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Line of defence


Trevi is building a complicated 2.2km long cut-off wall to prevent pollutants from a disused chemical plant reaching an Italian Alpine river.

Siting the ACNA chemical plant in a long sweeping curve of the Bormida River in north west Italy, close to a ready supply of water, probably made great sense in the late 19th century. But more than 100 years later, residual pollutants at the site in Val Bormida, near Cengio, and the potential to leak into the river comprises a great ecological and environmental threat.

The plant was originally established for the production of explosives but over its 120 year working life its output expanded to at least 374 known chemical compounds.

Unsurprisingly, significant heavy pollution occurred, largely as a result of leakage from on site waste lagoons. In particular these have affected surface alluvial deposits and the groundwater.

Geologically, the area, in the Alpine foothills about 400m above sea level, is characterised by surface alluvial deposits overlying marl at between 4m to 9m. Once through an upper weathered zone, the marl, which can be seen outcropping on some stretches of the nearby mountain slopes, generally has a low permeability and this provides a suitable layer in which to create a cut-off and so contain the pollutants at the site.

Some remedial work was carried out in the early 1990s, including installation of a slurry trench with an HDPE liner and gravel drainage trenches. The work was designed primarily to prevent large scale spread of pollutants from the lagoons, but there is still a significant risk of pollutants being mobilised, particularly during periods of heavy flooding.

Italian contractor Trevi is now involved in this next phase of environmental protection works.

The 2.2km long continuous cut-off wall is being built outside the boundary of the chemical plant and parallel with the curve in the river. Because the wall lies mostly within the river's flood plain, it is designed to accommodate flooding of 1.75m a one in 200 year flood. As a result the top of the wall will extend between 8m to 10m above the level of the flood plain.

This is achieved by forming a embankment to provide an elevated position from which the cut-off is constructed.

Design is complicated by plans in the future to excavate the embankment and remove the contaminated alluvial materials down to bedrock on both sides.

With the need for the cut-off to be stable during and after this excavation, design is much more involved than a conventional cutoff, which is laterally restrained and contained by the ground.

The final design has been drawn up by Milan-based consultancy Studio Geotecnico Italiano (SGI); while Trevi is working as part of a joint venture with Aquater. Trevi is responsible for the special geotechnical works with Aquater supervising earthworks and other construction subcontractors.

All activities are performed under the supervision of an 'extraordinary commissioner' appointed by the Ministry for the Environment.

For most of the cut-off, Trevi's work involves constructing two parallel concrete diaphragm walls down to and slightly penetrating the bedrock to provide lateral support and protection for the cut-off.

The sealing element comprises a slurry wall, complete with HDPE liner, built between the concrete diaphragms and taken deep into the impermeable marl.

The penetration of the HDPE composite wall into the marl varies from 4m to 12m depending on the condition of the marl which can be weathered and fractured.

Along some sections of the cutoff, the depth calls for a Berlinstyle anchored minipile wall as an alternative to the double concrete diaphragm.

As a further line of defence, a drainage trench immediately upstream of groundwater flow intercepts and collects chemically polluted groundwater for treatment.

Work is scheduled for completion at the end of next year - but at the end of the contract Trevi will have achieved a solution that it claims offers: 'final and full security for the environment from the contamination of the factory.'

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