What it costs: Pervious paving

Education and health buildings are invariably associated with hectares of hard landscaping in the form of playgrounds, car parks and drives that can and should be built with sustainable rainwater principles in mind.

Pervious paving is a form of sustainable urban drainage system (SUDS), which encompasses a range of constructions to managed rainwater. There are many design options available:

• The porous sub-base attenuates and filters water
• Water may be collected for reuse
• Sediments may be removed, directing the water to a settling pond with the additional benefit of creating another ecosystem.
• When paving costs alone are considered, pervious paving may be more expensive than impervious, however, savings may be made by not having to install a drainage system.

Pervious paving systems

A pervious pavement comprises a paving layer, an optional geotextile membrane and a sub-base, which is made up of aggregates with a low fines content to maximise voids, as this layer is the first to absorb rainwater. These layers are laid on the sub-grade which is the underlying ground, also known as the formation level.

There are two general types of pavements: porous ones, which allow water through their entire surface area, and permeable pavements, which are made up of impermeable units with gaps between joints.

There are a number of infiltration options:

• Type A: total infiltration. All rainfall passes through the paving and infiltrates the soil below. No discharge is expected from this system, but it is prudent to include features to deal with overflow if there is a risk of the paving silting up or heavy rain
• Type B: partial infiltration. This system includes perforated pipes at formation level to remove rainwater that cannot be absorbed by the sub-grade
• Type C: no infiltration. These systems catch the rainwater in an impermeable flexible membrane laid on the formation level. Perforated pipes or fin drains take the water to an outlet. This system is used where soils are impermeable, where water is to be retained, where there is a high water table or if the soil is contaminated.

There are a range of options for the pavement layer, which can be specified depending on the strength required and the overall look and feel of the scheme:

• Porous asphalt. These are generally proprietary products and are mainly used for lightly trafficked surfaces, but there are types available for main roads. A 40-65mm wearing course is available in a range of colours
• Porous concrete. This comprises only aggregate (10-20mm), water and Portland cement. Sand or fine aggregate is excluded. The quantities of cement and water are carefully controlled to obtain a thick coating around the aggregate. Porous concrete has a lower strength than conventional concrete
• Reinforced grass surfaces. This is based on a plastic mesh laid on the soil, through which grass grows. Concrete-based grass surfaces may be constructed from precast, unreinforced concrete grass paving blocks or insitu reinforced concrete grass paving construction with grass growing though voids in the surface
• Concrete or clay blocks or pavers. The standards associated with blocks do not deal expressly with permeable paving but do give a guide to their performance: BS EN 1338 (formerly BS 6717) for concrete blocks and BS EN 1344 for clay pavers. BS 7533 offers design and laying guidance.

Design and installation issues

It is essential to get the right structural design for the expected loading on the pavement and the bearing capacity of the sub-grade. This may include removing sub-grade softspots. The sub-base is laid in layers and compacted, taking care not to crush the aggregate and reduce the voids.

It is also important to ensure that the surface infiltration rate is greater than the expected rainfall intensity. This means having sufficient infiltration capacity or subsurface storage or discharge.

Take care with planting to limit the risk of root damage. Design the edge and restraint details to ensure the pervious paving is well restrained. The pavement over services routes such as gas or electricity may not need to be pervious. If geotextiles are used they must be protected from ultraviolet light.

Maintenance

Regular inspection and maintenance are essential. For block paving this means at least three sweep and vacuum sessions a year. Other maintenance should be instigated when required.