Treatment MeasuresPorous Paving : Design Details
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Porous paving can provide excellent stormwater discharge and water quality management from street and car park surfaces. Design of porous pavements should address the reasons for past failures including:
- clogging with sediments
- limited infiltration capacity
- possibility of groundwater contamination.
Sediment Inputs
The majority of porous paving failures in the past have been due to partial or total clogging of the paving with sediment or oil, during construction and over the life of the pavement. The clogging problem can be overcome by designing suitable measures to ensure that the paving:
- does not receive runoff from areas that are likely to contribute sediment and debris
- is not constructed adjacent to areas subject to significant wind erosion
- is carefully protected from sediment inputs during construction
- does not receive high vehicular traffic volumes or regular use by heavy vehicles
- receives pre-treated runoff through the careful placement and design of sediment traps, vegetated filter strips or specially designed gutter systems.
In cases where clogging does occur, sediment can be removed by vacuum sweeping or high pressure hosing.
Limited Infiltration Capacity
The limited infiltration capacity of some porous paving can be attributed to clogging caused by sediment, soils with insufficient infiltration capacity and designs with insufficient storage volume. The problem of limited infiltration capacity can be overcome by using modern design practices to:
- provide a retention trench below the sand/gravel sub-base
- provide an overflow to the street drainage system or other stormwater management measure
- limit the runoff area contributing stormwater to the porous paving surface.
Risk of Groundwater Contamination
Porous paving can introduce a small risk of contamination to shallow aquifers by toxic materials derived from asphalt and vehicular traffic. This risk can be eliminated by following these design principles:
- Do not construct porous paving over shallow aquifers.
- Do not use porous paving on streets with high traffic volumes.
- Capture contaminants by laying concrete or plastic porous pavers on a sand sub-base over a retention trench lined with geotextile fabric lining.
- Maintain the structural integrity of the porous paving.
Slopes
Porous paving should not be constructed on slopes greater than 5%
Unsuitable Soils
Porous paving should not be placed on:
- wind blown or loose sands
- clay soils that Shrink/Swell and/or disperse when in contact with water
- soils with a hydraulic conductivity of less than 0.36 mm/hour.
Soil assessment and permeability testing should be undertaken as part of the design of porous paving
Rock and Shale
Porous paving should not be placed over rock that has little or no permeability. Studies have shown that infiltration is possible in severely weathered or fractured rock (for example sandstone). Engineering testing is essential in these circumstances to ensure that the rock will accept infiltration. In the case of shallow soil cover, testing is required to ensure that seepage does not cause hazard or nuisance to downstream sites.
Suitable Locations for Porous Paving
Porous paving can be utilised in streets with low traffic volumes, car parks and for paving within residential and commercial development.
Heavy vehicles do not impair the structural integrity of porous paving. If properly installed, porous pavements have similar load bearing and design life performance to conventional pavements.
