In the UK, 25 million car tyres and 3 million truck tyres are discarded each year. Of this 18% is used for retreading, 6% for regeneration (eg granulation, carbon/oil extraction and steel recovery) and 9% is incinerated. This leaves 67% which is stockpiled, landfilled or illegally dumped, although the problem is far worse in the US. The best way to reduce the environmental and health hazards associated with scrap tyres is to minimise, and ultimately eliminate, stockpiling.
Waste tyre material has been used successfully on playgrounds and football fields and in paving materials. A potential use can be in soil stabilisation. Shredded tyre rubber can be mixed with different soils to improve their engineering properties for specific applications.
The main concern is the possible leaching of heavy metals from the tyre into the soil and groundwater. Very few studies have evaluated the potential engineering applications of shredded tyre in soil-tyre mixtures, which could include obvious uses such as part of a lightweight fill and less obvious ones such as in barriers and landfill liner and cover materials.
In relation to the latter application it has been observed that shredded tyre has a capacity to absorb petroleum-based hydrocarbons and in so doing may swell by up to three times its original volume. This applies swelling pressure on the soil, causing the permeability of the soil-tyre mixture to decrease. This is potentially a very useful finding because when permeating through soils organic chemicals, particularly hydrocarbons, increase permeability by up to three orders of magnitude as they cause the diffuse double layer to contract. Hence the presence of tyre in the landfill liner material could counterbalance this effect.
From a design point of view, the swelling pressure should be high enough to be effective but it should not exceed the overburden pressure on the liner so as not to jeopardise the integrity of the liner. Shredded tyre has also been found to sorb a variety of hazardous organic compounds, thus reducing the amount of leaching from a landfill into the soil and groundwater.
The level of removal of such compounds was found to be far more effective than earthen liners and geomembranes. Shredded tyre was also found to be effective as part of the drainage layer in a landfill leachate collection system and it was therefore postulated that shredded tyre would provide significant benefits as part of a landfill liner system.
An extensive research programme was initiated four years ago at Birmingham University on undergraduate and postgraduate levels (O Blackwell, 1995; SA Porter, NJ Bagley and T Aravinthan, 1996; EC Sowter and B Sampurno, 1997) and continued at Cambridge University (B Perry, 1998 and
P Waterfall and C Cogswell, ongoing).
These projects examined various geotechnical and geo-environmental aspects of the behaviour of soil-tyre mixtures. This included investigation of properties such as compaction, strength, stress-strain behaviour, permeability, compressibility, leachability, swelling and swelling pressure, staining and cracking. It included variables such as soil type, its moisture content and degree of compaction, use
of additives such as lime, tyre size and content, and contaminant type and concentration.
Apart from obvious effects such as decrease in dry density and strength, some of the observations made are:
Degree of cracking due to rebouncing of the compacted tyre depends on moisture content, compactive effort, soil type and tyre content.
Permeability reduction in the presence of certain hydrocarbons, such as gasoline and paraffin, by up to 50 times.
Reduced initial stiffness and prolonged strain range at maximum stress level.
Different leachability of heavy metals such as Cu and Ni for TCLP and NRA leaching tests reflecting the effect of extraction fluid pH and solid:liquid ratio used.
Leachability is significantly reduced due to the presence of lime.
Swelling of up to 40% and swelling pressures up to 600kPa were developed in the presence of hydrocarbons such as paraffin.
Figure 1 shows kaolin-tyre samples with 15% 4mm-8mm tyre shreds. The moisture content is optimum in the sample on the left and increased progressively in the other two samples. The figure shows reduced cracking as the moisture content is increased, indicating more effective interaction between the clay and tyre. The samples also show staining which was found to increase and change in colour with time.
Current research work includes the development of new mixtures using sand for more homogeneous mixing in practice, such as sand-cement-tyre mixtures and investigation of the absorption capacity of tyre.
Planned future work includes centrifuge testing to examine full-scale behaviour and applications and long term mechanical properties and leachability of soil-tyre mixtures.