When is solid not solid, when what you’re walking on is considered pervious or permeable. Pervious and permeable pavements and more specifically pervious pavers are one of the answers to run-off problems impacting many areas. Cities and towns across the country are fighting with increased flooding due to unusual weather patterns and increased levels of development. The increased levels of development have resulted in more water going directly into streams and rivers and less water infiltrating into the ground. Cities and towns have responded to the flooding by starting to limit the amount of impervious area on project sites. The green industry has answered this by creating more sustainable pervious pavements, these paving systems allow water to run through them directly into the soils below. This increases onsite infiltration and reduces the impervious coverage on a project.
Several types of permeable pavements are used and readily available, including pervious concrete, pervious asphalt, concrete grid pavers, plastic grid pavers and resin coated gravel. Look for more information on pervious concrete and pervious asphalt in future blogs. Each of these green types of permeable pavement has its own benefits and recommended uses. A Landscape Architect can help you determine which system is best for you and can help you with the design.
Pervious pavers are a type of permeable or porous pavement system created using precast concrete pavers or clay pavers that allow water to infiltrate through the joints between the pavers into a deep gravel base and the underlying soils. In this blog we will review the benefits and the reasons to use pervious pavers. Municipal, corporate, private developments and even residences can utilize these green paving systems to reduce run-off.
Some of the advantages of pervious pavers are that they can appear to be indistinguishable from non-porous pavers. They are a very attractive option for use on paths, terraces, driveways, parking areas or low speed low-volume roadways. They come in a wide variety of styles, shapes and colors that can enhance your project site. They are as attractive as other styles of precast pavers. The biggest benefit is that they allow for storm water and snow melt to percolate and infiltrate through the paving system into the underlying soils. These systems are an alternative to creating an impervious (solid ) surface, thereby reducing run-off as part of an overall surface water management system.
Using pervious or permeable pavers is an important element in low impact development. These non-point discharge systems reduce erosion and siltation by limiting runoff and the high volumes and velocities found in point discharge systems. These systems also reduce the turbidity and temperature of the run-off. Pervious pavers can controls pollutants by allowing water and pollutants to seep into underlying soils where they are naturally filtered out by the soil profile and neutralized by natural processes. Using this type of paver provides additional benefits such recharging groundwater, capturing heavy metals, elimination of point discharges and lower heat island effects. Providing more porous surface near tree roots allows air and water into root zone to create healthier more vibrant trees and plantings is another benefit of pervious paving.
In urban or heavily developed areas, use of pervious pavements can reduce the local heat island effect. Many pavers have a high albedo, reflecting heat back into the atmosphere keeping the pavers and the area cooler. The absorption of the water through the system keeps pavers cooled and the localized area is thereby also cooled. Additionally, there is known to be a reduction of overall irrigation demands when using pervious pavements, this will reduce your water usage and costs. Projects pursuing LEED certification can realize benefits in several credit areas depending on the product selected and its use, including storm water design quality and control, heat island effect non-roof, recycled content, regional materials and potentially innovation in design.
While pervious pavers are a great product and an innovative sustainable solution to run-off issues, they are not suited to every situation and have some limitations in their application. In most cases, pervious pavers cannot manage storm water alone and they need to be part of a storm water management program and system. Some of the limitations on the pavers that restrict their use are: permeable pavers are not ideal in heavy traffic areas like loading docks, they should not be used in areas that experience high speed traffic and they should not be used in areas that experience high cornering loads. On sites with steeply sloped areas this will limit infiltration and require additional measure in the base to prevent failure. Areas with very tight soils that are poorly drained will require a deeper pavement base and outlet for relief if the storage volume exceeds the infiltration capacity. Frost heaving can occur with some types of porous pavement if the base is not prepared correctly or does not infiltrate as expected.
Pervious pavers should not be placed too close to a well. If there is not sufficient distance to allow for the water to seep into the ground and be purified through the soil profile before infiltrating into the ground water system, and eventually your well, it is possible for contaminants to get into the well. A landscape architect can work with you and your local health district to help plan your project appropriately.
The use of permeable pavers requires more frequent to maintenance to maintain the system’s optimal performance. Since grit or gravel can block the open pores it is important to remove the deposits to maintain the proper infiltration and drainage. This is accomplished by the use of vacuums and wind rakes. In cold climates, sand and road salt may plug pores and reduce infiltration, careful attention must be paid to the winter maintenance program to reduce problems which might require additional maintenance or reduced performance. It is recommended to use straight salt in areas that require it. Often times the pervious systems will require less salt due to the fact that water does not move across the surface.
The cost of permeable pavers may be more than conventional paver systems, due to additional costs for the increase in materials for the base preparation and under drainage systems. This is usually offset by reduction in traditional drainage systems and structures, increased development potential, an increased “green impression” of the project and credits toward LEED certification. A landscape architect can help determine the best pervious pavement application for your project and budget
Permeable pavers for vehicular areas are typically 3-1/8” thick, while in pedestrian areas they are 2-3/8” thick. This allows for both pervious and standard impervious paving systems to be installed next to one another without any additional preparation. The installation of pervious pavers is the same as for other pavers.
Pervious pavers can be solid precast concrete or clay pavers that are designed to be laid out on flat or gently pitched surfaces with tabs that create small openings or joints between the individual pavers. These openings which are typically 5-25% of the paving surface area, the gaps are filled with a porous aggregate material to allow water to flow thru them into the gravel base below. The pavers are set on a pervious bedding layer of mixed crushed stone aggregate which is set over 3-4 inches thick of ¾ inch stone. Below this aggregate layer is the sub-base reservoir of larger 1-3 inches clean crushed stone. This is where the water is stored while it infiltrates into the ground. The thickness of this water storage layer depends on several factors including, the capacity of water storage needed, specific site conditions and infiltration capacity of the underlying soils. Under drains are often placed in this layer when there are low infiltrating soils, thereby allowing any excess water to be discharged out of the reservoir. It is also important to place a geotextile between the gravel sub-base and the underlying sub-soils to prevent the base from getting degraded.
Pervious pavers are an attractive solution to storm water issues that allow you to have an attractive well designed project that is also green. A landscape architect can determine the best pervious pavement application for project and budget.
We can help you plan ahead to make your dream project landscape a reality, call Rock Spring Design Group, LLC 203•268•6979 or email us at firstname.lastname@example.org today for a free one hour consultation to start planning your upcoming landscape project. www.rockspringdesign.com.