Like most brownfield sites, the Greenwich peninsula posed many challenges for the geotechnical engineers. Dave Parker reports.
By January 1997 it had become obvious that ground conditions on the Greenwich site were rather less promising than had originally been assumed. Nuttall's decontamination work (see page VI) had left up to 4m of contaminated made ground under a topping of good clean fill. Over much of the area where the Dome would stand this made ground was mostly 'foul lime', a soft compressible byproduct of town gas production, with the water table 3m below the surface. Below this were the soft alluviums which had formed the original marshland, up to 5m thick in places.
These at least formed a barrier to leachates percolating down to the layer of terrace gravels that were to be found 12m to 16m below the surface. Below all this was a band of London Clay up to 20m thick.
Buro Happold geotechnical manager Peter Scott says the foul lime effectively ruled out the obvious foundation option for structures within the Dome. 'We originally favoured pad foundations for what are essentially temporary structures.
'But the foul lime is so fine it would have been very hard to stabilise. Foundation settlements of up to 200mm were predicted, so piling was the obvious answer for most situations.'
Unfortunately, piling down to the terrace gravels or the London Clay had several drawbacks, not least the risk of bringing contaminated material to the surface. More serious was the presence of massive buried foundations scattered across the site. The real challenge, however was the southbound bore of the Blackwall Tunnel, which runs 13m below the south western side of the Dome's projected footprint.
'Three of the masts had to be founded very close to the line of the tunnel,' Scott reports. 'The Highways Agency was very supportive, but it originally set a 10m exclusion zone around the tunnel to protect it against vibration.'
Buro Happold's solution to the tunnel problem was relatively straightforward. Massive 25m long concrete beams would span across the line of the tunnel, supported by 750mm diameter piles founded deep in the London Clay below the tunnel invert. Continuous flight auger piling was chosen, despite the problem of disposing of the contaminated material that would inevitably be brought to the surface, simply because the low levels of vibration meant the piles could be located within 3m of the tunnel.
The ring of 36 temporary anchorages for the mast erection also used CFA piles, and the Highways Agency allowed these to be situated as close as 5m to the tunnel.
Elsewhere the design team favoured cast insitu concrete piles founded in the terrace gravels, as this involved no removal of contaminated material.
'We did consider re-using some of the existing foundations,' says Scott. 'The problem was that we had absolutely no idea what was going inside the Dome. All we could be certain of was the position of the masts and anchorages and the location of the core buildings.'
Whatever the solution, it had to offer the same degree of flexibility to the Zone designers as the Dome itself. In the end it was decided to install a grid of 350mm diameter 12m long piles at 3m centres right across the Dome site. Most of the existing foundations had to be broken out or punched through.
Similar 480mm diameter raking piles supported the nine masts away from the line of the Blackwall Tunnel. Horizontal forces from the anchor blocks for the main roof cables are taken by a simple 320m diameter concrete ring beam (see page X). Uplift forces are resisted by a variety of ground anchors, type depending on location.
When Keller Ground Engineering arrived on site in late June 1997 it was anticipated that 5,000 piles would be needed. By the time Keller finished its 13 week contract, dead on time, it had driven nearly 8,000, a measure of how fast the overall project design was evolving.
Construction of the massive pilecaps, anchor blocks and ring beam was down to John Doyle Construction. The ring beam alone, 8m wide by 600mm deep, consumed 5,000m3 of concrete, made, like all 12,000m3 in the ground, with sulphate-resisting Portland cement.
Most of John Doyle's work was well above the contaminated ground lurking beneath the capping layer. O'Rourke Civil Engineering was not so fortunate.
'We were responsible for all the concrete inside the outer ring beam,' says O'Rourke project manager Neil Henderson. 'The ground floor slabs and the concrete frames of the core buildings were no real problem - in fact much of it was made easier because of the weather protection from the Dome roof.
'But the 16,000m of trenches and tunnels and the 6m deep central shaft were a very different matter. They were into the contaminated ground.'
O'Rourke had to carry out a detailed sampling and testing programme before appropriate licensed tips could be found to receive the excavated material. Site staff wore protective clothing and underwent medical monitoring. With the water table 3m from the surface, excavations like the central shaft and the 5m deep chambers under the 12 distinctive service drums had to be carried out inside sheet pile cofferdams. 'And the contaminated groundwater had to be tankered off site,' Henderson points out.
Despite all these obstacles, and the inevitable last minute design changes, progress kept to target, and no Zones were delayed because of serious underestimates of their foundation needs.