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Kicking out contamination A vacuum drainage system more commonly used for accelerating consolidation in soft clays is helping to remove harmful gases from the World Cup stadium at the Stade de France.

Centrepiece of this summer's football World Cup will be the spectacular Stade de France in northern Paris. Before its transformation to the FFr2bn (£203M) home of French football, the site used to be home to a range of chemical and industrial works. These left a legacy of pollution and the need for a major environmental clean up in advance of stadium construction.

Between 1882 and 1969, the Cornillon site saw a succession of industries that caused considerable contamination of the ground, including a production and storage area for coal gas, a chemical factory and a road tar factory. And the nearby Saint Denis plain was a dumping ground for industrial waste and served as a storage area for solvents and hydrocarbons.

Before the site was handed over to the construction consortium, Grande Stade Construction (GSC), owner Gaz de France carried out shallow remediation. Some 7,000m3 of soil, polluted with polycyclic hydrocarbons, was dug out and sorted. Material was either dumped or cleaned and re-used on site.

But in July 1995, it became clear that the contamination problem was far more serious, with large amounts of pollution to some depth. So GSC commissioned Menard-SEFI to install a peripheral cut-off wall to prevent movement of pollutants from both internal and external sources.

The cut-off is an impervious cement bentonite slurry diaphragm wall surrounding the stadium, designed to trap hydrocarbons floating on top of the groundwater. The 1.1km long, 0.6m thick and 14.5m deep wall is keyed in the more permeable Saint Ouen horizon (limestone and marl) which allows clean groundwater to continue to flow under the stadium. It was built in just two and a half months.

The wall was installed in two stages, first down to 8m with backhoe excavators and then to full depth with a clamshell grab operated from a crawler crane. The grout, SEFISeal, was specially adapted to the particular pollutants in the soil. Strict quality control was enforced on site to ensure an almost perfect seal, with connections between primary and secondary panels being re-drilled after panels had cured and grouted a second time to complete the cut-off.

In 1995, GSC awarded the design and construction tender for a permanent solution to the problem of continuous gas leakage from the contaminated ground to a consortium of ANTEA and Menard Soltraitement.

This is based on the Menard vacuum system more usually applied as a ground improvement method in soft compressible clays. In this case gas is intercepted by a continuous horizontal drainage system which consists of a 1mm thick sealing membrane laid over a network of drains, partially filled with water.

The gas is drawn off by six variable speed blowers to a treatment area equipped with two active charcoal units and powerful fans that break down the volatile organic carbons.

Menard-Sefi technical manager Charles Spaulding explains that consolidation of the ground is avoided as only a very low depressurisation is applied - less than 10kPa, which has very little effect on pore pressure (vacuum systems usually operate at 70kPa to 80kPa).

Removal of pollutants in groundwater was carried out by selective pumping to skim off active hydrocarbons using a combination of eight 15m deep,1.200mm diameter shafts and deep draining trenches. Hydrocarbons dissolved in the water were separated on site in an effluent treatment plant before water was disposed. Other pollutants were stored on site before being sent to specialist companies for disposal.

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