Cornerstone speaks with Jim Pillsbury
What are the differences between pervious and impervious paving?
Pavement is necessary to support the weight of vehicles and to provide a hard, firm driving surface. Without pavement, we would have mud in wet seasons and dust in dry times, much like road conditions 100 years ago. Most types of pavement are designed to be impervious, which means that they do not allow any water to pass through them into the soil below. In contrast, pervious pavement is made of material that supports the weight of vehicles while simultaneously allowing rainfall to penetrate into the subsoil.
Describe the pervious pavement systems used where you work.
The Westmoreland Conservation District’s parking lot, a project sponsored by Growing Greener grants, showcases seven different types of pervious paving systems. We built a 50-foot by 60-foot, ten-space parking lot, along with a driveway. The seven various paving materials are divided among an equal number of sections over this surface.
The subsurface soil was leveled with a bulldozer, a geotextile laid above it and then ten inches of clean, crushed stone was placed on top of that. The lot was “paved” with three types of concrete pavers and four types of plastic paving. The former varieties included interlocking circular and triangular concrete blocks; rectangular, lattice-like blocks; and concrete bricks with plastic spacer strips. The latter plastic systems included an expandable plastic web; two kinds of grid-type paving blocks; and one roll-out, grid-like product. Two of the plastic products are made of recycled material and each of the seven pavers has spaces, filled with small pieces of gravel, to allow rain water to pass through to the crushed stone base underneath. The three concrete pavers support the weight of vehicles directly; the four plastic pavers contain the small gravel and prevent it from spreading or moving; and the small gravel supports the weight of the vehicles.
What was the cost?
Our pervious parking lot was installed by a contractor who put in the heavy concrete blocks and by Boy Scouts who installed the plastic paving materials. The cost, between purchase and installation, was about five or six dollars a square foot. This figure is quite a bit more than regular asphalt parking lot pavement, and our long-term hope is that this type of pavement will become more widely accepted and used, bringing down the cost.
What are the benefits of pervious pavement?
The benefits are numerous and include the infiltration of rainfall into the subsoil; recharging of groundwater; treatment or capture of vehicle pollutants; elimination of runoff and the need for curbs, storm drains, pipes and a detention pond; reduction of the urban heat island effect; and aesthetic benefits.
During the heaviest of rainfalls there is no runoff whatsoever from our parking lot; therefore, we consider that it has achieved its purpose. We are in the process of making a set of descriptive signs and a brochure describing the process to the public. So far, reaction to our lot has been largely positive.
How well does your system perform in the Western Pennsylvania climate?
Our system has now survived two winters—one mild and one quite severe. The plastic pavers are not affected by the freeze-thaw process because they are durable and flexible, while the concrete pavers aren’t hurt either because there are so many individual pavers with joints between them. In freezing rain conditions the pavers become slick with ice build-up since salt falls into the spaces between the pavers and therefore doesn’t melt the ice on top.
Snowplowing was performed with impunity this winter by trucks, tractors and a large rubber-tired excavator. A careful operator is imperative to snow removal from this type of surface and, since the blocks cannot be completely cleared, the surface remains somewhat snowy and lumpy.
What would you do differently next time?
We probably should have filled in all of the blocks with angular crushed stone instead of round river gravel, which would have made a firmer surface. Also, since one section of the plastic pavers did not interlock well and was too flexible, it had to be replaced with another kind of plastic block that is more rigid and locks together better. When vehicles drove over the initial pavers, they separated and began floating up through the pea gravel, creating a hazardous, uneven surface.
Ladies don’t like the blocks because they are hard to walk on while wearing high-heel shoes, and some people don’t like the fact that, as already mentioned, snow cannot be completely removed from the surface.
Jim Pillsbury is a hydraulic engineer for the Westmoreland Conservation District, where he has worked for 15 years. He specializes in storm water runoff and water control, streams and wetlands. He has a BSCE from Penn State University and is a Pennsylvania registered professional engineer.