Existing piles have been reused to support a new steel frame on the £60m commercial scheme that marks AXA’s return to the City of London’s office market.
Project: 60 London
Client: AXA Real Estate
Project value: £60m
Contract value: £50m
Main contractor: Balfour Beatty
Engineer: Capita Symonds
Start date: March 2011
Completion date: June 2013
The Holborn Viaduct on the western fringes of the City of London opened in the 1860s and was built to carry traffic across the valley of the River Fleet into the city itself.
Since then the cityscape has continually changed around it and now the venerable old bridge is set to get a new neighbour on its doorstep.
The 60 London development for client AXA Real Estate is under construction on the north-east side of the bridge and marks the developer’s first foray into the London office market for several years.
Timing reflects fertile market
“We feel that the time is right for this scheme,” says AXA Real Estate director of development Harry Badham. “We haven’t done much in the City for the past couple of years and we took the view that if we could get it up and running quickly we would be building into an undersupplied market.”
“It has been heavily modified, primarily to make sure we are getting the best, most efficient floor plan and making it more energy-efficient”
Jeremy Wilkins, Capita Symonds
With speed being of the essence, the site at Holborn was identified and snapped up by AXA, with planning permission to demolish the existing 1960s reinforced concrete building and redevelop the site already in place.
Purchased in March 2011, the design team were straight onto the site developing the stage C design. “It has been heavily modified – primarily to make sure we are getting the best, most efficient floor plan and making it more energy-efficient – both of which should make the offices more lettable,” says Capita Symonds project director Jeremy Wilkins.
The building that had been granted planning permission had been designed to Part L 2006 standards, but now the team has beefed up this design to meet the more stringent 2010 revision of Part L (Conservation of Fuel and Power).
With prime City rents being what they are, AXA has put a definitive completion date on the scheme. There can be no overruns and the project must be handed over on the 23 June 2013.
That tight a target has seen contractors and specialists brought onto the site by the client to try to cut down time lost during the tendering process.
Pile reuse saves time
With the old building demolished to ground-floor level and enabling works contracts then drawn up with contractor Skanska, it was the project team that took the decision to re-use as far as possible the existing network of piles to support the new building.
The final pile design includes 107 of the original building’s 15 m-deep, 900 mm-diameter to 1.5 m-diameter under reamed piles that are being reused alongside an extra 63 new straight shafted bored piles 1.5 m in diameter at depths of up to 27 m.
“A completely new piling scheme would have meant the rest of the contract being squeezed for time, so there were concerns”
Harry Badham, AXA Real Estate
Specialist subcontractor Balfour Beatty Ground Engineering was brought in to install the pile design. There were a few sleepless nights for Mr Badham and the rest of the project team before BBGE confirmed the piles and issued warranties for them.
Since then it has been full speed ahead on a project that has seen Balfour Beatty move in on the scheme under a £50 million design-and-build contract for the main work, which, added to the £7m contract sub-structure work, should see AXA’s overall project investment peak at £60m.
“A completely new piling scheme would have meant the rest of the contract being squeezed for time, so there were concerns,” Mr Badham says.
“We did have several specialists look into the re-use of the piles and there was a major testing regime, so we were very well informed. The new building is a very much reduced load, too.”
Of course, one of the downsides of incorporating existing piles into a new project is that invariably they are not exactly in the optimum location for the structure. At 60 London this meant the site team needed to install a raft over the top of the piles rather than pile caps.
The depth of these ranges from 800 mm to 1.8 m as the raft lifts over the site, with the basement and lower ground floor levels similarly cast in reinforced concrete.
Steel over concrete
“The steel springs from ground-floor level and there are around 2,200 tonnes being installed across the project,” says Balfour Beatty project manager Andy Clarke.
“There was never any question of using concrete for the frame – it never got a look in”
Andy Clarke, Balfour Beatty
And the use of steel to construct the frame significantly reduced the overall loading of the structure, ensuring those 107 existing under reamed piles are actually carrying a much smaller load than they were first designed for back in the 1960s.
“There was never any question of using concrete for the frame,” he says. “It never got a look in. Here we are creating an open floor plate on nine levels above the Holborn Viaduct. Reinforced concrete would have been just too heavy.” The primary grid for the 600 mm-deep steel beams is 12 m by 9 m, but with interim beams at 3 m centres.
A range of cladding solutions
The building frame is clad in a variety of systems. Precast concrete by specialist supplier Techrete has been inched into position on the eastern elevation, Portland Stone for the recreated gatehouse on the Holborn Viaduct (see box below) and a unitised aluminium/glass glazing system on the remaining facades (see box, bottom).
These systems have been brought onto site using the two 40 m-radius luffing tower cranes capable of easily lifting the 8-tonne end wall Techrete precast units.
It has been an exacting year since the team first arrived on site and the swift pace of the build shows no sign of letting up. But with the project delivery date indelibly inked in to the contract, there will be no rest for the project team until the keys to the building are handed over.
Rebuilding one of Holborn’s gatehouses
As part of the planning permission for the scheme, the team on the project have had to reinstate one of the four original gatehouses. These stood at each corner of the Holborn Viaduct that contained stair and lift access for pedestrians from the viaduct itself down onto Farringdon Street below.
Although there are three of these standing, only two are original, with the one on the north-western corner reconstructed a few years ago.
As well as glazed facades and brick cladding within, these gatehouses boast ornate Portland Stone cladding, with gargoyles, statues and figureheads on their outer elevations – all of which is a bit of a departure from the norm for the site team.
“It is something a little bit different for us,” Mr Clarke says. “Not what you would normally expect on a project like this.”
Working right alongside the listed structure of the Holborn Viaduct presents different challenges too – particularly when the foundations of its 12 m-high brick-built vaulted abutments and service tunnel are at the same level as the foundations to the new building.
“We have had to be very careful when working alongside them,” he says. “We’ve been casting walls right next to them – it has been difficult.”
Glazed cladding sections turn the corner
Across the main structure, architect KPF has specified a modular glazed system with steel fins. Their effect is to make the building catch the sunlight at different angles while also acting as a brise soleil. But in many sections and particularly on the upper floors these feature glass curved at amazing angles.
These units are supplied by specialist Seele and were procured in a two-stage appointment process, begun by Capita Symonds in April 2011, in a bid to help squeeze the lead-in time.
“Initially there were five companies that expressed an interest, but we whittled that down to two after the first stage, before Seele took it,” Mr Wilkins says.
Getting the subcontractors on board before the main contractor took over is a different approach for the design-and-build contract, but Mr Clarke is relaxed about it.
“To be honest, at this level there are only a few specialists that can supply, and most are on our supply chain anyway,” he says.