Permeable paving

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

Overview

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

The following are different types of permeable paving:

• Permeable interlocking concrete pavers
• Plastic or concrete grid systems
• Previous concrete; and
• 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:

• interlocking blocks with infiltration spaces between, or
• a poured 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:

• an underdrain system

Planning considerations

Geometry and Site Layout

Permeable paving can be used for entire parking lot areas or driveways or can be designed to receive runoff from adjacent impervious surfaces For example, the parking spaces of a parking lot may be permeable pavers while the drive lanes are impervious asphalt. In general, the impervious area should not exceed 1.2 times the area of the permeable pavement which receives the runoff.

A hybrid permeable pavement/soakaway design can feature connection of a roof downspout directly to the stone reservoir of the permeable pavement 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.

Landscaping

Landscaped areas must drain away from permeable pavement to prevent sediments from running onto the surface. Urban trees will benefit from being surrounded by permeable pavement 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.

Design

Modeling permeable paving in the Treatment Train Tool

Permeable paving: TTT

Materials

Stone Reservoir

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. This is a collection of three articles with the common theme of being aggregate products for various applications in LID.

Underground construction aggregates

For reservoirs

Note the uniform size and angularity of this clear stone sample. Note also that the fragments all appear to have a film of fine particles adhering; this material would be improved by being washed prior to use.

This article gives recommendations for aggregate to be used to store water for infiltration. This is usually called 'clear stone' at aggregate yards.

To see an analysis of Ontario Standard Specifications for granular materials, see OPSS aggregates.

For advice on decorative surface aggregates see Stone

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Gravel used for underdrains in bioretention, infiltration trenches and chambers, and exfiltration trenches should be 20 or 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a porosity of 0.4.

The clean wash to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the porosity). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles . See jar test for on-site verification testing protocols.

Gravel with structural requirements should also meet the following criteria:

• Minimum durability index of 35
• Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions

Standard specifications for the gradation of aggregates are maintained by ASTM D2940

• 

  The highest porosity is found in uniformly graded aggregate, as there are no smaller particles to occupy the inter-particle pores.

• 

  Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity.

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For choking/choker layers

medium sized granular, free from fines

In bioretention systems a choker layer of ≥ 100 mm depth is the recommended method to prevent migration of finer filter media into the underlying storage reservoir aggregate. These same mid-sized granular materials are recommended for use in Stormwater planter underdrains and may be useful in the fine grading of foundations courses for permeable pavements.

Suitable materials include:

High performance bedding (HPB)

Clean, angular aggregate screened to between 6 and 10 mm. Widely available and designed specifically for drainage applications. Free from fines by definition.

HL 6

Is a clean, angular aggregate screened between 10 and 20 mm. Free from fines by definition.

Pea Gravel

Rounded natural aggregate, screened between 5 and 15 mm, and washed free from fines.

In most scenarios, a geotextile layer is unnecessary and has been associated with rapid decline and clogging in some circumstances.

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OPS Aggregates

Of the standard granular materials in the standard OPSS.PROV 1010 only Granular O is recommended as a substitute for clear stone in LID construction.

Where Granular O is substituted for clear stone in underground reservoir structures, the porosity used in design calculations shall be 0.3 unless laboratory testing proves otherwise.

Examples of BMPs with underground reservoirs include Underdrains, infiltration trenches, permeable pavements, infiltration chambers, exfiltration trenches.

All other mixes must be avoided for free drainage or storage as they are permitted to contain a higher enough proportion of fines to reduce permeability below 50 mm/hr.

For more information see OPS aggregates

Landscaping aggregates

This rain garden in a school yard uses stone as both decorative edging and for erosion control.

This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

For advice on aggregates used in underdrains, see Reservoir aggregate.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions on the surface of an LID practice.

Stone for erosion control

Aggregates used to line swales or otherwise dissipate energy (e.g. in forebays) should have high angularity to increase the permissible shear stress applied by the flow of water. However, in some surface landscaped applications there may be a desire to use a rounded aggregate such as 'river rock' for aesthetic reasons. Rounded stones should be of sufficient size to resist being moved by the flow of water. Typical stone for this purpose ranges between 50 mm and 250 mm in diameter. The larger the stone, the more energy dissipation.

• Stone beds should be twice as thick as the largest stone's diameter.
• If the stone bed is underlain by a drainage geotextile, annual inspection and possible replacement should be performed as there is a potential for clogging of this layer to occur.

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  Stone lining the ponding zone of this rain garden. Image credit California Native Plant Society

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  Coarse angular stone laid onto a geogrid and geotextile. Image from wikimedia commons

Stone mulch

Finer inorganic mulch materials can be of value applied in areas with extended ponding times i.e. in the the centre of recessed, bowl shaped bioretention, stormwater planters, trenches or swale practices. Inorganic mulches resist movement from flowing water and do not float. Applying a thin layer of inorganic mulch over the top of wood based mulch has been shown to reduce migration of the underlying layer by around 25% . Inorganic mulches which may be available locally, include:

• Pea gravel
• River rock/beach stone
• Recycled glass
• Crushed mussel shells

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On-site verification

Steps in conducting a jar test to detect fines in construction materials

Specifying that aggregates for the construction of LID practices must be free from fines is important. But checking that the delivered materials meet specification is essential to reduce problems with construction and longer term performance.

When possible, Construction Managers should observe the offloading of materials to watch for dust clouds. Aggregates or sand for LID construction should not give rise to clouds of dust when dumped.

A simple jar test can be used to gauge the proportion of fines in an aggregate product before acceptance.

Apparatus:

• A large wide-mouthed jar - glass or clear plastic are both fine,
• Tap water, and
• The aggregate to be tested.

Method:

1. Collect approximately 5 cm of material in the jar (or at least two complete layers of 50 mm clear stone),
2. Add water to around 3/4 full,
3. Secure cap and shake,
4. Leave for at least 30 minutes and until the water is clear - plan to run the test overnight when possible,
5. Examine the layer of sediment - if > 3 mm has been washed from 5 cm of product, the material should be rejected,

Note that the sediment may collect on top of, or at the bottom of the construction material.

External references

• Heger, S. (2014). Critical Aspects During System Installation and Inspection General equipment considerations. In PMSA conference. Retrieved from http://www.psma.net/pdf/14/conference-presentations/Keys_to_Installation_(Heger).pdf
• Manitoba. (2010). Onsite Wastewater Management Systems: Field Reference Guide - JAR TEST. Retrieved from http://www.gov.mb.ca/sd/envprograms/wastewater/pdf/jar_test_reference_03_2010.pdf

Sizing Stone Reservoirs

The following calculation is used to size the stone storage bed (reservoir) used as a base course for designs without underdrains. It is assumed that the footprint of the stone bed will be equal to the footprint of the pavement. The following equations are taken from the ICPI Manual (Smith, 2006). The equation for the depth of the stone bed is as follows:

$db=[Qc\times R+P-I\times T]/V$ where:

• db = Stone bed depth (m)
• Qc = Depth of runoff from contributing drainage area, not including permeable paving surface(m)
• R = Ac/Ap = Ratio of contributing drainage area (Ac) to permeable paving area (Ap)
• P = Rainfall depth (m)
• i = Infiltration rate for native soils (m/day)
• T = Time to fill stone bed (typically 2 hr)
• Vr = Void ratio for stone bed (typically 0.4 for 50 mm dia. stone)}}

Note that the contributing drainage area (Ac) should not contain pervious areas. For designs that include an underdrain, the maximum depth of the stone reservoir below the invert of the underdrain pipe can be calculated as follows:

              dr max = i * ts / Vr 
       Where: 
              dr max = Maximum stone reservoir depth (m) 
              i = Infiltration rate for native soils (m/hr) 
              Vr = Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone) 
              ts = Time to drain (design for 48 hour time to drain is recommended) 

The value for native soil infiltration rate (i) used in the above equations should be the design infiltration rate that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation On highly permeable soils (e.g., infiltration rate of 45 mm/hr or greater), a maximum stone reservoir depth of 2 metres is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water. If trying to size the area of permeable paving based on the contributing drainage area, the following equation may be used:

              Ap = Qc * Ac / [Vr * dp – P + i * T] 

Geotextile

The properties of geotextiles vary widely.

See Clogging for notes on their application in LID structures.

Geotextiles can be used to prevent downward migration of smaller particles in to larger aggregates, and slump of heavier particles into finer underlying courses. Geotextiles are commonly used on low strength soils (CBR<4). The formation of biofilm on geotextiles has also been shown to improve water quality:

• By degrading petroleum hydrocarbons
• By reducing organic pollutant and nutrient concentrations
• When installing geotextiles an overlap of 150 - 300 mm should be used.

Material specifications should conform to OPSS 1860 for Class II geotextile fabrics . Note when expansive clays are present, a non-infiltrating design may be necessary. If used, geotextile socks around perforated pipes should conform to ASTM D6707 with minimum water flow rate conforming to ASTM D4491 (12,263 L/min/m² at 5 cm head).

• Fabrics should be woven monofilament or non-woven needle punched.
• Woven slit film and non-woven heat bonded fabrics should not be used, as they are prone to clogging.

In choosing a product, consider:

1. The maximum forces that will be exerted on the fabric (i.e., what tensile, tear and puncture strength ratings are required?),
2. The load bearing ratio of the underlying native soil (i.e. is the geotextile needed to prevent downward migration of aggregate into the native soil?),
3. The texture (i.e., grain size distribution) of the overlying and underlying materials, and
4. The 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.

Recommended criteria for selection of geotextile fabric

Percent soil/filter media passing 0.075 mm (#200 sieve)	Non-woven fabric apparent opening size (AOS, mm)	Woven fabric percent open area (POA, %)	Permittivity (sec-1)
>85	≤ 0.3	-	0.1
50 - 85	≤ 0.3	≥ 4	0.1
15 - 50	≤ 0.6	≥ 4	0.2
5 - 15	≤ 0.6	≥ 4	0.5
≤ 5	≤ 0.6	≥ 10	0.5

Performance research

http://www.mdpi.com/2073-4441/7/4/1595/htm

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Performance

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

• 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.

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 and low temperatures (City of Portland, 2004; U.S. EPA, 1999). 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

Like all other stormwater practices, permeable pavement requires regular inspection and maintenance to ensure that it functions properly.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.

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 savings and benefits may also be realized, including reduced need for storm sewer pipes and other stormwater practices, less developable land consumed for stormwater treatment, and ancillary benefits such as 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.

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

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

A perforated pipe used to assist the draining of soils.

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.

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

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.

Soil particles with a diameter less than 0.050 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.

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.

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

An underground water storage reservoir into which stormwater is directed and allowed to percolate into the underlying native subsoil.

An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

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.

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.

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

Penetration of water through the ground surface.

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.

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 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.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Includes the protection of soil from dislocation by water, wind or other agents.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

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.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

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.

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

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)

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)

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

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.

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 engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

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

Penetration of water through the ground surface.

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.

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

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

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