Leachate drainage layers are necessary in most waste landfill sites to minimise the accumulation of leachate within the site and they reduce the risk of contamination of surrounding ground and groundwater. A cheaper and environmentally preferable option is be the use of scrap vehicle tyres, but is their use permissible and what happens to them under pressure? A paper in the proceedings of Waste 2004 by A.P. Hudson, R.P. Beavan, and W. Powrie helps us to understand this.

Normally layers of whole or shredded tyres exhibit excellent drainage properties, but if tyres are used as the main drainage layer at the base of a landfill the concern exists that they may compress under the overburden stress from the weight of the waste above and cease to act as an effective drainage layer.

The results of a series of tests undertaken by the University of Southampton are reported by the above researchers as presented in their paper examining the compressibility and changes in hydrogeological properties of shredded and whole tyres subjected to a range of stresses typical of landfill conditions.

In the UK over 400,000 tonnes of used vehicle tyres are produced each year (Hird et al. 2002). The problem of disposing of used tyres has been made worse by the EU Landfill Directive which prohibited the disposal of whole used vehicle tyres to new landfills from 16 July 2003. The disposal of shredded tyres to landfill will be banned on 16 July 2006. There is, therefore, a need to establish alternative methods of re-use, materials/energy recovery and disposal of tyres.

The Landfill Directive permits used tyres to be utilised as engineering material in landfills. Use of whole or shredded tyres are often a cheaper and environmentally beneficial alternative to aggregates for the construction of landfill drainage layers or trenches. However drainage layers at the base of landfills will be subjected to high overburden stresses from waste subsequently placed above.

There is little published research indicating i) the extent to which tyre drainage layers will compress under such stresses, ii) the reduction in hydraulic conductivity due to compression and iii) the effect of tyre shred size on the compressibility and hydraulic conductivity of tyre layers. However, these atters have been addressed in their paper in a large scale compression cell in order to investigate the above.

The data demonstrated that tyre layers will compress under stress and this will result in a reduction of drainable porosity and hydraulic conductivity. The construction of any leachate drainage layer using whole or shredded tyres within a landfill would need to take into account the compressive behaviour of the material under load.

Countries that have specified a minimum hydraulic conductivity for landfill drainage layers generally give values of between 1 x 10^-3 and 1 x 10^-4 m/s.

However, this group found that shredded tyres would easily comply with requirements as low as 1 x 10^-3 m/s at stresses up to 600 kPa, but would only meet the most stringent requirements of some nations at stresses below 400 kPa.

The data presented in this paper demonstrate that the hydrogeological properties of whole and shredded tyres change according to the applied stress. In general the data indicates that shredded tyres are suitable for use as a drainage medium in landfill applications.

Dense Asphaltic Concrete has been as a Landfill Lining membrane material for many years and although it is not commonly seen in the UK, it is used much more often in Europe, and particularly in Germany.

The Dense Asphaltic Concrete is formulated in a manner which ensures both very low permeability and possibly as low as 1 x 10^-11 m/s, and permeability to landfill gas is also lower than for clays.

This is an imporant point as the ability of clays to pass some methane gas is often overlooked for single clay liner designs. Let us not forget either that the primary motivation for the composite lining (clay/HDPE)systems which are the norm now throughout the British Isles, was originally the concerns about landfill gas movement through the clay membrane. It happens also to be far less likely that a pinhole in the HDPE will ever leak when in a composite arrangement that hole will be backed up by the clay geomembrane layer.

Asphaltic concrete is stable on steeper slopes than HDPE unless special measures are taken to support and/or reinforce the HDPE, and much less prone to the sort of slip plane development we often see between HDPE and clay and sand/HDPE on slopes and banking.

Another big assett when considering this alternative lining material is its robustness during the infilling of the first layer of waste. All CQA Engineers worth the name will have experienced HDPE lining damage which occurs when the top 300mm of leachate drianage stone is being emplaced, and then potentially can occur again when the compactor vehicle runs in with the first waste lift.

Resistivity checks completed once the leachate drainage stone over the membrane often identify small holes in the HDPE liner after the leachate stone has been spread and levelled. These tend to be caused by a moments lack of concentration which the driver may suffer. Unfortunaelty, one unfortunate jab downwords (as the backactor spreads the sand or gravel of the leachate drain over the HDPE geomembrane) can be all that it needs to created minor pin holes, and to see about a dozen occurring per hectare was not uncommon in CQA Reports, a few years ago.

A UK specialist contractor offering Dense Asphaltic Concrete in the UK is WALO UK. http://www.walo.co.uk/

In the design of municipal landfill leachate collection systems, some state regulatory agencies require carbonate content of leachate collection system aggregate not to exceed 15 percent by weight. This requirement comes from a legitimate concern about the possibility of aggregate degradation, or loss of mass due to contact with leachate.

Most involved in landfill design and development will have experienced as a result, the fact that in some areas it is difficult to find carbonate free stone within an reasonably economic distance from the site. Many potential aggregate sources have been eliminated for supplying drainage material, due to this stipulation in the specification, but is it really warranted?

While leachate in MSW landfills is capable of dropping to pHs of 6.5, and sometimes 6, it rarely falls below this other than for short periods. This does not seem to be so low that problems would necessarily be serious, and if any of the carbonate dissolved from the stone, the amount would presumably be low as the reaction would be self limiting due to the dissolved carbonate caused by the reaction being bound to raise the pH. High pH will not erode the carbonate so the problem is corrected.

There is not a huge amount of research work on this that we have been able to find. We would be very interested to receive comments if our readers have sources to research on this matter which are more authoritative than the paper I am about to refer to.

The best paper we have found which sets out to by experimentation over a reasonably extended time period (in this case just under 6 months) to investigate whether carbonate drainage stone, when submerged in leachate, will suffer damage, is the following paper:

Suitability of Carbonate Aggregate in Land fill Leachate Collection Systems; Christopher G. Rubak, PE John,O. Starke, PE William D. Upman, PG M. Merrill Stevens, PhD: Presented to the Nineteenth International Madison Waste Conference, September 25-26 1996, Dept of Engineering Professional Development, University of Wisconsin - Madison.

This paper summarizes a research project which evaluated the suitability of a carbonate aggregate with a municipal solid waste leachate. The tests were conducted over a 20 week period using site specific landfill leachate and collection aggregate. Laboratory bench reactors were constructed to simulate landfill conditions with leachate flowing through carbonate aggregate.

The reactors consisted of 12-inch diameter plexiglass cylinders each charged with 80 pounds of carbonate aggregate. Leachate was then circulated through the reactors. An anaerobic environment was maintained in the reactors by applying 0.5 Atmosphere of CO2.

Fresh leachate was added to the reactors on a regular basis to maintain a constant concentration level during the test. Leachate samples were analyzed to determine the change in dissolved solids throughout the test period. Aggregate material was measured before and after the test to determine net mass change. Chemical equilibrium speciation modelling was also performed and compared to the bench test results.

On the face of it this experiment showed that there was no need for concern about carbonate deterioration even down to the exceptional pH 3.0 (exceptional for an MSW landfill under good regulatory control, built to good current standards).

However, the strange thing about the experiment to the writer is that the leachate used was changed on only, I think, 3 occasions; other than on these occasions the leachate was simply recirculated.

I would have preferred to see results which would ensure that the natural circumstances of a landfill were replicated more closely, and that would have meant allowing fresh leachate to pass through the system all the time.

The views of our readers are encouraged. There is a commenting facility available on the Blog Site to enable you to very easily let us know your views on this.