Excavation work on Crossrail’s first station - Canary Wharf - is close to completion after just two years. Declan Lynch visited the site to see how work is progressing.
In May 2009 London mayor Boris Johnson drove the ceremonial first pile for the Crossrail Canary Wharf station. Great excitement surrounded the event as Europe’s biggest construction project got under way and a decades-old project slowly - finally - moved from drawing boards and computer screens to reality.
Now after two years, the world is looking rosy - excavation of the station box is close to completion; and the project has successfully navigated a change of government and last October’s Comprehensive Spending Review (CSR), which saw more than £1bn trimmed off the scheme’s total cost.
Rail minister Teresa Villiers said key to Crossrail surviving the CSR was proving to the government that the construction could “manage risk” better. The Canary Wharf station project team has done much to prove that this is possible.
“Adjusting the tension in the bars was like playing a harp. Once you adjusted the pressure on one tension bar, it affected the others around it”
James Brown, Arup
Canary Wharf is the furthest progressed new station build along the route, thanks in part to £150M provided by developer Canary Wharf Group. In 2008 the developer confirmed it would design and build the station at a fixed price of £500M, with the remaining funds coming from Crossrail.
“The finance is similar to a private finance initiative project and it was developed and signed off in just six months,” explains Canary Wharf Contractors operations executive Michael Bryant.
Station box excavation
Contractors are hard at work excavating the station box, which forms the outline of the station so the two tunnel boring machines can pass through next year. The box measures 256m long, 30m wide and 28m deep.
Construction gathered pace in June 2009, when foundations contractor Laing O’Rourke subsidiary Expanded Piling began installing 292 Giken tube piles to form the cofferdam wall in which the station sits.
Two Giken silent piling rigs installed the 18m long, 1.2m diameter piles, moving along like a “caterpillar” as described by construction workers. It is the first time the rigs have been extensively used in the UK, and they were an instant success, installing the piles to 5mm accuracy.
The piles were installed on the north, east and west sides to form the cofferdam with the forth wall provided by the existing cofferdam supporting the HSBC and Barclays towers with an additional secant piled wall.
Once completed in September 2009, Expanded installed a further 71 tension piles around the perimeter of the cofferdam to stabilise and prepare it for excavation.
“Installing the anchor piles eliminated the need for bracing and enabled the team to work inside the box without obstructions,” says Arup associate director James Brown. Arup is structural and building services consultant on the job.
Between each Giken and tension pile engineers then threaded a Macalloy bar to connect them. This allowed engineers to balance the box by adjusting the tension in each bar.
“Adjusting the tension in the bars was like playing a harp,” explains Brown. “Once you adjusted the pressure on one tension bar, it affected the others around it.”
Once balanced, about 100Ml of water was pumped out of the piled box structure to create a 256m x 30m cleared site ready for top down construction.
After a 1m layer of debris and sludge was removed from the bottom of the box, contractors installed 30, 80m tall plunge columns longitudinally down the centre. They then installed the first slab, which forms the top roof of the Crossrail station.
Around 300,000t of material was excavated to clear the box in March 2010. Around one-third of the material was reused on site with the rest taken by river barge to help transform a former landfill into a conservation site atHolehaven Creek on Canvey Island, Essex.
Once the ticketing hall level was excavated, a second slab was poured to form the roof of over platforms and the railway line.
A temporary blinding slab has been placed on the floor of the station box, which will be used to form the base of railway line.
“Installing the anchor piles eliminated the need for bracing and enabled the team to work inside the box without obstruction”
James Brown, Arup
Now that excavation is virtually completed, Canary Wharf Contractors is working towards handing the station over to Crossrail between August and October 2012.
Crossrail will then take over the site for about six to eight months as the twin TBMs will pass through the station from Woolwich and tunnel through to Farringdon. Once completed, the station will be handed back to Canary Wharf Contractors to complete the rest of the station’s construction.
Canary Wharf Group has already secured planning permission for a retail development above the station although it has not confirmed the final designs.
By 2018, Canary Wharf will have three major rail links to the rest of London, which its developers think is needed because the workforce is expected to double from today’s 93,000.
Once completed, the station will form part of the £14.5bn Crossrail network running 118km from Maidenhead and Heathrow in the west, through 21.5km of twin-bored tunnels underneath London, and on to Canary Wharf, Woolwich, Abbey Wood and Shenfield in the east.