EH Smith

Permeable Paving

Sustainable development presents a strategy for society to follow such that progress is in harmony with the natural world rather than in conflict with it.
Increasing pressure from planning agencies to develop ‘brown field’ sites in towns and cities is concentrating attention on the issues associated with sustainable rainwater disposal.

Rain falling on paved areas is normally drained via gulley outlets to sewerage systems that discharge into streams and rivers. Increased run-off from the growing number of roads, paved areas and residential and industrial buildings therefore increases the risk of sewers becoming overloaded, receptive watercourses flooding and possible damage to river habitats. At the same time water table levels may become depleted.

Surface water run-off is often thought to be clean. It can, however, contain a range of contaminants including oils, organic matter, heavy metals and toxic materials.

Even when highly diluted, these can adversely affect water quality. The protection of streams and rivers from the damaging effects of flooding and
pollutants can be eased by the implementation of Sustainable Drainage
Systems (SuDS). These present a series of design options that permit a designer to elect the best solution for a particular site.
SuDS include grass swales, filter strips, retention ponds, wetlands and permeable pavements.

SuDS offer a number of advantages over-conventional drainage systems:-
• They deal with run-off close to where rain falls
• They reduce the loading on existing sewerage systems
• They help to reduce instances of flooding
• They protect urban watercourses from accidental spillage and pollution.
• They enable new development in areas where existing sewerage systems are at or near full capacity
• They permit direct recharging of ground water resources where conditions are suitable
• They permit simpler and more cost effective drainage solutions

Permeable pavements allow ‘run-off’ to pass through the surface rather than running off it as would normally be the case. The pavement structure and the materials used must therefore be designed with this in mind. Depending on ground conditions, either a fully ‘tanked system’ or an ‘exfiltration system’ can be specified.

With tanked systems water is temporarily held in a reservoir structure under the pavement formed using an impervious plastic membrane. The water is then allowed to discharge slowly through a small diameter pipe to a watercourse or an existing drainage system. Exfiltration systems are formed in a similar way to tanked systems but should only be used over a suitably pervious subgrade. The impervious plastic membrane is replaced
with a woven geotextile material that allows water to pass through it into the subgrade. Subgrade soils suitable for use with exfiltration systems are assessed using a falling head permeameter. When filled to a depth of 150mm with clean water, the permeameter should empty in 20 minutes
or less.

Any additional costs in constructing permeable pavements are generally more than offset by the savings made by eliminating conventional drainage networks and the simplification of surface drainage profiles.

If you would like some specialist advice, please contact us on specifications@ehsmith.co.uk for further information.