Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

3.7    Porous Pavements

3.7.1    Description and Purpose

Porous pavements have the potential to be an effective ultra-urban BMP. Different types of porous pavements commercially available include porous asphalt and concrete surfaces, as well as several types of lattice-type pavers (hollow concrete blocks and paving stones).

While conventional pavement results in increased rates and volumes of surface runoff, porous pavements, when properly constructed and maintained, allow some of the stormwater to percolate naturally through the pavement and enter the soil below. This helps to retain the natural infiltration rate facilitating groundwater recharge and maintaining base flows in urbanized streams while providing the structural and functional features needed for the roadway, parking lot, or sidewalk.

The paving surface, subgrade, and installation requirements of porous pavements are more complex than those for conventional asphalt or concrete surfaces. For porous pavements to function properly over an expected life span of 15 to 20 years, they must be properly sited and carefully designed and installed, as well as periodically maintained. A failure to properly test for soil drainage capacity and water table height, and a failure to protect paved areas from construction-related sediment losses, can result in their premature clogging and failure. The proper operation of porous pavements may also be a problem in regions that apply large amounts of road salt and where heavy truck loads are a problem.

3.7.2    Design Alternatives

Porous pavement systems generally consist of at least four different layers of material. The top or wearing layer consists of either asphalt or concrete with a greater than normal percentage of voids (typically 12-20 percent in the case of asphalt). The wearing layer may also be comprised of lattice-type pavers (either hollow concrete blocks or paving stones made from solid conventional concrete or stone) that are set in a bedding material (sand, pea-sized gravel or turf grass).

Below the wearing layer, a stone reservoir layer or a thick layer of aggregate (e.g., 50 mm [2 in] stone) provides the bulk of the water storage capacity for a porous pavement system. In the pavement design, it is important to ensure that this reservoir layer retains its load bearing capacity under saturated conditions because it may take several days for complete drainage to occur.

Typically, porous pavement designs include two (or more) transition layers that can be constructed from 25 to 50 mm diameter stone. One transition layer separates the top wearing layer from the underlying stone reservoir layer. Another transition layer is used to separate the stone reservoir from the undisturbed subgrade soil. Some designs also add a geotextile layer to this bottom layer or some combination of stones and geotextile.

Porous asphalt pavement, for example, consists of open grade asphalt mixture ranging in depth from 50 to 100 mm with 16 percent voids. The thickness selected depends on bearing strength and pavement design requirements. This layer sits on a 25 to 50 mm transition layer located over a stone reservoir. The bottom layer completes the transition to the underlying undisturbed soil using a combination transition/filter fabric layer.

Modular paving stones are also used to create porous pavements. These pavements can be constructed in situ by pouring concrete into special frames or by using preformed blocks. The top layer for these porous pavements consists of conventional concrete, with the intervening void areas filled with either turf or sand. A transition or bedding layer is used to make the transition to the reservoir layer. These lattice-type pavers or hollow concrete blocks are often used in conjunction with turf grasses and are used in low-traffic parking lots, lanes, or driveways. Porous pavements using paving stones have similar construction, but can be designed to have a much higher load bearing capacity, and therefore have more widespread applicability. Construction guidelines and design specifications are available from the manufacturers of these products.