Rather than conveying runoff from a site, a number of systems are designed to let it simply sink in.

By Janis Keating

As any driver can tell you, water and asphalt don't mix. Water pools on the surface of typical American pavement, which has been graded and sealed to make it waterproof. On high-speed roads, water can contribute to vehicle hydroplaning, and on any paved surface, these pools of water also create runoff containing various pollutants, including asphalt detritus, heavy metals, tire scrapings, and grease and oil. Depending on the site, this runoff ends up in drainage ditches, gutters, tiled drains, storm sewers, and the like.

Recognizing the severity of nonpoint-source pollution and concerned that this runoff will foul natural waterways and drinking-water sources, many state and federal agencies are calling for, or mandating, cleaned or better-contained pavement drainage.

For some sites and applications, porous pavement might be the answer. Unlike conventional pavement, water runs through porous pavement, then percolates into the soil below. In some cases porous pavement eliminates the need for drains or storm sewers.

The Secret's in the Mix

Variously called "plant-mix seal coat," "open-graded mix," "gap-graded mix," "popcorn mix," or "porous friction course," porous pavement contains the same elements as conventional pavement. The main difference is in its "recipe." With an open-graded asphalt concrete mix, containing a high percentage (by weight) of aggregate larger than a No. 4 sieve, porous pavement is laid to a thickness of 3/4-1 in. over a porous base of large, open-graded gravel. The resulting pavement features a high void ratio and a coarse surface texture. The latter creates pressure-release channels that remove water when it's pressed between the pavement and a vehicle tire. A number of states have used different variations of this recipe since the 1970s.

In a 1980 report, "Porous Pavement Phase I Design and Operational Criteria," EPA evaluated a dozen sites in an effort to standardize the pavement's design. (Full text of the document is available at www.epa.gov/ednnrmrl/repository/abstrac2/abstra2.htm.)

For proper water percolation through the asphalt and down into the soil, attention must be paid to the strata beneath the porous pavement, as shown in a typical cross-section (Figure 1).

The EPA report noted that initial costs for porous pavement can be up to 35-50% higher than the cost of conventional paving. As water percolates straight down into the subsoil, however, porous pavement makes storm drains unnecessary in many applications. (Drains would be indicated, though, in soils with intrinsic drainage problems.) In addition, curbs are not necessary with porous pavement; less pore-clogging debris accumulates and the material operates more efficiently if curbs are eliminated. With these items factored out, the total cost of a project might be comparable to, or perhaps even cheaper than, conventional paving.

The Test of Time

But how does porous pavement hold up with normal use? Won't all that water break up the asphalt? The Walden Pond State Reservation in Concord, MA, installed a porous pavement parking lot in 1977, which is still in use today. Even with the extreme freeze/thaw cycles of typical New England winters, the porous pavement has done its job.

"We haven't needed to repave," notes Park Supervisor Michelle Dumas. "There are a few cracks or frost heaves, but considering our winters and the fact that we get 600,000 visitors a year, the lot has held up well over time."

Several different mixtures were used in the lot; the one deemed best was concocted of an asphalt-type AC-20 viscosity grade, with an asphalt content of 4.5–5.58. The mix composition and types of base course are shown in Table 1. The cross-section is shown in Figure 2.

Ideally, porous pavement should be vacuumed and washed down periodically to keep its pores free of clogs. That's not always feasible, however. "We don't have the equipment to vacuum the lot," says Dumas, "yet the lot experiences little to no runoff."

Hoping to inform visitors of porous pavement's benefits, Walden Pond State Reservation offers an educational brochure that describes its lot. A. Richard Miller, the brochure's author, is an enthusiastic proponent of porous pavement and can't understand why it's not used in more applications. Commenting on the pavement's recipe, Miller remarks, "Maybe the problem is that it's free; no one's making money off of it."

Noting how waterproof conventional pavement blocks moisture from the soil below, Miller offers a current worst-case scenario: "Boston's Back Bay area was once just that—a bay. They filled in the water to make more land and sunk wooden pilings into the fill to support the buildings. This system worked for many years. With cobblestone streets, enough water filtered down to keep the pilings intact. But now, with waterproof streets and drains, no rain gets down there, and the pilings are developing dry rot."

The Long and Porous Road

Porous pavement is a moderate hit in Virginia, where it's been used at more than 100 sites, mostly for parking, or light- to light-/moderate-usage roads.

"There are good environmental reasons for using it," states Larry Gavan, senior environmental engineer for the Virginia Department of Conservation and Recreation. "It tends to recharge groundwater and reduces runoff and pollution."

Because of its high porosity, Gavan cautions that there are only certain areas where porous pavement can be installed. "We try to site them 100 feet horizontal from a water supply and 10 feet below gradient of nearby building foundations. We also suggest posting weight limitations on porous pavement lots or placing ‘headache bars' over lot entrances to discourage large, heavy vehicles."

When speaking to various groups about the porosity of this pavement, Gavan is often met with skepticism until he shows them one of his favorite slides. "They look at the water stream and say, ‘Where's the drain?'" Gavan chuckles. "When I tell them there is none, they can't believe it!"

Gavan suggests some optimal sites for the product. "A stadium parking lot, one only used about 10 times a year, would be the best. Also, it's good for relatively flat areas with a low groundwater table." He also stresses the proper care of a porous pavement surface. "You can't put the snowplow all the way down; that will break up the surface. The plow needs to be up about an inch. The little snow that's left will eventually melt into the pavement. Also, you can add a little salt, but you might want to hold back on sand and cinders on this type of lot. They will clog the pores."

Winter climates can be brutal on porous pavements if they're not taken care of properly. "We have taken some of them out," notes Randolph J. Stowe of Natural Areas Ecosystem Management in Harvard, IL. "Some we removed because of damage, not product failure. It seems like they are not good for a winter climate. They don't respond well to snowplows. Maybe the contract plower didn't raise the plow. A company's own grounds crew would be more conscientious, I'd think."

Whether the pavement's used properly or not, Brant Keller, director of public works and stormwater utilities in Griffin, GA, doesn't like the product. His main concern is the pollutants drawn through the pavement to the soil below. "I'm not recommending it for Georgia or my city," Keller remarks. "Once it's blighted, from a pollution removal standpoint, it's hard to fix. The pavement must have a filter zone under it—number 57 stone or sand. Once the filter zone is full, it's blighted. Then you have to remove the pavement and the stone and start over. It's not cost-effective."

In his other capacity as executive director of the Georgia Association of Stormwater Management Agencies, Keller is still looking into safe ways to deal with road drainage. In the past, he used brick pavers with drainage holes for parking areas. "That did lessen runoff," he recalls, "although we didn't intend it to be stormwater management."

Porous, but Less Like Pavement

No drain - just porous pavement!

For parking-lot applications, some skip the "pavement" (asphalt) and concentrate on the "porous" aspect by using gravel in conjunction with a structuring underlayment. Gravelpave², a product of Aurora, CO's Invisible Structures Inc., has been used for high-traffic porous parking areas since the early 1990s.

To construct a Gravelpave² lot, one first puts in a gravel base course (depth specified by a soils engineer) to bear vehicle loading. Gravelpave², which consists of 100% recycled plastic rings molded onto nonwoven geotextile filter fabric, is then secured in the lot bed. The Gravelpave² course is then filled with decorative gravel of minus 5 mm (3/16 in.).

"We're heading into our third winter with this lot, and we've been really happy with it so far," notes Rick Moore, program officer for the Grand Canyon Trust, a Flagstaff, AZ, regional conservation group that seeks to protect the Grand Canyon.

"When you compare the environmental impact of this, I think it is good," states Moore. "We didn't use fossil fuels to construct this lot. We pride ourselves on being an innovative, rather than confrontational, group. When we wanted a variance to build a gravel lot, the county planning department said, ‘Go be innovative.' We found Gravelpave² on the Web."

The county's main objection to a gravel lot was the dust. Moore reports that, despite Arizona's dry conditions, typically no dust comes off it. "We made a deal with the county to report on the lot after two years. Now the county sends people to look at our lot and allows others to do this in some situations."

The Grand Canyon Trust building and its Gravelpave2 lot

With movable gravel, one would expect a lot of maintenance, but Moore says the Gravelpave² lot has been relatively maintenance-free. "We have to do a bit of raking about once a year, maybe four to six hours a year."

The first year, a few problems needed to be ironed out. "After our first winter, we had half a dump truck of pea gravel added, so maybe we put a full day into it the first year," Miller reports. "In two places we had little problems: right where the asphalt apron meets the gravel, the underlayment tended to lift up a bit. Then, where people parked, water from their tires would freeze and take up some gravel. So in those two areas we drove in some spikes, about every 2 feet. There's a place in the grid where you could do it, and it had been mentioned in the installation instructions. We don't have a lot of truck traffic, but we have some semis deliver recycled paper to us a couple times a month, and we haven't had problems with depressions and sinking."

Flagstaff gets more than 100 in. of snow a year, so the lot has to be plowed in the winter. "We had to train the snow-plow person to lift the blade when plowing the lot," Moore recalls. "We don't plow unless we get more than 3 or 4 inches. The lot doesn't get standing puddles. We drive on the snow that's left and pack it down; it melts and drains in. There's just no runoff from this lot at all."

The Grand Canyon Trust building's approximately 6,000-ft.² lot was filled with 3/16- to ¼-in. Prescott, a decomposed granite native to the area, which many homeowners also use for landscaping. "It does ‘track' a little bit," Moore says. "I always have a few pebbles in the floorboards of my car. And it's not good on high-heeled shoes, I've heard."

The natural setting of Dominican University's Gravelpave2 lot

Overall, the Grand Canyon Trust is satisfied with its Gravelpave² lot. "There's a fabric backing on this stuff that keeps the weeds out," Miller happily points out. "We have not had a single weed come up."

Conversely, in River Forest, IL, Dominican University did want something growing up through its Gravelpave² lot. "We would have lost 20 mature trees," notes Vice President of Business Affairs Amy McCormack, explaining the university's choice of Gravelpave² instead of asphalt. "This was an area in our campus of natural setting, yet we needed more parking for faculty, staff, students, and visitors. Also, we had to meet village requirements. We could not increase our runoff into the village water treatments, and since we were expanding, we were looking for a creative solution."

River Forest, an upscale suburb of Chicago, requires asphalt or concrete parking. Dominican had to secure a variance to build the lot. "They were supportive because of the water management and saving trees," McCormack observes. "The lot was completed this June. The trees are woven within the lot. We gave up parking spaces to have the trees."

Geoweb porous pavement with aggregate/topsoil mix

The 128,000-ft.² lot is not lined with curbing. Wheel stops made of recycled material indicate the end of the lot and each individual parking space. "So far, of 260 spaces, maybe one or two are lost because the lot is not lined," McCormack points out.

She reports a few problems with the lot thus far. "Maintenance has been greater than expected. We had to rake more than anticipated. We're happy with the lot but a little disappointed at the amount of maintenance, and we have talked to Invisible Structures about it.

"There are long aisleways for the lot," McCormack continues. "People seem to travel 30 miles per hour, even though we put up signs for 10 miles per hour. The gravel in the aisleways has not performed to my liking. We also have a daycare facility, and rush hour is the worst time for fast driving."

Although this winter is the lot's first snow test, McCormack notes that the beloved trees are the cause of some lot maintenance. "You do have to rake the leaves so they don't remain on the parking surface. We have to be careful to not pick up the gravel, but of course that straightens the gravel too."

Porous Without Pavement

Carlton Memorial Reserve wetlands prior to restoration and installation of the Geoweb system.

Installation of the Geoweb for parking/roadway.

Completed stormwater retention system.

In some applications, such as emergency vehicle access or low-use residential drives, one might skip the pavement altogether. Presto Products Company of Appleton, WI, through its Geosystems products, offers two products that reinforce turf areas to allow vehicle access. Geoweb, a cellular confinement system made of heavy-duty, expanding polyethylene sections, can be used for a variety of soil stabilization solutions, such as slope and channel protection, load support, and earth retention. Geoblock, a porous pavement system manufactured from recycled plastic, consists of a series of interlocking, high-strength blocks for supporting heavy or concentrated loads while also protecting turf. Because its blocks create a flexible structural bridge system within the topsoil layer to support and distribute concentrated loads, Geoblock has been used for emergency and utility access, auxiliary parking, golf-cart paths, driveways, trails, and other high-use areas.

When Sarasota County, FL, acquired 24,565 ac. of environmentally sensitive wetlands from which to build the T. Mabry Carlton Jr. Memorial Reserve, a public-use and education facility, the first phase of the project required building a 3,200-ft.² roadway to the building site and a parking area for 51 cars. As the system had to support loads over poor soils while also functioning as a stormwater retention system, Geoweb was specified and used as the road building material.

An 8-in. roadbed was dug, and a geotextile was placed over the muck soil. Six-inch-deep Geoweb was then placed over the textile and secured with stakes. The cells were infilled with No. 57 stone, and to bring the road flush with the rest of the terrain, another 2 in. of stone was placed on top of the Geoweb. Depending on the project's use and available materials, however, Geoweb can be infilled with other materials. "Nearly any type of aggregate can be used," states Presto's Pat Stelter. "Some parts of Florida use crushed shell."

Geoblock porous pavement offers the appearance of a lush, grassy expanse while also allowing vehicle access. Homestead Village apartment complex in Tukwila, WA, used Geoblock to ensure good drainage and an upscale appearance for a fire lane. And because pedestrian traffic can also wear down a lawn in record time, a Geoblock system has been installed on the mall near the Washington Monument.

Geoblock is made from 50% postconsumer recycled plastic. "Presto's core business is consumer bags, garbage bags, and the like," says Stelter. "There's a lot of scrap in these plants. We tried to recycle it into a new product."

Drainage for Existing Pavement

Geoblock at the Washington Monument

Millions of miles of conventional asphalt pavement cover the planet. Since it's unfeasible to replace it all with a porous substitute, what can be done to improve asphalt's drainage? For the past four years, Louisiana State University's John Sansalone has been working on a possible solution. Anticipating EPA's findings that stormwater runoff was polluting and states would have to keep it out of natural water sources, in 1994 Sansalone began his doctoral work at the University of Cincinnati's College of Engineering by developing a pilot project on partial exfiltration trenches to trap pollutants in highway runoff. Part of this research involved excavating the 2- to 5-ft.-deep trenches carefully to avoid compacting the surrounding soil, putting in a PVC drain tile, then filling the trenches with iron oxide-coated sand and capping them with a honeycomb porous pavement. The project's premise: When the highway runoff washed into the trench, the pollutants (nondegrading heavy metals such as lead, zinc, cadmium, and copper) would adhere to the iron oxide-coated sand and not percolate into the soil or groundwater. Sansalone continued this work while at the University of Cincinnati as a research faculty member. Now an assistant professor in civil and environmental engineering at Louisiana State University, he has been subcontracted to finish work on the Cincinnati project.

"I was up there in August," Sansalone reports. "The drainage is staying out of the groundwater, but the trench needs to be cleaned out. In essence, we built a trashcan for pollutants, and it has to be emptied. We'd hoped the trench would have a 10-year life."

Will cleaning the trench be difficult? "You can either clean the sand or landfill it," he explains. "Landfilling would be the easier solution, but with the right technique, you could clean the sand on-site and reuse it. The heavy metals could be reclaimed from the sand, but perhaps that's not economically feasible right now. If nothing else, maybe you could reclaim some of them for a smaller amount of eventual waste."

In addition to monitoring the Cincinnati sites, Sansalone has also established research sites in Louisiana. While the goals of the projects are the same, the geologic factors vary. "The Cincinnati sites mainly monitor snow pollution," he says. "The clay soil there offers some benefits. Although clay is less permeable than some soils, it tends to hold pollutants because it has a surface charge. In addition, the pH of Cincinnati's rain is about 4-sometimes as low as 3.5-and this acid rain ‘pushes' the metals off the pavement. "The problems in Louisiana are completely different. Not only does it have a silty soil, but also—because of the high water table here—most roadways are elevated and drain directly into water."

Sansalone hopes his research yields a feasible solution. "To correct its runoff pollution, California is considering treating its stormwater, and the price of wastewater treatment is phenomenal. There has to be a cheaper way!"

Janis Keating is a frequent contributor to horticultural magazines.