GeneralA pervious (permeable) pavement system is a system that allows stormwater to percolate through small pores or gaps in the pavement. The purpose of these systems is to encourage infiltration by reducing the amount of runoff that is produced from a site. Runoff soaks through the voids in the pavement and into a basin that is filled with gravel, a layer of filter fabric, and a stone reservoir. These layers work together to both support the pavement above it and to speed percolation into the subsoil.
There are several types of pervious pavement systems. They include: porous asphalt, porous concrete, modular perforated concrete block, and cobble pavers with porous joints. These systems can be used in any area with limited traffic flow, such as overflow parking lots and driveways. Heavy traffic causes the soil beneath the pavement to become compacted and obstructs the downward flow of the water, limiting the system’s effectiveness. These systems should be used in conjunction with other management practices to reduce the amount of sediment that reaches the pavement, as heavy loads of sediment can permanently clog the pores in the pavement, severely hindering the ability of the pavement to accept runoff.
- Reduces the need for additional BMPs by reducing runoff
- Reduces temperature of runoff when compared to traditional pavement systems
- Can be aesthetically pleasing
- Limits handicapped accessibility
- Ineffective in areas with high volumes of traffic
- May be damaged by snow removal activities
- Pores are easily clogged by sediment
- May increase the potential for groundwater contamination
SoilsProper soils are necessary for pervious pavement systems to work correctly. The soil present on the site should have a permeability rate of at least 2 inches per hour and should be at least 4 feet thick to ensure that the pavement and the basin drain properly.
Storage BasinPervious pavement is underlined by a storage basin that aids in the drainage of water from the pavement to the subsoil. The basin consists of a layer of woven geotextile filter, 2 layers of gravel, a stone reservoir, an overflow pipe, and an observation well.
A geotextile filter with a high flow rate is placed on the bottom and along the sides of the basin. It is used because finer sediments, such as those contained in the subsoil, have a tendency to shift upward into the voids in the gravel and stone, reducing the infiltration capacity of the basin. The geotextile filter prevents this process from occurring, yet allows water to move freely through it.
2 layers of ½ inch gravel are placed in the basin: the first is located on top of the geotextile filter and the other directly beneath the pavement and above the stone reservoir. Both layers serve as a base for the layer above it; the top layer supports the pavement while the lower layer supports the reservoir above it and prevents settling under normal conditions.
A layer of 1 ½ to 3-inch stone is located between the gravel layers. It acts as a reservoir for the runoff, cooling it as it slowly passes through the stone. A perforated overflow pipe is located at or near the top of the stone and helps prevent runoff from leaving the site during large storm events. When large amounts of runoff infiltrate the basin, the reservoir may not be large enough to handle all of the runoff produced. The overflow pipe, which has holes in the bottom of it, drains water from the top of the basin when the basin is filled, but allows water to percolate downward when it is not.
To ensure the basin is working properly, an observation well is incorporated into the basin. Stretching the entire depth of the basin, it allows site owners to check the water levels in the basin and ensure that the runoff is infiltrating into the subsoil. As an added safeguard, the basin should be larger than the pervious pavement that is placed on top of it. If the voids in the pavement become clogged, the added surface area provides an overflow for any runoff that occurs. After paving, the remaining, exposed basin is covered with a decorative stone.
PlacementPervious pavement systems should be placed in areas with limited traffic use and away from areas that see even occasional use by heavy machinery. Heavy machinery and large volumes of traffic cause the soil beneath the pavement to become compacted, causing the infiltration capacity of the soil to be reduced.
ConstructionPervious pavement failures occur most often during construction due to sedimentation that fills in the voids in the pavement. As a result, careful construction practices that significantly reduce the site’s contact with sediment from construction vehicles and from around the site are essential to the success of these systems. Successful construction requires the use of stone tracking pads to reduce the sediment brought on site by vehicles and any practice that reduces the amount of sediment that runs off onto the site; such as buffers, filter strips, berms, or diversions. In addition, constant contact should be maintained between the contractor and the engineer to ensure that all aspects of the system are installed properly. The combination of these factors greatly increases the likelihood of a successful pervious pavement system.
MaintenanceProper maintenance of pervious pavement is crucial to its operation, but is similar to that required with traditional pavement. The main difference is that pervious pavement should be vacuumed by using a Hi-Vac truck or other device rather than swept. Sweeping may actually expedite sedimentation by brushing sediments into the pavements voids, blocking the percolation of runoff. Vacuuming removes sediment and debris without spreading it around and is more efficient at removing sediment than traditional street sweeping equipment.
Vacuuming should be performed at least 2-3 times a year to ensure that water is infiltrating properly. In addition, signs should be placed at various locations throughout the site after construction is completed stating that pervious pavement is located on the site. The signs should warn heavy machinery and snow plow operators to avoid the area.