Act of union

Robbie Smith must have one of the hardest jobs in construction. As head of construction manager Heery International's Playfair project team, he is responsible for a £28m scheme to unite two of Edinburgh's most famous neoclassical galleries – the Royal Scottish Academy and the National Gallery of Scotland – by constructing an underground link building between the two.

Both buildings are grade A-listed (or grade I as it would be known south of the border), and sit prominently on tourist-thronged Princes Street. Together, they form part of a World Heritage site dominated by the city's castle.

Smith's job is to ensure that construction of the underground link and the two subterranean access corridors connecting it to the existing buildings does not damage these historic structures.

What's more, he is responsible for managing the restoration and refurbishment of one of the buildings – the Royal Scottish Academy – and ensuring that this part of the works is completed in time to host a Monet exhibition in July next year. "It's a tight timescale for such a major project," he says.

As if this were not a big enough challenge, dithering by the Scottish parliament has denied Smith access to part of the site, forcing him to throw away the original construction programme and to phase the work instead.

The Playfair project implements a competition-winning design by architect John Miller + Partners. The main feature of the scheme is the cavernous 2600 m2 underground link building, which will provide much-needed additional exhibition space, an education suite, a lecture theatre and the important revenue-generating additions of a cafe and shop. It will also form a spectacular new entrance to the newly amalgamated galleries. A new lift and staircase in each building will allow visitors access to the galleries from the link building.

The link building is being constructed beneath the pedestrian plaza separating the two listed neoclassical edifices. The entire extension is below ground, with the exception of the east elevation, which overlooks the grassy Princes Street Gardens. On this side, the site falls away, and the building intrudes into the public park as a 7 m wide strip.

The problem for Smith is that he cannot even begin to build on this strip of parkland until MPs pass a bill in the Scottish parliament to change its designation from park to development land. This bill was originally expected to pass in January 2002, but parliament has been slow to act and the earliest this will now happen is April 2003. "Unfortunately, we haven't got the whole site," Smith laments.

This procrastination by Scottish MPs has required Smith to reschedule the link building's construction. Unable to gain access to the entire site, but with the pressure on to complete the scheme, Smith has been forced into building the gallery's subterranean accommodation in two phases: the western two-thirds of the building will be completed first, followed by construction of the final, eastern third some time after April's deliberations. "We'll have to go back and, in effect, build everything twice," says Smith.

This reprogramming has pushed the link's completion date back by three months, from early July 2004 to the end of September. However, Smith says he has been able to mitigate the effects of the two-phase approach because of the type of contract. "Because it is construction management, we can reschedule works and issue revised programmes there and then," he explains. Even so, rescheduling the works will not completely alleviate the problems. As Smith points out: "It's Sod's law that all the plant rooms are in this section."

So far, so good
Back on site, work is progressing on the first phase of the link building, where construction commenced in January this year. There have been some significant achievements in getting to this stage of the programme. More than 20,000 m3 of earth has been excavated from the site and the new link building's reinforced concrete floor slab, roof slab and its supporting columns have all been constructed. However, the concrete works stop abruptly at a sloping bank of earth at the boundary line of the park. Beyond the bank, the ground remains glaringly undisturbed.

But even getting to this stage in the programme has not been straightforward. Digging an enormous 7 m deep hole adjacent to two of the city's most prestigious buildings presented significant technical challenges.

The first thing the contractor had to do was to isolate the two buildings from the construction site by installing a line of piles around the two buildings, forming a concrete barrier between the excavations and the listed 19th-century buildings. With the structures safely protected behind their subterranean ramparts, excavation was simply a matter of digging out behind the piles until the hole reached the required depth. The building's raft foundation was then formed by pumping 2500 m3 of concrete into this hole.

The next challenge was to install the shafts containing the lifts and spiral staircases in the two neoclassical galleries to transport art lovers up from the new subterranean link. This was a particular challenge on the National Gallery building, which is to remain open throughout the course of the works. The Royal Scottish Academy, on the other hand, is closed for refurbishment.

"With the public still in the gallery, it affected the way we could go about the works," says Smith. A section of the gallery was partitioned off to separate the site from the public. The main problem was noise, but the contractor also had to ensure there were no vibrations when breaking out the floors to form the lift shaft because they could damage the gallery's art collection. It did this by avoiding the use of mechanical breakers in favour of a "core and split" technique, whereby a core is drilled into the concrete and a hydraulic splitter is used to break it up. The contractor had to excavate beneath the gallery to create the access shaft, but in doing so it had to construct a piled concrete box to prevent damage to the rest of the building. However, following initial demolition works, a huge concrete box was uncovered in the prepared shaft, 5 m deep with 1.5 m-thick walls. This turned out to be result of alteration works undertaken in the 1970s. With concerns about vibration, this concrete is now receiving the attention of the core and split team.

A second concrete box, formed from 120 mini-piles, had to be constructed at the foot of the new stairs in the Royal Scottish Academy building. The cramped spaces meant the piling team could only use a small rig to construct the piled walls. This was much more difficult than anybody had anticipated because the rig's auger kept snapping on the timber piles supporting the main building.

It had been envisaged that the spiral staircases for both buildings would be constructed from precast concrete units. But following a buildability review, it was soon apparent that manhandling large precast units into position in the confined stairwells was unfeasible. "It was physically impossible to get the precast units into the building," explains Smith. Instead, the stairs had to be redesigned and constructed from concrete cast in situ.

To connect each staircase to the underground extension, a link corridor had to be constructed. This would allow visitors to pass beneath each building's colonnaded facade. However, before any excavation could take place, the huge porticos had to be supported. "This was very intricate work," explains Smith.

Supporting acts
The construction process for the subterranean corridors involved carefully sinking rows of piles between the columns of the portico in very cramped conditions. After topping the piles with concrete pile caps, steel beams were laid on top of the pile caps on either side of the columns parallel to the facade. A core was drilled through the columns' supporting base to form a hole about a third of each column's width. A beam was inserted through this hole to straddle the two beams. Then a second hole was drilled and another beam inserted.

This painstaking operation was carried out for each column to allow the weight of the facade to be transferred onto the steel beam lattice, and from there onto the piles. "It was a serious bit of underpinning," Smith explains. With the building's precious porticos thus supported, it was a relatively straightforward matter to dig out the corridors beneath them.

While construction of the link building was pressing ahead, work on restoring the interior and exterior of the Royal Scottish Academy was also taking place. The National Galleries of Scotland, the body that owns both museums, has used the construction of the link building as an opportunity to completely refurbish and restore the 19th-century Royal Scottish Academy building and bring it up to modern gallery standards. This involves the installation of new air-conditioning, a new lighting system and a new precast concrete domed roof, complete with rooflights and solar-controlled louvres.

Work on restoring the academy is on target to finish at the end of next month, in time for the Monet exhibition. With the heating boilers running, the building is drying out ready for installation of the timber floors and a final lick of paint on the newly finished gallery walls. However, it will be another two years before the link building is complete and visitors will be able to pass between the Royal Scottish Academy and the National Gallery of Scotland. Or at least that is the latest forecast – assuming Smith gets his strip of parkland in April, of course …