Difference between revisions of "Permeable paving"
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The equation for the depth of the stone bed is as follows: | The equation for the depth of the stone bed is as follows: | ||
| − | <math>db= [Qc\times R + P - | + | <math>db= [Qc\times R + P - i\times T ] / V</math> |
where: | where: | ||
| − | {{plainlist| | + | {{plainlist| |
| − | *''db'' = | + | *''db'' = Stone bed depth (m) |
| − | *''Qc'' = | + | *''Qc'' = Depth of runoff from contributing drainage area, not including permeable paving surface(m) |
| − | *''R'' = | + | *''R'' = Ac/Ap = Ratio of contributing drainage area (Ac) to permeable paving area (Ap) |
| − | *''P'' = | + | *''P'' = Rainfall depth (m) |
| − | *''i'' = | + | *''i'' = Infiltration rate for native soils (m/day) |
| − | *''T'' = | + | *''T'' = Time to fill stone bed (typically 2 hr) |
| − | *''Vr'' = | + | *''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. | 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: | 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: | ||
| − | + | <math>dr max = [i \times ts] / Vr </math> | |
| − | + | Where: | |
| − | + | {{plainlist| | |
| − | + | *''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 | 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. | 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: | + | If trying to size the area of permeable paving based on the contributing drainage area, the following equation may be used: |
| − | + | ||
| + | <math>Ap= [Qc \times Ac] / [Vr \times dp – P + i \times T] </math> | ||
Revision as of 20:05, 11 September 2017
Overview[edit]
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[edit]
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[edit]
Modeling permeable paving in the Treatment Train Tool
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[edit]
For reservoirs[edit]
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
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[1].
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 [2]. 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. [2]
Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity. [2]
For choking/choker layers[edit]
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.
OPS Aggregates[edit]
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[edit]
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[edit]
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. [3] 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.
Stone lining the ponding zone of this rain garden. Image credit California Native Plant Society
Coarse angular stone laid onto a geogrid and geotextile. Image from wikimedia commons
Stone mulch[edit]
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% [4]. Inorganic mulches which may be available locally, include:
- Pea gravel
- River rock/beach stone
- Recycled glass
- Crushed mussel shells
On-site verification[edit]
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:
- Collect approximately 5 cm of material in the jar (or at least two complete layers of 50 mm clear stone),
- Add water to around 3/4 full,
- Secure cap and shake,
- Leave for at least 30 minutes and until the water is clear - plan to run the test overnight when possible,
- 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[edit]
- 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:
where:
![{\displaystyle db=[Qc\times R+P-i\times T]/V}](https://wikimedia.org/api/rest_v1/media/math/render/svg/97c89bf85ed8cbc3580e07d3f2a19a9fcb6a6248)
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: Where:
![{\displaystyle drmax=[i\times ts]/Vr}](https://wikimedia.org/api/rest_v1/media/math/render/svg/05597b82f503d823df36e75e26fd5fe54130536e)
- 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:
Failed to parse (syntax error): {\displaystyle Ap= [Qc \times Ac] / [Vr \times dp – P + i \times T] }
Geotextile
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[5]
- By reducing organic pollutant and nutrient concentrations [6]
- When installing geotextiles an overlap of 150 - 300 mm should be used.
Material specifications should conform to OPSS 1860 for Class II geotextile fabrics [7]. 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:
- The maximum forces that will be exerted on the fabric (i.e., what tensile, tear and puncture strength ratings are required?),
- 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?),
- The texture (i.e., grain size distribution) of the overlying and underlying materials, and
- 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.
| 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[edit]
http://www.mdpi.com/2073-4441/7/4/1595/htm
Performance[edit]
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[edit]
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[edit]
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[edit]
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[edit]
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
Poured in place
- ↑ Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, http://www.stormtech.com/download_files/pdf/techsheet1.pdf accessed 16 October 2017
- ↑ 2.0 2.1 2.2 Judge, Aaron, "Measurement of the Hydraulic Conductivity of Gravels Using a Laboratory Permeameter and Silty Sands Using Field Testing with Observation Wells" (2013). Dissertations. 746. http://scholarworks.umass.edu/open_access_dissertations/746
- ↑ Roger T. Kilgore and George K. Cotton, (2005) Design of Roadside Channels with Flexible Linings Hydraulic Engineering Circular Number 15, Third Edition https://www.fhwa.dot.gov/engineering/hydraulics/pubs/05114/05114.pdf
- ↑ Simcock, R and Dando, J. 2013. Mulch specification for stormwater bioretention devices. Prepared by Landcare Research New Zealand Ltd for Auckland Council. Auckland Council technical report, TR2013/056
- ↑ Newman AP, Coupe SJ, Spicer GE, Lynch D, Robinson K. MAINTENANCE OF OIL-DEGRADING PERMEABLE PAVEMENTS: MICROBES, NUTRIENTS AND LONG-TERM WATER QUALITY PROVISION. https://www.icpi.org/sites/default/files/techpapers/1309.pdf. Accessed July 17, 2017.
- ↑ Paul P, Tota-Maharaj K. Laboratory Studies on Granular Filters and Their Relationship to Geotextiles for Stormwater Pollutant Reduction. Water. 2015;7(4):1595-1609. doi:10.3390/w7041595.
- ↑ ONTARIO PROVINCIAL STANDARD SPECIFICATION METRIC OPSS 1860 MATERIAL SPECIFICATION FOR GEOTEXTILES. 2012. http://www.raqsb.mto.gov.on.ca/techpubs/OPS.nsf/0/2ccb9847eb6c56738525808200628de1/$FILE/OPSS%201860%20Apr12.pdf. Accessed July 17, 2017