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| [[File:PP IWS.gif|frame|Conceptual diagram illustrating an adjustable storage underdrain configuration beneath permeable interlocking pavers]]
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| ==Overview==
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| 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.
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| Permeable pavement types include:
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| * Permeable interlocking pavers (concrete and composite materials)
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| * Permeable grid systems (concrete or composite materials)
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| * Permeable articulated block systems
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| * Pervious concrete (poured-in-place or pre-cast)
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| * Porous asphalt
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| {{textbox|Permeable paving is ideal for:
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| *Sites with limited space for other surface stormwater BMPs
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| *Projects such as low traffic roads, parking lots, driveways, pedestrian plazas and walkways}}
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| '''The fundamental components of a permeable paving system are:'''
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| * 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)
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| * Subsurface base storage reservoir of open graded aggregate
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| * Edge restraints
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| '''Additional components may include:'''
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| * an [[underdrain]] system
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| * geotextile, at the discretion of design professional
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| * impermeable geomembrane on sites where soil infiltration is not desired
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| * geo-grids for sites with heavy vehicle loading
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| * monitoring wells and clean-outs
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| ==Planning considerations==
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| ===Infiltration===
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| 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.
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| Systems located in low permeability soils with an infiltration rate of less than 15 mm/hr (i.e., hydraulic conductivity of less than 1x10<sup>-6</sup> 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)
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| ===Geometry and Site Layout===
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| [[File:Perforated concrete block alt.jpg|thumb|Parking lot with perforated pavers in stalls only, Singapore]]
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| 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.
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| A hybrid permeable paving/[[infiltration chambers|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.
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| ===Pretreatment===
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| 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.
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| ===Landscaping===
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| 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.
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| ===Risk of Groundwater Contamination===
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| *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)
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| *Prioritize infiltration of runoff from source areas that are comparatively less contaminated such as roofs and low traffic areas.
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| ===Heavy Vehicle Traffic===
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| [[File:PaveDrain.jpg|thumb|Many types of permeable surface are certified to ASSHTO-25, including PaveDrain PICP type paving, LSRCA headquarters, Newmarket, ON]]
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| 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.
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| ===Setbacks from Buildings===
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| 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.
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| ===On Private Property===
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| 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.
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| ==Design==
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| ===Finish course===
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| 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.
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| {{:Permeable paving: Specifications}}
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| ===Foundation aggregates===
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| [[File:Geogitter-4.jpg|thumb|Geogrids like these are sometimes incorporated into the layers of permeable pavement foundation aggregates to provide additional stability and conserve aggregate material]]
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| 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.
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| *[[Choker layer]]
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| *[[Reservoir aggregate]]
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| ===Geotextile===
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| [[Geotextiles]] are not always necessary and may be prone to [[clogging]]. Consider using courses of finer aggregates or sand instead.
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| ===Sizing stone reservoirs===
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| The stone reservoir must meet both runoff storage and structural support requirements.
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| The bottom of the reservoir should be level so that water infiltrates evenly.<br>
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| If the system is not designed for infiltration, the bottom should slope at 1 - 5% toward the underdrain.
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| '''[[Permeable paving: Sizing]]'''
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| ===Modeling permeable paving in the Treatment Train Tool===
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| '''[[Permeable paving: TTT]]'''
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| ==Gallery==
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| {{:Permeable paving: Gallery}}
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| ==Landscaping and grading==
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| Landscaped areas ''must'' be [[grading|graded]] drain away from permeable paving to prevent sediment from running onto the surface. <br>
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| 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.
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| ==Performance==
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| Permeable pavers can be classified into two categories according to the infiltration rate of the underlying subsoil:
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| *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
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| *Partial Infiltration: Partial infiltration designs with underdrains generate more runoff
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| 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. <ref>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.</ref><br>
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| 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 <ref>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</ref> <ref>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.</ref> <br>
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| 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.
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| ==Construction Considerations==
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| Construction of permeable pavement is a specialized project and should involve experienced contractors. The following general recommendations apply:
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| *'''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.
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| *'''Weather''': Porous asphalt and pervious concrete will not properly pour and set in extremely high or low temperatures <ref>City of Portland. 2004. Portland Stormwater Management Manual. Prepared by the Bureau of Environmental Services (BES). Portland, OR.</ref>. One benefit to using permeable pavers is that their installation is not weather dependent.
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| *'''Pavement placement''': Properly installed permeable pavement requires trained and experienced producers and construction contractors.
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| ==Inspection and Maintenance==
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| 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]]
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| ==Life cycle costs==
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| 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.
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| See also: [[Cost analysis resources]]
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| ==Proprietary Links==
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| {{:Disclaimer}}
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| ===Pre-cast with joints===
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| *[https://unilock.com/products/driveways/eco-optiloc/?region=1 Eco-Optioc, Unilock]
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| *[https://oakspavers.com/products/enviro-pavers Enviro Pavers, Oaks]
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| *[http://nilex.com/products/pavedrain Pavedrain, distributed by Nilex]
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| *[http://santerrastonecraft.com/landscape/paving-stones/terra-flo Terra flo, Santerra]
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| ===Pre-cast pervious===
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| *[http://hydropavers.ca/ Hydropavers pervious pavers]
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| *[http://www.storm-crete.com/ Stormcrete pervious pavers]
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| ===Cast in place===
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| *[http://www.lafarge-na.com/wps/portal/na/en/HydromediaDetailWCM_GLOBAL_CONTEXT=/wps/wcm/connectlib_na/Site_na/AllProductDataSheet_Concrete/ProductDatasheet_Concrete_1321037540751/Product_EN Hydromedia]
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| *[https://www.purepave.com/ PurePave]
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| ===Plastic grid===
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| *[https://www.ecorastergrid.com/ Ecoraster]
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| *[https://www.hahnplastics.com/ca/products/ground-reinforcement-and-surfaces/hanpave/ Hanpave]
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| *[https://www.hahnplastics.com/ca/products/ground-reinforcement-and-surfaces/heavy-duty-ground-grid/ HAHN heavy duty ground grid]
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