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.