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Pervious paving refers to a category of ground coverings designed to allow water to pass through to the underlying soil. The various types of pervious paving can be used in everything from public road and sidewalk construction to private patios, pavers, and other uses. Traditional impervious paving can create the problem of effectively capping off the soil and creating excessive runoff during rainy conditions. Pervious paving is often able to provide the same basic function while allowing rain water to simply drain into the soil. Though permeable paving materials are often less strong than their impermeable counterparts, sufficient strength is typically available for a variety of applications.
One common type of pervious paving is permeable concrete. This type of paving is designed to contain 15 to 20% void space, rather than being more or less solid like regular concrete. It can be used for other purposes as well, but it is typically used for roads, sidewalks, and driveways. The void spaces allow the concrete to act like a sieve, with water simply passing through it. Some types of pervious concrete may allow as much as 480 inches per hour (1,219 cm per hour) of water to drain through.
The ability of pervious paving to effectively manage runoff is directly related to the capacity of the underlying water table. Though pervious concrete may readily pass a large amount of water through a surface, if the water table rises too high, the result can be water pooling on the surface. This means that pervious paving typically cannot be relied upon to drain other impervious areas, and that storm drains or other management systems may still be required to handle excess runoff.
The other main drawback of permeable paving is the potential for the drain surfaces to become blocked by grit or dirt. This can create additional maintenance requirements for roads using these materials. Certain permeable paving materials will need to be cleared of such grit regularly, while others — such as pervious concrete — may actually benefit from the void spaces being filled with fine grit. This filling can improve the ability of the concrete to filter rain water as it passes through.
Pervious paving may also be used in site management applications for commercial locations, or as landscaping on residential properties. Both of these types of land use will often have coverage limits that dictate the percentage of the property that can be topped with impervious surfaces. Therefore, permeable solutions, like pervious concrete or grass pavers, may allow a commercial project to achieve more parking or driving surfaces without the need to install retention ponds or complex water runoff management systems.