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FISHERMAN'S FRIEND

INNOVATIVE GEOTECHNICS

Piling at a riverside site at Fingeringhoe in Essex is giving polymer a chance to prove its envorone=mental and cost credentials. Damon Schuman reports.

Polymers have been used by the UK piling industry for some years, but not as widely as in other countries around the world.

This may be about to change as suppliers seek to prove their worth in situations that are not easily tackled with more conventional techniques and materials.

One such site is Fingringhoe, near Colchester 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 Roman River 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 bentonite drilling fluids to stabilise the boreholes.

The ground is 4m of fill overlying clays and-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, which won the £250,000 piling contract, says using polymer halved the cost of the piling compared with the permanent casing option.

Alex Cartwright of independent piling consultant Acdevco adds that further 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 Environmental 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 this polymer gets spilt it won't cause any harm.'

'It's a tidal river and as soon as it goes into the water it will be flowing either upstream or downstream into a site of special scientific interest, ' Alderson says.

Cetco claims that because the polymer is anionically charged it does not build up on the gills of fish and cause 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 foundation 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 photodegradable, 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 deep bored piles through an 8m temporary casing to support the unconsolidated ground with its high water table. On the day of GE's visit the casing was withdrawn (during drilling) to repair buckling after it hit a rock.

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

Ibbotson says: '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 £0.9M project was delayed by a pair of nesting swallows who had taken up residence under the old bridge, but began in June and is due to finish in March 2005.

Piling started in early October and was due to finish last month.

Polymer or bentonite?

Cetco Europe's foundation manager Tim Ibbotson says Cetco's parent company, Amcol, is a world leader in bentonite mining and manufacture. This lends authority to his claim that polymers are preferable to bentonite for some piling jobs.

Austin Weltman, technical director of siste investigation consultant CL Associates says: 'I think polymers will replace bentonite and if used appropriately there are benefits all round.

'It's pretty safe environmentally.

We are where we were with bentonite in the 1970s. There were a lot of questions, but we are getting a good message from polymers.'

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