Building roads and access tracks over soft, waterlogged, or low-bearing-capacity ground is one of civil engineering’s persistent challenges. The conventional solution, deep excavation and replacement with competent fill material, is expensive, disruptive, and generates significant waste from the excavated material. Where the soft zone is deep or extensive, the volume of material to excavate and replace becomes prohibitive.
Tyre bales offer an alternative. As a low-density, high-void material with good load distribution characteristics, a tyre bale layer beneath a road pavement reduces the load transmitted to the soft subgrade, provides free drainage, and can be installed at a fraction of the cost and disruption of full excavation and replacement. The technique is well-established, supported by published research, and standardised through PAS 108, which provides the specification framework for the tyre bales used in these applications.
Understanding how and where tyre bale road foundations work, and what the specification requirements are, is the starting point for any engineer, contractor, or waste tyre processor looking to engage with this application.
The engineering principle behind tyre bale road foundations is straightforward. A conventional road pavement transmits load from the surface through successive layers to the subgrade. When the subgrade is soft or waterlogged, it cannot accept this load without deforming, which causes the pavement above to fail. The conventional response is to stiffen the subgrade through replacement, stabilisation, or deep improvement works.
A tyre bale layer takes a different approach. Rather than stiffening the soft zone, the bale layer reduces the load applied to it. Tyre bales have a bulk density significantly lower than conventional fill materials. A PAS 108-compliant tyre bale has a density typically in the range of 500 to 700 kg/m³, compared to 1,800 to 2,200 kg/m³ for granular fill material. Replacing a deep section of granular fill with a tyre bale layer reduces the total load on the subgrade beneath, allowing the soft ground to support the structure without the pavement improvement that would otherwise be needed.
The tyre bale layer also provides drainage. The void space within a bale layer allows water to move freely laterally, preventing the build-up of pore water pressure beneath the pavement that accelerates soft ground failure. This drainage function is particularly valuable in applications over waterlogged ground.
At the surface of the bale layer, a granular capping layer and pavement structure distribute wheel loads across multiple bales rather than concentrating them on individual units. The bale layer acts as a mattress: distributing load, providing drainage, and reducing the net vertical stress applied to the soft subgrade.
The technique is best suited to specific ground conditions and road types. It is not appropriate for all soft ground situations, and understanding where it works well is important for both engineers specifying the solution and recyclers supplying the material.
Access roads and haul roads on construction sites. Temporary access roads on construction sites frequently cross areas of disturbed, waterlogged, or low-bearing-capacity ground. Tyre bale foundations provide a cost-effective temporary access solution that can be decommissioned at the end of the project. The bales are excavated and the material managed as waste at decommissioning.
Rural roads and farm tracks over poor ground. Low-traffic rural roads and agricultural access tracks over peaty, silty, or seasonally waterlogged ground are a common application. The low traffic volumes mean the pavement loading is modest, and the tyre bale foundation is typically designed for this reduced load regime.
Permanent roads over areas of poor bearing capacity. Where poor ground conditions extend over a large area and the road is permanent infrastructure, a tyre bale foundation layer designed to PAS 108 can be a cost-effective permanent solution. The bale material is stable over long periods when properly installed and has been used in permanent road construction in a number of UK infrastructure projects.
Road widening and verge strengthening. Where an existing road is being widened or the verge needs to be strengthened to support heavier loading, tyre bale foundations allow the works to proceed over the soft ground in the verge zone without major excavation.
The design of a tyre bale road foundation is an engineering task requiring appropriate geotechnical input. The following are the key design considerations, presented as context for understanding the application rather than as design guidance.
Subgrade characterisation. The bearing capacity and compressibility of the soft subgrade determines how much load reduction the bale layer needs to provide. Standard geotechnical investigation methods (trial pits, boreholes, in-situ testing) provide the data needed for design.
Bale layer depth. A single layer of bales provides a certain load reduction. Multiple layers provide greater reduction but add cost. The design balances required load reduction against bale layer depth.
Pavement structure. The road pavement above the bale layer must distribute wheel loads adequately across the bale surface. An insufficiently robust pavement structure can cause differential settlement between bale joints or point loading on individual bales. Typical designs use a granular capping layer over the bales before the road pavement structure.
Water management. The free-draining nature of the bale layer is an asset, but water entering the bale layer must have a route to exit. Edge drains and outfall arrangements need to be incorporated into the design.
Differential settlement. Where the soft ground zone is not uniform (which is almost always the case), differential settlement between areas over soft ground and areas over competent ground needs to be managed. Transition zones and structural arrangements to limit differential movement at the boundaries are standard design features.
The published research base supporting tyre bale road construction, including WRAP guidance documents and peer-reviewed geotechnical research, provides the material property data needed for design. Engineers new to tyre bale applications can reference this material for design parameter values.
In road construction applications, PAS 108 compliance matters because the engineer’s design is based on the material properties that the standard defines. A tyre bale road foundation design uses values for bale density, compressive stiffness, and permeability derived from PAS 108-compliant bales tested in controlled conditions. If the bales actually installed are not compliant, the installed material may not behave as the design predicts.
The consequences of non-compliance range from minor (a bale layer that performs adequately but not optimally) to significant (a layer that deforms excessively under load, causing premature pavement failure). For road foundations carrying any significant traffic, the risk of non-compliant bales performing below the design assumption is not worth accepting.
For tyre recyclers supplying bales to road construction projects, PAS 108 compliance is typically a contractual requirement. The production of PAS 108-compliant bales from the MKII Tyre Baler at Gradeall’s tyre recycling equipment specification is the standard starting point for operations targeting this market.
For tyre recycling operations looking to supply the road construction market, there are several practical considerations beyond PAS 108 compliance itself.
Volume and delivery logistics. Road construction projects consume tyre bales in significant quantities. A small access road project might need 200 to 500 bales; a larger project could require several thousand. Production volume and delivery logistics need to match project requirements. Bales are typically delivered on flatbed vehicles and installed by excavator directly from the delivery vehicle.
Documentation. Civil engineering procurement contracts typically require production documentation demonstrating PAS 108 compliance for each batch supplied. Set up a production record system before supplying the construction market, not after the first order arrives.
Pre-processing for truck tyre bales. If your supply includes truck tyre bales, sidewall cutting with the truck tyre sidewall cutter before baling is essential for consistent PAS 108 compliance. Truck tyre bales without sidewall cutting are harder to consistently bring within the standard’s density requirements.
Bale consistency. Road construction engineers need consistent product. A batch of bales where half are at the upper dimension tolerance and half at the lower creates field installation problems. Monitor bale dimensions and mass from the MKII Tyre Baler against the specification throughout each production run and adjust loading protocols if drift appears.
Contact Gradeall International for guidance on equipment specification and production setup for PAS 108 bale supply to the civil engineering and road construction market. With nearly 40 years of manufacturing experience and equipment operating in over 100 countries, the Gradeall team can advise on the complete production setup for this application.
Tyre bale foundations have been used in permanent road applications including access roads for commercial and light industrial use. For major roads carrying frequent HGV traffic at high speed, the design requirements are more demanding and careful geotechnical assessment is needed. The application is most straightforward for low-traffic rural roads, site access, and haul roads where design loads are more modest.
Research and field monitoring of tyre bale road foundations in service indicates that PAS 108 bales are dimensionally stable over extended periods when properly installed. Published case studies report bale installations performing well over 10 to 15 years of service without significant degradation. Long-term stability is one of the reasons rubber is considered a durable material for civil engineering use.
The road construction itself typically requires consent; the use of tyre bales as a foundation material within an approved road construction project generally does not require separate consent. Where the tyre bales are placed in or near a watercourse, or on land with specific environmental designations, additional consents may be required. Confirm with the relevant planning and environmental authorities for each specific project.
The free-draining characteristics of a tyre bale layer are an advantage in areas where the water table fluctuates or the ground is seasonally waterlogged. PAS 108 tyre bales are not adversely affected by saturation; the void structure allows water to drain freely when the water table falls. This makes them particularly suitable for roads in flood-prone areas where ground conditions are otherwise poor.
Cost comparisons are highly site-specific and depend on the depth and extent of soft ground, the availability and cost of alternative fill materials, and the disposal cost for excavated material. Published case studies report cost savings of 30 to 60 percent compared to conventional full excavation and replacement in suitable applications. An engineer familiar with the technique can assess the likely saving for a specific site.
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