About Porous Asphalt
General Information Performance Data Water Quality Treatment Process Design


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Design

Installed in 2004, the porous asphalt lot was designed with a subsurface storage capacity similar to a retention pond. An impervious asphalt lot of the same size was installed next to it for comparison. While the impervious lot requires catchbasins and piping, the porous lot stores runoff in a reservoir base and infiltration basin. Water quality volume (WQ_V), channel protection volume (CP_V), and higher flows, such as those associated with a ten-year event (QP), are managed in the system and by subdrains. A gravel edge with curbing that surrounds the porous lot prevents sediments from washing onto the surface and prematurely clogging the system. Its design consists of four basic layers:

• The top is a four-inch layer of porous asphalt. Sand particles smaller than two millimeters were removed from the mix to create pavement with an 18 to 20 percent void space.
• The second layer is a four-inch choker course consisting of 3/4 inch crushed stone, which allows runoff to pass into the next layer and offers structural support.
• The third layer consists of 24 inches of poorly graded sand, or “bank run gravel,” which serves as a filter course.
• The fourth layer is 21 inches of crushed stone, with a six-inch diameter, elevated subdrain. This layer serves as an infiltration reservoir; its thickness protects against freezing and thawing, and makes it possible to locate this system in group “C” soils (sandy clay loam with low infiltration rates).

The system is lined on the bottom and sides with a non-woven geotextile fabric to prevent influx of fines. However, bottom lining is no longer recommended because it can lead to premature clogging. It is believed that stormwater fines are more problematic than those migrating in from surrounding soils.