Permeable paving

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Overview[edit]

Permeable paving is ideal for:

  • Projects which accommodate light vehicular traffic or pedestrian traffic,
  • Sites which cannot accommodate additional surface area for bioretention

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]

Sizing in TTT

TTT.png

Permeable pavements is found within the LID toolbox
Surface
Berm height (mm) This is the height of the curb which constrains the overland sheet flow of water. Where curb cuts or other outlet exists at the lowest point of the pavement, the suggested value is 0.
Surface roughness (Manning’s n) Lower numbers indicate less surface obstruction and result in faster flow.

Suggested range for pavement 0.01 – 0.02 [1]

Surface slope (%) Typically between 1 – 4% (>2% recommended Effective grading)
Pavement
Thickness (mm) This is the thickness of just the pre-cast blocks (or depth of asphalt/concrete poured in place).
Void ratio This most commonly refers to the jointing material used between precast blocks. Suggest 0.4 unless otherwise tested. Where a pervious product is poured in place, an appropriate figure should be obtained from the manufacturer.
Impervious surface fraction This is the proportion of the total pavement taken up with the pre-cast blocks and will vary between products; an example value may be 0.85.

Where a pervious product is poured in place, this value will be 0.

Permeability (mm/hr) This is the permeability of the joint material in block systems. Where the permeability of the bulk surface is measured and known instead, the impervious fraction can be adjusted to model a block surface as continuously permeable instead.
Clogging factor 0.5 to model a matured system?
Soil (Bedding layer?)
Thickness (mm) Depth of bedding layer
Porosity (fraction) Suggested value 0.4 unless otherwise tested (see OPSS aggregates)
Field capacity (fraction) Suggested range 0.10 - 0.12 for sand[1]
Wilting point (fraction) Suggested value 0.03 for sand[1]
Conductivity (mm/hr) Suggested range 100 – 250 mm/hr for sand
Conductivity slope Suggested value 45 for sand [1]
Suction head (mm) Suggested value 50 for sand [1]
Storage
Thickness (mm) Depth of all aggregate bases
Void ratio Suggested value 0.4 unless otherwise tested
Seepage rate (mm/hr) Infiltration rate of native soil
Clogging factor 0.5 to model a matured system?
Design drawdown time (hrs) Maybe 72 or 96 hours? See Drainage time
Drain (underdrain)
Flow coefficient Suggested value 1
Flow exponent Suggested value 1
Offset height This is the height from the base of the cell to the height at which the drain discharges. In some designs this may be the height of the perforated pipe within the storage layer. In other designs this height is adjusted by creating an upturn in the discharge pipe. Permeable pavements
  1. 1.0 1.1 1.2 1.3 1.4 Oregon State Univ., Corvallis. Dept. of Civil, Construction and Environmental Engineering.; Environmental Protection Agency, Cincinnati ONRMRL. Storm Water Management Model Reference Manual Volume I Hydrology (Revised). 2016:233. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.

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.

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.

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[1]
  • By reducing organic pollutant and nutrient concentrations [2]
  • When installing geotextiles an overlap of 150 - 300 mm should be used.

Material specifications should conform to OPSS 1860 for Class II geotextile fabrics [3]. 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[edit]

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


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

Poured in place

  1. 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.
  2. 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.
  3. 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