Skanska brings a circus to London's musical mecca

The contractor is dodging Crossrail tunnels and preserving remarkable history on a major redevelopment described as a 21st century Piccadilly Circus.

The contractor is dodging Crossrail tunnels and preserving remarkable history on a major redevelopment described as a 21st century Piccadilly Circus.

Project: St Giles CircusClient: Consolidated DevelopmentsSkanska contract value: £126mContract type: JCT Design and Build (shell and core)Main contractor: SkanskaArchitect: OrmsStructural engineer: EngenuitiPiling subcontractor: Cementation SkanskaStart date: July 2017Completion date: January 2020

On a tight spot in central London where musical history meets the future of transport, Skanska is carving out a multi-layered mixed-use scheme to satisfy both.

The St Giles Circus development is designed to bring live music back to a corner of the capital that lost the iconic Astoria venue to Crossrail's Tottenham Court Road redevelopment.

A 2,000-capacity space suitable for gigs as well as other events will be created underground, close to the Northern line escalator box and the eastbound tunnel of the forthcoming Elizabeth line.

A raft of retail, office, hotel and dining facilities will be created on top of the four-storey basement to increase the scheme's viability, while a striking public thoroughfare and events space aim to help make this a destination development.

A second area of the project (Zone B) will see a major refurbishment of buildings on Denmark Street, which forms the boundary of the development and features a blue plaque for its rich musical history that saw it labelled Britain's ‘Tin Pan Alley’.

Artists including David Bowie, the Rolling Stones and the Kinks recorded in one of the studios on the street, which is now crammed with music shops. Publications New Musical Express and Melody Maker also reportedly started out there.

Skanska has used buildings along one side of Denmark Street for site offices and access, but will return them as ground-floor music shops, first-floor offices and residential units following completion.

The biggest challenges, however, lie in Zone A, primarily in digging out a major basement close to Crossrail, the Northern line escalators and Oxford Street.

Skanska project director Paul Roberts says St Giles is "as complex a building project as is happening in the UK". As such, the contractor spent more than two years working in a preconstruction services role ironing out the most efficient way to sequence the scheme. Configuring the basement build was a key part of this work.

"If there was no Crossrail, we could have piled all the way down, put propping in and dug it out," Mr Roberts says. "But it would have been prohibitively expensive to put in the layers of props necessary to use this method and not move the underground assets."

Project architect Andrew McEwan moved down to London from Glasgow to help deliver St Giles Circus, which he describes as "a once-in-a-lifetime opportunity" and "a career-defining project".

"The whole scheme is anchored in the culture of the area," he explains. "Tin Pan Alley and Denmark Street are famed for music. The client is interested in the theatre of the area, so the louvres on the urban gallery are like stage curtains that open up to reveal the building behind. The technical expertise that goes into creating bespoke building elements like this is amazing. There is a bit of masterplanning, a renovation of listed buildings and bespoke cladding packages with craftspeople."

Patience has been required to get to this stage, however. "We had planning consent in 2012 and then had to wait for Crossrail works to complete as well as procure the right subcontractors for the complex job. We’ve been lucky to find the right people," he adds.

To avoid either causing ground heave outside the strict tolerances allowed by London Underground and Crossrail, or busting the budget with costly temporary works, the project team came up with a plan to cast elements of the permanent floor decks early to act as supports.

This process also allowed use of skyscraper-style construction at times during the build, with work happening above and below ground simultaneously, speeding the programme as well as helping the project stay within its other parameters.

"It's a complex sequence," Mr Roberts explains. "We looked at it from a ground movement and programme point of view and worked through all the permutations during the preconstruction period.

"If there was no Crossrail, we could have piled all the way down, put propping in and dug it out"

Paul Roberts, Skanska

"In the end we decided to build two floors down to B1 mezzanine level to allow us to get bigger machinery in than if we stopped at lower-ground floor. That slab had to go in alongside a number of walls, then we could go down to B1 and B2 before we did the lower-ground floor."

Once the project started on site last summer, demolition contractor H Smith carried out a range of tasks including removing those structures that had not already been taken down for Crossrail. H Smith also undertook archaeology work, removed existing foundations that would have been obstructions to the new project, put in a piling mat, and erected hoarding.

More than 350 piles were required, taking five months to install during the second half of last year. "The deepest pile goes down 50 m and they are up to 1,200 mm in diameter," Mr Roberts says. "The basement is deep and we have tension piles to staple Crossrail down, with male secant reinforced piles put in using rotary technique and female CFA piles to form the watertight wall."

The western third of the basement is supported on seven 2 m-diameter piles, installed some years ago to facilitate development above the Northern line escalator. After a capping beam operation came the ground-floor slab pour. This was unusual in being designed to work for what's below it as much as what's above.

"The ground-floor slab has mole holes to allow us to work below and bring materials up and down," Mr Roberts explains. "We also designed it so we could drive on it with wagons. It was massively important for logistics as well as locking in movement."

Skanska St Giles Circus 4 Andrew Parish

Source: Andrew Parish

Skanska project director Paul Roberts says St Giles is "as complex a building project as is happening in the UK".

There were also key structural reasons for getting the ground-floor slab completed, he adds. "Four giant transfer beams went in this January to bridge over the box-in-box basement, which has minimal columns within it for usability and aesthetics. The whole structural concept is transfer structures as you go up. Everything is hanging."

The lift and stair cores for buildings A and B – two of the four structures that will sit on the basement – were built before the intricate process began of digging down two levels of basement beneath the ground-floor slab.

A carefully sequenced excavation process is in operation on the project. This sees 360-degree excavators weighing up to seven tonnes digging at the face, before Bobcat machines take the material from there to a ‘mole hill’ where a telescopic grabber – known as Heidi – brings the material up to the trucks on the slab to drive it away. "We average 30 trucks a day," Mr Roberts says. "In total we will have 27,000 cu m of muckaway."

Working conditions under the ground-floor slab required adaptations to be made. "The machines are rigged up to prevent exhaust fumes, we have carbon monoxide monitoring and there is strict segregation of people and vehicles," he says.

The dig was continuing when CN visited, with the concrete slab for B1 – the third level down from the ground – having been cast, along with the mezzanine level above.

A steel frame will be fed down in pieces and assembled along with acoustic isolators. The roof of the basement will be built and cast at B1 level before being jacked up to the underside of the ground floor in a method likened by Mr Roberts to constructing a ship in a bottle.

Skanska St Giles Circus 7 Andrew Parish

Source: Andrew Parish

More than 350 piles were required, taking five months to install during the second half of last year

Once basement construction is complete, with work continuing upwards above ground, attention will turn to the next major challenge: construction of the urban gallery and public events space. This structure appears to rely on four columns like table legs, except one is for show, sitting above the tube escalator and thus hanging from above rather than sitting on piles. To add to the construction challenge, the ‘fake column’ has to hold an extensive openable facade.

"The urban gallery will be like a 21st century Piccadilly Circus with four storeys of LED lights and 230 sq m of screens," Mr Roberts enthuses. "It has 22 louvres weighing four tonnes each that pivot and slide. There is polished, ribbed black granite on the frame and the gold-coloured cladding is PVC-coated stainless steel."

A hotel at the back of the space will act as a base for the hidden cantilever, and movement of the steel will be finely modelled and closely monitored as construction takes place.

The project has one challenge after another, but if the contractor can succeed in creating a fresh destination for music lovers in Denmark Street, then – to paraphrase Bowie – Skanska could be heroes.

Piling to create the all-important basement for St Giles Circus, and to pin down the Crossrail tunnel during works, required removal of a heritage-protected blacksmith's shop from the 1700s.

This was overcome by moving the smithy wholesale 25 m to a temporary location during piling, then replacing it on its original site close to a 1680s building that survived the Great Fire of London.

"We got permission to demolish a Victorian building between the smithy and 26 Denmark Street so we could reveal the two more significant buildings in their original plan form," says Orms project architect Andrew McEwan. "We are building a glass link building to allow us to go below the smithy and reimagine the former 12 Bar Club [which stood on Denmark Street] as a 21st century grassroots music venue."

Mr Roberts describes the delicate lift of the historic building in straightforward terms. "The demolition guys put a slab underneath it, brought a 750-tonne crane in and lifted it to the side of the site so we could do the secant piling for the basement," he says.