The Pferdekoppeln (horse paddocks) to the left and right of the road are what gave the nearby Koppelwald wood its name. In earlier times, chalk was quarried here before the site found use as a waste tip.

Today, from the idyll of the wood, this presents a problem: The tip was closed thirty years ago and Nature is winning back the site - but the environment must not be endangered by the contaminated ground. "The main threat is that during storms rainwater will wash pollutants out of the soil, which could then enter the ground water," says Guido Lieser from the contractor constructing the works, Backes.

Three tasks have to be achieved in the Koppelwald wood near the Saarland town of Ormesheim. The contaminated waste tip must be sealed at the top. An extensive drainage system must be installed to carry away rainwater seepage. And because the ground slopes at an angle of almost 22 degrees, the surface sealing membrane must be protected against sliding parallel to the slope (slip failure). To do this an area of approximately 90,000 square metres had to be prepared in such a way that the neighbouring woodland could spread outwards again.

The three tasks were accomplished in Koppelwald starting at the end of 2002 using three HUESKER products. Contractor Backes Bauunternehmung GmbH based in Tholey-Theley installed about 100,000 square metres of NaBento® RL-C synthetic clay liner to seal the one-time chalk quarry. The advantages of this product: NaBento® RL-C meets the then-applicable approval criteria of German construction accreditation body, the Deutsche Institut für Bautechnik (DIBt), for a single layer sealing membrane instead of the normally required 2-layer solution. This means only one synthetic clay liner needed to be installed - a saving in time and costs.

"The old tip is sealed off by the synthetic clay liner so that the risk of water ingress is considerably reduced," says Guido Lieser.

The drainage mat adopted as the solution to the drainage problem - also proved to be very effective. Originally, a 300mm thick gravel drain had been planned - but a drainage mat, which had a higher water carrying capacity - could be installed quicker and at a lower cost.

And then there was the land slip problem: in order to contain as large a volume of fill as possible and thus make the best use of the area, the final outer slope of the tip had been inclined very steeply. This is the reason that today we have to prevent the surface sealing works from sliding and a slip failure occurring.

When constructing steep slopes, the angle of internal friction between the individual layers is often inadequate. The slope must therefore be secured against slip failure. This is where Fortrac® geogrid, in this case with tensile strengths of 200 to 350 kN/m, could play its part. The product's characteristics were particularly important to this project. The geogrids readily allow water to pass through them, avoiding a build up of water above the drainage mat, and they are cost effective. The overall good durability and high dimensional stability of Fortrac® - designed for this project to have a life of 120 years and low elongation - will allow Nature sufficient time to reclaim and cover the former tip.

Photo as jpg-File (jpg, 157 kb)

Photo as jpg-File (jpg, 113 kb)

Photo as jpg-File (jpg, 157 kb)