Creating an underground arts complex between and beneath two listed buildings has posed some interesting challenges. Dave Parker reports from Edinburgh.
Imposing landmarks they may be, but by the standards of the world's top art galleries the 19th Century Royal Scottish Academy (RSA) and National Gallery of Scotland (NGS) are well behind the times. Their collections may be of international importance but they lack the extra facilities now seen as essential: shops, restaurants, lecture theatres and the like.
Until now, however, their sensitive location below Edinburgh Castle ruled out many expansion options - above ground at least.
The answer is the £26M Playfair Project, at the heart of which is the Link Building: an underground structure which will eventually house all the missing facilities in a modern complex connected by new staircases and lifts to both the existing galleries. On the face of it, digging out the 18,000m 3void required should have been relatively straightforward. The site, after all, is on 18th Century fill, which was dumped into the valley that separates the Old and New Towns of Edinburgh during the construction of the latter to create the famous Mound.
At the bottom of the Mound, now a steep, curving road link between the Royal Mile and Princes Street, a restricted plateau supports the imposing Grade A listed 19th Century siblings (see box). To the east a 1970s retaining wall marks the boundary. Below is Princes Street Gardens, at the lower point of which run the main rail lines south to England.
'These pass under the Mound through tunnels well to the south of our site, so they had no significant effect on our operations, ' explains construction manager Heery International's project manager Robbie Smith. 'And as the fill was transported to the site in hand barrows or horse and cart, we didn't expect to find any large obstructions either.'
However, groundworks contractor Laing O'Rourke was not to have a completely free run, as Smith reports.
'There was a 1m diameter water main running diagonally between the two buildings. This had to be diverted down the Mound and along Princes Street, a major undertaking as there were so many obstructions in the ground.'
Later the remains of a much earlier equivalent were to cause some debate. A 1.8m diameter masonry tunnel was uncovered actually underneath the RSA, and identified as an old water conduit from a reservoir at the Castle. Eventually the decision was taken to pump it full of foamed concrete.
Main excavation began some 8m west of the retaining wall and headed towards the Mound. By last week around 13,000m 3had already been dispatched to 'landfill' at the nearby port of Leith - although the spoil is actually being used to reclaim land from the sea it still attracts Landfill Tax. The new Link Building's eastern facade will eventually replace a section of the retaining wall, but work to break out the wall is not possible until April next year.
In the meantime, however, as expected, the real challenge is underpinning the existing porticos and the formation of entrances below them into the spaces being created beneath the existing ground floors. Each of the two buildings had its own distinct problems on top of the usual difficulties of working so close to listed buildings.
'All piling, including the contiguous piled wall round the entire basement, had to be bored to minimise vibration, ' Smith reports. 'For the underpinning on both buildings' porticos we had to get the rigs in between the external columns, which on the RSA are two rows deep. This meant carefully wrapping each column in protective fabric and taking other precautions to protect the stonework.'
As it turned out, the southern and eastern porticos of the RSA were supported on a forest of pine piles with iron shoes 5-6m deep 'many in surprisingly good condition'. Their discovery during the main excavation explained why piling contractor Keller had had such a struggle getting its 12m deep, 330mm diameter piles in earlier.
'Keller lost umpteen casings and augers during the original piling, ' Smith explains. 'Piles just had to go wherever they could. Seeing the mass of timber piles was quite a revelation.'
A more recent obstruction was to dog the underpinning of the northern and part of the eastern porticos of the NSG. Heery construction manager Mike Smith explains: 'In the 1970s a staff stair leading to a new basement was slotted into the south eastern ground floor. We knew it was there, but quite frankly we've got better information from the 1850s than the 1970s.
'Our original plan was to sandwich the stone footings under the porticos between two rows of contiguous bored piles, one inside, one out, with the inner rows slotted down outside the stair well. But when we started trial holes, we got an unpleasant surprise.'
What confronted the team was mass concrete fill up to 1.5m thick and 4m deep, apparently placed by the 1970s contractors in a belt and braces policy to minimise the risk of subsidence associated with the new stairwell. 'It would have been stupid to break out perfectly good concrete to make room for concrete piles, ' Robbie Smith remarks.
'So we put down 12m deep, 330mm diameter piles outside, then needled through the footings and supported them with beams between the piles and the stair structure' Once the permanent underpinning was completed these temporary supports could be withdrawn and the mass concrete and existing stair broken out to make room for a new stair.
Working only a wall's thickness away from a major display area a stealthy specialist team spent four months chipping away the obstruction with diamond coring and hydraulic splitters.
A more pleasant discovery was a layer of lime concrete beneath footings and ground floor alike. Where this had remained dry it was still in good condition, and, again, no one could see the point of removing it. The only real problem, as Mike Smith reports, was the interface between the new supporting walls below and the irregular underside of the lime concrete above - 'like a roller coaster' is his description.
This posed some interesting conundrums for the steel fixers and concreting gangs.
Otherwise, concreting operations have been relatively straightforward. One late change agreed by the team was a switch to waterproof concrete using a Sika system for the roof slab. This will eventually carry road traffic again - mainly delivery vehicles - and no one involved really liked the original tanked solution proposed.
Designed by William Henry Playfair, the National Gallery of Scotland and the Royal Scottish Academy opened in 1850 and 1830 respectively.
The other major element in the Playfair Project, due to be completed in 2004, will be the refurbishment of the RSA itself, much of which involves reversing the modifications introduced in a major reconstruction in 1911.
Client: National Galleries of Scotland Construction manager: Heery International Project manager: Bovis Lend Lease Architect: John Miller and Partners Structural engineer: Anthony Hunt Associates Groundworks contractor: Laing O'Rourke