Avoiding solid - non-permeable - surfaces in driveways, walks and patios is better for the environment. Solid surfaces, such as driveways, walks and patios force storm water to run off and create a host of environmental problems. First, storm runoff can erode surrounding soil, damag lawns, wash away soil and cause flooding. Even more damaging, storm water can carry pollutants into sewer systems, surrounding waterways and water tables, posing serious health hazards.
[Commentary includes images] Solid surfaces, such as driveways, walks and patios, force storm water to run off, creating a host of environmental problems. First, storm-water runoff can erode surrounding soil, damaging lawns, washing away soil and causing flooding in streets and surrounding areas. Even more damaging, if allowed to run off unchecked, storm water can carry pollutants into sewer systems, and then into surrounding waterways and water tables, posing serious
health hazards.
A much better scenario is to prevent storm water runoff from these surfaces entirely. One way to accomplish this is to use what are called "pervious" paving systems. Such systems substitute a variety of materials, such as concrete paving blocks, wood paving blocks and others for these solid surfaces. The materials thus permit water to drain through the surface to the soil below.
In the opinion of Bruce K. Ferguson, Franklin Professor of Landscape Architecture at the University of Georgia in Athens, this represents a much superior solution to the problem of storm-water runoff. Ferguson, a Fellow of the American Society of Landscape Architecture, preaches the gospel of pervious paving — and he preaches it zealously.
“It is much better to filter rainwater through paving rather than allow it to run off, unchecked,” Ferguson says. “Not only does that reduce pollution and erosion by minimizing the amount of impervious surfaces around a home, it can even result in tax benefits.” Ferguson points out that many municipalities have codified reduced runoff by reducing storm-water fees for homeowners who replace hard-surfaced paving with pervious materials.
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A variety of materials
Pervious paving comes in a variety of forms, Ferguson points out. It can include concrete or asphalt material as well as concrete block, bricks and even wood blocks. Ferguson says that a case can be made for all of these, but that he feels that concrete blocks and bricks, especially those designed specifically to create pervious paving surfaces, provide the greatest advantages.
“Both concrete and asphalt can be made to be pervious,” Ferguson notes. He points out, however, that to create a pervious surface with either monolithic concrete or asphalt requires careful control. To do so, the materials must contain a single-size aggregate, usually 3/8 of an inch, to create voids that allow storm water to sift through the material rather than run off the surface. Typical concrete or asphalt contains a variety of aggregate sizes, including such fine particles as sand. This reduces the amount of void space in the material, making it more impervious.
Even if the material itself is formulated properly, improper installation techniques, such as compacting and troweling, can reduce the pervious nature of the materials, thereby reducing effectiveness.
“I believe that brick or concrete block represents a far better solution,” Ferguson concludes. “By filling the space between the blocks or brick with gravel of a uniform size, you create voids that allow the water to run through, rather than off of the material.” Such an installation creates openings throughout the surface. Even though these openings only occupy 12 or 13% of the surface area, they provide enough permeability to allow water to sift down through the driveway or walkway to the soil below.
Installation methods important to success
Ferguson cautions that even with brick or blocks designed specifically for creating pervious pavements, installation is still key to a successful project.
“I believe that blocks provide several advantages, not the least of which is appearance,” Ferguson says. But he cautions that proper preparation is the key to a good installation.
Such installation should include a 6-inch base course of a single-size aggregate. The reason behind having a single-size aggregate is to create void space between the particles in this course. Mixed sizes of aggregate tend to cause the material to compact, thereby reducing the void space so key to good drainage. Above that base course should be at least a one inch leveling layer of an aggregate of about 3/4-inches in size, Ferguson says.
The brick or block then is laid atop the leveling layer, and the spaces between the material filled with an aggregate of 3/8 inches in size to create spaces for storm water to run through to be absorbed first by the layers below and eventually to drain into the soil underneath.
Ferguson knows of several concrete block manufacturers who are making units for installation in pervious paving applications. He prefers those that interlock. “I’m not a big fan of the units that provide large openings that allow grass to grow up through them for residential applications,” he says. “I believe that they are difficult to walk on. I much prefer the interlocking materials. Not only do they provide the permeability desired, but they are terrific to look at.”
By Wayne A. Endicott
RESOURCES
American Society of Landscape Architects, www.asla.org
Versa-Lok, www.versalok.com
Uni-Group U.S.A, www.uni-groupusa.org
[Sidebar]
How does pervious paving reduce pollution?
The biggest pollutant present in residential locations is oil, according to Professor Bruce K. Ferguson. “Oil from vehicles is deposited on driveways and can be tracked onto walkways and patios. When storm water washes these materials off of hard surfaces, it goes directly into the storm sewer system and eventually downstream into waterways.”
Pervious paving changes the dynamic. As the water sifts through the pervious material, its passage is slowed considerably. This allows microorganisms that are present to attack the oil and render it harmless before passing it onto the soil below. Says Ferguson, “By the time that oil is washed into the soil, it no longer poses an ecological threat.”
Captions:
Photo 1
Interlocking pavers with aggregate filling the voids between the blocks not only allow water to pass through this walkway without running off, but also provide an attractive landscaping feature.
Photo courtesy of Versa-Lok Retaining Wall Systems.
Photo 2
Close-up of this sidewalk reveals the voids between the interlocking paving blocks that create a drainage system for storm water.
Photo courtesy of Versa-Lok Retaining Wall Systems.
Photo 3
Diagram illustrates the proper way to install pervious paving, including at least 6 inches of a base course and a leveling course before actual installation of the blocks. Spaces between the blocks are filled with a single-size aggregate to allow water to pass through to the layers below.
Diagram courtesy of Versa-Lok Retaining Wall Systems.
Photo 4
Interlocking concrete pavers with spaces filled with gravel provide a stable driving surface while allowing storm water to drain into the soil below without running directly into the sewer system. By filtering water down through the pavement into the sub-base, microorganisms are allowed to attack any petroleum deposits, breaking them down and rendering them harmless.
Photo courtesy of Uni-Group U.S.A.
Photo 5
Concrete blocks create both a driveway and walkway at this home.
Photo courtesy of Uni-Group U.S.A.
Learn More: www.ecologicalhomeideasonline.com
