Designers and engineers have been reluctant to introduce porous pavement technology to higher-traffic applications…until recently. 

The concept of porous pavement — one designed to take water into the pavement structure and slowly filter it into the soil below — has been around for decades. It offers many benefits, but its application has been limited primarily to low-traffic applications like parking lots.

“Streets have historically been designed specifically to keep water out of the pavement structure,” said Jim Huddleston, executive director of the Asphalt Pavement Association of Oregon. “Porous pavements do just the opposite, and people who have been trained to regard water as a culprit of pavement problems are sometimes skeptical of this approach,” he said. Huddleston served as a resource on porous pavement technology to the Pringle Creek Community development in Salem, Oregon, which represents the first residential development of significant size to use 100% porous asphalt in its streets.

Interest in “green paving” is on the rise in other locales as well, like Chicago, Illinois, where the Department of Transportation recently completed installation of five green alleys in support of an aggressive citywide sustainability initiative. “We’ve had many more inquiries about permeable pavement in the last eight months (since construction of the alleys was completed),” said Cindy Williams, the department’s quality assurance manager.

Pringle Creek

Pringle Creek Community is the first development of significant size to use porous pavement for all its streets. In all, there are approximately 9,000 linear feet of porous asphalt streets and alleyways that will serve 170 homes within the 32-acre sustainable development. It is believed to be the largest residential application of porous asphalt in the country.


Pringle Creek Community represents the first residential development of significant size to  use 100% porous asphalt in its streets.

Pringle Creek’s developers employed state-of-the art practices to achieve an eco-conscious community — one that features green streets and sidewalks, environmentally inspired drainage techniques, a reclaimed bridge from a state highway project, and the Northwest’s first LEED Platinum home (there are only five in the entire country).

Almost all of the rainwater that falls on the community will infiltrate the ground naturally, and porous pavement will contribute to that goal. “There will be less pollution and runoff, saving at least 90% of the rainwater and returning it to an aquifer,” said Huddleston. Bioswales and mixed-rock bands also contribute to the site’s eco-friendly stormwater management design.

In preparation for construction of Pringle Creek’s streets last fall, the soil was cut to grade and efforts were made to minimize compaction of the subgrade. This was important if the soil was to maintain its permeable qualities.

A geotextile was used to cover the partially completed streets during construction. Here, the geotextile is being removed prior to placement of the final layer of porous asphalt.

Next, an 8-inch layer of pit-run material (stone ranging from 1 to 4 inches in size) was placed as the sub-base, and serves to support traffic and protect the subgrade. On top of that went a 10-inch layer of porous rock. The porous rock layer consists of 1- to 2.5-inch stone, and contains approximately 40% air voids. It serves as a temporary storage basin for water that flows through the street surface, and has the capacity to contain up to 4 inches of rainwater until the ground is able to absorb it.

A third layer of capping material was applied prior to surface paving with a 3-inch asphalt treated permeable base layer. The ATPB has about 30% air voids and contains 3% asphalt binder.

The Pringle Creek installation had its first real test in November 2006 when record rainfall hit the Salem area. While impermeable streets filled with water and in some cases flooded, those in Pringle Creek remained clear of standing water. Huddleston notes that street flooding in the Salem area is exacerbated by tree leaves and needles that clog drainage systems, but Pringle Creek’s built-in redundancies alleviate such problems. 

The next challenge came with the need to drop utility lines and construct sidewalks, when the streets had not yet received their final lift of porous asphalt. As a solution, North Santiam Paving, the prime paving contractor for the project, covered all of Pringle Creek’s streets with geo-textile fabric. This allowed utility and construction crews to drive on the unfinished streets without introducing unnecessary debris to the porous pavement structure.

The fabric was removed in the spring after all sidewalks and utilities were in place. The streets were power-washed and vacuum-swept to clear away any residual debris, and the final 1.5-inch lift of porous asphalt, made with 0.375-inch aggregate with 6% asphalt binder, was installed.

So far, engineers are impressed with how the porous asphalt in Pringle Creek performed during a wet winter and under construction traffic. No soft or weak areas developed.

“We used the lower lift of the pavement to support construction traffic, and it performed very well, even with heavy truck traffic,” said Don Myers, Pringle Creek project manager and president of Sustainable Development in Salem. He added that the top lift shows no signs of cracking or sagging.

Because the streets at Pringle Creek are designed in layers, maintenance issues are expected to remain confined to the top few inches of the pavement structure, which can be milled and repaved as needed. There is little concern for clogging of the pavement’s lower layers because air voids in the pavement increase as the pavement deepens, so anything that is able to penetrate the upper layers should only move more freely as it passes through the lower layers.

“We expect the lifespan of this pavement to be at least as long as a traditional pavement,” Myers said. While the development will not be fully built out for approximately three years, its streets are already seeing some action. “We had 5,000 people onsite during our home tour — that means approximately 2,500 cars — and the streets held up quite well,” he said.


A paving crew places the final lift of porous asphalt on a Pringle Creek street.

For its innovative approach to sustainable community development, Pringle Creek earned the Green Land Development of the Year award of the National Association of Home Builders. “Sustainability is the real motivator for us,” said Myers. “Our goal is to be the greenest community in the world. People are drawn to it, and it supports our company’s philosophy of environmental stewardship,” he added.

Chicago’s pilot program

The City of Chicago has mounted a campaign to “build a sustainable city,” and even has a Department of Environment dedicated to the effort. As part of the initiative, the city launched a “green alley” pilot program last year, which provided five different test sites for evaluating porous pavement options.

Depending on the outcome of the pilot program, more of the city’s 1,900 miles of alleyways (the most of any city in the world) could be converted to eco-friendly pavements in the future. 

While the green alley pilot project supports the City of Chicago’s “sustainable city” initiative, it provides other specific benefits as well. It offers a way to test low-impact and recycled materials for potential use in more expansive city infrastructure applications, and provides an environment for evaluating potential solutions to stormwater management issues affecting the city’s alleyways.

“One of residents’ biggest complaints is flooding,” Janet Attarian told Better Roads magazine. Attarian is the project director of the City of Chicago’s streetscape and urban design program.

When heavy or prolonged rain events occur, alleys become flooded and overwhelm the sewer and stormwater system. Basements flood and contaminated water finds its way into the river as a result, Janet explained. Repaving with conventional methods can make flooding worse, and connecting sewer mains to improve drainage is cost prohibitive. 

Of the five alleys identified as candidates for the green alley pilot program, each suffered from maintenance issues and each exhibited ideal soil conditions for a porous pavement application.

Soil conditions are key to the success of a porous pavement. “Soil infiltration rates are determined by boring samples taken from the site,” said Cindy Williams, quality assurance manager with CDOT. “Anything with a rate of less than 0.5 inches per hour, for example, would require an underdrain,” she explained.

One of the five alleys in the pilot program was constructed with porous asphalt. It is located in the Beverly neighborhood of southwest Chicago, and was completed in the fall of 2006. Bigane Paving Company performed the construction work under contract to CDOT.

“This was a new project for us,” said Anne Bigane Wilson, president of the company. Aside from a sub-base for a hockey field, this was the first time Bigane Paving had constructed a porous pavement installation. The mix was new to them, as was the ground-tire-rubber component, which helped prevent draindown of the asphalt.   

“We’re typically targeting an air void percentage of 3 to 4%,” said Wilson, “but in this case, the target was 18 to 20%.” That also presents challenges with rolling patterns, she explained, because too many passes will compact the pavement too much. With insufficient air voids, water will not flow through the pavement at a high enough rate, and its drainage properties will be significantly diminished or lost completely.

Wilson stressed the need for a proper, well-draining sub-grade for the success of any porous pavement application. In this case the alley was excavated, then pitched and graded to direct stormwater to the center of the alley, which measures almost 100 square yards. For the sub-base, 12 inches of washed CA7 Dolamite stone (0.75-inch in size) was installed. The next layer consisted of 8 inches of porous asphalt.

After 10 months of use, including a fierce Chicago winter,  the alley is performing well. “There are no issues that I am aware of,” Wilson said. “It looks good, and we’ve had no complaints or warranty calls. It was a good project for us, and there was no downside.”

Cindy Williams of CDOT notes that some contractors are reluctant to bid projects like these because the terminology and specifications may be different from what they are accustomed to on more traditional paving projects. S.T.A.T.E. Testing was hired to translate American Society for Testing and Materials standards to Illinois Department of Transportation specs in order to facilitate the bidding process, she said. 

Today, all five porous alleys have survived a season of freeze/thaw conditions, and will be evaluated on a variety of criteria over the next two years. The city will determine, for example, how well the different installations maintained their permeable qualities.

“Evaluations performed in the spring showed no signs of decline in permeable attributes,” Williams said. A flow rate of six gallons per square foot per minute was maintained for the surface material, and 200 gallons per square foot per minute was maintained for the sub-base. “We’ve also seen no signs of deterioration or rutting,” she added. Evaluations were based on core samples and in-place observation.

If the alleys in the pilot program continue to perform well, other sites will be considered for future green alley construction or retrofitting. In addition to soil conditions of the site, candidates for green alley construction will be evaluated based on existing pavement construction, drainage, and slope. Adjacent elevations must also be considered, as well as proximity to potential pollutant threats. Williams said between 10 and 15 locations are currently under consideration.

“We are already including green options in term contracts for alley construction,” Williams said. The city is also working to establish local guidelines for porous pavement applications in an effort to facilitate the greening of Chicago’s alleyways. In addition, The Chicago Green Alley Handbook has been published to help educate city residents about the program. It offers tips on what they can do to further the benefits of green alley construction, including installation of rain gardens, shade trees, and green roofs. 

In summary

As cities and developers realize the benefits of green construction, they’ve become more willing to test porous pavements in increasingly significant applications. Beyond parking lots, we are beginning to see porous pavements incorporated into streets and alleyways, as highlighted here, and in other applications as well. Phoenix, Arizona, for example, has an 0.5- to 1-mile stretch of porous asphalt on an urban highway which is over 20 years old and still performing well. With further trials and continued successes, the application of this technology should become more prevalent, and its benefits be leveraged to their full potential.