Porous Asphalt — An Alternative Tool for Managing Stormwater

By Joanne Ray | September 28, 2010

Porous asphalt pavements offer developers and planners an alternative tool for managing stormwater. These pavements, used mostly for parking lots, allow water to drain through the pavement surface into a stone recharge bed and infiltrate into the soils below the pavement.

Such pavements have been proving their worth since the mid-1970s, and recent changes in stormwater regulations have prompted many consulting engineers and public works officials to seek information about these products.

Recently, the Danvers, MA-based J. Masterson Construction had the opportunity to do the site work for one of the first porous asphalt commercial parking lots in the eastern Massachusetts area, according to Project Manager William Peach.

"Basically, because of new stormwater regulations and because this company is near wetlands, they had to bring their lot up to current standards," said Peach. "We bid on the project and we were the successful bidder."

The parking lot belongs to the RREEF Company on Ballardvale Road, next to I-93 in Wilmington, MA. The site improvements and porous pavement system were designed by Merrimack Engineering Services from Andover, MA, under contract to the property owner. Merrimack is also providing field survey layout services.

The $750,000 project started in mid-July. The 15-acre job includes parking lot removal and replacement, a new drainage system (with catch basins, manholes and oil-water separators), new landscaping, 8,000 square yards of pervious asphalt pavement, and 2,000 square yards of concrete pavement.

"The porous asphalt includes two 2-inch layers for a 4-inch total, and the standard is 2-inches with 1-1/2 inches on top," Peach said. "Underneath the porous asphalt is a drainage system consisting of 4 inches of stone, 30 inches of gravel and 8 more inches of stone."

The design for the recharge bed under the porous asphalt consists of four lifts of various materials totaling 41 inches of total depth. Starting directly beneath the porous asphalt and descending to the native sub-base, the materials were: 4 inches of crushed stone as a 'choker' course beneath the asphalt, 30 inches of bank run gravel as a reservoir 'filter course'; 3 inches of pea stone to serve as a 'filter blanket'; and 4 inches of crushed stone directly above the sub-base as another 'reservoir course.'

J. Masterson was also successful in reclaiming the existing asphalt and reusing it on the site. They are also in charge of building new concrete loading docks. Early September found laborers underpinning the existing foundation to prep for the new loading docks.

The job is scheduled to be completed by the end of October.