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Porous Pavement

来源：epa.gov 发布日期：2006-11-10

Porous pavement is a special type of pavement that allows rain and snowmelt to pass through it, thereby reducing the runoff from a site and surrounding areas. In addition, porous pavement filters some pollutants from the runoff if maintained. There are two types of porous pavement: porous asphalt 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. Figure 1 illustrates a common porous asphalt pavement installation. 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.
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 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 few feet. The pavement should be inspected several times during the first few months following installation and annually thereafter. 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.