While porous asphalt parking lots are not a new idea, they have been rapidly catching on in recent years. The main impetus is environmental. Porous asphalt pavement allows rainwater to pass to a recharge bed made of coarse stone, which provides some treatment benefits. Water then re-enters the ground naturally and can flow back to the stream system.
By contrast, conventional practice is to build large detention basins that receive surface runoff from conventional, impermeable parking lot pavements. Most of that water does not infiltrate the soil and recharge the groundwater, as nature intended.
One consulting firm that has designed more than 40 porous parking lots with storm water recharge beds in the past 18 years is Cahill Associates, Inc., based in West Chester, Pa. A large majority of them have been topped with porous, open-graded asphalt pavements.
“We absolutely prefer asphalt for porous pavement systems,” said Wesley Horner, principal planner with Cahill. “We find that it’s very cost effective, and the fines can be screened out relatively easily and inexpensively. Most asphalt plants are able to satisfy the specifications that we write. Asphalt is not terribly temperamental; it gives you more flexibility in its placement. It’s not as time-sensitive as concrete.”
A prime example of these structures comes from Portland, Ore., where one of the nation’s largest porous asphalt parking lots went into place early last year. With Cahill Associates as its hydrology consultant, the $6.4 million project covers 46 acres of land. It is located at the Port of Portland on the Columbia River.
The parking lot is used to store Hyundai cars until they can be shipped to dealers. In areas totaling 11 acres—where delivery trucks travel heavily—the pavement is standard dense-graded asphalt. Over the remaining 35 acres, contractor Lakeside Industries of Portland paved a 3-in. course of open-graded porous asphalt.
The native river sand along the Columbia River, which easily absorbs water from the parking lot, made the choice of porous asphalt a natural, said Dave Dittmer, senior project manager for the Port of Portland.
“The porous structure we built is a little more expensive than standard pavement because the porous pavement has a 10-in. open-graded rock base, which doubles as a pavement base and water storage reservoir,” Dittmer said. “But we don’t have pipes to convey the storm water, we don’t have treatment devices and we save time (a year) and costs of environmental permit acquisition. For all of those reasons, porous asphalt was the best solution.”
Three permit agencies have jurisdiction over drainage infrastructure for conventional impervious pavements adjacent to the river: the U.S. Army Corps of Engineers, the Division of Oregon State Lands and the city of Portland. Because it is a navigable waterway, the Corps of Engineers controls the Columbia River. The state controls the land from the waterline to the high-water mark, and the city maintains jurisdiction from the high-water mark back for a distance of 40 ft.
“We were very pleased with the speed of construction with porous asphalt,” Dittmer said. “We could put down between 2,000 and 3,000 tons per night, which covered 3 or 4 acres per shift. At that rate, we could cover 20 acres in less than a week.”
Dittmer said Lakeside Industries, the paving contractor, would pave more than a dozen 1,100-ft pulls per night. “We had a Roadtec Shuttle Buggy out there to act as a surge bin, and we maintained a steady stream of trucks to the Shuttle Buggy,” he said.
Dittmer appreciated the relatively quick availability of the porous asphalt for parking. “We were parking cars on it within a week to 10 days of having it down,” he said.
Following a light excavation to clear the area of vegetation, Coffman Excavation of Oregon City, general contractor, lightly compacted the sand to a specified density. Coffman then placed a filter fabric down to prevent the open-graded stone in the recharge bed from mixing with the sand base. That could have closed off voids and slowed down drainage. Placement of the 2-in. top-size recharge stone came next. It has approximately 40% voids content. That was topped with a 1-in.-thick choker course of 1-in. top-size aggregate to form a paving base.
The recharge beds offer water treatment benefits, according to Horner. “In terms of water quality, these recharge beds filter out non-point source pollutants through an array of physical, chemical and biological mechanisms,” he said. “In that way, these systems achieve a high level of water-quality mitigation.”
To get full-time access to a 500-ton-per-hour asphalt plant, Lakeside Industries paved the project at night. The porous mix consisted of 0.5-in. top-size aggregate with 5.6% asphalt cement, said Larry Hansen, Lakeside’s project manager. Because the project was paved in early spring, temperatures were cool, so paving was suspended for some nights when the temperature dipped below 50°F.
“We normally worked 12.5 to 15 ft wide, and the pulls were fairly long,” Hansen said. “We needed a roller to run in front of the paver to smooth out the grade. Otherwise the base rock would rut under the weight of the trucks. We used a Shuttle Buggy to transfer asphalt into the paver from the side. The Shuttle Buggy had wide tires with a lot of flotation, so it worked fine. We never ran trucks right in front of the paver.”
Meeting the project’s production needs presented somewhat of a challenge, Hansen said. “We averaged around 2,000 tons per shift, which was about 10 to 12 hours,” Hansen said. “Paving at night actually helped, because there was a lot less traffic for the trucks to deal with.” The contract called for 28,800 tons of 0.5-in. open-graded hot mix and 12,800 tons of dense-graded asphalt.
The contractor took care not to over-compact the porous asphalt, “You reverse your thinking, compared to dense-graded asphalt,” Dittmer said. “You don’t want to close off the voids.” Compaction consisted of four to five passes at the most, then a pause, then a finish pass or two.
To put the finishing touches on the parking lot, and to further enhance the site’s innovative management of storm water, a firm called Green Works in Portland designed landscaping for three rainwater gardens, or swales. The largest of the gardens is approximately 110 by 15 ft long and runs along Marine Drive. The other two are 1,200 and 2,000 sq ft in size. All three gardens have red alder and Western red cedar trees, as well as creeping mahonia (a low ground cover) and California grey rush, a wetland plant. All three swales are connected by storm water overflow pipes.
“We’re very satisfied with the design,” said Jaime English, a landscape designer with Green Works.
From 1975 to 1990, Cahill designed one or two porous pavement projects per year, according to Horner. But in recent years, the firm has consulted for up to four such projects per year. “There’s been a very dramatic upturn in the popularity of these things,” he said. “We get more requests than we can handle.”
Much of the surge in popularity of porous pavement can be explained by storm water regulations backed by federal, state and local laws.
“People are taking both water quality and water quantity issues more seriously, so as a result this kind of technology is being embraced with great eagerness,” Horner said. “We have been designing these recharge beds for over 25 years now, and we have numerous projects that have achieved lasting success for that time period.”
In Portland, Dittmer said the project is working great so far. The pavement can absorb water from a 3-in. hose running full blast. Water covers approximately 12 sq ft of pavement before it flows downward into the recharge bed. “We just need to sweep it two or three times a year to keep the leaves and cottonwood seeds from plugging up the voids,” he said.
Porous pavement projects have been constructed for a wide array of clients, including corporate office complexes, industrial facilities and universities. “A lot of our clients want to be environmentally sophisticated and green,” Horner said. “This allows them to do that in a very cost-effective way.”