By Paul Cureton, P.E
From the June 2023 Issue
Permeable paver systems are gaining popularity for both residential and commercial projects due to rising municipal stormwater management concerns resulting in more stringent regulations. In many cases, permeable pavers can provide an all-in-one solution, meeting stormwater needs and reducing impervious cover at the same time. Contractors who understand how to leverage the advantages provided by permeable pavers can provide solutions competitors may not be presenting to their customers.
Permeable Paver System Basics
Permeable paver systems are significantly different from standard paver systems. Standard paver systems are built on a dense-graded road base (the same base material under asphalt pavement), with a setting bed of coarse sand placed on top. The pavers are set on the sand and the joints are filled with sand. Standard pavers typically have spacer bars on the side to create a 1/8″ joint that allows the sand to be placed between the pavers, increasing paver to paver interlock. The system is impervious and surface drainage is crucial to the overall success of the project. Water must be prevented from saturating the base and sand layers.
In contrast, permeable pavers have a different composition to encourage water to enter the system and flow through the pavement matrix. Here the water can be infiltrated, stored, or allowed to exit the system through a perforated pipe underdrain.
This is accomplished using open-graded stone. Open-graded stone, sometimes called gravel, uses only the larger stones without the fine material and dust. The result is stone materials which contain about 40% air. A good analogy would be a jar of only marbles versus a jar of marbles combined with sand. The jar of only marbles has a lot of space occupied by air similar to the opengraded stone.
Starting from the bottom, the first layer, or subbase, is placed on the soil in a 6″ thickness and is composed of 3″ diameter stone meeting ASTM No. 2 gradation. The base layer is then placed on top in a 4″ layer consisting of 3/4″ diameter stone meeting ASTM No. 57 gradation. The setting bed is then placed on top in a 2″ lift of 3/8″ stone meeting ASTM No. 8 gradation. The permeable pavers are then placed on the setting bed, and the joints are filled with the 3/8″ stone. The joints are created with spacer bars, but the joints are wider than standard pavers and range from 3/8″ to 1/2″. All the layers must be compacted, except the setting bed, in accordance with the manufacturer’s recommendations.
The composition of the permeable paver system allows not only a robust, aesthetically pleasing pavement, but also provides four important stormwater functions:
- Capture. The entire surface is a stormwater inlet replacing the need for most storm structures (catch basins, inlets, etc.)
- Conveyance. The open-graded stone allows water to flow from one side of the system to the other, eliminating the need for most storm pipes.
- Water Quality. Sediment is filtered out of stormwater as it passes through the stone-filled joints.
- Storage. The stone is 40% air and can be used to store stormwater (detention).
Impervious pavements, on the other hand, dramatically increase runoff which picks up pollutants from the pavement surface and carries them into our rivers and streams. As a result, cities and stormwater regulators are increasingly turning to permeable pavers to convert this stormwater negative into a positive. When an area that would have been impervious is replaced by permeable pavers, the stormwater return on investment is significant.
Not only does a permeable paver system eliminate runoff, the water leaving the system will likely meet water quality requirements. Additionally, water can be conveyed to the soil for infiltration or to an outlet, and storage can be provided where water can be released slowly. So how can this shift benefit a landscape or hardscape contractor?
Many municipalities across the U.S. have set impervious cover limits for residential lots. In other words, there is a maximum percentage of the property that can be covered by impervious surfaces—such as roofs, driveways, sidewalks, pools, patios, and other hardscape elements. The requirement varies from municipality to municipality, but it’s common to limit impervious coverage to no more than 30% of the lot area.
Since permeable pavers are typically considered permeable surfaces, they can often be used to exceed the impervious lot limit. (Some municipalities will explicitly state that permeable pavements are considered “impervious” for the purpose of calculating surface cover so it is important to check with the local jurisdiction.)
Permeable pavers, in most instances, would provide the homeowner a solution to expand their driveway without exceeding the impervious limit and provide the landscape or hardscape contractor a solution to present to the homeowner.
For example, if a homeowner really wanted to expand their driveway but had reached the impervious limit, the project could not move forward with an impervious pavement. Permeable pavers, in most instances, would provide the homeowner a solution to expand their driveway without exceeding the impervious limit and provide the landscape or hardscape contractor a solution to present to the homeowner.
In many planned developments and subdivisions, the limiting factor on impervious surface is not a local ordinance but the amount of impervious surface that was used to calculate the stormwater requirements that were planned for the entire development before the lots were constructed. This includes elements such as the stormwater ponds.
In many cases, the civil engineer estimates the impervious lot coverage using a percentage. As the individual lots are constructed and permitted, the regulatory agency checks to make sure the permit documents do not exceed the impervious cover used in the original calculations. Once again, permeable pavers can often be used in these cases to expand the hardscape area on that lot without increasing the impervious cover. And if the added development requires additional storage, then the permeable paver system can provide its own detention.
Because of their multi-functionality, permeable paver systems can provide significant benefits to commercial developments, such as lower construction and maintenance costs and increased development density. As a landscape/hardscape contractor, you may not always be involved in a project early enough to impact the stormwater strategy, but the opportunity could arise to present a solution using permeable pavers.
By using permeable pavers as the primary stormwater management system, commercial developments can eliminate most catch basins, inlets, water quality devices, and stormwater ponds. By replacing these functions with the permeable paver system, often the total stormwater management cost is lower than with conventional systems. This can offset the higher installation cost of permeable pavers versus conventional pavements.
Permeable interlocking concrete pavement (PICP) reduces stormwater runoff volumes and rates, while filtering pollutants. An opportunity for landscape contractors, millions of square feet of PICP need maintenance. Read more…
Permeable pavers also last longer and require less maintenance than conventional stormwater management systems. The life expectancy of the permeable paver system is over 40 years, which is twice the 20-year life of asphalt or concrete. When comparing the life-cycle costs of stormwater management systems where asphalt pavement is used, permeable systems almost always lower life-cycle costs reaching the breakeven point within the first 10 years.
Stormwater ponds, whether designed for detention or retention, take up large areas in residential subdivisions and multi-family developments. By providing stormwater storage within the permeable paver roads and parking lots, the stormwater basins can be replaced with more homes or multi-family buildings. Increasing the development density of the property and lowering both construction and maintenance costs can increase the project’s return on investment.
Gain An Edge
Permeable pavers have many uses in both residential and commercial projects providing a multi-functional solution as stormwater regulations become more restrictive. An understanding of the benefits and functions of a permeable paver system can provide contractors with a solution that could generate more project opportunities or salvage projects that would not be possible with conventional pavement. Permeable pavers can also provide a contractor with a unique solution that the competition may not be utilizing, which provides an opportunity to differentiate yourself from the competition.
Cureton is the manager, Paver Design Services for Oldcastle APG. He is a graduate of the University of Tennessee and a licensed Professional Engineer in Illinois. Prior to coming to work for Oldcastle, he was a consulting civil engineer for 20 years focusing on stormwater management and sustainability. Paul has extensive experience designing permeable pavements with over 2 million square feet of projects designed, permitted, and constructed as the engineer of record. Paul has also assisted in the design of over 21M square feet of permeable paver projects since joining Oldcastle.
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