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BALEFUL EXPRESSION

CONTAMINATED LAND

TRL has been studying potential construction applications for tyre bales. Mike Winter reports.

About 38.7M tyres, or 440,000t, are scrapped annually in the UK. Scotland alone contributes 2.8M, or 32,000t. Most of these are sent for energy recovery, stockpiled, disposed of in landfill or dumped illegally.

In June 2003 the EC Landfill Directive outlawed the sending of whole tyres to landfill and this year will outlaw the disposal of shredded tyres to landfill.

However, there is no provision in the directive to allow the use of tyres shred for engineering purposes in landfill. Clearly alternative means of disposal are required.

It is expected many of the UK's used tyres will be consumed by energy recovery and waste-to-energy plant, including pyrolysis (decomposition through high temperatures) and burning in cement kilns. But a significant proportion will probably be available for alternative uses.

Potential applications for waste tyres include retreading, material recovery (mainly crumbing) for incorporation in new rubber products and civil engineering uses.

The latter take many forms including landfill engineering, lightweight fill, soil reinforcement, drainage, erosion control, artificial reefs, hydrocarbon retardation in ground barriers and noise barriers as well as thermal insulation.

Contractors can use varying forms of tyre material, from small crumb and shred particles up to whole tyres, depending on the application's technical requirements.

A more recent development is the fabrication of tyre bales. Waste management licensing has been a concern for some time as various regulators have taken differing views on whether tyres (and tyre bales) either constituted or should be treated as a waste.

However, late last year, UK regulators took the view that tyre baling and the use of such materials in construction was a low risk activity and that waste management licences would not be required.

A bespoke baling machine compresses 110 to 120 tyres that are restrained with tie-wires. The process produces a rectilinear bale of about 1.3m by 1.53m by 0.82m.3 density.

Tyre bales are much less dense than most conventional construction products, making them highly suitable for works over soft ground, and are also highly permeable.

These two features mean tyre bales are likely to play an increasing role in construction work. Additionally, the baling process is a low energy option compared with other tyre recovery processes.

TRL (Transport Research Laboratory) recently completed a project on tyre bales in construction and has published two reports.

PPR045 presents a series of case studies from the UK and the US. It covers a range of applications including the construction of road foundations over soft ground, slope failure repair and flood defence works.

PPR080 is the final project report and offers guidance on the design, construction and specification of tyre bales in construction. One of its main purposes was to help the tyre baling industry meet the need for the increasing volumes being used in construction while helping raise the value of those schemes.

The report presents possible approaches to the design and construction of a wide range of uses such as road foundations over soft ground, slope failure remediation, lightweight embankment fill, gravity retaining walls, drainage layers, storm water management systems and rainwater soakaways and environmental barriers.

These guides are supported by an extensive study of key attributes and behaviours including dimensions, volume, mass, density, mechanical properties, hydraulic properties and behaviours in relation to durability, contamination potential, fire resistance and health and safety.

They also present issues common to multiple applications including supply and construction of bales, handling, alignment and layout, fill around the bales, maximum height of tyre bale fill, cover depth and stability, drainage, contamination, typical costs and end of service life.

The bale manufacturing process is described in some detail and, with other information, used as the basis for an engineering specification.

During this project tyre bales were used in successful applications including road foundations over soft ground, slope failure remediation and soakaway construction.

Research was funded by Onyx Environmental Trust, Inverness and Nairn Enterprise and Scottish Executive. The TRL reports referred to are available from www. trl. co. uk

Mike Winter is regional manager (Scotland) for TRL.

Future research

The need for further research is clear. High quality mechanical tests should be a priority and include frictional response, stiffness and creep.

Tests to date have been either fit-for-purpose or conducted on bales in the US.

The fit-for-purpose tests have generally been designed to suit a project-specific purpose and may be more or less transferable to more general uses.

Tyres in the US are generally larger than those in Europe and so bales contain up to 17% fewer tyres. This will affect the internal structure of the bales and their properties but to what extent is a relative unknown.

TRL also considers pilot scale or full scale tests a priority. These may take the form of structures that are specifically constructed to be monitored in a controlled environment over the long-term or may be intended for test to failure.

Production of a specification which can be accepted by all quarters of the construction industry should also be a priority.

This will need to be supported by detailed technical information.

Monitoring of tyre bale structures for deformation, leachate and temperature in the long term would be a welcome opportunity to gather data of great value in con'rming behaviours implied largely from desk study information.

The lack of data should not preclude further use of tyre bales in construction, nor should it prevent the development of new applications. However, the availability of data should encourage and accelerate the take-up of tyre bales and the development of the applications and markets for them.

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