4.6.1 Applications & Limitations

Typical applications for permeable paving include industrial site employee parking, commercial parking, sidewalks, pedestrian and bike trails, driveways, residential access roads, and emergency and facility maintenance roads. Grid pavers are not intended for streets but are often used for emergency access lanes and intermittently used (overflow) parking areas. All other types of permeable paving can withstand loads from the number of trucks associated with local roads. Specialized engineering expertise is required for designs for heavy loads and cold weather considerations.

Thoroughfares, highways, and other roads that combine high vehicle loads and high speed traffic are generally not considered appropriate for permeable pavements. However, porous asphalt has proven structurally sound and remained permeable in a few arterial and highway applications (Hossain, Scofield, and Meier, 1992) and pervious concrete and permeable interlocking concrete pavement have been successfully used in industrial settings with low speeds and high vehicle loads.

Water quality treatment

Currently, Ecolgoy does not offer a water quality treatment credit for stormwater passing through a standard permeable pavement wearing course or the aggregate base. However, treatment can be attained using one of the following design approaches:
  • Infiltrate the runoff into subgrade soils that have a cation exchange capacity of ≥ 5 milliequivalents/100 grams dry soil, minimum organic matter content of 0.5 percent and a maximum infiltration rate of 12 inches per hour (short-term or measured rate). The soil must have the above characteristics for a minimum depth of 18 inches.
  • Design a treatment layer into the aggregate base that has the characteristics described above for subgrade soils.

Freeze-thaw conditions
Properly designed permeable paving installations have performed well in the Midwest and Northeast U.S. where freeze-thaw cycles are severe (Adams, 2003 and Wei, 1986). Cold weather design guidance from the University of New Hampshire is recognized in many areas (e.g., Michigan, Ohio, etc.) as the foremost guidance for the design of permeable pavements in cold weather climates (UNHSC, 2009). Design strategies for freeze-thaw conditions, such as use of under-drains to limit subsurface saturation, are presented below.

Permeable pavement should not be used (unless additional engineering analysis and design is conducted) where:
  • Excessive sediment is deposited on the surface (e.g., construction and landscaping material yards).
  • Steep erosion prone areas are upslope of the permeable surface and will likely deliver sediment and clog pavement on a regular basis, and where maintenance is not conducted regularly.
  • Concentrated pollutant spills are possible, such as gas stations, truck stops and industrial chemical storage sites, and where infiltration will result in the transport of pollutants to deeper soil or groundwater.
  • Seasonally high groundwater is within 1 foot of the bottom of the aggregate base (interface of the subgrade and aggregate base).
  • Fill soils, when saturated, cannot be adequately stabilized.
  • Sites receive regular, heavy applications of sand (such as weekly) for maintaining traction during winter.
  • Steep slopes where water within the aggregate base layer or at the subgrade surface cannot be controlled by detention structures (e.g., check dams) and may cause erosion and structural failure, or where surface  runoff velocities may preclude adequate infiltration at the pavement surface. Note that permeable pavement has been used successfully on slopes up to 10 percent with subsurface detention structures and at 8 percent slopes without subsurface structures.

Slope restrictions result primarily from flow control concerns and to a lesser degree the structural limitations of the permeable paving. Steep gradients increase surface and subsurface flow velocities and reduce infiltration capability and storage capacity of the pavement system. Detention structures placed on the subgrade and below the pavement can be used to detain subsurface flow and increase infiltration and maximum slope recommendation. In general, detention structures should be considered for permeable pavement on slopes ≥ 3 percent. All permeable pavement surfaces should have a minimum slope of 1 to 2 percent to allow for surface overflow in extreme rainfall. General recommendations for maximum slopes for permeable pavement are as follows:
  • Porous asphalt: 5 percent.
  • Pervious concrete: 12 percent.
  • Permeable interlocking concrete pavers: 12 percent (Smith, 2011).
  • Concrete and plastic grid systems: maximum slope recommendations vary by manufacturer and generally range from 6 to 12 percent (primarily a traction rather than infiltration or structural limitation). Contact the manufacturer or local supplier for specific product recommendations.

Previous Section  |  Next Section