TEMPORARY RETAINING works for the new Scottish Parliament building in Edinburgh had to deal with highly variable ground as well as unexpected obstacles.
Amec Piling, working for the Scottish Office and management contractor Bovis construction, recently completed installation of 305m of temporary piled retaining wall for the basement excavations for the new structure.
A combination of an anchored hard/soft secant piled wall and a cantilevered contiguous piled wall was used on the £1.7M contract.
Temporary support was needed on threequarters of the site perimeter to form a watertight excavation. Piles, typically between 600mm and 750mm diameter, were installed to depths of 15m through the highly variable ground of fill and stiff glacial till overlying weak to very strong siltstone, mudstone and sandstone, with up to 10m deep rock sockets.
Several igneous intrusions complicated matters.
Amec worked closely with Bovis and engineer Ove Arup & Partners at the tender stage, employing consultant Gibb in April 1999 to design the retaining system and to provide a flexible approach to piling design and construction.
The contract increased significantly when the wall along the eastern basement was added to the requirements for the western basement.
Potential difficulties of forming rock sockets efficiently in the strong sandstone layers underlying the eastern part of the site presented a particular challenge for the team.
After field drilling trials, Amec and Gibb offered an alternative shear pin design for these areas. This was included within the lump sum contract.
Two Soilmec R622 rotary rigs were used to install the piles while heavy duty drilling and coring equipment formed the 750mm diameter rock sockets. The walls are generally propped by up to three rows of temporary ground anchors, with internal bracing used in some areas.
Cantilever walls had to be used in one area because of a large gas main. Here wall design had to limit deflection to avoid potential damage to the gas main.
'In common with many old city centre sites, unexpected obstacles that had not been revealed by the desk study frequently interfered with the wall layout, 'said Gibb project manager John Whitworth.
'Discovery of a significant old culvert in the eastern basement area caused a few headaches before a multi-anchor solution was found.'
Temporary work, which included addit ional sect ions of cont iguous wall, bearing piles and minipiles, was completed in December and excavations are now under way.