Tyre Bale Physical Properties: Porosity, Permeability, and Shear Strength

By:     Conor Murphy

Civil engineering design is quantitative. An engineer specifying tyre bales in a road foundation, retaining wall drainage layer, or slope stabilisation structure cannot do so reliably without access to measured physical property data for the material. The design calculations for settlement, stability, and drainage performance all require numerical inputs that represent how the material actually behaves under load and in water contact.

The engineering property data for PAS 108-compliant tyre bales has been established through laboratory testing and field monitoring conducted over the period since WRAP began its research programme in the early 2000s. The published data, available through WRAP technical publications, academic journal papers, and BSI’s supporting documentation for PAS 108, provides the numerical values engineers use in design. This guide summarises the key engineering properties of PAS 108 tyre bales, explains what each property means for design, and indicates the range of values reported in the published literature.

All property values given here are from published research on PAS 108-compliant tyre bales. They are indicative ranges, not design values. Engineers should refer to the primary published sources and apply appropriate factors of safety for each specific application and project context.

Bulk Density: The Fundamental Property

Bulk density is the mass of the tyre bale divided by its total volume. It is the most fundamental property of a tyre bale as an engineering material because it determines the load that a tyre bale layer applies to the ground beneath it, and it influences almost every other engineering property of the material.

Published measurements of PAS 108-compliant tyre bale bulk density range from approximately 500 to 700 kg/m³, with most studies reporting values in the 550 to 650 kg/m³ range for car tyre bales. This compares to:

• Granular fill: 1,800 to 2,200 kg/m³
• Expanded polystyrene (EPS): 15 to 30 kg/m³
• Water: 1,000 kg/m³
• Saturated soft clay: 1,600 to 1,900 kg/m³

The low bulk density of tyre bales relative to granular fill is the property that makes them useful as lightweight fill in embankments and road foundations over soft ground. The load-reducing benefit of tyre bale fill compared to granular fill is directly proportional to the density difference.

Bulk density is determined by the tyre bale production process. The PAS 108 density requirement defines the minimum compression needed to achieve the material properties used in design. Under-compressed bales with density below the PAS 108 minimum have lower stiffness and higher creep deformation than the design values assume.

Porosity: The Void Space Within the Bale Layer

Porosity is the proportion of the total volume of a tyre bale fill layer that is void space (air or water) rather than solid rubber material. Porosity is critical for the drainage function of tyre bale structures, determining how much water a bale layer can store and how readily water moves through it.

The porosity of a tyre bale fill layer differs from the porosity of individual bales because there is void space both within each bale (between tyres within the compressed mass) and between adjacent bales in the layer. Published research reports total porosity values for tyre bale fill layers in the range of 30 to 50 percent.

For comparison, compacted granular sub-base fill has a typical porosity of 25 to 35 percent. The porosity of a tyre bale layer is similar to or slightly higher than granular fill, but the void structure is fundamentally different. Granular fill has small, interconnected pore spaces between individual particles. A tyre bale layer has large void spaces between bales, and the internal void space of each bale comprises the gaps between compressed tyres. Water moves more freely through the larger void structure of a bale layer than through the smaller pores of granular fill.

The 30 to 50 percent porosity of a tyre bale layer is what makes it useful as attenuation storage in SUDS applications: a 100 cubic metre bale fill zone can store 30 to 50 cubic metres of water. For drainage applications, the large void spaces ensure free drainage without the clogging that affects finer materials over time.

Permeability: Hydraulic Conductivity of Tyre Bale Fill

Hydraulic conductivity (permeability) describes how readily water flows through a material under a hydraulic gradient. High permeability means water moves through freely; low permeability means water is retarded. For tyre bale drainage applications (leachate drainage layers, retaining wall backfill, SUDS attenuation), permeability determines whether the material can convey the expected water flows without pressure build-up.

Published measurements of the hydraulic conductivity of tyre bale fill layers are typically in the range of 10⁻² to 10⁻¹ m/s. This is extremely high compared to most civil engineering drainage materials:

• Tyre bale fill: 10⁻² to 10⁻¹ m/s
• Coarse gravel drainage blanket: 10⁻² to 10⁻³ m/s
• Well-graded granular fill: 10⁻³ to 10⁻⁴ m/s
• Sand: 10⁻⁴ to 10⁻⁶ m/s
• Compacted clay: 10⁻⁸ to 10⁻¹⁰ m/s

The very high permeability of tyre bale fill means it will not become the limiting factor in any drainage system design under normal conditions. Water flows through a tyre bale layer far more readily than through the surrounding soil, which means the bale layer acts as a preferential drainage pathway in the way that a drainage blanket is intended to.

This high permeability is also why tyre bale drainage layers maintain their function over long service periods. The large void sizes are not susceptible to the fine particle migration clogging that affects some granular drainage media. Provided a geotextile filter is placed between the retained soil and the bale zone, the permeability of the bale layer is maintained over the service life of the structure.

Compressive Stiffness and Settlement Behaviour

The compressive stiffness of a tyre bale fill layer determines how much it deforms under the load of the overlying fill, pavement, or structure. For road foundation and embankment fill applications, the stiffness of the bale layer affects the total and differential settlement of the structure above it.

Tyre bales are not rigid materials. Under sustained compressive load, they exhibit an initial elastic compression followed by a creep component that continues over time. The elastic component recovers when the load is removed; the creep component does not. For permanent applications, the long-term settlement of a tyre bale fill layer must be calculated and compared against the tolerable settlement for the structure.

Published constrained modulus values for PAS 108 tyre bale fill range from approximately 200 to 600 kPa, significantly lower than the stiffness of well-compacted granular fill (typically 20,000 to 80,000 kPa). This means tyre bale fill deforms more under load than granular fill, which is a key design consideration for road and embankment applications.

The lower stiffness is not disqualifying for most applications because the load applied to the bale layer is also lower than the load that would be applied in an equivalent granular fill design. The soft ground benefit (reduced load on the subgrade) and the softer response of the bale layer combine in a way that must be considered together in design, not as separate factors.

Long-term creep behaviour of PAS 108 tyre bales has been studied through laboratory testing and field monitoring. Published data indicates that the majority of creep settlement occurs within the first few months of loading, with the rate of further settlement reducing significantly over time. Well-designed tyre bale fill structures reach a stable settled condition that does not continue to deteriorate over the design life.

Shear Strength: Stability Against Lateral Movement

Shear strength is the resistance of a material to sliding or shearing failure along a plane within or through it. In slope stabilisation applications, the shear strength of the bale fill determines how it resists the downslope gravitational forces that would otherwise cause movement.

The shear strength behaviour of a tyre bale fill layer depends on two mechanisms: the friction between adjacent bale surfaces (the inter-bale friction angle) and any interlocking or connection between bales that provides additional resistance to relative movement.

Published inter-bale friction angle values for tyre bales typically range from approximately 25 to 35 degrees, comparable to the friction angle of medium-dense granular fill. The texture of the tyre rubber surface and the contact geometry between adjacent bale faces both contribute to this friction angle.

Where bales are connected by threading wire through the tie wire loops (a common practice in slope and coastal applications), the connection provides additional resistance to inter-bale sliding that increases the effective shear strength of the structure. Design approaches that rely on connected bales use higher effective shear strength values than those for unconnected bale layers.

Thermal Properties: Relevant for Embankment Applications

Tyre bales have a lower thermal conductivity than most mineral fill materials, which means they provide some insulation within an embankment structure. This property is not typically the primary design consideration, but it may be relevant in applications where frost penetration into the subgrade is a concern.

The air-filled void spaces within a tyre bale fill layer provide additional thermal insulation compared to a solid rubber mass. The practical effect is a slight reduction in frost penetration depth beneath the bale layer, which may modestly reduce freeze-thaw effects on the subgrade in vulnerable locations.

For the full range of applications where these properties support design, Gradeall’s MKII Tyre Baler produces PAS 108 bales with the consistent density that these published property values are based on. Pre-processing with the truck tyre sidewall cutter for truck tyre bales and appropriate intake sorting ensure the production consistency that makes these design property ranges reliably achievable. See the full tyre recycling equipment range or contact Gradeall International for equipment specification guidance.

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