Porous pavement

From PlanningWiki

Contents 1 Description 2 Applicability 3 Advantages and disadvantages 4 Design criteria 5 Performance 6 Operation and maintenance 7 Implementation 8 Best practice 9 Definitions 10 References 11 See also 12 External links

Description

Porous pavement is a special type of pavement that allows rain and snowmelt to pass through it, reducing the runoff from a site and surrounding areas. Porous pavement also filters some pollutants from the runoff if maintained.

There are two types of porous pavement: porousasphalt and pervious concrete. Porous asphalt pavement consists of an open-graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected voids to make it highly permeable to water. Pervious concrete consists of specially formulated mixtures of Portland cement, uniform, open-graded coarse aggregate, and water. Pervious concrete has enough void space to allow rapid percolation of liquids through the pavement. The porous pavement surface is typically placed over a highly permeable layer of open-graded gravel and crushed stone. The void spaces in the aggregate layers act as a storage reservoir for runoff. A filter fabric is placed beneath the gravel and stone layers to screen out fine soil particles.

Two common modifications made in designing porous pavement systems are (1) varying the amount of storage in the stone reservoir beneath the pavement and (2) adding perforated pipes near the top of the reservoir to discharge excess storm water after the reservoir has been filled.

Some municipalities have also added storm water reservoirs (in addition to stone reservoirs) beneath the pavement. These reservoirs should be designed to accommodate runoff from a design storm and should provide for infiltration through the underlying subsoil.

Applicability

Porous pavement may substitute for conventional pavement on parking areas, areas with light traffic, and the shoulders of airport taxiways a runways, provided that the grades, subsoils, drainage characteristics, and groundwater conditions are suitable. Slopes should be flat or very gentle. Soils should have field-verified permeability rates of greater than 1.3 centimeters (0.5 inches) per hour, and there should be a 1.2 meter (4-foot) minimum clearance from the bottom of the system to bedrock or the water table.

Advantages and disadvantages

The advantages of using porous pavement include:

• Water treatment by pollutant removal.
• Less need for curbing and storm sewers.
• Improved road safety because of better skid resistance.
• Recharge to local aquifers.

The use of porous pavement may be restricted in cold regions, arid regions or regions with high wind erosion rates, and areas of sole-source aquifers. The use of porous pavement is highly constrained, requiring deep permeable soils, restricted traffic, and adjacent land uses. Some specific disadvantages of porous pavement include the following:

• Many pavement engineers and contractors lack expertise with this technology.
• Porous pavement has a tendency to become clogged if improperly installed or maintained.
• Porous pavement has a high rate of failure.
• There is some risk of contaminating groundwater, depending on soil conditions and aquifer susceptibility.
• Fuel may leak from vehicles and toxic chemicals may leach from asphalt and/or binder surface. Porous pavement systems are not designed to treat these pollutants.
• Some building codes may not allow for its installation.
• Anaerobic conditions may develop in underlying soils if the soils are unable to dry out between storm events. This may impede microbiological decomposition.

As noted above, the use of porous pavement does create risk of groundwater contamination. Pollutants that are not easily trapped, adsorbed, or reduced, such as nitrates and chlorides, may continue to move through the soil profile and into the groundwater, possibly contaminating drinking water supplies. Therefore, until more scientific data is available, it is not advisable to construct porous pavement near groundwater drinking supplies. In addition to these documented pros and cons of porous pavements, several questions remain regarding their use. These include:

• Whether porous pavement can maintain its porosity over a long period of time, particularly with resurfacing needs and snow removal.
• Whether porous pavement remains capable of removing pollutants after subfreezing weather and snow removal.
• The cost of maintenance and rehabilitation options for restoration of porosity.

Design criteria

Porous pavement - along with other infiltration technologies like infiltration basins and trenches - have demonstrated a short life span. Failures generally have been attributed to poor design, poor construction techniques, subsoils with low permeability, and lack of adequate preventive maintenance. Key design factors that can increase the performance and reduce the risk of failure of porous pavements (and other infiltration technologies) include site conditions, construction materials and installation methods.

Site evaluation

• Take soil boring to a depth of at least 1.2 meters (4 feet) below bottom of stone

reservoir to check for soil permeability, porosity, depth of seasonally high water table, and depth to bedrock.

• Not recommended on slopes greater than 5 percent and best with slopes as

flat as possible.

• Minimum infiltration rate 0.9 meters (3 feet) below bottom of stone reservoir:

1.3 centimeters (0.5 inches) per hour.

• Minimum depth to bedrock and seasonally high water table: 1.2 meters (4

feet).

• Minimum setback from water supply wells: 30 meters (100 feet).
• Minimum setback from building foundations: 3 meters (10 feet) downgradient,

30 meters (100 feet) upgradient.

• Not recommended in areas where wind erosion supplies significant amounts

of windblown sediment.

• Drainage area should be less than 6.1 hectares (15 acres).

Traffic conditions

• Use for low-volume automobile parking areas and lightly used access roads.
• Avoid moderate to high traffic areas and significant truck traffic.
• Avoid snow removal operations; post with signs to restrict the use of sand,

salt, and other deicing chemicals typically associated with snow cleaning activities.

Design storm storage volume

• Highly variable; depends upon regulatory requirements. Typically design for

storm water runoff volume produced in the tributary watershed by the 6-month, 24-hour duration storm event.

Drainage time for design storm

• Minimum: 12 hours.
• Maximum: 72 hours.
• Recommended: 24 hours.

Construction

• Excavate and grade with light equipment with tracks or oversized tires to

prevent soil compaction.

• As needed, divert storm water runoff away from planned pavement area before

and during construction.

• A typical porous pavement cross-section consists of the following layers: 1)

porous asphalt course, 5-10 centimeters (2-4 inches) thick; 2) filter aggregate course; 3) reservoir course of 4-8 centimeters (1.5-3-inch) diameter stone; and 4) filter fabric.

Porous pavement placement

• Paving temperature: 240° - 260° F.
• Minimum air temperature: 50° F.
• Compact with one or two passes of a 10,000-kilogram (10-ton) roller.
• Prevent any vehicular traffic on pavement for at least two days.

Pretreatment

• Pretreatment recommended to treat runoff from off-site areas. For example,

place a 7.6-meter (25-foot) wide vegetative filter strip around the perimeter of the porous pavement where drainage flows onto the pavement surface.

Performance

Porous pavement pollutant removal mechanisms include absorption, straining, and microbiological decomposition in the soil. An estimate of porous pavement pollutant removal efficiency is provided by two long-term monitoring studies conducted in Rockville, MD, and Prince William, VA. These studies indicate removal efficiencies of between 82 and 95 percent for sediment, 65 percent for total phosphorus, and between 80 and 85 percent of total nitrogen. The Rockville, MD, site also indicated high removal rates for zinc, lead, and chemical oxygen demand. Some key factors to increase pollutant removal include:

• Routine vacuum sweeping and high pressure washing (with proper disposal of removed material).
• Drainage time of at least 24 hours.
• Highly permeable soils.
• Pretreatment of runoff from site.
• Organic matter in subsoils.
• Clean-washed aggregate.

Traditionally, porous pavement sites have had a high failure rate - approximately 75 percent. Failure has been attributed to poor design, inadequate construction techniques, soils with low permeability, heavy vehicular traffic, and resurfacing with nonporous pavement materials. Factors enhancing longevity include:

• Vacuum sweeping and high-pressure washing.
• Use in low-intensity parking areas.
• Restrictions on use by heavy vehicles.
• Limited use of de-icing chemicals and sand.
• Resurfacing.
• Inspection and enforcement of specifications during construction.
• Pretreatment of runoff from offsite.
• Implementation of a stringent sediment control plan.

Operation and maintenance

Porous pavements need to be maintained. Maintenance should include vacuum sweeping at least four times a year (with proper disposal of removed material), followed by high-pressure hosing to free pores in the top layer from clogging. Potholes and cracks can be filled with patching mixes unless more than 10 percent of the surface area needs repair. Spot-clogging may be fixed by drilling 1.3 centimeter (half-inch) holes through the porous pavement layer every meter. The pavement should be inspected several times during the first few months following installation and annually afterwardsd. Annual inspections should take place after large storms, when puddles will make any clogging obvious. The condition of adjacent pretreatment devices should also be inspected.

Implementation

Best practice

Definitions

References

• United States Environmental Protection Agency, 1999, Storm Water Technology Fact Sheet: Porous Pavement.
• United States Environmental Protection Agency, 1981. Best Management Practices Implementation Manual.
• Field, R., et al., 1982. “An Overview of Porous Pavement Research.” Water Resources Bulletin, Volume 18, No. 2, pp.265-267.
• Metropolitan Washington Council of Governments, 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs.
• Metropolitan Washington Council of Governments, 1992. A Current Assessment of Best Management Practices: Techniques for Reducing Nonpoint Source Pollution in a Coastal Zone.
• Southeastern Wisconsin Regional Planning Commission, 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures, Technical Report No. 31.

See also

Porous pavement grid systems

External links