Credit: Chicago DOT
Measuring the cross-slope of an existing pervious green alley. Green alleys are sloped to the middle to allow excess flow to go to the end of the alley, away from adjacent back yards. This alley has a keyway cut down the centerline to help collect and route excess flow. Photo: Chicago DOT
4) Do think about snow plowing.
If snow removal is part of your department’s annual routine, think twice about permeable pavement. Steel blades scrape up the road, leaving a “salt and pepper” surface.
Fortunately, this is primarily an aesthetic issue and, based on observation and anecdotes, long-term performance isn’t significantly affected. The aesthetic issue can be addressed by using a carbon fiber or polyurethane blade or a darker aggregate in the surface.
5) Do consult a landscape architect. There are a couple of factors at work here. Because they retain more water on site, permeable pavements expand your tree and shrubbery options. But on the other hand, more deciduous trees and shrubs increase your maintenance load, so maybe conifers are better. Also, you may need to worry a bit more about future root growth, and how it affects pavement.
For these reasons, and for reasons an engineer or materials scientist might not know about, it’s a good idea to consult a landscape professional.
It’s hard to be authoritative about a solution as new as pervious pavement; most of the above relies on observation and on conversations with the relatively small number of contractors and designers who’ve been working in this area for a while. But don’t be discouraged by any of the above caveats.
After all, a similar list of do’s and don’ts could easily be assembled for just about any roadway technology, and pervious asphalt is working out extremely well for most cities, most of the time. And it may very well be the right solution for your project.
— Jay Behnke (email@example.com) is president of S.T.A.T.E. Testing LLC (www.statetestingllc.com) in East Dundee, Ill. Formed in 1999, the firm specializes in alternative paving technologies such as ground tire rubber and warm mix asphalt, including several Chicago DOT permeable pavement projects (see "The trickle-down effect" on page 36 of our May 2007 issue).
What about the Hamburg wheel test?
The Hamburg wheel test — where a weighted steel wheel is rolled up to 20,000 times over pavement samples submerged in hot water to test for rutting and stripping — has proven to be a very useful for conventional pavements.
It is potentially very useful for pervious pavements as well, but it would be wrong to be too definite about that. More results must come in first.
One confounding factor is the amount of water that works its way into the voids of pervious pavement during the test.
In the Hamburg test, the water saturates the pavement sample and stays there for the duration of the test. In actual use conditions, rain flows through pervious pavement and into the ground, and pavement dries out quickly once the storm passes.
So the comparison isn’t exactly apples-to-apples.
That said, something is being measured, and maintaining stability under Hamburg wheel testing is a solid indicator of performance. As more research is conducted, better specifications for Hamburg wheel testing of pervious asphalt will emerge.