Permeable Block Paving


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Permeable Block Paving

Permeable block paving helps to reduce flooding

Permeable block paving helps to reduce flooding

In response to the Government’s SUDS (Sustainable Urban Drainage Systems) policy, most block paving manufacturers have introduced a range of permeable block paving.

The blocks themselves are manufactured only slightly differently from standard ones, with a larger spacer nib on the side so that once installed, they sit further apart. This is so that they create wider gaps between them for drainage. The major difference between installing standard concrete block paving and permeable block paving is the use of open-graded stone for the sub-base and a course joint filling material rather than fine kiln-dried sand.

The sub-base is designed to allow the flow of surface water through to the soil sub-grade or in to a land drain or water harvesting system. The individual concrete blocks used for permeable block paving may be very similar to those used for standard concrete block paving but the structure of the installation and its performance is very different.

How much is permeable block paving?

The final, installed cost of permeable block paving can be as much as 20% to 50% more expensive than standard concrete block paving. This is not only due to the additional depth of the sub-base which not only takes longer to prepare but the fact that it also requires more aggregate material. It is also due to the economies of scale enjoyed by the vast quantity of standard blocks manufactured compared to the much smaller quantity of permeable blocks.

Blocks used in permeable block paving have nibs on the side to space the blocks further apart, aiding drainage

Permeable block paving blocks

Apart from the wider gaps between the blocks in permeable block paving compared to standard block paving, the appearance is identical. Although permeable concrete blocks are available in a wide choice of colours, textures, shapes and sizes, most manufacturers will offer a limited range of products compared to their standard range. Only the most popular blocks will be made due to high manufacturing set up costs.

How Does Laying Permeable Block Paving Differ from Standard Blocks?

Standard block paving can be used, without a planning application, when it is possible to provide rainwater run-off from a driveway into a border, soak away or rain garden contained within the boundary of the property. Standard concrete block paving sits on a compacted layer of hard core or Type 1 MOT (crushed carboniferous limestone with particle size from 20 mm to dust), with a further layer of moist sand above, and once the blocks are installed the gaps between them are filled with kiln dried or silica sand to strengthen the surface and hold the blocks firm.

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The sub-base does not allow water through and since SUDS legislation was introduced, surface water must be directed to an area within the boundary of the property to drain slowly into the ground – not to the domestic drainage system. Surface water can be directed through either concrete dished channel blocks or aco / linear drainage towards a soak away, a rain garden or to be collected for re-use.

Permeable block paving should be used where it is not possible to control water run-off without it being directed by gravity on to the public highway and into the public sewer drainage system. Permeable block paving relies on the porosity of the sub-base to allow surface water to infiltrate through to the sub-grade or soil and the type of soil can determine, in part, the design of the various layers of sub-base. There are a number of different ways that the sub-base can be designed and these are determined by the type of soil and whether the surface water will be allowed to infiltrate into the sub-grade or soil, or be collected.

Permeable block paving should sit on Type 3 MOT sub-base which has aggregates ranging from 20 mm down to 4 mm. It is clean, crushed open-graded carboniferous limestone which allows water to be stored in the spaces between the aggregate. Both the laying course and the joint filling material is clean, crushed gravel with a particle size from 6 mm to 2 mm, again to allow water to pass through. Conventional jointing sand and other fine materials are not used as they would clog up the pores between the open-graded aggregate and prevent water infiltrating into the sub-grade material.

Methods of Laying Permeable Block Paving

Total Infiltration

This method is used where the sub-grade, or soil, is capable of absorbing all the surface drainage water which infiltrates through the crushed gravel between the blocks and then through the open-graded sub-base material.

The sub-base material is still compacted in the same way as for standard block paving to provide a firm foundation for the blocks. The open-graded texture of the crushed stone without fine material will typically provide a 33% void for water storage to slow down the infiltration of surface water into the soil below.

The concrete blocks sit on various layers of sub-base which consist of 50 mm bedding course aggregate (2 mm to 6 mm crushed angular gravel or rock), on up to 250 mm of Type 3 MOT (4 mm to 20 mm crushed course aggregate), on a geotextile membrane. The membrane is positioned directly onto the soil and up the sides of the sub-base materials and the blocks – once the paving is installed it can be trimmed off at surface level. The thickness of the main sub-base will be determined by the stability of the sub-grade soil and the amount of water storage required.

Partial Infiltration

This is used when the existing sub-grade, or soil, is capable of absorbing some but not all of the surface water which infiltrates down to it, such as in the case of partly clay soils.

The sub-base specification is similar to that for total infiltration but without additional measures the sub-grade layer could become waterlogged. A perforated pipe or land drain is embedded within the main sub-base layer to take the excess water which the sub-grade cannot cope with. Water collected by the perforated pipe can legally be directed to public drainage systems but the ‘peak discharge rate’ must first be agreed with the relevant Local Authority.

Tanked Systems

Tanked, or non-infiltration systems are used when the sub-grade soil is almost incapable of absorbing any surface water which infiltrates through the sub-base. They can also be used when the sub-grade is of such poor quality that water ingress would cause it to deteriorate to such a point that it could break up or be washed away, causing subsidence of the sub-base and blocks.

The specification is the same as the partial infiltration system above, except that the permeable geotextile membrane is replaced by an impermeable flexible geo-membrane and it sits on 50 mm of dense sub-grade capping layer of stone with surface fines to protect the membrane.

The term tanked system refers to the fact that all of the new products installed, including the blocks and the aggregates are enclosed within an impermeable tank, created by the impermeable geo-membrane. The open pores of the sub-base become waterlogged by surface water filtered from above and the task of the perforated pipe is to collect this water and direct it to a drainage system, thus providing space for more water from the next downpour.

Permeable Sub-base Replacement Systems and Rainwater Harvesting

These are proprietary sub-base systems consisting of a series of hollow plastic crates or boxes which fasten together to form a raft and replace all or part of the aggregate sub-base. The crate structures are geo-cellular boxes which form rainwater attenuation tanks (water storage devices) whilst also providing support for the bedding course and blocks above.

Whilst an open-graded aggregate sub-base will make available around 33% of its overall volume for water storage in the pores between the course stone particles, these replacement crates provide up to 95% of their inner space for water to infiltrate into, from between the concrete blocks.

The water collected in these geo-cellular crates can be legitimately discharged into household drains as it satisfies the SUDS requirements of slowing down the water underground prior to it flowing into public sewers. However, once the water has been collected it could be piped to a separate tank and re-used to water the garden or even to flush toilets if clean enough.

The collected water will have already been filtered through the course gravel bedding course before entering the geo-cellular crates but will probably need to be filtered again prior to use. Rainwater harvesting systems can include the collection of rainwater from roofs and other areas around the house and though they are not inexpensive to install, will at least save on water bills if you have a metered supply. They will also contribute to overall water efficiency.

The Polystorm cellular unit manufactured by Polypipe measures 1 metre x 500 mm x 400 mm and has a 95% void ratio, storing almost 0.2 m³ of water

Polystorm cellular unit used in permeable block paving

Polystorm cellular unit used in permeable block paving

Benefits of Permeable Block Paving
  • Permeable block paving is suitable for a wide range of domestic, commercial and industrial applications.
  • It provides a structural and durable paving surface whilst allowing rainwater to infiltrate the lower layers for temporary storage.
  • The Infiltration process removes a wide range of pollutants, preventing them from entering the public drainage system.
  • It allows infiltrated water to be harvested for re-use, saving on water bills for a metered supply and contributing to water efficiency. Rainwater harvesting systems can be adapted to deal with water from roofs and other areas around the property.
  • It optimises land use by combining two functions – structural paving plus water storage.