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PLASTIC FANTASTIC

Plastic piles are being used in difficult ground at a bioremediation test site in central England. Damon Schünmann reports.

A site contaminated by various industrial chemical spillages over the past 50 years is the testbed for a new remediation technique known as Sabre - source area bioremediation (see technical note, page 32).

'We're one-and-a-half years into the four-year project, ' says engineer Catherine Wesley of White Young Green Geoenvironmental at the site, the exact location of which is confidential.

'The three-week piling phase will form a test cell within which the British Geological Survey will install an array of water monitoring boreholes to about 7m depths.' The boreholes will be used to monitor the remediation once the active phase begins within a Ushape containment cell formed from plastic sheet piles.

The BGS has a laboratory set up on site. It initially did site investigation to defi ne the contaminated area and to determine the test cell's location.

The Universities of Edinburgh and Sheffi eld are also involved in the project.

WYG is providing services for the chemical works including site monitoring and supervision of the piling phase. There will also be an uncontained area outside the 30m long by 4m wide test cell, for a larger scale pilot trial, Wesley says.

Archon Environmental Consultants engineer Lawrence Houlden says: 'It's a link project coupling industry with academia for research and development work, with central government funding 50%.

'A number of industrial partners also fund the project. It's a pooling of resources to advance cleanup technologies for a chlorinated solvent.' In this case the contaminant is a dense non-aqueous phase liquid (DNAPL). 'This is a widespread problematic solvent and cleaning up dissolved plumes has relatively cost effective solutions, ' Houlden says.

'What we are trying to demonstrate here is that the technology will be effective in DNAPL source areas.' The original plan was to use HDPE piles but US supplier CMI (Crane Materials International) could not deliver them in time.

This meant a design change to a composite glass fibre material as the organic compounds in the ground would dissolve a PVC pile.

Steel was a non-starter as it would interfere with later geophysical resistivity tests.

'We could have used a bentonite slurry wall but this potentially would have interfered with the contaminants, ' says Houlden.

'We believed plastic piles had not been used in such dense ground in the UK. But in the US plastic piling is a much more advanced industry, so that's where we got the piles from. Strictly speaking they are a composite of glass fibre and polyester resin.' Sheet Piling UK is doing the installation under licence from Texas based Stab Cat using the mandrel known as a Stab Cat Stompper.

Houlden says: 'We have medium dense ground with made ground to about 2m down, over alluvium, over river terrace deposits. These are a mixture of sands and gravels (with up to 80% gravels) including cobbles up to 125mm diameter.

'You cannot normally drive a cheap plastic pile in these ground conditions so we drive them with the mandrel. Anyone doing a lot of steel piling, say in a marine application, could save an awful lot of money using plastic piles.' The mandrel runs down the back of the pile to transfer vibratory driving to hinged teeth that run along the pile's bottom edge. Once the pile has reached depth, the teeth swivel on their hinges and are withdrawn with the mandrel.

However this technique needed a little help because of unexpected ground conditions. Sean Nolan, contracts manager for principal contractor Sheet Piling UK, says: 'We pre-augered along the pile line as we were hitting concrete slabs left in the ground from the old works.' The piles are driven to refusal in competent mudstone below weathered clay at about 6.5m to 7m down and the interlock between each is sealed with a hydrophilic sealant to resist the groundwater contaminants.

A grout barrier behind the piles seals off groundwater. 'The grout is 25% bentonite and 75% PFA. It will ensure any imperfections in the piles - like declutching from one another or not toeing into the mudstone properly - are sealed off with secondary containment, ' says Houlden.

Nolan adds: 'The wall is formed by pumping grout, which includes the PFA to give a neutral PH, as the mandrel withdraws. It's the thickness of the mandrel, about 10mm to 12mm.' Natural groundwater will come into the open end of the cell with the closed end preventing it escaping.

Afterwards it is pumped out to induce through-flow into the cell.

'The upper two-thirds of the cell has the greatest contamination and we inject treatment chemicals for the trichloroethylene at the open end, ' says Houlden.

'It is a contained cell where we can do a very controlled series of tests on the contamination. We can measure what goes in, what comes out and what happens in between.' Piling was just finishing at the time of GE's spring visit.

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