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Crossrail: On the blocks

Construction of the mammoth £14.5bn Crossrail project is on the cusp of full-blown construction, with the first tunnel boring machine ready to launch. Mark Hansford updates on progress.

Late last year Crossrail programme director Andy Mitchell told NCE that 2012 was set to be the year of tunnelling for Britain’s megaproject (NCE 15 December 2011). Then, Mitchell and all the team were eagerly anticipating the arrival of the first tunnel boring machine (TBM).

It has now arrived, is built, named, christened, and ready to launch from Royal Oak portal.

London mayor Boris Johnson and transport secretary Justine Greening carried out the christening earlier this month marking what Crossrail chairman Terry Morgan described as a “hugely significant” and “symbolic” milestone.

Eight TBMs

Over the next three years, eight Herrenknecht TBMs will construct a total of 21 km of twin-bore tunnel under the capital.

Johnson and Greening christened the first 150m long earth pressure balance (EPB) TBM Phyllis - after Phyllis Pearsall who created the London A-Z. It begins its journey from the Royal Oak Portal in west London this week, tunnelling 6.4km east to Farringdon via Bond Street and Tottenham Court Road.

Johnson and Greening named sister TBM Ada, which will follow slightly behind Phyllis and create the second bore to Farringdon.

“The sight of these mighty tunnelling machines is a significant step forward in the construction of this vital project”

Boris Johnson, London mayor

“The sight today of these mighty tunnelling machines primed for action is a significant step forward in the construction of this vital infrastructure project,” said Johnson.

“It’s exciting that we’ve reached this landmark for this world-class testament to our engineering excellence,” added Greening

Phyllis will take the lead in the race to Farringdon; once she has progressed sufficiently, the second TBM will be launched from Royal Oak.

Second machine

When the second tunnel boring machine has reached Paddington, both machines will progress forward through the Paddington station box to Bond Street and on to Farringdon.

The Bam Nuttall/Ferrovial/Kier joint venture has the £500M western tunnels contract to construct the tunnels between Royal Oak and Farringdon. The 148m long EPB machine with its 7.1m diameter cutter head will progress at about 100m a week, with its 980t weight driven forward by a thrust force of 58,000kN. Ten hydraulic steering rams will guide the TBM within millimetre precision of its designated alignment.

Later this year, Dragados/John Sisk joint venture will launch the second pair of EPB machines from Limmo in Docklands driving 8.3km west towards Farringdon via Whitechapel and Liverpool Street under a £500M contract.

In the winter, two more TBMs will be launched from Plumstead travelling 2.6km and under the River Thames to North Woolwich. The different geology beneath the Thames means that slurry TBMs will be used.

The final two TBMs will bore 2.7km from Pudding Mill Lane to Stepney Green, before one of these is re-used to drive the two 9oom bores between Victoria Dock and Limmo Peninsula in 2014.

To reduce the likelihood of settlement while the tunnels are constructed, the TBMs will have to run nearly 24 hours a day, seven days a week, 365 days a year. Stopping the 980t TBMs in the London Clay would increase the risk of ground settlement caused by the load imposed by the machines on the tunnel floor. To minimise this risk there will be scheduled breaks to allow for maintenance of the TBMs.

Tunnel gang

Each TBM will be operated by a tunnel gang comprising around 20 people - 12 on the TBM itself and eight working from the rear of the machine to above ground.

Excavated material will be removed from the tunnel face at a controlled rate and deposited onto a series of belt conveyors on the EPB machines or into pipes on the slurry machines. These remove the excavated material through the TBM and out of the tunnel.

As the TBMs advance, precast concrete segments will be built in rings behind the TBMs.

The TBM advances by using a series of hydraulic rams at the back of the cutter shield which push against the last completed precast concrete lining ring.

Construction of lining segments at the Old Oak Common concrete segment factory that will supply the western running tunnels started last month.

The plant will manufacture over 70,000 segments for the western tunnels. In total, 250,000 concrete segments will be needed for the Crossrail project, with a facility due to open in Chatham, Kent in the next few months. This will produce segments for the eastern tunnels.

Hanson is supplying the plant with concrete. With 12 plants along the Crossrail route, Hanson has already been supplying key stations on the route, including Canary Wharf (see box).

Bond Street postponed

Elsewhere on the project, the £200M construction contract for Crossrail’s Bond Street station has been postponed until the beginning of next year.

Sprayed concrete linings will be used to build the larger platform tunnels at stations and the shorter passenger circulation tunnels. The sprayed concrete lining is applied in layers to the excavated ground. This method of lining is slower to construct than using a TBM but is more suited to larger and more irregular shaped tunnels and tunnel connections. The tunnel lining will have a design life of at least 120 years.

Crossrail had said in December that the remaining central London station main works contracts — at Bond Street, Tottenham Court Road and Liverpool Street — would be awarded in 2012.

However, according to a report prepared by Crossrail for the London Assembly Transport Committee dated 3 February 2012 the award of Bond Street is now scheduled for the first quarter of 2013.
In the running are Laing O’Rourke, Lend Lease and joint ventures of Bam Nuttall/Ferrovial/Kier and Costain/Skanska.

The remaining station contracts for Tottenham Court Road is due to be let this year and there is a plethora of contractors chomping at the bit.

Return to Canary Wharf

Canary Wharf was where the first Crossrail spade hit the ground back in May 2009 and this week Canary Wharf Group completed construction of the £500M station’s platform level five months ahead of schedule. It is another significant milestone and makes the station ready to receive the two eastern tunnel boring machines in 2013.

Canary Wharf Contractors Limited, the construction arm of Canary Wharf Group, has created a station box that is more than 250m long and 30m wide, approximately the same size as One Canada Square laid on its side, Britain’s tallest operational building. Twenty-eight metres below the surface of the dock, twin 7.6m diameter rings are now in place at both ends of the station ready to receive the eastern tunnel boring machines.

Since May 2009 the construction team has driven over 1,000 piles and pumped nearly 100 million litres of dock water - the equivalent to 40 Olympic swimming pools. Approximately 300,000t of material has been excavated from beneath the dock bed and almost 375,000t of concrete poured.

This has been aided through the use of the UK’s only floating concrete batching plant. It was supplied by Hanson, and is located within metres of the construction site. The aggregate and sand used to produce the specialist concrete mixes have then been delivered by barge along the river Thames from Hanson’s Dagenham depot.

“By supplying the concrete from our floating plant we have been able to keep lorries off the already congested local roads and have helped to reduce the overall carbon footprint of the project,” says Hanson technical services manager Jasen Gould. “This principle of reduced lorry movements and increased investment in safety awareness is a key part of Hanson’s commitment to sustainability.”

The concrete has been designed by Hanson’s in-house technical team to comply with Arup’s specification for the project. All blends contain Hanson’s Regen ground granulated blast furnace slag (GGBS), a cement replacement that can reduce the greenhouse gas emissions associated with the manufacture of concrete by up to 50%.

In addition Hanson has supplied 30,000m3 of concrete for the piling works - C16/20 for the female piles and C32/40 for the male piles. In addition it has supplied around 60,000m3 for the station including a watertight concrete mix containing 70% GGBS, and a C45/55 high strength concrete for the columns.

After the TBMs pass through, Canary Wharf Group will fit out the station with work due to complete in 2015. Work will continue to construct the levels sitting directly above the station box.

 

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