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GEOSYNTHETICS - An excavator wobbling on a waterbed may be an odd image. Add a sewage works and the picture becomes stranger still, and smellier.Damon Sch³nmann went to Leeds with a peg on his nose to make sense of it all.

Like so many of the important industrial processes, the water treatment works at Knostrop, northern England, goes on with its work largely unnoticed by the rest of the world. Clean water comes out of the surrounding areas taps, and everybody's happy.

But there is more to the procedure than just dirty water in, clean water out. The process produces a gelatinous gooey byproduct, commonly referred to as 'sludge' that needs to be disposed of one way or another.

At the Knostrop works in West Yorkshire, the sludge has been emptied into a large lagoon separated into a northern and southern area by a bund, rendering the land pretty much unusable. But now Yorkshire Water has focused its attention on developing the 9.5ha area into a recreational landscape.

Yorkshire Water turned to a joint venture of consultant Mott MacDonald and main contractor Bentley (MMB) to come up with a solution.

Surface tension on the highly viscous mess means the depression from a foot pressed down on the crust will come straight back up again, like a pudding jelly, although not a particularly mouthwatering one.

So in turn the JV awarded a £500,000 subcontract to geotextile supplier Tensar International, to provide a capping solution. Tensar northern area business manager Colin Thompson says: 'There is probably no industrial use planned, as [the capping solution] is not designed for it.' The company had developed and already used its proposed capping technique on a previous MMB sludge lagoon project at Dowley Gap, near Bingley, also in West Yorkshire.

It is based on its biaxial geogrid SS30G, which has a geocomposite fabric on the back that allows water to pass but will stop fine silts rising through.

'The sludge in the lagoon never completely dried out and we believe it's 10m to 12m deep, ' says MMB site manager Colin Root. He points out the plant working on top of the sludge saying: 'You can see a wave created by the slewing of the excavator.' The lagoon is being capped in four phases with differing aggregate layer thicknesses and types of grid.

The first section uses Tensar SS30G, underlaying 140mm fill, another layer of SS30G and finally 215mm fill. Future phases will involve different grids with twin layers of SS30 and 215mm fill.

Once complete, MMB will lay a 4mm geocomposite gas dispersal and protection layer, a 1mm HDPE capping membrane and then a 6mm geocomposite drainage and protection layer on top.

Through this triple layer will run 150mm diameter gas monitoring pipework with a perforated base sitting in single size stone, extending through the upper layers of 850mm subsoil and 150mm topsoil that will bring the site to final level.

But before work could begin the problem was how to build the necessary access roads for the excavators and dumpers on the huge gelatinous mass. MMB needed a material that would fl oat on top of the sludge and provide a working platform for the machines. As chance would have it, the answer was not just close to hand, it was already onsite.

Another byproduct of the water treatment plant is a fine ash produced by the onsite incinerator.

'The ash was the only material that would sit on the sludge and take the weight of our plant, ' says Root.

So in June this year, Bentley began putting down a Terram geogrid on the sludge and then added a layer of ash up to 1m thick.

These access roads, which are spaced up to 31m apart, allow a dumper running on big load spreading balloon tyres to supply an excavator with recycled aggregate, although the excavator noticeably wobbles as it works.

Root says: 'We're doing the capping nice and easy, otherwise dumping aggregate in one area would just displace the sludge somewhere else. It's like spinning 25 plates and making sure it's done evenly across the site.' However, by the time the project completes the aggregate and geogrid solution will cover the entire site, including the access roads.

Work on the £6M project is due to finish in spring 2008.

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