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Mother of all clean-ups

Before the party could begin, the dome site's 100-year history of industrial use had to be confronted.

The Millennium Dome - or The Millennium Experience as its promoters call it - is the centrepiece of the UK's 2000 celebrations and sits atop the biggest remediation project in the country.

Built on a former British Gas site on the Greenwich peninsula in south east London, construction of the dome was managed by a joint venture of McAlpine Laing. Its costs of £220M, funded by the Millennium Commission, include £27M for the dome structure, £18.4M for the foundations and £10M for ground remediation.

The 120ha site was originally managed by BG Properties and was used as a gasworks for nearly a century before it was decommissioned in the mid 1970s. In 1988 WS Atkins began a series of investigations which included more than 2000 boreholes and revealed a legacy of a variety of contaminants including solid ferrous cyanide, coal tars, mineral oils, benzene, polychromatic hydrocarbons, phenols and heavy metals.

Two thirds of the site needed cleaning up. Remediation began in 1996, with contractor Edmund Nuttall excavating nearly 220,000m3 (about 700,000t) of contaminated material which was removed to designated landfill by waste management contractor Shanks & McEwan. This material included contaminated fines from soil washing carried out on site. Clean fill was then placed, including material reclaimed from soil washing.

Remediation contractor Fluor Daniel also carried out soil vapour extraction to remove benzene and petrol. The system comprised 300 wells in six treatment cells, and used blowers to push air through the ground, volatising contaminants and drawing the vapour to the surface.

Where contamination was low and did not affect development, it was left and a marker layer of orange mesh laid. This was capped with a capillary layer which allows gaseous exchange between the ground and air but prevents contaminants rising to the surface hydraulically.

Foundation contractor Keller Ground Engineering installed a slurry wall to prevent any remaining contaminants leaching into the Thames.

Piling for the 100m masts that support the dome's fabric roof started in June 1997 and took three months. Nearly 8000 piles were installed using up to 14 rigs on site at once.

Foundation design was by structural and civil consultant Buro Happold, whose geotechnical group had anticipated using spread footings in low loaded areas until extensive areas of foul lime - a byproduct of purifying gas by passing it through crushed chalk - were identified during site investigation. Periodically this chalk was replaced, with contaminated material dumped in loosely filled unmarked pits up to 4m deep.

The dome is mostly founded on vertical and raking driven cast insitu piles up to 12m deep. Driven cast insitu enlarged head piles support the exhibition floor area while temporary tension anchorages were needed for support cabling during mast erection.

Driven cast insitu piling was selected partly because of the potential number of buried obstructions. It also meant very little contaminated material was brought to surface, a highly significant factor to the project programme.

A further complication was the southbound Blackwall Tunnel which runs below the dome. CFA piles were used in this area to avoid piling-induced vibration damage within the tunnel structure.

Initially, driven piles were excluded from within 25m of the tunnel. After monitoring this was reduced to 15m. Careful consideration was also given to potential transfer of load on to the tunnel lining.

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