Massive geotechnics and intricate logistics have ensured one of London Underground’s most complicated schemes keeps the project team on its toes.
Project: Tottenham Court Road East redevelopment
Client: London Underground
Contract value: £300m
Contract type: NEC Option C – Target cost
Main contractor: Bam Nuttall
Main contractor: Taylor Woodrow
Start date: January 2010
Completion date: September 2016
If you’ve ever been shopping on London’s Oxford Street in the days leading up to Christmas then you’ll have some idea just how busy this part of the capital can get.
Thronged with frenzied bargain hunters, last-ditch present purchasers and stressed-out businessmen, the street is the epitome of crowded city life.
At one end of Oxford Street lies Tottenham Court Road underground station, which plays its part in ensuring the shopping masses reach their respective retail terminus.
More than 145,000 people pass through the station on an average day, and more than 40 million each year.
According to London Underground figures, five people are in competition for every square metre of station at peak travel times.
This extremely high level of footfall is only going to increase when the station becomes an important staging post on the Crossrail route.
A £480m revamp
Thankfully the £480m revamp of the station currently under way will allay any overcrowding fears and engineers are well on their way to delivering one of the key milestones for the project.
A joint venture partnership between Vinci’s civil engineering arm Taylor Woodrow and Bam Nuttall has been working on the scheme since 2010, which will see the new station and ticket halls increased to six times their current size, providing step-free access and easy ticketing for both the Northern and Central underground lines as well as the construction of a concrete box.
Known as the Goslett Yard, this section of the site is the embryonic Crossrail Eastern Ticket Hall. Eventually this will become the connection point between the new ticketing hall and the future.
Giant-sized ground engineering
The new London Underground station, which will be linked to the Crossrail station, has seen a series of plunge columns and huge diaphragm walls installed. With a depth of 45 m and a steel ‘H’-section weighing more than 60 tonnes in each, these plunge columns are bigger than those installed at The Shard skyscraper and were placed through a bespoke steel guide frame lowered down the pile borehole and set to the plunge columns tolerance.
The H-section was then lowered into the guide frame, which holds the section at the correct level to give 10 m of embedment into the concrete pile. An array of shear studs on the web and flanges of the steel section helps transfer the loading.
Similarly, the diaphragm walls themselves have been installed to 45 m in 3 m-wide, 1 m-thick panel sections, which feature geothermal pipes that future developments will be able to use.
On the corner of the site, where the new entrance to the tube station will draw travellers down into the ticket hall, the team has installed 300 mm-thick precast concrete beams that form the roof. A thin layer of concrete was placed in a single 200 cu m pour on top of the 17.5-tonne beams at 2.5 m centres to complete the roof slab for the ticket hall.
Installing these beams is just one aspect of the project that needed to be controlled by the robust site delivery and logistics programme set up for the site.
“It’s a very congested site and we are bringing material through the centre of London”
Chris Usher, Taylor Woodrow/Bam Nuttall
“Moving materials has been the biggest challenge,” says Chris Usher, project director for the Taylor Woodrow/Bam Nuttall joint venture. “It’s a very congested site and we are bringing material through the centre of London.”
Thankfully the team has a holding bay a few miles to the east of the project at Holborn and is able to call material off when needed.
Casting the escalator box
Further into the station, the 8.1 m-wide, 6.6 m-high and 1.1 m thick 30 deg sloping box for the escalators, which will eventually connect at the southern end of the Northern line platforms, has been cast within the 1,050 mm-diameter secant pile walls of the escalator box itself.
These are dwarfed by the 2 m-diameter piles for the over-site development, which have been pushed 62 m through the thick layer of London Clay and bear onto the layers of solid chalk way beneath the main part of the construction site.
At the bottom of the escalator box the tunnel widens into the 11.5 m-diameter machine chamber. This will become the plant room for the escalators and provide the entrance to the cross-over tunnels to the Northern line platforms.
Diaphragm wall breakthrough
In one corner of the escalator box, the team has had to break through a section of the diaphragm walls into the Goslett Yard where the team are working on the Crossrail section of work that has been wrapped up as part of the scheme.
This is to allow materials to be moved in and out of the area and will be reinstated once work has completed.
Further on into the seemingly never-ending labyrinth of tunnels, lift sumps are being eked out of the clay and site workers are completing the connection chambers. It still seems a long way from the day that will see passengers using the new Oxford Street Plaza entrance to the station.
“Pedestrians will be able to use the new Oxford Street entrance toward the end of 2014. The official programme date to hand over the first phase is 5 January 2015 and we are well on target to achieve that,” Mr Usher says.
From then the focus will be on delivering the second phase of the tube station project and tying it in with the first, before the underground station is fully completed in September 2016.
And with the first Crossrail services set to begin two years after that, there will still be plenty of work continuing beneath Oxford Street’s shoppers.
Tunnel drives the site
Tunnelling underneath Central London is a delicate affair. The team at Tottenham Court Road is using the side wall drift method to excavate the 10 m-diameter tunnels beneath Charing Cross Road that will provide the Northern line concourse tunnels and the 5 m cross passages between the tube lines.
“Fortunately we are well into the layer of London Clay, which is ideal for tunnelling – there has been no need for any ground treatment,” says London Underground programme manager Les Hamillton.
The team is using the side wall drift tunnelling method where the cross-section of the tunnel is split into a total of six sections, three on each side of a vertical divide through the 10 m diameter.
These smaller sections are called the top heading, bench and invert and are excavated in 1 m ‘pulls’, with the top heading advancing first and the bench and invert pulls following.
Each pull is supported with spray-applied concrete, which creates a temporary sidewall between the two halves of the tunnel. This final section is removed when the two halves of the tunnel are fully excavated and creates the tunnel’s full bore.
Initially the plan had been to use steel propped and mesh-applied lining throughout the tunnels, but a change in process has brought increased productivity.
The JV team is using a two-stage reinforced lining on the tunnels, with the primary lining being a 300 mm-thick spray-applied steel fibre reinforced concrete lining.
It is then followed up with a 3-6 mm-thick waterproofing membrane and the 350 mm-thick secondary cast in-situ lining. This secondary lining is poured in three stages – base to knee, then the pour to tunnel shoulder and finally the crown. Reinforcement steel within the lining itself is at 150 mm centres and 16 to 20 mm in diameter.
“The soffit shutter is on hydraulic jacks and we pump from ground level,” Mr Usher says. “The distance we have had to pump varies but it’s probably been no further than 150-200 m.”
Northern line shows the way
One of the most important aspects of the work at Tottenham Court Road underground station project was probably the least noticeable for the travelling public.
The work took eight months to complete and meant the station was closed to Northern line passengers during that period.
But it’s fair to say that passengers using that section of the station now probably wouldn’t notice the complicated engineering work which was going on during that time.
Where the two Northern line tunnels enter and exit the station they run parallel to and are at the same level as one another.
With the access tunnel set to run longitudinally between the two, the engineers needed to steal an extra few centimetres of room to squeeze in the access tunnel.
This they managed by straightening the circular section of the tunnels at their shared side, striking a vertical chord through the circular section of the existing tunnels to provide enough room for the bore of the new access tunnel to pass through.
Exiting iron tunnel segments have been removed – 195 in total – and replaced with more than 1,000 tonnes of specially manufactured straight-sided ‘D’-shaped segments.
On the Central line platforms, two new access tunnels that bridge the existing platform have been excavated beneath one of Sir Joseph Bazalgette’s famed Victorian London sewers.
A raft of spiles, formed from an adjacent tunnel, protected the sewer while the overbridges were completed. This was achieved by building their decks through the platform tunnels’ upper arches during weekend possessions.