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The Impact of Permeable Segmental Pavement in Urban Areas

By Leo Fellin

When permeable systems are implemented by municipalities and private owners they can reduce the size or, in some cases, eliminate the need for stormwater retention or detention facilities by using the pavement itself as the retention facility. This in turn, serves to reduce the size and cost of the drainage infrastructure needed to handle the volume of stormwater run-off created by urban development. The gradation and void ratio of this component determines the infiltration rate. Typical sizing is a #8 or a #9 gradation.
Photos courtesy of Pavestone

Permeable segmental paving systems began to emerge in Germany and Austria some 25 years ago, and they have subsequently spread rapidly to gain popularity worldwide as part of water sensitive urban design and low impact development. The increase of impervious pavements associated with the spread of urbanization greatly increases the volume of stormwater run-off and stormwater pollutants that enter drainage systems.

Managing and mitigating the influx of urban stormwater run-off has increasingly become more of a major concern to design professionals and government authorities in all major urbanized societies. To address these concerns, designers and governmental communities have started to implement permeable segmental pavement systems as part of sustainable urban design that aims to manage stormwater run-off, stormwater pollution, and in turn achieve sustainability at the site level. As the concept of sustainable urban design evolves from a pioneering design philosophy to urban design guidelines, permeable segmental pavements will be essential to the future of urban development by offering a proven pavement system that reduces site run-off, removes run-off pollutants, and provides an ecologically friendly solution to conventional impervious pavements.

Under LEED sustainable site (SS) credits, permeable pavements can also reduce the heat island effect in both roof and non-roof applications. Segmental permeable pavements contribute to LEED material and resources (MR) credits by providing solutions to construction waste management, resource reuse, recycled content, and regional manufacturing and extraction.

Surface run-off rates from impervious pavements have increased peak flows, run-off volumes, and in turn, have overloaded stream channels causing erosion and stream bank instability. In an effort to minimize these effects of urbanization, municipalities and design professionals have started to implement permeable segmental pavements. These permeable pavements allow stormwater to infiltrate through the pavement, thus considerably reducing surface run-off. All permeable segmental paving systems consist of three major components. The first component is the pavement surface itself, which is composed of concrete pavers that either have void spaces cast into their surface or interlock in such a way to create apertures in between individual pavers.

The second component is the surface open voids are aggregate infill. The third component is the structural base constructed out of uniformly open graded crushed aggregate. The rate of infiltration through the permeable paving and volume of storage is achieved through the elimination of the finer aggregates. The lack of these finer aggregate sizes creates voids in the aggregate base, which allows for temporary storage until run off can infiltrate into the subgrade, when high permeable soils are present, or is removed via drainage pipes to an existing drainage infrastructure when low permeable soils are present. Furthermore, constructing permeable pavements will greatly reduce peak discharge of surface run-off, which translates to major reductions in downstream flooding and stream channel erosion.

In addition to increasing surface run-off, urbanization has also been detrimental to the quality of urban surface water systems. The EPA reports that nearly 40 percent of America's surveyed waterways are too polluted for fishing and swimming and that 90 percent of the nation's population resides within 10 miles of these bodies of water. With statistics like these, governmental agencies, like the EPA, have come to the conclusion that alternative pavement systems are going to have to be utilized to counteract the effects of non-point sources of pollutants created by urbanization.

According to the Environmental Protection Agency (EPA), non-point sources of pollutants, such as stormwater run-off, are reported to cause the majority of water pollution problems today. Sediments, pesticides, salts, metals, and chemicals are deposited into rivers, lakes, and estuaries from non point sources.

Permeable segmental pavements are such alternatives that have been recommended by federal and local government agencies to mitigate pollutants carried by surface run-off at the site level. Permeable pavements can reduce the quantity of pollutants reaching receiving waterways because of increased infiltration through the porous pavement. Surface run-off is filtered and pollutants are removed through the apertures between individual pavers and also through void spaces in the open graded aggregate base of the pavement. Research has proved that permeable pavements reduce pollutants in surface run-off like sediments, pesticides, chemicals, and heavy metals on average by 60 percent to 80 percent compared to conventional impervious pavements.

While permeable concrete pavers offer an alternative solution to reduce stormwater run-off quantity and quality, they can also contribute to green building design that strives to achieve resource efficiency and sustainable site design. Design communities have embraced sustainable design through the adoption of LEED (Leadership in Energy and Environmental Design). Developed by the U.S. Green Building Council (USGBC) in 1998, LEED is a voluntary system of design for buildings and sites that provides a rating system, which encourages the use of technologies that reduce energy and conserve non-renewable resources. This rating system is based on points that are earned by fulfilling requirements and prerequisites as specified in five principal LEED categories. These categories are sustainable sites (SS), water efficiency (WE), energy and atmosphere (EA), material and resources (MR), and indoor environmental quality (IQ). An additional category, innovation and design process (ID) recognizes expertise in green building and accounts for measures that are not covered under the five principal categories.

Permeable segmental pavement can contribute up to fourteen LEED points under the sustainable sites (SS), material and resource (MR), and innovation and design process (IQ) credits. They can contribute to sustainable site (SS) credits under stormwater design by incorporating permeable interlocking paver systems that reduce the rate and quantity of surface run-off and additionally reduce pollutants such as total suspended solids (TSS) and total phosphorous (TP).

"The EPA reports that nearly 40 percent of America's surveyed waterways are too polluted for fishing and swimming and that 90 percent of the nation's population resides within 10 miles of these bodies of water."

With all of these advantages, the popularity of permeable pavement systems continuous to increase every year. Despite this popularity, these systems are still new to the North American market. This raises concerns about reliable and readily available information to contractors and designers regarding the design and installation of these permeable pavements. Having the knowledge to know exactly how these systems should be engineered and constructed is paramount to the success of any permeable pavement installation and design life.

As permeable pavements increase in popularity and as the industry matures, information pertaining to these systems will be more readily available to engineers, architects, contractors, and end users. Meanwhile, designers and contractors can inquire about segmental permeable systems at organizations like USGBC (U.S. Green Building Council) and ICPI (Interlocking Concrete Paver Institute) to learn more about permeable paving specifications. In addition to these organizations, manufactures of permeable concrete pavers, like the Pavestone Company, can also assist and provide any technical information to inquiring designers, contractors, and end users.

Although permeable segmental paving systems have only been implemented for the past twenty five years, they have proven time and time again that they can be part of sustainable urban design that minimized the effects of urbanization by reducing surface run-off and removing run-off pollutants.

Furthermore, permeable segmental paving has also proven its ability to out perform impervious concrete and asphalt pavement under a myriad of soil and climate conditions. The empirical data has proven that these systems are not an anomaly of the pavement and erosion control world, but a leader in innovative design, creative solutions, and is a pioneer in ecologically sensitive pavement systems.

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December 9, 2019, 10:23 am PDT

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