Full of Holes

June 27, 2012

It’s one of the better-known green infrastructure techniques, but not all of us recognize it when we see it: Permeable pavement looks, to the untrained eye, much like the regular kind. It’s been around in various forms for decades (the category includes not only porous or permeable asphalt, but also porous concrete, interlocking concrete blocks, geocells, and other forms). During that time, many beliefs, some of them incorrect, have sprung up about how and where it can and cannot be used. Some of these are slowly being disproven or dispelled. In some cases, the changes have come about because of advances in technology; in others, because information about the proper construction and maintenance of porous pavement installations is more widespread.

A once-common perception, for example, was that porous pavement was generally unsuitable for cold climates, because the freeze-thaw cycle would damage it as ice crystals in the void spaces within the pavement expanded. Bruce Ferguson addresses this issue in some detail in his comprehensive 2005 book Porous Pavements. An article from our September 2008 issue, “Pervious Pavements: New findings about their functionality and performance in cold climates,” reports on research from the University of New Hampshire Stormwater Center showing that even in cold climates, open-graded, well-drained porous pavement can actually have a longer lifespan than a traditional asphalt parking lot. (The same article reports, incidentally, that application of salt can be reduced by 75% on a porous asphalt lot, compared to a similar dense-mix asphalt lot.)

Another common belief–this one generally true–has to do with the conditions under which permeable pavement can be used. It’s considered suitable for low-traffic, low-speed areas like parking lots, alleyways, access roads, and the like, but not for heavily trafficked thoroughfares. This is because the drainage system beneath a porous asphalt installation usually consists of coarse stone, perhaps a layer of sand or gravel, and possibly drainage pipe depending on the soil type. What absolutely should not lie beneath it is compacted soil. Unfortunately, the very thing that allows water to infiltrate–spaces in the fill under the road’s surface–is the same thing that will destabilize a roadbed.

But what if it were possible to make just the shoulder of the highway porous? How would that change the nature of highway stormwater management, in terms of both volume of runoff and pollutant removal? New research from at least two different sources indicates that, despite the need for highway shoulders to sometimes bear heavy vehicles, the use of “full-depth permeable pavement” might be feasible. The topic will be the subject of a 90-minute panel discussion at StormCon in Denver (it takes place at 8 a.m. on Wednesday, August 22). An article in our next issue by Masoud Kayhanian, the panel moderator, will discuss the issue in greater detail and give you a sense of what to expect during the discussion. I hope to see you at the conference; this topic is a potential game-changer and something to watch closely in the months to come.

You can find more information on events at StormCon at www.StormCon.com. Forester University also has an ongoing master class series of webinars on permeable pavement–design, construction, performance, and case studies. Past classes are available via webcast. You can find details here.

About the Author

Janice Kaspersen

Janice Kaspersen is the former editor of Erosion Control and Stormwater magazines.