Big Ben and other famous London buildings hardly moved while the tunnels of the Jubilee Line Extension were driven below them - thanks to compensation grouting. Now the technique, which involves the controlled injection of grout into the ground below foundations to control settlement, has been adapted to fill thousands of voids left by rotting timber piles beneath the Royal Scottish Academy art gallery in the centre of Edinburgh.
Geotechnical contractor Keller Ground Engineering, working for client National Galleries of Scotland and its consultant WSP Group, is close to completing its £1.5M grouting contract. While the building is not in immediate peril, the work should ensure the future safety of one of Scotland's most important art galleries.
Built in the 1820s on the Mound, an artificial slope between Princes Street and the Royal Mile created by dumped glacial and boulder clay fill from construction of Edinburgh's New Town in the 1780s, the building's foundations consist of mass stone footings that sit on about 3,500, 150mm and 200mm square timber piles driven at 450mm to 600mm centres to depths of between 3m and 6m into the fill. Between the footing and the top of each pile there is a timber grillage, originally between 75mm and 150mm thick, a timber cross piece and a stone Hailes pavement (actually a guide for driving the piles).
Three site investigations carried out since the 1970s have revealed that the majority of the timber piles had decayed along 20% to 100% of their length, leaving perfectly square voids up to 3m deep. A little surprisingly, settlement is not believed to be caused by these voids, but simply because the original foundations were inadequate. In particular there has been settlement of the clay fill and decay and squeezing of the timber grillage between the stone footings and the Hailes pavement, which is now just millimetres thick in places.
The reason for the present work is concern that any future development will not be adequately supported by the foundations. Large scale lab tests by the Building Research Establishment in 1994 concluded that there was a risk of collapse and settlement of the fill if there was water ingress, additional loading or further decay of the foundations. Remedial work was needed if any refurbishment was to go ahead.
WSP Group associate director Ken Guthrie says a number of options were considered, including mass concrete underpinning, filling the pile voids with concrete, piling and jet grouting. All were thought to be too messy, disruptive and potentially risky and piling would have had to been drilled or jacked down in some way.
Keller suggested using its patented Soilfrac grouting system, which uses tubes-amanchette to inject grout beneath the building to fill the voids. Its proposal closely resembles the set-up commonly used in compensation grouting schemes. An important advantage of this approach was that access was not required from inside the building, which will remain fully open to the public throughout the project.
The first step was the construction of a large box outside the RSA building from which grouting operations would be carried out. By the beginning of November 1999, Keller had built a 34m long, 4.2m wide and 5m deep contiguous piled box using continuous flight auger piles right next to the eastern wall of the RSA.
Some 76 Soilfrac pipes have been installed in two levels, in places just 2m below the stone.
These are up to 41m long and extend 2m beyond the building's western edge. A cased auger system is used, with the casing up to 1m ahead of the auger to minimise any settlement effects.
After the tube-a-manchette pipe is installed, the casing is withdrawn and grout can be injected through ports covered by temporary rubber sleeves.
Grouting began in the middle of February and is due to be finished by the end of May, well in time for the annual Edinburgh Festival in the summer.