Specifying Tyre Bales for Construction: Engineer’s Guide to PAS 108

Updated on: 27th February 2026 By:     Conor Murphy
Expert review by:     Kieran Donnelly

Specifying tyre bales in UK civil engineering projects has a strong track record, from road foundations to retaining walls, yet the specification process still catches engineers out. Not because the material is difficult to specify, but because the relevant standards, design parameters, and procurement requirements are spread across multiple documents that aren’t always easy to find in one place.

This guide pulls it together. If you’re writing tyre bales into a construction contract for the first time, or tightening up a specification you’ve used before, you’ll find the PAS 108 compliance requirements, WRAP design parameter sources, procurement documentation, and on-site verification process covered in a single, practical reference.

Writing Tyre Bales Into Your Specification

Engineers who have used tyre bales in civil engineering applications know they work. Engineers encountering the material for the first time face a question that should not require significant effort to answer: how do I specify tyre bales properly in a construction contract, and what do I need to verify to be confident that what I receive matches what I specified?

The answer is straightforward once you understand the PAS 108 framework, the design parameter data available through WRAP publications, and the contractual and verification mechanisms that apply to specification compliance. This guide is written for practising civil and geotechnical engineers to provide that understanding in one place, covering specification language, procurement requirements, on-site verification, and design parameter sources.

The Specification Starting Point: PAS 108

Every structural tyre bale application in UK civil engineering specifies PAS 108 compliance as the material requirement. This is the standard published by BSI that defines bale dimensions, minimum mass, minimum density, tie wire specification, and quality management requirements.

When you write “PAS 108-compliant tyre bales” into your specification, you are referencing a defined product standard that the supplier must demonstrate compliance with. This is the correct approach. Do not specify “recycled tyre bales” without the PAS 108 qualifier; the unqualified description has no enforceable specification content and allows the supply of bales of any quality.

The specification clause for tyre bales in a bill of quantities or material specification document should state:

• Material type: PAS 108 Tyre Bales
• Reference standard: PAS 108, current edition
• Compliance evidence required: production records documenting bale dimensions, mass, and tyre input type for each batch supplied
• Verification: as specified in the project quality plan

Include the current edition date of PAS 108 in the specification. Standards are periodically updated; specifying the edition ensures you and the supplier are working to the same version.

Design Parameter Sources

The material property values you need for geotechnical design with tyre bales are not in PAS 108 itself. PAS 108 is a product specification standard; the design parameters come from published research.

The primary sources for tyre bale design parameters are:

WRAP Technical Guidance Documents. WRAP has published a series of technical guidance documents on tyre bale use in civil engineering, covering road foundations, embankment fill, drainage applications, and slope stabilisation. These documents tabulate design parameter values (density, compressive modulus, hydraulic conductivity, friction angle) derived from research programmes, and provide guidance on their application in design calculations. These are the first-stop reference for design parameter values.

Academic Research Literature. Peer-reviewed papers on tyre bale geotechnical properties have been published in Geotechnique, the Quarterly Journal of Engineering Geology and Hydrogeology, and conference proceedings of the British Geotechnical Association. For research-quality data or for applications where WRAP guidance values need supplementing, the academic literature provides more detailed test data.

Environment Agency Guidance. For applications involving potential water contact (drainage, SUDS, coastal), the Environment Agency has published guidance on the environmental assessment of waste-derived materials in civil engineering, which addresses the leachate question for tyre bales specifically.

Before using any design parameter value, confirm the PAS 108 compliance requirement on which it is based. Design parameters derived from research on PAS 108-compliant bales are not valid for non-compliant bales. This is the technical reason why PAS 108 compliance in supply is a design requirement, not merely a contractual preference.

Procurement Specification: What to Require from the Supplier

A robust procurement specification for tyre bales should require from the supplier:

Pre-supply documentation:

• Confirmation of PAS 108 compliance and production capability
• Description of the production quality management system
• Evidence of bale weighing capability in production
• Sample production records from recent production

With each delivery:

• Delivery note stating tyre type, production date, and quantity
• Production records for the batch showing bale dimensions and mass for a representative sample (or all bales if the system supports this)
• Compliance declaration by the producer against PAS 108 requirements

Supplier capability:

• Confirmation that the baling equipment is specified and maintained to produce PAS 108-compliant bales
• Confirmation of pre-processing capability (rim separation, tyre sorting, sidewall cutting for truck tyres)

Requiring this documentation from the outset, before the first delivery, establishes whether the supplier has a genuine quality management system or is claiming compliance without the production infrastructure to support it. Gradeall’s MKII Tyre Baler is designed for PAS 108 production; operations using Gradeall equipment with appropriate pre-processing should be able to provide this documentation without difficulty.

On-Site Verification of Delivered Bales

Even with a documented supplier quality management system, on-site verification of delivered bales is good practice for any project where bale quality is critical to structural performance. The verification process:

Dimensional checking. Measure the length, width, and height of a sample of bales from each delivery. A sample of 5 to 10 per cent of the delivery, or a minimum of five bales, is typical for medium-scale projects. Measurements should be taken with a tape measure on newly delivered bales before any handling that might deform them. Compare against PAS 108 dimensional targets and tolerances.

Mass verification. Weighing individual bales requires either a suspended load cell on the site’s lifting equipment or a weighbridge capable of sequential weighing of bales. On-site weighing of every bale is not practical on most sites, but a sample check of 5 to 10 bales per delivery provides confirmation that the batch is within the mass specification.

Tie count and placement. Visual inspection of the tie count and placement on a sample of bales. Count the number of ties on each sampled bale and check that they are distributed across the bale face rather than clustered at one position.

Visual quality assessment. Check for obvious defects: bales that are visibly expanding due to tie failure, bales with unusual geometry suggesting incorrect loading, or bales showing signs of having been stored wet for extended periods before delivery.

Non-conformance action. Establish in the project quality plan before delivery commences what action will be taken if on-site verification identifies non-compliant bales: rejection of the delivery, acceptance of a specific non-compliant proportion, or notification to the designer for assessment. Do not install bales that have failed on-site verification without the designer’s assessment.

Design Considerations: Applying Published Parameters

When designing with tyre bales, the following practical notes support the correct application of published design parameters:

Use PAS 108 material property values only. Design parameters derived from test data on PAS 108-compliant bales are not valid for non-compliant bales. The specification and supply verification process described above is the mechanism that ensures the installed material matches the design assumption.

Apply appropriate factors of safety. WRAP guidance values are presented as test-derived data ranges. Apply factors of safety appropriate to the specific application, design consequence, and the variability of the published data range. Geotechnical design with tyre bales should follow the same safety factor approach as design with any other geomaterial.

Consider long-term behaviour. Tyre bales exhibit creep deformation under sustained compressive load. For embankment and road foundation applications, calculate long-term creep settlement and compare against the tolerable settlement for the structure. Published data indicate that the majority of creep occurs within the first few months of loading, but for long-design-life structures, the full creep component should be estimated.

Drainage design is not optional. For applications where the tyre bale layer is performing a drainage function (retaining wall backfill, SUDS attenuation, leachate collection), the drainage design is a structural element of the scheme. The base drain capacity, outlet control, and outfall arrangement must all be designed for the expected flows. A tyre bale drainage layer without a functioning drainage outlet does not drain.

Environmental Consent and Regulatory Compliance

For applications that may require environmental consent, the engineer’s specification documents should address the regulatory requirements that apply to the use of waste-derived materials in civil engineering.

The Environment Agency’s position on tyre bale civil engineering use and the end-of-waste assessment framework for waste-derived materials should be reviewed for each application type. For straightforward road foundation and embankment fill applications on standard construction sites, the regulatory position is well-established. For applications near watercourses, in groundwater protection zones, or on environmentally sensitive sites, early engagement with the Environment Agency and reference to their guidance on waste-derived materials in engineering is needed before specification is finalised.

Contact Gradeall International for supplier information, production documentation examples, and guidance on equipment specification for PAS 108 production. The tyre recycling equipment range covers the full production line from pre-processing through baling.

Frequently Asked Questions

Tyre bale specification raises practical questions that aren’t always answered clearly in the standards themselves. Here are the ones engineers ask most often.

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