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Porous Asphalt Pavement With Recharge Beds 20 Years and Still Working

With the right soil conditions and careful design, installations retain their ability to infiltrate.
By Michele C Adams, Stormwater E-Magazine May-June 2003

Is it possible to have a stormwater best management practice (BMP) that reduces impervious areas, recharges groundwater, improves water quality, eliminates the need for detention basins, and provides a useful purpose besides stormwater management? This seems like a lot to expect from any stormwater measure, but porous asphalt pavement on top of recharge beds has a proven track record.

First developed in the 1970s at the Franklin Institute in Philadelphia, PA, porous asphalt pavement consists of standard bituminous asphalt in which the aggregate fines (particles smaller than 600 ┬Ám, or the No. 30 sieve) have been screened and reduced, allowing water to pass through the asphalt (Figure 1 on website). Underneath the pavement is placed a bed of uniformly graded and clean-washed aggregate with a void space of 40%. Stormwater drains through the asphalt, is held in the stone bed, and infiltrates slowly into the underlying soil mantle. A layer of geotextile filter fabric separates the stone bed from the underlying soil, preventing the movement of fines into the bed (Figure 2 on website).

Porous pavement is especially well suited for parking-lot areas. Several dozen large, successful porous pavement installations, including some that are now 20 years old, have been developed by Cahill Associates (CA) of West Chester, PA, mainly in Mid-Atlantic states. These systems continue to work quite well as both parking lots and stormwater management systems. In fact, many of these systems have outperformed their conventionally paved counterparts in terms of both parking-lot durability and stormwater management.

Installations Old and New

One of the first large-scale porous pavement/recharge bed systems that CA designed is in a corporate office park in the suburbs of Philadelphia (East Whiteland Township, Chester County). This particular installation of about 600 parking spaces posed a challenge because of both the sloping topography and the underlying carbonate geology that was prone to sinkhole formation. The site also is immediately adjacent to Valley Creek, designated by Pennsylvania as an Exceptional Value stream where avoiding nonpoint-source pollution is of critical importance. Constructed in 1983 as part of the Shared Medical Systems (now Seimens) world headquarters, the system consists of a series of porous pavement/recharge bed parking bays terraced down the hillside connected by conventionally paved impervious roadways. Both the top and bottom of the beds are level, as shown in Figure 3, hillside notwithstanding. After 20 years, the system continues to function well and has not been repaved. Although the area is naturally prone to sinkholes, far fewer sinkholes have occurred in the porous asphalt areas than in the conventional asphalt areas, which the site manager attributes to the broad and even distribution of stormwater over the large areas under the porous pavement parking bays.

Other early 1980s sites, such as the SmithKline Beecham (now Quest) Laboratory in Montgomery County, PA, and the Chester County Work Release Center in Chester County, PA, also used the system of terracing the porous paved recharge beds down the hillside to overcome the issues of slope. At the DuPont Barley Mills Office complex in Delaware, the porous pavement was installed specifically to avoid the construction of a detention basin, which would have destroyed the last wooded portion of the site. More recently (1997), the porous parking lots at the Penn State Berks Campus were constructed to avoid destroying a wooded campus hillside. The Berks lots, also on carbonate bedrock, replaced an existing detention basin and have not experienced the sinkhole problems that another campus detention basin has suffered.

To view complete article and figures, please visit website.