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LID SWM Planning and Design Guide β

Permeable paving

Overview

Permeable paving allows stormwater to drain through the surface and into a stone reservoir, where it infiltrates into the underlying native soil or is temporarily detained.

The following are different types of permeable paving:

  • Permeable interlocking concrete pavers (PICP)
  • Plastic or concrete grid systems
  • Pervious concrete
  • Pervious asphalt

Permeable paving is ideal for:

  • Sites with limited space for other surface stormwater BMPs
  • Projects such as low traffic roads, parking lots, driveways, pedestrian plazas and walkways

The fundamental components of a permeable paving system are:

  • interlocking blocks with infiltration spaces between
  • Precast pervious slabs or pavers
  • a cast in place surface without fines, so that the finish is pervious to water
  • a bedding course to stabilize the surface
  • underground storage layer of aggregate

Additional components may include:

Planning considerations

Permeable pavements are surfaces that encourage infiltration. They can be used in place of conventional asphalt or concrete pavement. These alternatives contain pores, spaces and joints for allowing stormwater to pass through to a stone base, where it infiltrates into underlying native soils or is temporarily detained. Types of permeable pavement include:

  • Pervious concrete
  • Porous asphalt
  • Permeable interlocking concrete pavers (PICP, or just permeable pavers)

Geometry and Site Layout

Parking lot with perforated pavers in stalls only, Singapore

Permeable paving can be used for entire parking lot areas or driveways and can be designed to receive runoff from adjacent impervious surfaces. For example, the parking spaces in a parking lot may be permeable pavers while the drive lanes themselves are impervious asphalt. In general, the impervious area should not exceed the area of the permeable paving which receives the runoff. A hybrid permeable paving/infiltration chamber design can feature connection of a roof downspout directly to the stone reservoir of the permeable paving system, which is sized to store runoff from both the pavement surface and the roof drainage area.

Pretreatment

In most designs, the surface acts as pretreatment to the stone reservoir below. Periodic vacuum sweeping and preventative measures like not storing snow or other materials on the pavement are critical to prevent clogging. Another pretreatment element is to have a choking layer above the coarse gravel storage reservoir.

Landscaping

Landscaped areas must drain away from permeable paving to prevent sediments from running onto the surface. Urban trees will benefit from being surrounded by permeable paving rather than impervious cover, because their roots receive more air and water. Interlocking pavers used around the base of a tree may be removed as the tree grows.

Risk of Groundwater Contamination

  • Stormwater infiltration practices should not receive runoff from high traffic areas where large amounts of de-icing salts are applied (e.g., busy highways), nor from pollution hot spots (e.g., source areas where land uses or activities have the potential to generate highly contaminated runoff such as vehicle fuelling, servicing or demolition areas, outdoor storage or handling areas for hazardous materials and some heavy industry sites)
  • Prioritize infiltration of runoff from source areas that are comparatively less contaminated such as low traffic roads and parking areas.

Heavy Vehicle Traffic

Many types of permeable surface are certified to ASSHTO-25, including PaveDrain PICP type paving, LSRCA headquarters, Newmarket, ON

Permeable paving is not typically used in locations subject to heavy loads. However, some permeable pavers are designed for heavy loads and have been used in commercial port loading and storage areas.

Setbacks from Buildings

Permeable paving should be located downslope from building foundations. If the paving does not receive runoff from other surfaces, no setback is required from building foundations. Otherwise, a minimum setback of 4 m down-gradient from building foundations is recommended.

On Private Property

If permeable paving systems are installed on private lots, property owners or managers will need education on their routine maintenance needs, understanding the long-term maintenance plan. They may also be subject to a legally binding maintenance agreement. An incentive program, such as a storm sewer user fee based on the area of impervious cover on a property that is directly connected to a storm sewer, could be used to encourage property owners or managers to maintain existing practices.

Design

Finish course

Consult the manufacturer for the design specifications of their product. In pervious concrete and porous asphalt systems, the concrete or asphalt mix specifications and construction procedures are key to proper functioning. These systems require well-trained and experienced contractors for installation.

Specifications for pervious concrete, porous asphalt, and permeable pavers
Material Specification Quantity
Pervious concrete
  • 28 day compressive strength = 5.5 to 20 MPa
  • Void ratio = 14% - 31%
  • Permeability = 900 - 21,500 mm/hr
Thickness will range from 100 mm - 150 mm depending on the expected loads
Porous asphalt
  • open-graded asphalt mix with a minimum of 16 % air voids
  • Polymer can be added to provide additional strength to heavy loads
Thickness will range from 50 mm - 100 mm depending on the expected loads
Permeable pavers
  • ASTM No. 8(5 mm dia) crushed aggregate is recommended for fill material in the paver openings.
  • Pavers shall meet the minimum material and standard specification for precast concrete pavers.
  • Pigment in concrete pavers shall conform to ASTM C 979 ACI Report No. 212.3R provides guidance on the use of pigments.
  • Maximum allowable breakage of product is 5%
  • For vehicle applications the minimum paver thickness is 80 mm
  • For pedestrian applications is 60 mm and joint widths should be no greater than 15 mm.

Foundation aggregates

Geogrids like these are sometimes incorporated into the layers of permeable pavement foundation aggregates to provide additional stability

Most OPSS aggregates are not recommended for use in permeable paving systems. The exception being type 'o' with a default void ratio of 0.3.

Geotextile

Geotextiles are not always necessary and may be prone to clogging. Consider using courses of finer aggregates or sand instead.

Sizing stone reservoirs

The stone reservoir must meet both runoff storage and structural support requirements. The bottom of the reservoir should be level so that water infiltrates evenly.
If the system is not designed for infiltration, the bottom should slope at 1 - 5% toward the underdrain.

Permeable paving: Sizing

Modeling permeable paving in the Treatment Train Tool

Permeable paving: TTT

Gallery

Landscaping and grading

Landscaped areas must be graded drain away from permeable paving to prevent sediment from running onto the surface.
Urban trees benefit from being surrounded by permeable pavement rather than impervious cover, because their roots receive more air and water. Block pavers around the base of a tree can be removed as the tree grows.

Performance

Starting after TRIECA (end March) members of STEP will be undertaking a literature review on the performance of our most popular BMPs. The results will be combined with the information we have to date from the development of the Treatment Train Tool and agreed performance metrics established. Until then, please feel free to continue to ask questions via email or the feedback box below.

Permeable pavers can be classified into two categories according to the infiltration rate of the underlying subsoil:

  • Full Infiltration: Full infiltration designs are more effective, because little if any of the pollutants generated on the impermeable surfaces leave the site as surface runoff
  • Partial Infiltration: Partial infiltration designs with underdrains generate more runoff

Studies in North Carolina have shown the average curve number of permeable pavements to range from a low of 45 to a high of 89. [1]
Partial infiltration designs with underdrains generate more runoff, and as a result, are often used in studies investigating the water quality impact of permeable pavements on surface waters. These studies show load reductions above 50% for total suspended solids, most metals and hydrocarbons [2][3]
As with all stormwater infiltration practices, risk of groundwater contamination from infiltration of runoff laden with road de-icing salt constituents (typically sodium and chloride) is a significant concern. Chloride ions are extremely mobile in the soil and are readily transported by percolating water to aquifers.

Construction Considerations

Construction of permeable pavement is a specialized project and should involve experienced contractors. The following general recommendations apply:

  • Sediment Control: The treatment area should be fully protected during construction so that no sediment reaches the permeable pavement system and proper erosion and sediment controls must be maintained on site.
  • Weather: Porous asphalt and pervious concrete will not properly pour and set in extremely high or low temperatures [4]. One benefit to using permeable pavers is that their installation is not weather dependent.
  • Pavement placement: Properly installed permeable pavement requires trained and experienced producers and construction contractors.

Inspection and Maintenance

Permeable pavements require regular inspection and maintenance to ensure proper functioning. The limiting factor for permeable pavers is clogging within the aggregate layers, filler, or underdrain. Ideally, signs should be posted on the site identifying permeable paver and porous pavement areas. This can also serve as a public awareness and education opportunity. See: Permeable paving: Maintenance

Life cycle costs

Initial construction costs for permeable pavements are typically higher than conventional asphalt pavement surfaces, largely due to thicker aggregate base needed for stormwater storage. However, the cost difference is reduced or eliminated when total life-cycle costs, or the total cost to construct and maintain the pavement over its lifespan, are considered. Other potential savings and benefits include reduced need for storm sewer pipes and other stormwater practices, less developable land consumed for stormwater treatment, and ancillary benefits (improved aesthetics and reduced urban heat island effect). These systems are especially cost effective in existing urban development where parking lot expansion is needed, but there is not sufficient space for other types of BMPs. They combine parking, stormwater infiltration, retention, and detention into one facility. See also: Cost analysis resources

Proprietary Links

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Pre-cast with spaces

Pre-cast pervious

Cast in place

Plastic grid


  1. Bean, E.Z., Hunt, W, F., Bidelspach, D.A. 2007a. Evaluation of Four Permeable Pavement Sites in Eastern North Carolina for Runoff Reduction and Water Quality Impacts. Journal of Irrigation and Drainage Engineering. Vol. 133. No. 6. pp. 583-592.
  2. Legret, M and V. Colandani. 1999. Effects of a porous pavement structure with a reservoir structure on runoff water: water quality and fate of metals. Water Science and Technology. 39(2): 111-117
  3. Pratt, C.J., Mantle, J.D.G., Schofield, P.A. 1995. UK research into the performance of permeable pavement reservoir structures in controlling stormwater discharge quantity and quality. Water Science Technology. Vol. 32. No. 1. pp. 63-69.
  4. City of Portland. 2004. Portland Stormwater Management Manual. Prepared by the Bureau of Environmental Services (BES). Portland, OR.