Structural Stormwater Technologies
Porous Asphalt with Subsurface Infiltration/Storage Bed
Porous Asphalt is a bituminous paving mixture which has all of the structural properties of conventional asphalt, but which is constructed of an aggregate (gravel) mix in which fine particles have been kept to a minimum. Eliminating the fine particles allows rainfall to drain through the pavement, rather than running off the surface.

Beneath the pavement is a bed containing uniformly graded, clean stone aggregate, designed to receive all rainfall from the pavement as well as inflow from other impervious areas such as rooftops and driveways. 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 sediment or fines into the bed. The subsurface stone bed serves as either a storage/infiltration structure, or a simple subsurface detention basin, depending on site conditions.

Porous asphalt pavement is especially well suited for parking lot areas, though our designs have also been incorporated into play courts and pathways. Cahill Associates has developed several dozen large, successful porous pavement installations, including some that are now 20 years old. 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.

Many of our installations occur in areas with cold weather climates, sloping topography or carbonate geology that is prone to sinkhole formation. To overcome these design challenges, Cahill engineers carefully analyze site conditions before design so that the final recommended solutions compliment the site features and limitations. For example, in the sloping topography of the Piedmont Plateau region, CA designed a stormwater system consisting of a series of porous parking bays terraced down the hillside connected by conventionally paved impervious roadways for Shared Medical Systems, now Seimens. Even though the site is underlain by carbonate geology, our infiltration design distributes stormwater over a wide area, thus minimizing the potential for sinkhole formation.

The key to every successful Porous Asphalt with Subsurface Infiltration / Storage Beds system is a well-planned design followed by careful construction. Cahill Associates engineers routinely schedule site visits during critical construction stages to inspect and provide construction oversight.

Click here for a pictorial construction sequence (PDF) showing the major steps in a Porous Asphalt with Subsurface Infiltration / Storage Bed installation.

To watch a 7-minute video on porous pavement for managing stormwater, please go to the Green Works Web site.

For more information on Porous Asphalt Pavement, see the May/June 2003 issue of Stormwater Magazine and read the Porous Asphalt Article written by one of our principal engineers, Michele Adams, P.E.

For more information on Sustainable Stormwater Management, please see the Nov/Dec 2003 issue of Stormwater Magazine to read an article with contributions from Tom Cahill, President of Cahill Associates.

Please contact Cahill Associates for more information on Porous Asphalt.
Examples

Porous asphalt pavement conceptual cross-section detail.

Rainfall flows off the standard pavement (center driveway)and over the porous pavement under the parking bays.

Porous asphalt can be utilized in sidewalks and pathways, as shown here at Swarthmore College.