Water: Best Management Practices
Permeable Interlocking Concrete Pavement
Permeable interlocking concrete pavement (PICP) consists of manufactured concrete units that reduce stormwater runoff volume, rate, and pollutants. The impervious units are designed with small openings between permeable joints. The openings typically comprise 5% to 15% of the paver surface area and are filled with highly permeable, small-sized aggregates. The joints allow stormwater to enter a crushed stone aggregate bedding layer and base that supports the pavers while providing storage and runoff treatment. PICPs are highly attractive, durable, easily repaired, require low maintenance, and can withstand heavy vehicle loads. Figure 1 shows installed pavers in a Seattle, Washington residential neighborhood.
PICP can be used for municipal stormwater management programs and private development applications. The runoff volume and rate control, plus pollutant reductions allow municipalities to meet regulatory water quality criteria. Municipal initiatives, such as Chicago's Green Alley program, use PICP to reduce combined sewer overflows and minimize localized flooding by infiltrating and treating stormwater on site. In addition, the Chicago Department of Transportation is experimenting with a photocatalytic cement coating on the concrete paving units that absorb nitrous oxide air pollutants, a component of photochemical smog. Green alley pilot project designs in the City of Richmond, Virginia and Los Angeles, California anticipate storing and slowly releasing water to reduce peak flows as well as filtering pollutants. Private development projects use PICP to meet post-construction stormwater quantity and quality requirements. Public and private investments in PICP can potentially reduce additional expenditures and land consumption for conventional collection, conveyance, and detention stormwater infrastructure.
PICP can replace traditional impervious pavement for most pedestrian and vehicular applications except high-volume/high-speed roadways. PICP has performed successfully in pedestrian walkways, sidewalks, driveways, parking lots, and low-volume roadways. The environmental benefits from PICP allow it to be incorporated into municipal green infrastructure and low impact development programs. In addition to providing stormwater volume and quality management, light colored pavers are cooler than conventional asphalt and help to reduce urban temperatures and improve air quality. The textured surface of PICP also provides traffic calming and provides an aesthetic amenity.
PICP should not be confused with concrete grid pavements (i.e., concrete units with cells that typically contain topsoil and grass). These paving units can infiltrate water, but at rates lower than PICP. Unlike PICP, concrete grid pavements are generally not designed with an open-graded, crushed stone base for water storage. Moreover, grids are for intermittently trafficked areas such as overflow parking areas and emergency fire lanes.
Siting and Design Criteria
PICP should be designed and sited to intercept, contain, filter, and infiltrate stormwater on site. Several design possibilities can achieve these design aspects. For example, PICP can be installed across an entire street width or an entire parking area. The pavement can also be installed in combination with impermeable pavements to infiltrate runoff and initiate a treatment train. Several applications use PICP in parking lot lanes or parking stalls to treat runoff from adjacent impermeable pavements and roofs. This design economizes PICP installation costs while providing sufficient treatment area for the runoff generated from impervious surfaces. Inlets can be placed in the PICP to accommodate overflows from extreme storms. The stormwater volume to be captured, stored, infiltrated, or harvested determines the PICP scale required. Figures 2 and 3 illustrate some PICP design variations.
The concrete pavers with permeable joint material comprise the surface layer of PICP. Pavers are typically 80 mm (3 1/8 in.) thick for vehicular areas. Pedestrian areas may use 60 mm (2 3/8 in.) thick units. Additional subsurface components of this treatment practice are illustrated in Figure 4 and include the following (NCSU, 2008):
- Open-graded bedding course - This permeable layer is typically 50 mm (2 in.) thick and provides a level bed for the pavers. It consists of small-sized, open-graded aggregate.
- Open-graded base reservoir - An aggregate layer immediately beneath the bedding layer. The base is typically 75 to 100 mm (3 - 4 in.) thick and consists of crushed stones typically 20 mm down to 5 mm (3/4 in. to 3/16 in.). Besides storing water, this high infiltration rate layer provides a transition between the bedding and subbase layers.
- Open-graded subbase reservoir - The stone sizes are larger than the base, typically 65 mm down to 20 mm (2½ in. to ¾ in.) stone. Like the base layer, water is stored in the spaces among the stones. The subbase layer thickness depends on water storage requirements and traffic loads. A subbase layer may not be required in pedestrian or residential driveway applications. In such instances, the base layer is increased to provide water storage and support.
- Underdrain (optional) - In instances where PICP is installed over low-infiltration rate soils, an underdrain facilitates water removal from the base and subbase. The underdrain is perforated pipe that ties into an outlet structure. Supplemental storage can be achieved by using a system of pipes in the aggregate layers. The pipes are typically perforated and provide some additional storage volume beyond the stone base.
- Geotextile (optional) - This can be used to separate the subbase from the subgrade and prevent the migration of soil into the aggregate subbase or base.
- Subgrade - The layer of soil immediately beneath the aggregate base or subbase. The infiltration capacity of the subgrade determines how much water can exfiltrate from the aggregate into the surrounding soils. The subgrade soil is generally not compacted.
Specific Design Considerations and Limitations
The load-bearing and infiltration capacities of the subgrade soil, the infiltration capacity of the paver surface, and the storage capacity of the stone base/subbase are the key stormwater design parameters. To compensate for the lower structural support capacity of clay soils, additional subbase depth is often required. The increased depth also provides additional storage volume to compensate for the lower infiltration rate of the clay subgrade. Underdrains elevated above the subgrade clay soil (see Figure 4) are often used in PICP further making it suitable for many clay soils by infiltrating some water, and filtering and draining the remainder.. In addition, an impermeable liner may be installed between the subbase and the subgrade to limit water infiltration when clay soils have a high shrink-swell potential or there is a high depth to bedrock or water table (NCSU, 2008).
Measures should be taken to protect PICP from high sediment loads, particularly fine sediment. Appropriate pretreatment BMPs for run-on to pavers include filter strips and swales. Preventing sediment from entering the base or permeable pavement during construction is critical. Runoff from disturbed areas should be diverted away from the PICP until they are stabilized.
Several factors may limit PICP use. It is not appropriate for stormwater hotspots where hazardous materials are loaded, unloaded, or stored or where there is a potential for spills and fuel leakage. For slopes greater than 2%, terracing of the soil subgrade base may likely be needed to slow runoff from flowing through the pavement structure.
There are many PICP paver designs on the market. While most pavers are ADA compliant, units with large openings filled with aggregate may not be appropriate for some paths or parking areas used by disabled persons, bicycles, pedestrians with high-heels, and the elderly (SPU, 2009). Such areas can be paved with solid interlocking concrete pavements (see Figure 5).
|Key Siting and Maintenance Issues:
The most prevalent maintenance concern is the potential clogging of the openings and joints between the pavers. Fine particles that can clog the openings are deposited on the surface from vehicles, the atmosphere, and runoff from adjacent land surfaces. Clogging will increase with age and use; but while more particles become entrained in the pavement surface, it does not become impermeable. Studies of the long term surface permeability of PICP and other permeable pavements have found high infiltration rates initially, a decrease, and then a leveling off with time. With initial infiltration rates of hundreds of inches per hour, the long term infiltration capacity remains high even with clogging. When substantially clogged, surface infiltration rates usually well exceed 1 inch per hour, sufficient in most circumstances to effectively manage stormwater. Permeability can be increased with vacuum sweeping or in extreme circumstances, replacing the aggregate between pavers.
In cold climates, sand should not be applied for snow or ice conditions and snow plowing can proceed as with other pavements. PICP has been found to work well in cold climates as the rapid drainage of the surface reduces the occurrence of freezing puddles and black ice. However, plowed snow piles should not be left to melt over the paver joints and openings as they can receive high sediment concentrations that can clog them more quickly. While all permeable pavements do not treat chlorides from road salts (SPU, 2009), deicing material use can be reduced with PICP. In addition, snow plowing is reduced due to surface snow melting and infiltrating. By eliminating ice forming, there can be a reduction in potential liability from slips and falls.
PICP is an on-site stormwater management practice that reduces the volume and rate of stormwater runoff as well as pollutant concentrations. PICP transforms areas that were a source of stormwater to a treatment system and can effectively reduce or eliminate runoff that would have been generated from an impervious paved area. Because it reduces the effective impervious area of a site, PICP should receive credit for pervious cover in drainage system design. The infiltration rate of the pavers and base generally exceed the design storm peak rainfall rate; the subsoil infiltration rate and base storage capacity are the factors determining stormwater detention potential. Table 1 provides monitored reductions in stormwater volumes via storage and infiltration.
|Table 1. Volume Retention of PICP|
|Application||Location||Soil Type||Underdrain||Volume Retention|
|Residential street||Auckland, New Zealand||Clay||Yes||60%|
|Field and laboratory tests||Guelph, Ontario, Canada||---||---||90%|
|Parking lot||Swansboro, NC||Sandy soil||No||100%|
|Parking lot||United Kingdom||Impermeable liner installed||Yes||34% - 45%|
|Parking lot||Renton, WA||---||No||100%|
|Parking lot||Kingston, NC||Clay||No||99%|
|Sources: (Fassman and Blackburn, 2006); (Bean, et al., 2005); (Pratt, 1999); (Booth and Leavitt, 1999); (Brattebo and Booth, 2003); (Collins, et al., 2008)|
PICP reduces pollutant concentrations through several processes. The aggregate filters the stormwater and slows it sufficiently to allow sedimentation to occur. The subgrade soils are also a major factor in treatment. Sandy soils will infiltrate more stormwater but have less treatment capability. Clay soils have a high cation exchange capacity and will capture more pollutants but will infiltrate less. Also, studies have found that in addition to beneficial treatment bacteria in the soils, beneficial bacteria growth has been found on established aggregate bases. In addition, PICP can process oil drippings from vehicles. Table 2 provides measured pollutant removals from PICP.
|Table 2. Monitored Pollutant Removals of PICP|
|Driveways||Jordan Cove, CT||67%||Cu: 67%
|Parking lot||Goldsboro, NC||71%||Zn: 88%||TP: 65%
|Parking lot||Renton, WA||---||Cu: 79%
|Parking lot||King College, ON||81%||Cu: 13%
|Sources: (Bean, et al., 2004); (Clausen and Gilbert, 2006); (Van Seters/TRCA 2007)|
Van Seters (TRCA 2007) compared pollutants in soils under and next to six PICP sites 3 to 16 years old in Ontario. There were no increases in oils (PAHs), iron, lead, zinc, copper and iron in soils under the PICPs compared to soils adjacent to them. Chlorides saw some increase under the PICP sites and would be expected under all permeable pavements subject to snow and deicers. Like other permeable pavements, PICP drains snowmelt, offering significant opportunity to reduce deicing material use. Van Seters also documented the condition of the six PICP sites and found that they were providing adequate structural support after years of use.
PICP water quantity and pollutant reduction characteristics such as 80% TSS reductions can qualify it to earn credits under green or sustainable building evaluations systems such as Leadership in Energy and Environmental Design (LEED®) and Green Globes. Credits also can be earned for water conservation, urban heat island reduction, and conservation of materials by utilizing some recycled materials and regional manufacturing and resource use.
Several factors influence the overall cost of PICP:
- Material availability and transport - The ease of obtaining construction materials and the time and distance for delivery.
- Site conditions - Accessibility by construction equipment, slope, and existing buildings and uses.
- Subgrade - Subgrade soils such as clay may result in additional base material needed for structural support or added stormwater storage volume.
- Stormwater management requirements - The level of control required for the volume, rate, or quality of stormwater discharges will impact the volume of treatment needed.
- Project size - Larger PICP areas tend to have lower per square foot costs due to construction efficiencies. Mechanized installation of the paving units (shown in Figure 6) is often used for larger projects thereby reducing construction time.
Costs vary with site activities and access, PICP depth, drainage, curbing and underdrains (if used), labor rates, contractor expertise, and competition. For vehicular applications over 15,000 square feet, costs generally range from $4 to $8 per square foot for the pavers, jointing and bedding materials. Base and subbase can vary in thickness and price depending on the design.
E. Z. Bean, et al., A Monitoring Field Study of Permeable Pavement Sites in North Carolina, 8th Biennial Conference on Stormwater Research & Watershed Management, 2005.
E. Z. Bean, et al., Study on the Surface Infiltration Rate of Permeable Pavements, 1st Water and Environment Specialty Conference of the Canadian Society for Civil Engineering. Saskatoon, Saskatchewan, CA. June 2-5, 2004.
D. B. Booth and Jennifer Leavitt, Field Evaluation of Permeable Pavement Systems for Improved Stormwater Management, American Planning Association Journal, Vol. 65, No. 3, pp. 314-325, 1999.
B. O. Brattebo and D. B. Booth, Long-Term Stormwater Quantity and Quality Performance of Permeable Pavement Systems, Water Resources, Elsevier Press, 2003.
J. C. Clausen and J. K. Gilbert, Stormwater Runoff Quality and Quantity From Asphalt, Paver, and Crushed Stone Driveways in Connecticut, Water Research, Vol. 40, pp. 826-832, 2006.
K. A. Collins, et al., Hydrologic Comparison of Four Types of Permeable Pavement and Standard Asphalt in Eastern North Carolina, Journal of Hydrologic Engineering (accepted), 2008.
Elizabeth Fassman and Sam Blackbourn, Permeable Pavement Performance for Use in Active Roadways in Auckland, New Zealand, University of Auckland, 2006.
Wayne Huber, et al., Low Impact Development Design Manual for Highway Runoff Control, National Academy of Sciences - National Research Council, National Cooperative Highway Research Program, NCHRP Report 565, 2006.
Interlocking Concrete Pavement Institute, Permeable Interlocking Concrete Pavements - Design, Specification, Construction, Maintenance, Third Edition, 2000.
Interlocking Concrete Pavement Institute, Permeable Interlocking Concrete Pavement (PICP) - Municipal Officials Fact Sheet, 2008.
North Carolina State University and North Carolina A&T State University Cooperative Extension, Urban Waterways - Permeable Pavement: Research Update and Design Implications, E08-50327, 2008.
C. J. Pratt, Use of Permeable, Reservoir Pavement Constructions for Stormwater Treatment and Storage for Re-Use, Water Science Technology, 39 (5), 145-151, 1999.
Seattle Public Utilities, Green Stormwater Infrastructure Manual, 2009 (in publication).
T. Van Seters, Performance Evaluation of Permeable Pavement and a Bioretention Swale Seneca College, King City, Ontario, Interim Report #3, Toronto and Region Conservation Authority, Downsview, Ontario, May 2007.