Neat trick

Appearances can be deceptive. A quick look at the outside of the £34.6m Waverley Gate project and you'd think not much has changed since Building last went on site more than a year ago (see 17 January 2003, pages 52-54). From Edinburgh's busy Princes Street you can see the same mass of scaffolding interspersed with red structural supports, giving no clue as to the progress inside.

This is the largest retained facade project in Europe. It will involve a glass-walled box, designed by local architect Hugh Martin Partnership, being placed inside the facade of a 1861 grade A-listed former post office. The 215,000 ft2 office is to have two central cores, eight floors of office space and another two floors for car parking and plant. It is a steel-framed structure founded on a mix of piled foundations and foundations dug into the rock and reinforced with concrete. And all this has to be done by Christmas 2004, and work is progressing 24 hours a day.

No room to swing a Caterpillar
Balfour Beatty, the main contractor, is 51 weeks into the construction stage, with another 39 to go. It now faces two challenges. The first is that the site is bordered by three main roads and Edinburgh's central train station, so absolutely all the work must take place within the building footprint. Since the floor was excavated 10 m to create two new floors, it has not even been possible to drive vehicles onto the site, so everything has to be brought on and off site by crane. Three were needed to cope with the quantities involved. "We have no room for manoeuvre," says Mike Kidd, Balfour Beatty's project director. "There's no extra space around the building for storage."

Inside that footprint the internal structure and roof were demolished within a year; Balfour then signed up for the next stage of the project, excavating two floors below the basement level using contiguous piled walls and constructing the new building inside the shell of the old.

Before any of this work could start, however, the contractor had to solve the second of its main problems: how to secure the ornate stone facade. It was vital that the listed stonework remain stable throughout the demolition, excavation and construction work, so a huge and complex facade retention system had to be set up. Altogether, the supports weigh 600 tonnes – and so far, they seem to be working. Kidd's engineers take measurements around the site every two weeks to monitor the facade's position. "We're passed the point where movement was most likely – for example when we removed the old building's cross-walls and floors, and when we excavated 10 m below the original floor level. All those things could have caused problems," says Kidd.

But they are not out of the woods yet – they still have to remove the supports. Kidd says: "The logistics of getting rid of all of those is really quite something. We can't remove them until the floors are poured and the facade walls are tied in using steel frames, which are bolted onto the new structure. The first pieces will start to come out in three weeks' time, and it will all be out by July. Our completion date is Christmas so it's a tight programme."

Erecting the temporary support system has been problematic. No stress can be put on the neighbouring train station roof, so on that side of the building the supports have been cantilevered to put all their weight on the pile caps.

Kidd explains: "We put beams out of the facade's windows, which are tied to beams internally, which in turn are fixed to the pile caps so that all the weight is taken off the station roof. It's not difficult to do; the complication is when we take it out again – we'll be threading it into a live building so there's no room for error."

Danger: High voltage The station has caused other problems – construction and restoration work on that side of the facade can only be undertaken once a working scaffolding platform is constructed above the busy railway line. And this is proving tricky, as construction can only be carried out when the live rail is turned off, and that happens only between 1am and 4am. "Once we get the scaffold up they'll let us work during the day, but we have to get it up first," Kidd says. "There's a third of it up now, it will take another four to six weeks of night work to complete it. Then there's three months' restoration work to be done. And then we take all the scaffolding down again slowly."

Core blimey
Retention and restoration of the facade is only part of the story – in addition to all this, the business of constructing the building must be addressed. There are two central cores, created using slip-forming to speed up the process. This involves erecting a frame and pouring concrete inside, leaving it to set, then jacking the frame up a level and adding more concrete. It took just nine days to complete the first core – a feat achieved by working 24 hours a day. The external frame and facades will be pinned to the cores for support, so it is vital they are completed not just quickly but perfectly.

Balfour hit a stumbling block two weeks after starting the second core, however, when their subcontractor went into receivership. Kidd then took a brave decision: "We decided to take the work on ourselves – we employed some of the subcontractor's staff and got it finished quickly. We saved about four weeks by doing this rather than finding another subcontractor."

Keeping beauty
In addition to the jobs of demolition, excavation and construction, Balfour Beatty is undertaking restoration. The facade must not merely be retained, but also be repaired and restored to its original glory. "There's quite a bit of work to be done on the outside of the building," says Kidd. "We saved all the facade's windows during the demolition. We're refurbishing any windows we can, and for the others we're making replicas. The new windows have been inspected by the planners – you really can't tell the difference."

Kidd will oversee a complicated exercise in stonework later in the project – changing the position of several of the original windows on the east side of the facade. "The client wanted to have more floors in this building than there were in the original, so the windows in the facade don't match up with the new floor levels," he says. "So the new building has been designed as a glass box within the original walls. There's a 2 m gap, with a glass roof, between the new box and the old facade that extends most of the way around the building [see site plan]. But there are no gaps between the original wall and the new building on the east side, so we've got lots of work to do to alter the position of the windows. They'll be moved up or down to fit in with the position of the new floors, but it will look just the same in the end – we'll just move the stones around."

Supporting their heritage
With so much to pack into the project's remaining nine months, Kidd is busy. "The biggest challenge for us at the moment is to remove the facade retention system without damaging anything. There's also 6600 m2 of glass curtain walling to go up, and we're going to reinstall the original chimneys once the roof is on. They won't be connected of course, but they're in keeping with the external walls." There will also be a gravelled roof garden with spectacular views over Edinburgh Castle and the city.

What does Kidd think that Waverley Gate's original engineers would make of today's project? "I think they'd be happy to see the building being put back to use, and to see how well it has stood up. It's a tribute to their skills. And I think they'd be impressed with how we build up to this height these days."

Kidd is passionate about doing justice to the building and its creators. "It was a mess two years ago. It's very well built – we have checked the tolerances and so on. They knew what they were doing in those days."

Kidd and the team seem confident they too know what they're doing and will be able to finish Waverley Gate with the historic facade intact. But with the trickiest stage in construction – the removal of the supporting system – now beginning, they're in for a testing few months.