It is almost Christmas and Oxford Street is a maelstrom of shoppers and sightseers, pretty lights and petrol fumes. But 24m below the hustle and bustle, deep inside the cavernous tunnels of Crossrail, the only sound is the drone of ventilation machines, punctuated by the clatter of metal and the buzzing of hand drills.
I’m walking through one of the passenger tunnels being constructed as part of the £1bn Crossrail station at Tottenham Court Road and it is difficult to communicate the awesome scale of the space. It feels like I’m standing inside a whale — the cylindrical sprayed concrete walls are covered by a rib-like framework of metal cladding rails and, at 10m across, the tunnel is almost double the diameter of those on the London Underground.
Engineer design Arup/Atkins
Main contractor Laing O’Rourke
Urban realm designs Atkins/Gillespies/AHMM
Oversite development partner Derwent London (One Oxford Street)
Tottenham Court Road is arguably the most complex and challenging of the 10 new Crossrail stations in central London being constructed for the £14.8bn Elizabeth line, running between Abbey Wood and Paddington and due to begin operation by the end of 2018.
The project involves construction of a new station entrance and ticket hall to the west, on Dean Street, and an integrated ticket hall to the east, below St Giles Circus, at the intersection of Oxford Street and Charing Cross Road (under the famous Centre Point tower).
The integrated ticket hall will be six times larger than the original London Underground station on the site, designed to absorb the estimated 200,000 people expected to use the station each day, 50% more than today. The station will boast the longest and only curved platform tunnel on Crossrail – measuring 230m, the length of three football pitches – and the longest escalator.
“Londoners do not know what is going to hit them, the scale of this project is enormous. I think most people are expecting another Jubilee line, but this is immense,” says Harbinder Singh Birdi, partner at lead architect Hawkins\Brown, which designed the station in collaboration with Crossrail, London Underground and consulting engineers Arup, Atkins and Halcrow.
As heavy civil construction work draws to a close, main contractor Laing O’Rourke and its small army of subcontractors are getting their teeth into a major fit-out project involving a wealth of cladding systems, electrical and communications cabling, control systems, signalling and other equipment.
Phil Jones, construction manager at Crossrail, told CM: “Three quarters of the overall project is complete, but the final 25% is the most condensed period of the programme. The challenges have really changed risk-wise, with a greater emphasis on health & safety – there’s a lot more work at height and manual handling – and managing multiple trades in the same area.”
The cylindrical sprayed concrete walls of the passenger tunnels are covered in a rib-like framework
The project was a lot less advanced back in the summer of 2015, when tunnelling and portal and shaft excavation work drew to a close and heavy civil construction began.
Behind the hoardings at the western ticket hall, at Dean Street, operatives would have been staring into a gaping six-storey hole, criss-crossed with a network of hydraulic mega-props, required to prevent the surrounding streets from caving in.
Brian Keane, site manager for the western ticket hall, told CM: “The biggest civil engineering challenge was to prevent the diaphragm walls from collapsing. The giant 1.2m-wide hydraulic props had to be installed at different angles throughout the space and monitored for any movement or loss of pressure.”
This end of the development was built from the bottom of the excavation up, mainly to facilitate access for the tunnelling work. The station box under St Giles Circus, meanwhile, was constructed top-down, to reduce the requirement for back-propping needed to reduce ground movement and the potential for damage to adjacent buildings.
The concrete frame for the former was built one level at a time: first, the columns and floor slab above were cast, then the lattice of mega props below were “burnt out” (cut into sections), hoisted out onto a truck and removed from site. It took a team of up to six men around three weeks to clear each level of props.
Building on a constrained site has been a challenge
Almost all concrete structures on the project were built in-situ, despite Laing O’Rourke’s preference for the use of prefabrication, explains Julian Robinson, head of architecture for Crossrail: “The original engineering design was for in-situ and couldn’t be changed by the time Laing got involved. The way forward, on schemes like Crossrail 2 and other projects, is to have the contractor on board early and get buy-in from engineers and the client to prefabricate everything, even the primary structure.”
One key challenge, he adds, is working out how to build huge structures in constrained sites very quickly and efficiently.
Construction of the tunnels began shortly after the giant boring machines were removed. A layer of concrete was poured across the base of the cylindrical hole to create the track bed and adjacent platforms. A mid-section of the concourse tunnels will permit the future intersection of the £32bn Crossrail 2 project, the first tenders for which are due shortly.
Each platform is designed to handle 200m-long trains that are able to carry up to 1,500 passengers in nine interconnected carriages. They incorporate concealed vents (the height of a 6ft 4in journalist!) that channel searing hot air produced by trains along to the ends of platforms and up to the surface through huge ventilation shafts. The shafts will be fitted with 9.5m-diameter fans and run through new buildings constructed at ground level above each ticket hall.
Full-height glass safety screens, similar to those installed on the Jubilee line, will separate the tracks from the platforms. The platform doors will be fitted trackside from a special train truck, in an innovative automated process intended to simplify logistics. This “tilt and fix” procedure is being rehearsed at a mock-up Crossrail platform created at a facility in Milton Keynes.
Keane explains: “When Laing O’Rourke’s contract ends, our delivery ‘mole holes’ from the surface will be closed up, so any large deliveries for follow-on contractors, including the platform doors, must come in via rail.”
The project is three-quarters complete
The passenger tunnels and concourse will feature a limited palette of naturally self-finished materials, used throughout Crossrail, designed to be durable and require little maintenance. These include precast concrete, glass, anodised aluminium, stainless steel, and sleek white glass fibre reinforced cement (GFRC) cladding.
The 40mm-thick GFRC, a mixture of solid and perforated units intended to improve acoustic absorption, is just a third of the thickness of the precast concrete alternative and will flow across the tunnel walls and the complex curved junctions between passageways.
The fluid geometry was made possible by the sprayed-concrete tunnel construction technique – super-heated material machine-sprayed onto surfaces at around 350mph – there are no sharp angles and the cladding fits tight to tunnel walls on a secondary subframe.
Computer modelling the cladding was a time-consuming exercise, says Robinson: “With tunnelled structures you often have to deal with non-standard geometries, where the tunnel shifts position to avoid underground obstacles. That creates a number of unique conditions that require unique panel shapes.”
As installation progresses, some sections of wall are already covered with cladding, while other areas are marked with spray paint where the concrete had to be stripped back to enable cladding rails to fit snugly.
Cladding at the Dean Street entrance
As passengers travel up escalators from the underground concourse to the ticket halls, they will transition from the homogenised Crossrail aesthetic into spaces bespoke-designed to respond to the local context of each ticket hall.
The eastern ticket hall is light and bright, taking cues from the Centre Point building directly above and other 1960s architecture in the locality. The neighbourhood’s heavily gridded street plan is reflected in an exposed structural concrete soffit with its large downstand beams.
Striking twisted glass-and-steel canopy entrances form part of an area of urban realm, designed to create a new pedestrian connection to Covent Garden, where the Underground station is close to capacity and unable to expand due to its Grade II-listed status.
A tradition for integrated art in Tottenham Court Road Underground is continued inside the ticket hall with colourful and monochrome artworks by French artist Daniel Buren. In addition, the concrete soffit forming the ceiling of one of the escalator boxes will be covered by an applied gold-leaf artwork by Turner Prize-winner Richard Wright.
The ticket hall as it will look when complete
The opposite end of the station will feature mostly black and gold finishes, intended to evoke the vibrant night time culture of Soho. Internal walls will be covered by 1.9 tonne slabs of black reconstituted stone cladding.
The process of installing the around 90 slabs on the wall next to the escalator, each measuring 3m high by 2.2m wide, is an ongoing challenge as operatives work from the steep escalator incline.
Lifting chains from a crane positioned on the floor above feed down through holes drilled through the concrete soffit and deliver panels to operatives working from an incline hoist positioned on the escalator slab. The panels are manually fixed to steel wind posts attached to the concrete walls.
Keane comments: “We had to get engineer buy-in to the idea that, by drilling through the 750mm-thick slabs, we were not weakening structural integrity.”
Harbinder Singh Birdi, Hawkins/Brown
The same cladding will be used on the facades of a new six-storey residential block, due to be constructed above the ticket hall when the contract to build the station is completed. This block will enclose the ventilation extract tower and feature projecting window reveals and pleated “gold” anodised aluminium spandrel panels.
The ground floor of the western ticket hall is a condensed intersection of different trades – cladding installers work alongside steel fixers, carpenters and operatives striking out the concrete decking.
The high level of congestion requires a strong emphasis on health & safety culture and regular meetings with subcontractors to plan the phasing and re-phasing of work packages.
“A key part of that is how we manage materials coming on site,” says Crossrail’s Phil Jones. “We don’t have a huge storage area, so the emphasis is on just-in-time deliveries and installing straight away. Cabling is a significant task in its own right, particularly around preventing damage. A tiny nick in a 100m length of cable can cause serious communication problems. All these things add to the complexity of interfaces we have to manage.”
Although the project is engineer led, engineer, consultant and contractor teams are co-located in the same office, to improve collaboration and design coordination. This set up will be key when it comes to the overarching task of connecting the station with nine other new stations, and the wider network of 43 external stations and all their related systems, due to integrate into Crossrail.
As Crossrail’s Julian Robinson puts it: “There’s a lot of wire and a lot of pipe, a lot of kit that has to be in the right place, connected to the right things, at the right time.”
It’s a Herculean job, made all the more impressive by the fact it will all be carried out unbeknownst to the legions of shoppers, tourists and workers on the streets above.
Once the stone cladding on the escalator wall is in position, work will begin on the escalator itself, the longest on Crossrail, with a 47m incline.
The escalator will be delivered by lorry in 11 separate truss sections, each one craned to the top of the incline, then lowered down using a winch. The bottom-up installation will begin with the tension carriage (the bottom truss section), followed by the incline sections and the main drive section.
An incline hoist will deliver sections weighing less than three tonnes, including the controller inverters, the balustrade, step chain, steps and floor plates.
Marijan Harris, section engineer for the western ticket hall, told CM: “Lifting in the escalators is a big challenge as the sections are so heavy. We are currently working to get the floating concrete slabs in the ticket hall cast so work on the escalators can begin.”