Permeable Parking Practice


5,500 square feet of parking area in front of the building is surfaced with pervious concrete pavers. The pavers resemble standard paving blocks, except their corners have been cut to give the surface approximately 10% void space. The voids are filled with angular, gap-graded gravel, which is extremely porous and allows stormwater to soak down between the pavers into an 8” gravel reservoir below. The water then flows through perforated underdrains to the vegetated floodplain on either side of the parking area.

Because the high-traffic lanes are paved in standard asphalt, the pervious pavers are separated from the drive aisle by a concrete header curb. Without this curb, the asphalt would flake off at the paver/asphalt interface as vehicles drive onto the pavers. The header curb provides a rigid transition which will extend the life of the driving surface.

More Information on WSSI’s Pervious Pavers

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When the building was first constructed, GravelPave2 (Invisible Structures, Inc.) was installed in the majority of the parking areas. Many of those areas have since been converted to other types of permeable parking, but 3,280 square feet of GravelPave2 parking still remains. GravelPave2 is a grid of plastic rings fused to a filter-fabric backing. The product is laid over a gravel base, and the rings are filled with small, angular, gap-graded gravel. The gravel is held in place by the rings and fabric, and stormwater can soak through the filter fabric and into the gravel base below. Perforated underdrains within the gravel base then allow the water to drain to the vegetated floodplain.

WSSI’s use of this product in areas frequently driven on by large trucks and dirty field vehicles has highlighted several potential limitations for its applicability. Construction vehicles weighing several tons can generate sufficient torque with their tires to tear the product, particularly if trying to navigate tight turns. Accumulation of fine particles between the gravel and filter fabric is also a concern, as it forms an impermeable layer preventing stormwater from reaching the gravel base below the product. This process is accelerated if the GravelPave2 is regularly driven on by vehicles carrying mud or sediment on their tires. Another special consideration is snow removal, which must be done in such a way as to not tear or pull up the fabric. A traditional plow blade can be used if the gravel and filter fabric are frozen, but otherwise use of a snowblower or blade outfitted with skis is recommended to prevent damage.

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Between May of 2007 and November of 2009, the Virginia Ready-Mix Concrete Advisory Council replaced 11,800 square feet of GravelPave2 surface near the WSSI kennel with pervious concrete. The project was intended to demonstrate the advantages of pervious concrete for contractors, government officials, and interested parties. The concrete is 5” thick on top of a 12” gravel base course. A 6’ header curb of standard concrete separates the pervious concrete from the adjacent porous asphalt parking area. A narrower header curb could have been used, but the 6' width ensured that the concrete did not clog with gap-graded gravel in the past when the pervious concrete was next to Gravelpave2 parking areas.

Pervious concrete is made in approximately the same manner as standard concrete, except the fine material is removed and an admixture is added to help it cure properly. Removing the fine material gives the concrete very high porosity and permeability, allowing water that would run off of standard concrete to soak straight down through. Water that seeps through the pervious concrete enters into the gravel bed detention area.

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In 2010, following the installation of pervious concrete at the WSSI office, Chemung Contracting installed 8,120 square feet of porous asphalt parking surface. The porous asphalt was installed in a 4” layer on top of a 12” gravel base course, located near the back of the site, behind the rain garden. A 6’ header curb of standard concrete separates the porous asphalt and pervious concrete parking areas.

Much like pervious concrete, porous asphalt differs from regular asphalt in that its contents include less fine material. Instead, fibers are added that act as a bridge between the large aggregates, adding strength and reducing the risk of cracking. The porous asphalt surface can be swept or plowed as necessary, and can even be salted like regular asphalt to prevent freezing in winter.

More Information on WSSI’s Porous Asphalt