Inside job

Brian McGee collects pictures of historic Edinburgh. He even has a small selection of prints in his basement site office close to the eastern end of the city's famous Princes Street. There are prints of some of the city's grand historic buildings in various stages of construction, along with Victorian and Edwardian street scenes.

McGee, dressed neatly in a two-piece suit, carefully places the prints on the large wooden table at the back of his cavernous office, drawing attention to one in particular. It is captioned: "1861: The prince consort laying the foundation stone of the new general Post Office." It shows a group of Victorian dignitaries, including the prince consort himself, celebrating the start of the building's construction. This engraving is especially pertinent to McGee's role as project manager for Balfour Beatty Construction.

Now, more than 140 years after the artist captured the moment when construction of the Post Office in Waterloo Place commenced, it is print collector McGee who is in charge of the building's partial demolition. His task is to demolish all eight floors of the building's core while ensuring its grade A-listed (the Scottish equivalent of grade I) neo-renaissance facade remains intact. When demolition is complete, a separate contract will be drawn up to construct a glass-walled office within the confines of this stonework shell.

The listed facades mean that this is no smash-it-and-skip-it demolition project. To ensure the historic stonework survives intact McGee's team has had to erect an assortment of steelwork, trusses and other temporary propping systems to create the largest facade retention scheme in Europe. "It's a belts-and-braces approach because we could take no chances with the facade," says McGee. But it is not just the scale of the task that makes this a difficult project; the construction site is hemmed in by a series of busy roads, pedestrian routes, a road bridge and a major railway station, all of which have combined to make this one of the most challenging demolition projects anywhere in the country.

Before starting on its partial destruction, Balfour Beatty attempted to cobble together a potted history of the building to better understand its construction. The contractor knew the original Post Office had been added to over the years. Its research included visits to Edinburgh's numerous bookshops and antique dealers in the quest for old prints and ancient publications featuring the historic building. It was a worthwhile exercise; the research revealed the building had been constructed over three phases – something that would have to be taken into account in demolition programming in order not to damage the facades. "The whole building is a series of extensions," explains McGee.

The original Post Office building, which Prince Albert helped construct in 1861, was L-shaped in plan and included the facades along Princes Street to the north and North Bridge to the west. However, this building forms only part of the Post Office's current footprint. In 1890, the original structure was extended by infilling the L to form a rectangle. Then, 18 years later, the building was extended again. This time, a new block was attached to its eastern elevation and additional floors were stacked on top of the original 1861 building. It remained in this form until the Post Office vacated the building in 1995.

In addition to the documentary research, an on-site investigation was also undertaken. The team was fortunate in that the building had been vacated some years earlier. This allowed what McGee terms "a substantial coring operation" to be carried out. This involved drilling into the facades to take core samples from the walls, so as to discover how the walls had been constructed and determine the most appropriate facade retention system. Fortunately, the walls were well built with a sandstone outer skin covering an inner layer of brickwork, varying in thickness from 1.5 m to 3 m. "It is pointless having a facade retention system tying into the walls if the walls are not sound," explains McGee.

Demolition specialist Dem-Master started on site in February 2002. Its first task was to design and assemble the facade retention system so that when it removed the building's core, the walls were still supported. The system had to be able to limit movement in the facades to a maximum of 20 mm – quite a challenge given that some of the walls are more than 42 m high.

Dem-Master appointed structural engineer McLay Leonard to help design a scheme. However, the propping system's design was not straightforward: the tight city-centre site meant that the easy option of shoring up the facades by erecting temporary steelwork around the outside of the building was only possible on some elevations. Where external access was limited, or nonexistent, as in the facade above Waverley Railway Station, the contractor had to design a system of temporary trusses and internal props to support the facades.

Because of the weight of the facades, supporting the propping system itself was a major challenge. Balfour Beatty had to sink hundreds of piles into the ground to carry the weight of the temporary structures and the loads placed on them by the facades. "The interesting thing about this facade retention system is what goes on below ground," says McGee.

By far the largest number of piles, 130 in total, had to be installed beneath the propping system on the east wing's facade. The piles here vary in length from 8.5 m to 24 m depending on the depth of the bedrock below. The contractor was fortunate that the retention system for the east wing could be accommodated in a small car park at the rear of the building. They were less fortunate with the remainder of the south facade, which rises directly up from Waverley Station. Here, 50 piles have been sunk inside the building to carry the three steel towers to support the 42 m-high facade. And on the north elevation, 74 piles were sunk to carry a series of external vertical frames to support the Waterloo Place facade. McGee explains: "There are more piles on his scheme than we originally anticipated because of the design of the facade retention system and the ground conditions."

Providing the support for the elevations that face onto Carlton Road and North Bridge was much more difficult. There was no room to install an external supporting system on either elevation. Instead, these facades are supported from a series of internal trusses installed parallel to the facades they support, which span the full width of the building. The facades are clamped to the retention system using threaded tie rods. "It takes a long time to support the facade and a short time to demolish the core," says McGee.

With the retention system in place, demolition could at last begin. The demolition is being undertaken in two phases. An initial "soft strip" is carried out first. "This involves removing everything but the structure," says McGee. This is also the time that anything of any value is reclaimed and sold on as architectural salvage; for the Post Office building this included slates from the roof, scores of Victorian cast iron radiators and even pieces of stonework that will be reused later to repair damaged areas of the facade. The completion of the soft strip phase of demolition is signified by the arrival of heavy plant ready for the second major demolition phase – removal of the building's structure.

On site now, almost a year after demolition first started, the building's steel structure, internal walls and assorted floors are finally beginning to come down. Already much of the pitched area of the roof has disappeared; all that remains is a line of blackened steel trusses, silhouetted against a grey Edinburgh sky heavy with rain. Stacked neatly at the foot of the trusses are piles of salvaged slate, awaiting removal from site. McGee seems bizarrely gleeful about the prospect of approaching rain: "It helps keep the dust down and so stops us being a nuisance to our neighbours," he explains.

However, it is not the roof but the core of the building where demolition is most advanced. Two enormous shafts have been punched vertically through the building, one on the eastern side, one on the west, to allow the erection of a pair of giant tower cranes. These are being used to lift demolition plant and equipment over the precious facades and into the building. Between times, the cranes serve to support the weight of massive sections of the building's heavy steel structure while the demolition contractors slice it into manageable pieces using acetylene torches.

A series of vertical shafts – similar to those created for the tower cranes – have also been knocked through the building to form what McGee calls "drop holes". Demolished materials are thrown down these holes onto a carpet of old car tyres before being segregated into its different constituents, with metal and timber being sent for recycling. Separating the materials "makes economic sense", according to McGee – it cuts down on the cost of disposal.

At the eastern end of the building, removal of the floors and walls has proceeded in places as far down the building as the ground floor (although there are still four levels of basement yet to be demolished below). But, even though the floors and walls have mostly disappeared, the building's enormous steel structure remains firmly in place, helping support those sections of the building still to be demolished.

"If we're taking out a floor we have to have permission," explains McGee. Permission is obtained through a permit-to-demolish system to ensure critical sections of the structure are not removed out of sequence, causing the facades to collapse. First, Balfour Beatty produces a method statement defining the floor area to be destroyed and stating how it proposes to demolish it. This proposal has then to be approved by structural engineer Arup before work can start. McGee admits: "We're already on our fifth volume of method statements explaining how each section of the building is going to be tackled."

With demolition now under way, and a strong relationship established with the client, McGee is optimistic that Balfour Beatty will win the contract for the new office to slot into the void it is busy creating. "All that's left to do now is sign on the dotted line," smiles McGee.