South of the Dome, tracts of bare earth hint at the mammoth ground works recently carried out on the Greenwich peninsula. But few among the Dome-visiting hordes realise that only four years ago the landscape was pock-marked with abandoned industrial structures and laced with contamination.
The area was home to one of the largest gasworks in Europe. Alongside production of gas through the coking of coal, concentrated at its northerly tip, the 118.6ha site took in tar distillation processes, a benzol plant and a chemical works. At its heart stood a power station and to the south a steel process plant. Domestic waste had been landfilled there. All had left a legacy of pollution - coal tars, polyaromatic hydrocarbons, phenols, benzene and other volatile organics, cyanides and heavy metals, sulphur compounds and ammonia.
During the late 1980s the site's owner, British Gas, began looking at ways of promoting development. A plan was drawn up and site investigation carried out by consultant WS Atkins, appointed as lead engineer in 1988. Crashing property values laid British Gas' ambitions to waste the following year.
In 1995 plans for a national millennial exhibition created an opportunity to turn the peninsula from liability to asset. Negotiations secured a regeneration grant from development agency English Partnerships.
WS Atkins, still acting for British Gas, appointed architects Richard Rogers Partnership to draw up a plan for the site, with JMP Consultants as transport planner and Gardener & Theobald as cost consultant.
The millennial exhibition would be housed at the north of the site, the driving element in a mixed-use scheme of housing, parkland, retail and leisure. Greenwich peninsula was to have new transport, new services and new life. In 1996 came the fateful decision in favour of Greenwich.
Under terms of the Landfill Act, British Gas was required to undertake statutory remediation. With a millennium exhibition to build, no time could be lost. Contractor Edmund Nuttall started work on the £21.5M contract in late 1996.
ICE 6th contracts were used, with all risk borne by the contractor for a fixed price.
Following detailed desk studies, extensive ground investigation had been carried out between 1988 and 1996, recalls WS Atkins projects director Alan Micklam. More than 1,000 trial pits and more than 100 bore holes had been used to gauge ground conditions on a 25m grid across the site.
Groundwater monitoring had been carried out since the early 1990s. 'That provided sufficient data for statutory remediation,' says Micklam. He notes that continuing groundwater monitoring shows 'nothing adverse happening at the moment'.
Statutory works were designed to tackle contamination that would pose long term risk, Micklam adds. This was assessed by cross-referencing Interdepartmental Committee for the Redevelopment of Contaminated Land (ICRCL) guidelines, Dutch toxicological clean-up criteria and the Government's draft Statutory Guidance on Contaminated Land.
The toxicity of the contaminant, its mobility, pathways and the sensitivity of receptors to pollution were key considerations. WS Atkins identified the River Thames and a small aquifer deep below the site as sensitive receptors; and at all costs humans visiting the exhibition and living on the peninsula had to be safe.
Cleaning the site started at the north and progressed south. Across the site hotspots were removed. After remediation, to protect against residual contamination, an orange polythene mesh marker layer was laid. Above this a 200mm capillary break layer - typically of crushed concrete from the vast number of structures left in the ground - prevented upward migration of water-borne contamination. The hole was capped with a low permeability layer of London clay, allowing a 1.5m layer of clean soils to be placed on top.
Where contamination was most acute, at the peninsula's northerly tip, a bentonite slurry cut-off wall was constructed, set 15m back from the river, to contain contaminants on site. Slurry walls have also been placed either side of all deep drainage to ensure the structure cannot become a migration path for rainwater or ground water.
Gas production had left an 8m deep, 36m diameter tar well close to the Blackwall Tunnel on a part of the site now covered by the Dome. It had been founded on gravels underlying alluvial clay and over years tar had 'pancaked' out into the surrounding ground. It was necessary to remove the tar well structure itself and also to extract the hydrocarbon plume being dispersed via groundwater through the gravels.
First a shallow bentonite slurry cut-off wall, 100m in diameter was constructed around the well. It was keyed into the clay to prevent contaminated perched water migrating into the excavation once removal of the well began. Through the bentonite wall a deep sheet-pile wall was placed, preventing movement of water from surrounding gravels. The well and its contents were removed off-site.
Meanwhile, perched groundwater from within the cut-off wall was pumped out and stored in a lagoon before being treated for contamination and discharged into public sewers (in the order of 50,000m3 contaminated groundwater were treated across the site).
An estimated 250,000m3 soils were removed to landfill during the statutory remediation phase. But to minimise the volume of material requiring disposal off-site Nuttall screened excavated material. 'There simply wasn't enough capacity in local landfill sites,' says Nuttall project manager Ian Parish.
Parish notes: 'Coarser fractions tend not to contain contamination.' Some 150,000m3 of material was dry-screened for re-use; contaminated fines alone were landfilled. Nuttall and Atkins also developed bespoke soil washing plant. 'With the tar tank excavations, gravels underneath were washed and tarry fines extracted,' says Parish.
He adds that the system extracted all particles below 2mm diameter. 'In Holland the cut-off would be 63 microns; safeguards against residual contamination were far more onerous here.'
Tackling volatile organic compounds like benzene, toluene, ethyl benzene and xylenes, soil vapour extraction was deployed. The VOCs were concentrated mainly in made ground, consisting of 1m to 4m of granular material; they had spread laterally on contact with underlying clays. Ex-situ treatment would have required removal of massive volumes of overlying clean soils.
More than 300 wells were sunk and pumps set to work. While groundwater contaminated with dissolved phase VOCs was removed, air being sucked through the soils also encouraged volatilisation of free phase compounds. At the same time organic degradation of hydrocarbons was accelerated. Measurement showed a 90% reduction in contamination, says Micklam.
Development remediation, carried out by English Partnerships followed directly behind the statutory works, starting in July 1997. English Partnerships assumed ownership and role of client for this second phase. Work included installation of infrastructure and landscaping, packaged into 30 contracts with a combined value of £180M. Nuttall won 16 contracts worth £115M and seamlessly moved from the first project phase into the second.
'Statutory remediation doesn't need to assume any end use,' remarks English Partnerships technical director Simon Wright. 'Development remediation is all about end use.' Development remediation set about adjusting residual contamination levels to suit different uses as defined in the RRP plan.
Having capped and isolated contaminants below and behind impermeable barriers, a major concern was to construct infrastructure with minimal disturbance, or ideally none at all. 'We wanted to create clean paths that could be dug out,' says Atkins resident engineer Martin Lass.
Trenches are lined with impermeable membranes, trees and shrubs have been planted in clay-lined trenches with 1.2m of topsoil beneath their roots. An automatic, comprehensive irrigation network supplied via a new borehole by the chalk aquifer keeps roots from growing down into the capping layer.
Surface drainage has been placed using microtunnelling. to minimise contaminated arisings. Wright notes: 'In a cut and cover trench you have to control ingress of rainwater and disposal of that water'. The prime goal of the second phase has been to create an impermeable site, he says. To this end, to keep the horizontal clay barrier intact, once construction started above ground all foundations were either above the orange warning layer or driven piles.
English Partnerships' approach to remediation was governed by factors of safety, time and, as a public agency, best value for money. 'We were balancing environmental benefit against economic cost,' Wright says. More on site and insitu treatment could have been achieved, he says. 'But we believed that taking more contamination away from the site would increase its economic value.'