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Porous asphalt pavement with stone recharge beds—another tool for storm water management
Porous asphalt pavements with stone recharge beds are a tool to help reduce runoff and associated pollutants from developed sites. Unlike typical paved surfaces, these pavements allow rainwater to pass through the surface to a stone recharge bed constructed using clean, single-size, crushed aggregate with high voids that allow room to store water. The water can then infiltrate through the soil for groundwater recharge. At the same time, the infiltration of rainfall into the soil mantle removes virtually all of the petroleum hydrocarbons, metals, organic matter and NPS pollutants attached to the fine soil particles, such as phosphorus. When properly sized, the infiltration bed also mitigates the peak runoff in the same fashion as a detention basin, which can now be eliminated. Because the beds are designed for infiltration they also can reduce the total volume of runoff. Another benefit is that when properly designed and constructed they provide cost-effective, attractive parking lots with a life span of 20 years or more.
A typical porous asphalt pavement consists of an open-graded asphalt surface (2.5 in.), a top filter course of small aggregate, a deep stone reservoir course and a bottom of filter fabric placed on uncompacted natural soils. The depth of the stone recharge bed is determined by frost depth, soil infiltration rate and the total impervious area drained to the bed. Rooftops and contiguous impervious roadways and streets usually have to be figured into the total impervious area.
The design process requires evaluation of a number of factors, including soil type, infiltration rate, depth to bedrock and depth to water table.
The location of the porous pavement infiltration system should be considered very early in the concept stage of the design process. storm water infiltration systems perform best on well-drained upland soils, which are usually occupied by buildings and pavement. Therefore, porous pavements can often be placed at the optimum location for infiltration. Thoughtful site design also can integrate a mixture of large and small infiltration systems throughout the parcel to reduce or even eliminate conveyance systems of inlets, gutters and storm sewers.
The following is a summary of design guidelines for subsurface infiltration:
Before any infiltration system is designed, a soil investigation must be done. This consists of two steps. First, a simple “test pit,” 6 to 8 ft in depth, is excavated with a backhoe and the soil conditions are observed. Next, infiltration measurements are performed at potential bed-bottom elevations. Simple percolation tests, while not very scientific, can provide initial evaluation.
For the final design, infiltrometer readings should be used. While the EPA recommends minimum infiltration rates of 0.5 in. per hour, rates between 0.1 in. and 0.5 in. per hour are acceptable if the bed is sized accordingly. For poor soils, infiltration may occur slowly over a two- to three-day period, which is poor for volume control but ideal for water quality improvement.
The storm water component of the system should be designed by an engineer proficient in hydrology and storm water design. Essentially, the bed acts as an underground detention basin in extreme storm events, albeit one that also reduces volume. A storm can be routed through the bed using the same calculation methods employed to route detention basins to confirm peak rate mitigation.
In order to assure that storm water is always able to reach the subsurface bed, even if the surface becomes clogged or repaved, an unpaved stone edge, or catch basin, is used to assure discharge into the bed. This “belt-and-suspenders” approach provides redundancy, in case the porous pavement is paved over, forgotten or clogged.
Additionally, in case the bed-bottom clogs, the underlying bed systems are designed with a “positive overflow.” During a storm, as the water in the underlying stone bed rises, it must never be allowed to saturate the pavement. Catch basins with a higher outlet than inlet can be used to provide positive release, so the bed also serves as an underground detention basin.
The stone recharge bed is the heart of the porous pavement. It provides temporary storage of storm water falling directly on the pavement as well as from other impermeable surfaces if desired. It uses uniformly graded (i.e., screened) 1.5-in. to 2.5-in. clean-washed crushed stone, such as an AASHTO No. 3. Depending on local aggregate availability, both larger and smaller stones have been used successfully.
The important requirement is that the stone be uniformly graded (to maximize void space) and washed clean. The void space between the stones provides the critical storage volume for the storm water and therefore the void content of the aggregate should be confirmed. Stones that are dusty or dirty may clog the infiltration bed and must be avoided. The depth of the stone reservoir should be such that it drains completely within 72 hours. This allows the underlying soils to dry out between storms (improving pollutant removal) and also preserves capacity for the next storm.
The bottom of the recharge bed is excavated to a level surface and must not be compacted by heavy machinery. The level surface will allow storm water to distribute and infiltrate evenly over the entire bed bottom. Compaction of the soils will prevent infiltration, so it is important that care be taken during excavation to prevent this.
A layer of nonwoven geotextile at the bottom of the bed allows the water to drain into the soil while preventing the soil particles from moving into the stone bed. Often, the underlying stone bed also can provide storm water management for adjacent impervious areas such as roofs and roads. To achieve this, storm water is directly conveyed into the stone bed, and then perforated pipes in the stone bed are used to distribute the water evenly.
A thin layer, 1 to 2 in. thick, of clean, single-size 0.5-in. stone is placed on top of the stone reservoir. Often referred to as a filter course, this layer is placed to lock up the stone surface of the reservoir bed, providing a firm paving platform.
A 2- to 4-in. layer of open-graded asphalt is used for the surface. As the name implies, the gradation of the mix is open, with only a small percentage of sand. There are a number of state and federal standards that may be used to specify open-graded mixes. The National Asphalt Pavement Association (NAPA) publication Design, Construction, and Maintenance of Open-Graded Asphalt Friction Courses (order number IS-115, available at www.hotmix.org) provides guidance on the design and construction of open-graded mixes. Using a standard local or state department of transportation open-graded friction course (OGFC) mix, if available, is recommended, because contractors will be familiar with the design, production and construction of these mixes. The mix requirements should be checked to assure the following requirements are met:
Open-graded asphalt mixes used for porous pavements have in the past used unmodified binders with good results. However, because open-graded pavements are more susceptible to scuffing, it is recommended that the binder be one or two grades stiffer than that used for conventional mixes. Polymer-modified asphalt, asphalt rubber and fiber additives have proven beneficial for OGFCs used as thin surfaces on highways and may prove beneficial for porous pavements.
When comparing the costs of porous pavements to conventional pavements it is important that the comparison also include the costs of the alternative storm water management strategies. The cost of the porous pavement and stone recharge bed will typically be higher than that for a conventional dense-graded pavement, primarily because of the amount of material required for the stone recharge bed. However, this cost difference is generally offset by the significant reduction in storm water pipes and inlets. Additionally, because porous pavement is designed to fit into the topography of a site, there is generally less earthwork and no deep excavations.
There are specific best practices to follow when constructing porous pavements to assure optimum long-term performance:
All porous pavement surfaces should be vacuum-swept twice per year with an industrial vacuum sweeper. Unfortunately, like many storm water maintenance requirements, this advice is often overlooked or forgotten. Fortunately, even without regular maintenance, these systems continue to function.
When runoff is conveyed from adjoining areas or roof surfaces into the bed, a drop inlet box or other structure can be used to reduce the amount of detritus and sediment that would be conveyed to the bed. This structure also requires regular removal of sediment and debris.
Freezing has never been an issue, even in very cold climates such as Detroit. Sand or gravel for deicing should never be used. Salt may be used, however, and the surface may be plowed if needed. Most sites have found that light plowing eliminates the need for salt because the remaining snow quickly drains through the asphalt. This has the added benefit of reducing groundwater and soil contamination from deicing salts.
Where it doesn’t work
Porous asphalt is not recommended for slopes over 6%. There also are locations where the threat of spills and groundwater contamination is quite real. In those situations (such as truck stops and heavy industrial areas), systems to treat for water quality (such as filters and wetlands) before any infiltration occurs are necessary. The ability to contain spills also must be considered and built into the system.
Finally, it is not recommended in areas where the pavement is likely to be coated or paved over due to a lack of awareness, such as individual home driveways.
Porous asphalt for parking lots has proven to be one of the most effective and affordable techniques for storm water management. It performs in both hot and cold climates and in a variety of situations. To date, the installations include pavements at schools and universities, corporate offices, industrial sites, shopping centers, parks, libraries, a prison and even fast-food restaurants. Porous asphalt is cost-effective, long-lasting and an ideal solution to many site design challenges. More information on the design and construction of porous asphalt pavement is available in the NAPA publication Porous Asphalt Pavements (order number IS-131, available through www.hotmix.org) and on the website of Cahill Associates Environmental Consultants (www.thcahill.com).