material, a major supplier of GM – geosynthetic clay liners – puts the case for using this manufactured product on landfill capping systems.

As the pressure for the restoration of closed landfill sites or completed phases of existing sites increases, the site operator or authority responsible for restoration is faced with a number of new challenges:

1 The lack of good quality capping clay in some parts of the country. The general downturn in construction activity in the recession of the early 1990s together with a longer-term trend towards the depletion of good quality clay sources have been the main factors operating. Good quality clay is defined here as clay which when worked and compacted will give a consistent permeability of less than 1 x 1 0 9m/sec.

2 More stringent demands on the quality of engineered clay caps, in line with the general improvement in quality standards of all lining and capping systems required by the Environment Agency. In practice, this has meant increased testing and other CQA, costs for compacted clay caps and more frequent quality rejections.

3 The increasing value of landfill void space, particularly since the introduction of the Landfill Tax. This has two main effects on landfill cap design:

a A tendency to optimise void space by using thinner capping systems such as GCLs (geosynthetic clay liners) or geomembranes rather than the traditional 1 metre of compacted clay.

b The re-profiling of caps to optimise void space. The end result of this is often a design

with steeper side slopes than previously.

4 The effect of the Landfill Tax on the quantity and quality of cover materials coming on to sites. The Landfill Tax (as was intended) has had the effect of reducing the quantity of soil wastes (eg from major earthworks projects) arriving at landfill sites. These materials are now often being used either to landscape the projects from which they originate or are being given to farmers who wish to reprofile their land (filling hollows etc). The consequence of this is that good quality cover materials are now scarce on some landfill sites.

The use of GCLs as a solution to some of these problems is an attractive option. The following is a list of the main benefits which GCLs could bring to address some of the above concerns:

GCLs are readily available throughoutthe country. Their equivalence in hydraulic conductivity terms to 1 metre of compacted clay is easily demonstrated especially at the low hydraulic heads applicable to most caps.

One truckload of GCL covers the same area as 300 truckloads of clay hence minimising traffic movements and on-site storage requirements.

Because GCLsare manufactured items, they can be supplied with MQA data and onsite conformance testing can be relatively simple compared with a compacted clay cap.

GCLs can optimise void space when compared with 1 metre of compacted clay. Some grades have excellent frictional and settlement characteristics ideal for use on steeper slopes.

The self-sealing characteristics of GCLc allows them greater flexibility than geomembranes to be used with sub-grade and cover materials containing a proportior of stone (which might otherwise have been screened out). This list is by no means exhaustive. The ease of installation and simple seaming method for GCLs are also advantageous as is their ability to be installed in less than perfect weather conditions at rates of typically 2,000m2/day. These advantages have recently been taken up by site operators throughout the UK and CETCO Europe’s project list for GCL caps now covers most areas of the country.

In terms of economic positioning, it should, be noted that the installed cost of a GCL cap is easily competitive with the alternatives of a metre of compacted clay (imported onto site) or bentonite enriched soil. Likewise, when final installed costs are taken into consideration GCLs can compete with geomembrane caps, depending on sitespecific factors such as quality and availability of cover material and site profile

Author David Rogers is a Senior Area Manager with CETCO Europe Ltd

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