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Going underground

Work to remove 20,000m3 of material from below Stepney Green Park is set to get underway this autumn as Crossrail constructs huge caverns to house crossover tunnels for the new link.

37196_Stepney_Green_Shaft

With Stepney Green in the running to be one of the largest Crossrail construction sites, you wouldn’t expect the main sounds in the area to be the bleating of sheep and braying of donkeys.

But even as work is about to begin on excavating four large caverns deep below the ground, it is still the sounds from the next door Stepney City Farm that fill the air.

Stepney Green is one of the largest packages of work on Crossrail because it is one of the most complex sections of the new rail link - the caverns will house the crossover tunnels that will allow trains to connect to both eastern terminals.

The interchange will allow passengers from the Essex end of Crossrail in Shenfield to connect with the Abbey Wood station in north Kent as well as destinations on the central and west sections of the new rail line.

Considerable effort has been made to liaise with the community about what is happening on the contract and mitigate the noise from the site

“Contract C305 involves taking the tunnels from Pudding Mill Lane through Stepney Green to Farringdon - known as the Y-drive and measuring 8.3km, driving the Z-drive tunnels 2.7km from Pudding Mill Lane to Stepney Green and the 900m G-drive from Limmo to the Victoria Dock Portal, plus seven cross over passages,” says Crossrail project manager Peter Main.

But it is not just the engineering that makes this a difficult site - the site is close to residential properties and schools, and sits partly on the farm’s land and Stepney Green Park.

Considerable effort has been made to liaise with the community about what is happening on the contract and mitigate the noise from the site, particularly as the cavern excavation work will operate 24 hours a day, seven days a week.

In addition to fitting secondary glazing to nearby properties and installing noise monitoring systems around the area, Crossrail has recently unveiled the results of a £1M investment programme to upgrade the next door farm.

The funding has helped to provide an improved layout, new barn and workshops at the farm - negotiated in return for using part of the site during construction - and is the first major community project to be delivered by the project.

Work in Stepney Green started in March last year and the result is a 15m wide, 60m long and 34.5m deep shaft. Diaphragm walls have been constructed to 52m depth by Carillion and Spie to create the shaft using 1.2m thick diaphragm wall panels excavated by a grab.

42011_Stepney_Green_July_2012

The shaft will be the main access for constructing the caverns and will house M&E equipment when the rail line is completed. The shaft is heavily reinforced and Main says it contains as much steel reinforcement as a nuclear bunker.

In total 1,100t of steel reinforcement was used for the diaphragm wall construction and around 4,000m3 of concrete has been used to construct the shaft and the temporary props, walers and base slab.

Around 500t of temporary props have also been used to provide support while the 31,000m3 of material was excavated from the shaft to take the work to the cavern construction level.

The shaft will also allow for intervention of the TBMs on the various drives that will pass through the site (see box). “We have done really well to get to this stage of the work but the work is only really just beginning,” says Main.

Ground conditions at the site comprise London Clay and work is underway to dewater the area ahead of the cavern construction. A network of 45 boreholes will be constructed by WG Groundwater.

A further 60, 55m deep boreholes have been constructed and instrumented to monitor the area for ground movement as the excavation of the caverns progresses.

“We have done really well to get to this stage of the work”

Main says the ground conditions preclude the use of a pilot tunnel for excavation of the caverns, so the site team will be using Brokks to excavate the material using top heading benches with side wall drift, moving towards double side wall drift as the excavation widens.

In the lead up to starting the excavation work in the next few weeks, the designs of the sprayed concrete lining (SCL) that will be used for the caverns is being finalised.

When GE visited the site a Meyer spray machine had recently completed spraying trial mixes onto test panels at the head of the shaft to check the mix design for strength and workability.

A concrete batching plant is also being set up on the site to ensure a steady supply of material for the SCL work and to prevent a stream of delivery trucks from causing noise and congestion on local roads.

Main says excavation of the caverns is due to start in October and he expects it to take a year to complete the 20,000m3 excavation and application of the SCL. “We are currently ahead of schedule,” he says and is confident that the careful planning already undertaken means the work will stay on track too.

42010_Stepney_Green_July_2012

Work to insert TBMS starts

Elsewhere on Crossrail’s C305 contract activity is also ramping up. Two TBMs - Elizabeth and Victoria - are being constructed at the Limmo peninsula and were due to be lowered into the ground in October using a 1400t crane.

Two shafts have been constructed at Limmo for the launch of the TBMs. The cutter heads and drive units will be lowered into a temporary 27m diameter, 37.5m deep auxiliary shaft to start a flying launch towards the main 30.5m diameter shaft. Once installed both will be launched towards Stepney Green.

One TBM will be driven from Pudding Mill Lane towards Stepney Green before being removed and taken to Limmo to complete the G-drive. A second TBM will then be launched to drive the second tunnel from Pudding Mill Lane to Stepney Green.

More than 120,000 precast concrete tunnel lining segments will be needed to line the tunnels and these will be brought to the Limmo site by barge from the specially developed precasting factory in Chatham, Kent.

The return journey for each barge, carrying a single ring, will be four hours but the barges are being shared with contract C807 so will return from Limmo carrying excavated material.

There is space set aside at Limmo to stockpile the concrete segments but the rings will be brought in by road if construction rates outstrip the ring supply.

“The journey is only two hours by road but by using barges we are removing 30,000 lorry trips from the road,” says Crossrail project manager Peter Main.

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