Signs of a slowdown in the UK property market have done little to dampen demand for developments in prime city centre locations. Seven apartments in a block of 30 taking shape beside the River Wear in Durham have already been sold.
No wonder the site's developer and main contractor Newton Moor Construction wants to make the most of this steeply sloping piece of land.
The £8M project, overlooking the city's cathedral and castle, will include five floors built into the slope. The block will stand at the foot of a new vertical earth bank, supported by a 7m high contiguous piled retaining wall which is being installed by geotechnical contractor Roger Bullivant.
The retaining wall allows the development to occupy a larger footprint than would otherwise have been possible. The apartments will sit lower down the slope and not interrupt views of Durham castle from a nearby vantage point.
Project architect Andrew Riley of Stephenson Johnson Riley says: 'Durham is a world heritage site and we have had to ensure that views of the castle are maintained from Crossways, a street behind the site that leads down to the river.' The retaining wall (and others on site) is made up of 175, 600mm and 450mm diameter CFA piles, secured by 12 ground anchors. The piles go through a few metres of made ground which overlies clay before reaching the underlying mudstone.
The augers were powered by a modifi ed Klemm 709 rig with an extended arm which installed piles at 500mm and 675mm centres to depths of 6m-13m.
Two installation methods were used. In firmer ground the team used open holing, with concrete poured after the auger had been withdrawn, and in less stable areas concrete was pumped through the auger shaft.
Bullivant's area manager Paul Manning says: 'The developer had originally asked for a sheet piled wall, but I suggested that a contiguous piled retaining wall would be more suitable. Recent increases in steel prices would have pushed the works over budget had sheet piling been used.' Pile installation was followed by excavation of ground from the side of the wall on which the block of flats will be built. The level was lowered 3m to allow the installation of anchors between the piles that will counter the overturning forces of the wall.
Each 13m anchor was installed 30¦ from the horizontal at 2m centres using the Klemm rig. Further muckshift will follow to lower the level of the ground another 6m to the final construction level.
Basement apartments facing the retaining wall will receive natural light from a light void, a 4m wide shaft extending to the surface.
CFA piles are also being used to create a retaining wall for an underground car park. The cutting edge facility will automatically deliver and retrieve apartment owners' vehicles.
The architect worked with Durham City Council planners to make sure the new building does not look out of place this historic area. Much of the city's housing started off as small properties that were extended over the centuries and the new building has been designed to mimic this.
The apartments will be in three separate buildings linked by bridging units to give the impression of staged extensions.
Split levels and different roof pitches should add to the illusion.
Project manager Franz Hutcheson says the first of the three apartment buildings should be ready in April, with the development scheduled for completion by summer 2006.
Foundation work under Bullivant's £165,000 subcontract began in early December 2004 and finished at the end of January.