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Polymers Fisherman's friend

Foundations - Piling at a riverside site in Essex is giving polymer a chance to prove its environmental and cost credentials, discovers Damon Schunmann.

Although polymers have been used in the UK piling industry for a number of years, they have never quite taken off. But this may be about to change as suppliers seek to prove their worth in situations that are not easily tackled using more conventional techniques and materials.

One such site is Fingringhoe in Essex where J Breheny Contractors is replacing a bridge whose origins date back to at least Roman times.

Using bentonite piling for the crossing over the appropriately named River Roman ran the risk of spillage into the watercourse or leachate from the unlined portion of the pile boreholes.

The piles, which are being installed within sheet-piled cofferdams, pass through a number of water-bearing layers at depths that would normally require either long lengths of casing, or the use of bentonite drilling fluids to stabilise the boreholes.

The ground is 4m of fill overlying clay-sand-clay-sand layers, and even though the river is only a few metres wide, it has a 3m tidal range. Permanent fulllength casings were considered at tender stage but would have proved costly in the 23.5m holes.

Mowlem Piling and Foundations secured the piling contract for £250,000, and says using polymer halved the cost of the piling compared with the permanent casing option.

Alex Cartwright of independent piling consultant Acdevco adds that savings arise because less plant is needed to handle and clean the polymer than with bentonite.

Mowlem chose Cetco Europe's Shore-Pac GCV polymer and use has been approved by the client bodies - Essex County Council, the Environment Agency and Anglian Water Group. Mowlem claims this is the first time polymer has been used so close to a river.

Regional contracts engineer Mark Alderson says: 'If it gets spilt it won't cause any harm.'

'As it's a tidal river, as soon as it goes into the water it will be flowing either upstream or downstream into a Site of Special Scientific Interest.'

Cetco claims that because the polymer is anionically charged it does not pose a danger to fish by building up on their gills and causing suffocation, like some pollutants.

Cartwright says: 'Using bentonite would have been an environmental nightmare. You would have needed a physical barrier around it. Using the polymer so close to a river is a big tick in its box'.

Even more significant is the potential, pending approval, to dispose of the polymer straight into the river. After reductive treatment the polymer is broken down to harmless materials.

Cetco Europe's foundations manager Tim Ibbotson says:

'This alleviates disposal problems and the soil from the site can go directly to landfill.

You're effectively left with virgin soil as the polymer is both chemically and photo degradable which means huge savings with the new Landfill Directive.

'It's oilfield technology that has been manipulated over the years to suit civil engineering needs.'

Mowlem is installing the 35, 1,200mm diameter bored piles through an 8m temporary casing to support the unconsolidated ground with its high water table. When NCE visited the site the casing was withdrawn during drilling to repair buckling after it hit a rock.

Because of the a high tide the river was about 1m below the head of polymer in the casing. But even though the tidal flow meant the river was carrying high-density salt water, the polymer prevented the 16m deep hole collapsing.

Ibbotson: 'I would put money on a bentonite hole collapsing in that situation because it probably wouldn't have had time to form a filter cake.'

Work on the £900,000 project was delayed by a pair of nesting swallows that took up residence under the old bridge but it is due to complete in March 2005. Piling is due to finish this month.

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