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Conceptual diagram illustrating an adjustable storage underdrain configuration beneath permeable interlocking pavers

Overview

Permeable pavements are an alternative to conventional impervious pavement that allows stormwater to drain through the surface and into a stone reservoir, where it infiltrates into the underlying native soil or is temporarily detained. Depending on the native soil properties and physical constraints, the system may be designed with no underdrain for full infiltration, with an underdrain for partial infiltration, or with an impermeable liner and underdrain for a non-infiltrating, or detention and filtration only practice.

Permeable pavement types include:

• Permeable interlocking pavers (concrete and composite materials)
• Permeable grid systems (concrete or composite materials)
• Permeable articulated block systems
• Pervious concrete (poured-in-place or pre-cast)
• Porous 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:

• Pavement surface: interlocking pavers with open joints, grid systems, precast pervious slabs or pavers, poured in place asphalt or concrete Bedding layer as a levelling course (for precast systems)
• Subsurface base storage reservoir of open graded aggregate
• Edge restraints

Additional components may include:

• an underdrain system
• geotextile, at the discretion of design professional
• impermeable geomembrane on sites where soil infiltration is not desired
• geo-grids for sites with heavy vehicle loading
• monitoring wells and clean-outs

Planning considerations

Infiltration

Permeable pavements can be constructed over any soil type, but hydrologic soil group (HSG) A and B are best for achieving water balance objectives. Native soil infiltration rate at the proposed facility location and depth should be confirmed through in-situ measurements of hydraulic conductivity under field saturated conditions. A liner will be required if native soils are contaminated.

Systems located in low permeability soils with an infiltration rate of less than 15 mm/hr (i.e., hydraulic conductivity of less than 1x10^-6 cm/s), require incorporation of a perforated pipe underdrain. Pavements require an impermeable geomembrane where the underlying soils are contaminated or the pavement is within two year time-of-travel wellhead protection area (see local drinking water source protection plan)

Space

Parking lot with perforated pavers in stalls only, Singapore

Permeable pavements can be used for the entire parking lot, driveway or road surface, or be designed to to receive runoff from adjacent impervious surfaces. For example, the parking spaces of a parking lot or road can be permeable pavements while the drive lanes are conventional asphalt. The impervious area draining on to the permeable pavement should not exceed the area of the permeable pavement itself. Relatively clean drainage areas such as roofs may be up to 4 times the size of the pavement but should include filtration pretreatment to remove coarse debris prior to being conveyed directly to the storage reservoir.

Permeable pavements should be located down-gradient from building foundations. If the pavement does not receive drainage from other surfaces, no setback is required. If the pavement receives drainage from other surfaces a minimum setback of four metres down-gradient is recommended. A smaller setback may be permissible where foundations are protected by a geomembrane.

Site Topography

Permeable pavement surface slope should be at least 1% and no greater than 5%. Steeper slopes may require additional features (see subgrade section below). Pervious areas should not drain onto permeable pavements.

Groundwater

Maintaining a separation of one metre between the elevations of the base of the practice and the seasonally high water table, or top of bedrock is recommended. Lesser or greater values may be considered based on groundwater mounding analysis. See Groundwater page for further guidance and a spreadsheet tool.

To protect groundwater from possible contamination, runoff from pollution hot spots should not be treated by permeable pavements designed for full or partial infiltration. Facilities designed with an impermeable liner (filtration-only) can be used. Also see the Site considerations page.

Private Sites

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

Subgrade

For infiltrating pavements, subgrade slopes should be minimized so that runoff will be able to infiltrate evenly through the entire surface. For steeply sloped sites (>5%), check dams, berms or weir structures on the native soils of the pavement should be considered. If the system is not designed for infiltration, the bottom should be sloped at 1 to 5% toward the underdrain. Subgrades should be compacted to 95% Standard Proctor Density. If a lesser value is desired to promote infiltration, a thicker sub-base should be considered. Subgrade soils should not be scarified.

Pretreatment

In most designs, the permeable surface acts as pretreatment to the storage reservoir below. Since sediment accumulates in the permeable areas of the pavement, periodic vacuum sweeping and preventative measures like not storing snow, construction materials or landscaping materials on the pavement are critical to prevent clogging.

Underdrain

The diagram below displays permeable pavement cross sections showing full and partial infiltration designs. See Underdrains page for further guidance.

Access Structures

Access structures are used for periodic inspection and flushing. It could be a maintenance hole or vertical standpipe connected to the perforated pipe. For accessing the underdrain pipe with camera or cleaning equipment, 45° couplings should be used for standpipe connections.

Overflow

Porous asphalt system with overflow edge draining to a reservoir. Source: Pennsylvania Department of Environmental Protection (PDEP). 2006.

All designs require an overflow outlet connected to a storm sewer with capacity to convey larger storms. This is normally achieved with a catch basin outlet, but water may also be directed to a downstream practice (e.g. bioretention, swale). Another option is a gravel diaphragm or trench along the downgradient edge of the pavement that drains to the storage reservoir below.

The diagram to the left is an example of a porous asphalt system with overflow edge draining to a reservoir. A flush curb extending into the base may be required as an edge restraint for some permeable pavement types. Decorative stone is lined with a geotextile to prevent migration of fine sediment into the pavement base and facilitate maintenance.

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	NO4-RG-S7 mix with air entrainment proven to have the best freeze-thaw durability after 300 freeze-thaw cycles. 28 day compressive strength = 5.5 to 20 MPa. Porosity between 14% to 31%. Permeability = 900 to 21,500 mm/hr. Proprietary pre-cast slabs meeting required specifications are also available.	Thickness will range from 100 mm - 150 mm depending on the expected loads.
Porous asphalt	Open-graded asphalt mix with a minimum porosity of 16%. Polymers can be added to provide additional strength to heavy loads. The University of New Hampshire Stormwater Center has detailed design specifications for porous asphalt.	Thickness will range from 50 mm - 100 mm depending on the expected loads.
Permeable Interlocking Pavements	Pavers shall meet the minimum material and standard specifications for precast concrete pavers (CSA A231.2 in Canada; ASTM C936 in United States). Open space between pavers (i.e. joint space) typically ranges between 5 and 15% of the total surface area. ASTM No. 8 (5 mm dia.) crushed aggregate is recommended for fill material in the paver joints (typically HPB). For narrow joint pavers, a smaller sized aggregate may be used. Narrow joints are required where pavement must be AODA compliant.	For vehicle applications the minimum paver thickness is 80 mm For pedestrian applications the minimum paver thickness is 60 mm and joint widths should be no greater than 15 mm.
Stone Resevoir	All aggregates should meet the following criteria: (i) maximum wash loss of 0.5%, (ii) minimum durability index of 35, (iii) maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions Most OPSS aggregates are not recommended for PP, with the exception of ‘granular O’. The granular subbase material shall consist of granular material graded in accordance with ASTM D 2940. Material should be clear crushed 50 mm diameter stone with void space ratio of 0.4.	The granular base material shall be crushed stone conforming to ASTM C 33 No 57. Material should be clear crushed 20 mm diameter stone. The granular bedding material shall be graded in accordance with ASTM C 33 No 8. Typical bedding thickness is 40 to 75 mm. Material should be 5 mm diameter stone or as determined by the Design Engineer. In Ontario, high performance bedding (1 to 9 mm diameter) or equivalent is often used. See LID Planning and Design Wiki (Permeable Pavements: Sizing) to size aggregate bed depth and multiply by application area to get total volume.
Geotextile	Should be woven monofilament or non-woven needle punched fabrics. Woven slit film and non-woven heat bonded fabrics should not be used as they are prone to clogging. Primary considerations for Geotextile are: suitable apparent opening size (AOS) for non-woven fabrics, or percent open area (POA) for woven fabrics, to maintain water flow even with sediment and microbial film build-up maximum forces to be exerted on the fabric (i.e tensile, tear and puncture strengths required) load bearing ratio of the underlying native soil (i.e. is geotextile needed to prevent downward migration of aggregate into the native soil?), grain size distribution of the overlying aggregate material, and permeability of the native soil.	Geotextiles are not always necessary and may be prone to eventual clogging. Consider using sand or fine aggregates instead. Where they are necessary (typically on low strength soils of CBR <4) between stone reservoir and native soil, material specifications should conform to Ontario Provincial Standard Specification (OPSS) 1860 for Class II geotextile fabrics. Geotextile socks on pipes should conform to ASTM D6707 with a min. water flow rate conforming to ASTM D4491 (12,263 L/min/m2 at 5 cm head).
Underdrain (optional)	PVC or HDPE, continuously perforated with smooth interior and a minimum inside diameter of 200 mm. Perforations in pipes should be 10 mm in diameter. A standpipe from the underdrain to the pavement surface can be used for monitoring and maintenance of the underdrain. The top of the standpipe should be covered with a screw cap and a vandal-proof lock.	Pipes should terminate 0.3 m short from the sides of the base.

Foundation Aggregates

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

All aggregates should meet the following criteria:

• Porosity of 0.4
• Maximum wash loss of 0.5%
• Minimum durability index of 35
• Maximum abrasion of 10% for 100 revolutions and maximum 50% for 500 revolutions

Most OPSS aggregates are not recommended for use in permeable pavements, with the exception of 'Granular O' type.

Granular Subbase Should be clear, crushed 50 mm diameter stone, graded in accordance with ASTM D2940. 'Rain ballast' aggregate (63 mm) would also be suitable

Granular Base Should be clear, crushed 20 mm diameter stone conforming to ASTM C33 No 57

Bedding Should be clear, crushed 5 mm diameter stone, graded in accordance with ASTM C33 No 8, High Performance Bedding (1-9 mm diameter), or equivalent. Typical bedding thickness is between 40 mm and 75 mm.

For more information, also see Aggregates page.

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

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  Porous asphalt, Coultice Park, Whitchurch-Stoufville, ON

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  Laneways retrofit program, Toronto ON

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  Permeable concrete at Lakeside Park, Mississauga ON

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  Permeable pavers on residential driveways, Brampton ON

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  Permeable pavers on a shared street, Georgetown ON

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  Permeable pavers in parking bays, Mississauga ON (note inspection of bioretention occurring in the background)

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  Although this permeable pavement at Belfountain Conservation Area was not maintained, it remains structurally sound after over 30 years of constant use.

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  Pre-cast pervious concrete slab.

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  Permeable Interlocking Concrete Pavement (PICP) on a narrow road between buildings in Chicago (Source: ICPI)

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  Different colours of permeable pavers in parking lot, CVC headquarters, Mississauga ON

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  Crews pour pervious concrete into a parking lot (Source: Washington State Dept of Transportation on Flickr)

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  Inspection wells should be located at the edges or even off the side of permeable pavement areas

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  Water quality sampling in permeable pavement study, Mississauga ON

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  Porous asphalt parking lot (Source: Villanova Urban Stormwater Partnership)

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  Placement of permeable pavers during construction.

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

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.
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
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 . 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 joints

• Eco-Optioc, Unilock
• Enviro Pavers, Oaks
• Pavedrain, distributed by Nilex
• Terra flo, Santerra

Pre-cast pervious

• Hydropavers pervious pavers
• Stormcrete pervious pavers

Cast in place

• Hydromedia
• PurePave

Plastic grid

• Ecoraster
• Hanpave
• HAHN heavy duty ground grid

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A perforated pipe used to assist the draining of soils.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

The natural ground material characteristic of or existing by virtue of geographic origin.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

The technique of removing pollutants from runoff as it infiltrates through the soil.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The natural ground material characteristic of or existing by virtue of geographic origin.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A parameter that describes the capability of a medium to transmit water.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

The water below the surface, and typically below the groundwater table.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

A perforated pipe used to assist the draining of soils.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the practice.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Credit Valley Conservation

Layer of rock or soil that holds or transmits water.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47