Porous surfaces

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Porous surfaces are constructed using materials containing small or large voids that allow water to pass through, but are strong and durable enough to support the weight of users. Porous surfaces are used as part of sustainable drainage systems (SUDS) to control storm water at source. Rain falling onto a porous surface is stored and released into the ground in a controlled way rather than producing surface water runoff. This process filters out harmful pollutants and reduces the flow and volume of water runoff compared with impervious surfaces such as bitmac or asphalt.

Porous surfaces are a relatively recent introduction to path management, but their use is likely to rise in the coming years. The base layer needs to be free draining to allow water to drain through the stone to reach the underlying ground. Open graded aggregate, such as Type 3 could be used, which is composed of a limited range of stone sizes with open voids between the individual stones to provide free draining properties. The base layer must be designed so that the water level never rises into the porous surface. The surface of porous paths do not need a camber or cross fall for shedding water as water drains directly down through the constructed layers.

Materials

Porous asphalt

Asphalt can be laid as a porous surface by removing the sand from the aggregate mixture to create voids so water can drain freely through. It is generally even more expensive than standard hot rolled asphalt, and is laid in the same way, to a high specification by paving machine and has a coarser texture. It has a long service life and can be recycled.

Resin bound paving

resin bound gravel

A mixture of resin binder and washed single sized aggregate (size range 6mm – 12mm) is laid and spread over open graded stone base layer to provide a durable and porous surface. Each aggregate particle is fully coated with UV resin before laying. The mixture is spread and levelled by hand held screeding equipment. The aggregate particles adhere to one another and to the prepared base layer surface whilst leaving voids between each aggregate particle for water to pass through.

 

Specification
icon Bound Gravel Path - Standard Detail Drawing & Specification Details

 

Reinforced grass / gravel paving

reinforced grass paving

reinforced gravel paving

Interlocking and continuous plastic ‘cellular’ paving systems provide strong structural load bearing surfaces that will take the weight of users. These paving systems have a large amount of surface void space allowing rain water to drain through easily and quickly. For porous grass paving, the cells are filled with free draining soil (or sand / soil mixture) and seeded with grass mix. For porous gravel paving, the cells are filled with clean angular gravel over a free draining sand bedding layer. The individual cells contain and prevent the gravel substrate moving laterally. For both systems, the cells are supported on an open graded stone base layer, covered with a permeable geotextile. This prevents the bedding layer from being washed into the voids between the large stone particles, maintaining the free draining base layer. A second geotextile may be required below the base layer to separate the free draining stone from the formation layer, preventing soil clogging and moving upwards through the open voids between the large stone particles which will eventually stop water draining through and back into the ground below.

To find out more about reinforced grass paving have a look here.

 

Specification
icon Reinforced Grass Path - Standard Detail Drawing & Specification Details
icon Reinforced Gravel Path - Standard Detail Drawing & Specification Details

 

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