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Permeable Tarmacadam
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Permeable Tarmacadam Driveways

Permeable tarmacadam or asphalt is also known as porous or pervious, though in the trade it is almost always referred to as open-graded tarmacadam and in simple terms is merely the standard hot-mix product with reduced sand or fines (finely graded aggregates) which enables water to drain through it.

The reduced fines in the permeable tarmacadam mix provides interconnected but stable air pockets within the layers of tarmacadam which allow rainwater to flow through it. To be effective however, permeable tarmacadam relies on the aggregate sub-base layers also being porous whilst still supporting the tarmacadam. Properly installed permeable tarmacadam will reduce both the rainwater run-off volume and run-off rate, and also reduce pollutants entering storm water drainage systems.

Permeable tarmacadam systems can be designed to collect or harvest rainwater for later use, or allow the surface rainwater simply to infiltrate the ground. In terms of a basic installation there should be no real difference in cost between a permeable tarmacadam driveway or patio and a standard one, and both offer similar durability in a domestic installation.

Permeable Tarmac
Permeable Tarmacadam
Flow test on permeable tarmacadam
Flow test on permeable tarmacadam

Layers of Permeable Tarmacadam

To provide an effective porous paving system, permeable tarmacadam cannot be installed without consideration to the sub-layers to ensure that the whole system works. The same equipment is used for mixing and laying permeable tarmacadam as for the standard product but less pressure is used when compacting, otherwise the pore space can be reduced. In a typical situation, the specification for a domestic driveway (working from the top) would be: –

  • Wearing course of approximately 30 mm of open-graded tarmacadam with 6 mm to 10 mm aggregate, bonded together with bituminous material.
  • The reduced quantity of fine aggregates would normally allow air voids amounting to around 15% of the volume to in order to permit water to flow through.
  • Sub-base of porous bituminous macadam containing 20 mm open-graded aggregate, compacted down to approximately 50 mm thickness.
  • Open-graded aggregate base, at least 100 mm thick consisting of crushed stone graded from 20 mm down to 5 mm.

This layer should be capable of not only storing water but should also act as a transition between the bituminous macadam layers and the sub-grade or soil, to help spread the load.

Permeable Tarmac - Perforated land drain

Perforated Land Drain

Where the sub-grade or soil is not capable of permitting rainwater to filter through it adequately such as with clay soils, a land drain may be required – this is a perforated pipe which collects and stores water for later filtration and takes it away to a soak away or rain garden.

Where a land drain is used, the aggregate sub-base will normally be more than 100 mm thick

Permeable Tarmacadam – Other Considerations

Design of Permeable Tarmacadam Driveways

The key design parameters of a permeable tarmacadam surface are the load bearing ability and infiltration capacity of the soil, the storage capacity of the aggregate sub-base and the porosity of the two layers of tarmacadam.

Clay Soils

Often, clay soils provide neither sufficient load bearing ability nor an adequate infiltration rate for the rainwater and in these circumstances a thicker aggregate sub-base should be installed. This will compensate for both of these problems by providing more structural support for the tarmacadam and a greater storage capacity for surface water.

Surface Sediment

All types of permeable paving rely on free movement by gravity of surface water through their various layers and they are often criticised for clogging up the pores which allow such water infiltration.

The truth is that most permeable paving solutions have such efficient infiltration systems, that long term studies have found high initial infiltration rates, followed by a decrease, then a levelling off with time. But even when clogged with sediment they still manage acceptable levels of filtration of water down to the sub-grade soil or in to land drains.

However, wherever there is a potential issue with clogging up the pores within a permeable tarmacadam driveway or patio area, surface sediment should be kept to a minimum. The permeable tarmacadam surfacing area should be swept clean of sediment regularly and certainly sand should not be used as a treatment for snow and ice.

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