Parking Lots That Leak

June 2004 » Feature Articles
Design of stormwater management systems is changing dramatically, with volume reduction by infiltration and water quality considerations dictating the methods and materials used. Porous asphalt pavements with sub-surface infiltration beds offer site planners and public works officials the opportunity to minimize impervious surfaces and manage stormwater in a cost-effective and environmentally friendly way.
Thomas H. Cahill, P.E.

Design of stormwater management systems is changing dramatically, with volume reduction by infiltration and water quality considerations dictating the methods and materials used. Porous asphalt pavements with sub-surface infiltration beds offer site planners and public works officials the opportunity to minimize impervious surfaces and manage stormwater in a cost-effective and environmentally friendly way.

Since the initial development of this material in the early 1970s by the Franklin Institute of Philadelphia, engineers have successfully designed parking lots and some roadways in a variety of climates and topographies. These stormwater management systems provide cost-effective, attractive parking lots with a life span of 20 years or more. At the same time, infiltration of rainfall into the soil mantle removes virtually all of the petroleum hydrocarbons, metals, organic matter, and non-point source pollutants, such as phosphorus, attached to fine soil particles. When properly sized, an infiltration bed also can mitigate a site's peak runoff in the same fashion as a detention basin.

A typical porous asphalt pavement (shown in Figure 1) consists of an open-graded, asphalt surface (2.5 inches thick); a top filter course of small aggregate; a deep stone reservoir course; and a bottom layer of filter fabric, placed on uncompacted natural soils. Water drains through the open-graded asphalt surface and the various layers, then slowly percolates into the soil at a rate estimated by soil data and confirmed by field tests. The slower the rate, the deeper the bed required for storage. The bed size is determined by assessing the total impervious area to be drained, which usually includes rooftops and contiguous impervious roadways and streets.

Design factors

The design of porous asphalt pavements requires evaluation of a number of factors, including site design, soil type, infiltration rate, depth to bedrock, and depth to water table.

The most important factor is to consider the location of the porous pavement infiltration system early in the concept design process. Traditionally, the architect or developer lays out a desired building program that covers most of the site with structures, and the civil engineer is asked to design a stormwater management system to collect and convey runoff to the lowest point. Unfortunately, this is usually the wettest location on the site, often adjacent to streams or wetlands where there are poorly draining soils.

Perforated pipes at the bottom of an infiltration bed distribute water from adjacent impervious surfaces.

By comparison, stormwater infiltration systems perform best with well-drained, upland soils, which usually are occupied by buildings and pavement. Thus the design method (infiltration) and material (porous asphaltic concrete) are built in the optimum location for both functions. Site designers also can integrate a mixture of large and small infiltration systems throughout the parcel to reduce, or even eliminate, conveyance systems, such as inlets, gutters, and storm sewers.

Following is a summary of design guidelines for subsurface infiltration.

  • Select infiltration opportunities within the immediate project development area.
  • Consider past site uses and appropriateness of infiltration design with porous pavement. Avoid conveying stormwater long distances.
  • Consider runoff sources. (Roads are many times dirtier than rooftops.) Incorporate sediment-reduction techniques at inlets as appropriate, or plan to sweep the surfaces.
  • Perform site tests to determine depth to seasonal high water table, depth to bedrock, and soil conditions, including infiltration capabilities. Maintain the infiltration bed's bottom elevation 3 feet above the high water table and 2 feet above bedrock.
  • Avoid excessive earthwork. Design with the contours of the site.
  • Do not infiltrate on compacted fill.
  • Avoid compacting soils during construction.
  • Maintain erosion and sediment control measures until the site is stabilized because sedimentation during construction can cause infiltration systems to fail.
  • Create the largest feasible infiltration area. Avoid concentrating too much runoff in one area. A good rule of thumb is a 5-to-1 ratio of impervious area to infiltration area. A smaller ratio is appropriate in carbonate bedrock areas.
  • The bottom of the infiltration area must be level to allow even distribution.
  • The slope on which the porous pavement is placed should not exceed 5 percent. Use conventional pavement in steep areas that receive vehicular traffic.
  • Provide thorough construction oversight.

Conduct a two-step soil investigation before designing any infiltration system. First, a simple test pit, 6 to 8 feet deep, is excavated with a backhoe in order to observe the soil conditions. While some designers prefer an auger, there is no substitute for physically observing and describing the soil horizons. Next, measure infiltration at potential bed bottom elevations. Simple percolation tests, while not very scientific, can provide initial evaluation, but for the final design, use infiltrometer readings. The U.S. Environmental Protection Agency recommends minimum infiltration rates of 0.5 inches per hour, however, rates between 0.1 and 0.5 inches 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.

Geology is important in locations that are underlain by carbonate formations. More detailed site investigations may include borings and ground-penetrating radar, or other geophysical methods, to confirm sub-surface conditions. Contrary to popular belief, sinkholes are formed by the concentration of runoff, as with a detention basin, and not by properly designed infiltration systems. A number of infiltration systems designed by Cahill Associates during the past 20 years were situated in carbonate areas, several of which were adjacent to areas where detention basins previously had created sinkholes.


Table 1: Example filter and reservoir course gradations


Figure 1: Pavement profile

To ensure a positive flow pathway from the surface to the sub-surface bed, even if the surface becomes clogged or repaved, an unpaved stone edge or catch basin is installed that discharges into the bed. Additionally, in case the bed bottom clogs, the underlying bed systems are designed with a positive overflow. During a storm event, 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.

An engineer proficient in hydrology and stormwater design should design the stormwater  component of the system. 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.

The stone recharge bed is the heart of porous pavements. It provides temporary storage of  stormwater falling directly on the pavement, as well as from other impermeable surfaces if desired. It uses uniformly graded, 1.5- to 2.5-inch, clean-washed, crushed stone, such as an AASHTO No. 3. Depending on local aggregate availability, both larger and smaller size stones have been used. The void space between the stones provides the critical storage volume for the stormwater, and therefore, the void content of the aggregate should be confirmed. Avoid dusty or dirty stones that may clog the infiltration bed. 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 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 allows stormwater to distribute and infiltrate evenly over the entire bed bottom area. Compaction of the soils prevents infiltration, so it is important that care be taken during excavation to prevent this. A layer of non-woven geotextile at the bottom of the bed allows water to drain into the soil while preventing soil particles from moving into the stone bed.

Often, underlying stone beds provide stormwater management for adjacent impervious areas such as roofs and roads. To achieve this, stormwater is conveyed directly into the stone bed through perforated pipes that distribute the water evenly.

A 1- to 2-inch-thick layer of clean, single-size, 1/2-inch stone is placed on top of the stone reservoir. Often referred to as a filter course, the real purpose of this layer is to lock up the stone surface of the reservoir bed, providing a firm paving platform.

Table 2: Recommended gradation for open-graded friction course

A 2- to 4-inch-thick 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 in the mix (see Table 2). 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 IS-115, “Design, Construction, and Maintenance of Open-Graded Asphalt Friction Courses,” provides guidance on the design and construction of open-graded mixes. It is recommended that open-graded asphalts be designed with 18 percent minimum air voids to ensure they drain freely and with 6.0 percent minimum asphalt content by weight of mix for durability.

Open-graded asphalt mixes used for porous pavements historically 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 fibers have proven beneficial for open-graded friction courses used as thin surfaces on highways and may prove beneficial for porous pavements.

Calculating costs

Porous pavement does not cost more than conventional pavement. On a yard-by-yard basis, the asphalt cost is approximately the same as the cost of conventional asphalt. The underlying stone bed is usually more expensive than a conventional compacted sub-base, but this cost difference generally is off-set by the significant reduction in stormwater pipes and inlets, and associated costs. Additionally, because porous pavement is designed to fit into the topography of a site, there generally is less earthwork and no deep excavations.

When cost savings provided by eliminating detention basins are considered, porous pavement is always an economically sound choice. On those jobs where unit costs have been compared, the porous pavement always has been the less expensive option. Current jobs are averaging $2,000 to $2,500 per parking space for parking, aisles, and stormwater management.

The Mustang lot at the Ford Motor Company Rouge River facility in Dearborn, Mich., is made of porous asphalt that drains to vegetated bioswales. The system is designed to improve water quality.

Sites with limited infiltration

Despite the need for infiltration, not all sites and soils are suitable. In those situations, porous pavement systems can be used in conjunction with other water quality improvement programs to reduce impervious surfaces. For example, porous pavement parking lots recently constructed at the John Heinz National Wildlife Refuge near the Philadelphia Airport are located on a wet, low-lying site that has been subject to fill over the years. The soils are not well drained. In this situation, a trench was excavated to a lower gravel layer to facilitate infiltration; however, the parking lots primarily serve to avoid the creation of new impervious surfaces.

Another example is the Ford Motor Co. Rouge River Facility in Dearborn, Mich., where the use of porous pavement is an important part of Ford's commitment to sustainability. The project site is in a low-lying, wet area and has been subject to a century of industrial use. In 1999, Ford constructed a porous parking lot that slowly discharges stormwater stored in beds beneath the paved surface to support a series of planted wetland swales. The system is designed specifically to improve water quality.

Construction and maintenance

When an infiltration Best Management Practice (BMP) fails, it generally is because of difficulties and mistakes in the construction process. This is true for porous pavement, as well as for other infiltration BMPs. Carelessness in compacting subgrade soils, poor erosion control, and poor-quality materials all are causes of failure. Therefore, detailed specifications regarding site protection, soil protection, and system installation are required.

For infiltration BMP installations to be successful, consultants must take a hands-on approach during construction. They must meet with the contractor before construction to discuss the need to prevent heavy equipment from compacting soils and sediment-laden waters from washing on to the pavement, as well as the need to use clean stone and other specific materials. Also, consultants must review the installation process with the project foreman. They routinely need to stop by the site or to provide construction advice. Proactive approaches such as these can avoid failures.

Because construction sites are inherently messy places, it is best to install porous pavement toward the end of a project's construction period. On many projects, the stone bed area is excavated to within 6 inches of the final grade, and the empty bed area is used as a temporary sediment basin and stormwater structure. For these situations, take care to prevent heavy equipment from compacting soils; however, sediment can accumulate in the area. In later stages of the project, the sediment is removed, the bed is excavated to final grade, and the porous pavement system is installed. Using the bed area in this way avoids the need for a separate sediment basin during construction.

After construction, all porous pavement surfaces should be cleaned twice each year with an industrial vacuum sweeper. Unfortunately, like many stormwater maintenance requirements, this advice often is overlooked or forgotten. Fortunately, even without regular maintenance, these systems continue to function.

When runoff is conveyed from adjoining areas or roof surfaces into a bed, a drop inlet box or other structure can be used to reduce the amount of detritus and sediment conveyed to the bed. Such structures also require regular removal of sediment and debris.

The infiltration bed beneath a porous pavement must be excavated without use of heavy machinery, which can compact the bed bottom.

Where it doesn't work

Porous asphalt is not recommended for slopes greater than 6 percent. 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, are necessary before infiltration occurs. The ability to contain spills also must be considered and built into the system. Porous pavement also is not recommended in areas such as private residential driveways where the pavement is likely to be coated or paved over because of a lack of awareness.

Freezing, however, has never been an issue in the use and performance of porous pavement, even in very cold climates such as Detroit. Obviously, sand or gravel used for deicing is detrimental to the porous surface. Salt may be used, 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.

Summary

Porous asphalt pavement for parking lots is one of the most effective and affordable techniques for addressing stormwater management. Although Cahill Associates has used other materials, such as porous concrete, for sidewalks and parking areas, asphalt is less expensive and easier to install, and remains the first choice. It performs in both hot and cold climates. Even in hot, Southern climates, for example at the University of North Carolina in Chapel Hill where two large commuter parking lots recently were installed, porous asphalt and porous concrete have performed quite well.

To date, 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 long lasting and an ideal solution to reduce the negative environmental effects of constructing paved sites traditionally.

Thomas H. Cahill, P.E., is president; Michele Adams, P.E., is principal engineer; and Courtney Marm is a biologist with Cahill Associates, West Chester, Pa. They can be contacted at 610-696-4150. Kent Hansen, P.E., is director of engineering for the National Asphalt Pavement Association, Lanham, Md. He can be contacted at 301-731-4748.

Upcoming Events

See All Upcoming Events