill Lane.
These will pass through ground which is predominantly London clay, sand and gravels. The Thames Tunnel, which is predominantly constructed through chalk, will use Slurry TBMs.
“The EPB cutter heads on the front scrape and excavate the clay, which goes into a contained chamber behind, taken out by augur at the back of machine, onto a conveyor, and away in excavated material cars,” explains Chris Dulake, Crossrail’s chief engineer.
“This is a controlled process, so that we know exactly, how much material is coming out of the face, and how much volume is taken away, to balance the pressures in the face.”
Tunnelling
The machines will work 24 hours a day, continuously digging out spoil from the tunnel face, constructing the concrete tunnel lining.
“Once they've started, they'll drive at an average of about 100m a week” says Dulake.
“The overhead associated with the TBMs is high so driving tunnels quickly is important in keeping costs down.”
Each TBM will be operated by a ‘tunnel gang’ of around 20 workers – 12 on the TBM itself and eight working from the rear of the machine to above ground.
To help reduce the chance of settlement while the tunnels are constructed, the TBMs have to run nearly 24 hours a day, 7 days a week, 365 days a year. There will be scheduled breaks to allow maintenance on the TBMs to take place.
The eight tunnel boring machines will be used as follows:
- Royal Oak to Farringdon (Drive X) – 2 x earth pressure balanced TBMs
- Limmo to Farringdon (Drive Y) – 2 x earth pressure balanced TBMs
- Pudding Mill Lane to Stepney Green (Drive Z) – 2 x earth pressure balanced TBMs
- Limmo to Victoria Dock Portal (Drive G) – 1 x earth pressure balanced TBMs re-used from the Stepney Green to Pudding Mill Lane drive
- Plumstead to North Woolwich (Drive H) – 2 x slurry TBM
Western Tunnel
The western tunnel drive, between Royal Oak and Farringdon, will be the first drive, and will be the point where existing Network Rail services on the western section of the route enter the tunnels under central London.
The first two tunnel boring machines will embark from the portal at Royal Oak towards Farringdon, travelling 6.4 km. They will pass through the station sites at Paddington, Bond Street, Tottenham Court Road, and on to Farringdon east where the cutter heads will be removed and the machines dismantled.
The Royal Oak Portal, completed in September 2011, will be used for the construction of the new twin-bore tunnels, and in the future will provide the western entrance and exit for trains to the underground sections of Crossrail.
The portal site has now been handed over to the tunnelling contractor, a JV between Bam Nuttall, Ferrovial Agroman, and Kier Construction.
At Bond Street and Tottenham Court Road stations, spray concrete lining works will need to progress simultaneously with the main tunnelling. Where the TBMs pass Bond Street, a cross-over will be installed, to divert excavated material out of one tunnel bore and allow the creation of the station platform in the other.
Excavated material from the platform and running tunnels will emerge at Royal Oak Portal rather than be transported by lorry through London’s busy streets.
Eastern Tunnels
Tunnelling for the eastern tunnels – Limmo Peninsular to Farringdon (Drive Y), Stepney Green to Pudding Mill Lane (Drive Z) and Limmo Peninsular to Victoria Dock – starts slightly later than the western tunnels. The contractor here is a joint venture between Dragados and Sisk.
Limmo Peninsular to Farringdon (Drive Y) will be the longest of the 5 Crossrail tunnel drives, at 8.3km in length.
Two TBMs will launch from the Limmo shaft which will then head towards Farringdon east, through Canary Wharf, Whitechapel and Liverpool Street stations. This tunnel is scheduled to complete in Q3 2014 when the TBMs reach Farringdon station.
When the two TBMs for Drive Y pass Stepney Green, another pair of TBMs will start out from Pudding Mill Lane Portal near Stratford to tunnel towards Stepney Green (Drive Z). This 2.7 km stretch of tunnel is due to complete in Q3 2014.
While Drives Y and Z progress, two other tunnel drives will bore between the Limmo shaft and the Victoria Dock Portal (Drive G) in the Docklands area.
Thames Tunnel
To deliver Crossrail to Abbey Wood in Kent, a new tunnel will be built underneath the River Thames between Plumstead and North Woolwich – the Thames Tunnel. The contractor is a Hochtief/Murphy JV.
The Thames Tunnel will be 2.6km long and about 15m below the existing river bed.
It will have portals constructed at either end – North Woolwich and Plumstead – both of which begin this year. The first TBM will begin its drive in late 2012, reaching the North Woolwich Portal in 2014.
Connaught Tunnel
To connect the Crossrail branch of Abbey Wood to the Victoria Dock Portal, a major proportion of the construction work involves reusing disused rail infrastructure on the former North London Line branch to North Woolwich. This includes the Connaught Tunnel and disused tracks to Custom House, which both closed to passenger traffic in December 2006.
Vinci Construction has the contract to refurbish this infrastructure for use by Crossrail.
The Connaught Tunnel was constructed to 1878. It allowed the railway to be diverted under the Connaught Passage, a water link which connected the Victoria and Albert Docks.
The tunnel is around 550 metres long and runs between Royal Victoria Dock and Royal Albert Dock close to London City Airport. It will need to be enlarged to accommodate Crossrail trains and overhead line equipment.
Around 100 metres of tunnel wall will be removed and replaced with a new tunnel lining. The existing brick arches, part of the tunnel approaches, will be retained and repaired.
The roof of the central section of the Connaught Tunnel will be reinforced. It has been exposed since the Royal Victoria Dock was deepened in 1935 to allow larger ships to enter the dock.
A large concrete base will be installed on the dock floor to ensure the safety and stability of the Connaught Tunnel. Vinci will dredge a wide passageway in between the Royal Albert Dock and Royal Victoria Dock and install a 1m thick, reinforced concrete protection slab, measuring 1000m3 on the bed of the passageway. The slab will be placed using marine pontoons and with the help of specialist divers.
During this operation, the remainder of the brick-lined tunnel with be grouted and filled with a low strength foam concrete to ensure the integrity of the existing tunnel.
These tunnels will then be enlarged by boring through the foam creating tunnels large enough to allow Crossrail trains to pass. Precast concrete tunnel segments will be installed in this section of the tunnel.
Endings
If all goes to plan, the TBMs will complete their drives in the second quarter of 2014.
Let's hope they're given names befitting of the journey which lies ahead. If you have any ideas, Crossrail's TBM naming competition runs until 6 February.
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