Permeable Pavement: Does It Make the Transportation System More Resilient?

Michael Fitch, Ph.D., Deputy Director / Research Council, Virginia Department of Transportation

It is difficult to look at any recent transportation journal or trade magazine and not find an article or two focusing on resiliency and all its associated challenges. Although significant work has been done for years under the resiliency umbrella, recent prominent weather events have further highlighted the potential damage to infrastructure resulting from what appears to be more frequently occurring hurricanes, heat waves, flooding events, etc. As a result, the Virginia Department of Transportation supports a number of research efforts and initiatives to address some of the anticipated changes in both weather and climate to ensure its system is resilient to these changes.

Water is the Challenge

So exactly how does increased resiliency look? What form does it take? Currently, most of the immediate resiliency challenges facing Virginia are related to water. As the perceived increases related to precipitation intensity, frequency and duration become quantified and the locations most likely to be affected by these increases are identified, the real work begins—making changes to the transportation infrastructure to increase its resiliency. VDOT has long been required to address stormwater runoff coming from its facilities. To meet environmental regulations and design proper drainage for these facilities is a given, so dealing with more water to protect infrastructure certainly seems like a challenge that can be met.

Typically, proper management of runoff water is accomplished by one or more structural best management practices (BMPs), also referred to as stormwater control measures. These can take many different forms, such as stormwater basins, underground manufactured filtering or hydrodynamic devices and other low-impact development (LID) practices. LID practices are a subset of BMPs designed to behave hydrologically similar to undisturbed, natural land surfaces by allowing water to infiltrate into the soil and do so as close as possible to its original source rather than its original source being collected and conveyed offsite. Permeable pavement systems (PPSs) are one type of LID.

Figure 1: Average Infiltration Rates and Standard Errors Across All Sampling Sites for the 4-Year Monitoring Period.

Permeable Pavements Are Resilient

There are several types of PPSs: pervious concrete, permeable pavers and porous asphalt are the most common. Regardless of type, these function similarly by allowing water to infiltrate through the voids in the pavement matrix and then move into the base material. The voids in the pavement structure are created by the absence of fine aggregates, as compared to conventional pavements. In addition to the interconnected voids resulting in the pavement being permeable, the base for these systems serves as a foundation to support the pavement and as a reservoir for the infiltrating water. As such, the base is typically composed of large, clean aggregate (e.g., No. 57 or No. 2 stone) or something similar, having a void space of approximately 40%. The depth of this base layer is designed to accommodate the needed volume of water that will be treated under certain site-specific conditions. Depending on the type and design of the system, water is allowed to infiltrate from the base into the underlying soil or is discharged to an outlet by way of an underdrain. PPSs help remove nutrients from the runoff and reduce the peak runoff volume by allowing infiltration and then temporary storage of the infiltrated water. This reduction in peak runoff results in PPSs serving as an option for making transportation infrastructure more resilient.

VDOT’S Experience

VDOT, along with the Virginia Asphalt Association, developed guidelines for the use of porous asphalt pavement structures. These guidelines were developed based on the Virginia Department of Environmental Quality’s (DEQ) Stormwater Design Specification No. 7, Permeable Pavement, Version 1.8 (1). The development of these guidelines allowed VDOT to begin investigating ways that PPSs could be used as yet another stormwater BMP as early as 2013. However, though there was an abundance of research documenting the pollutant removal efficiency of PPSs and the runoff volume reduction that could be expected from these systems, little was known at the time about the associated maintenance requirements and costs. This prevented the direct comparison of PPSs to other BMP options based on a life-cycle cost analysis.

In response, VDOT undertook a study to better understand the long-term infiltration performance and the associated maintenance requirements of a porous asphalt PPS based on operating and maintaining a parking facility constructed using this material. The study’s objectives were to determine the most effective maintenance practices for keeping the infiltration rate of porous asphalt at an acceptable level, to calculate the costs of these practices and to estimate how long the porous asphalt parking lot would serve a functional stormwater BMP. Having this performance, maintenance and cost information would help VDOT decide which BMPs to use in the future.

The parking facility monitored was constructed in the spring of 2013. It is approximately 1 acre in size and serves as the overflow lot for a larger park and ride lot adjacent to the site. The area under the entire lot was constructed to serve as a reservoir and subbase, composed of 18 to 24 inches of No. 2 stone placed on top of a woven subgrade stabilization geotextile fabric. A 3-inch base of a permeable asphalt mixture with a nominal maximum aggregate size of 19.0 mm was placed directly on top of the subbase. This was overlaid with 1.5 inches of a permeable asphalt surface mixture with a nominal maximum aggregate size of 9.5 mm. Binders used for the base and surface mixtures were PG 70-22 and PG 76-22, respectively.

The lot was divided into four sections to evaluate the effectiveness of different maintenance practices, each receiving a different type of maintenance treatment or maintenance interval. Field infiltration measurements were taken in each section using multiple embedded single-ring, falling head infiltrometers. Values were captured after construction and before and after each maintenance treatment for approximately four years.

The study found that regardless of the maintenance practices used, the average permeability of the porous asphalt lot dropped significantly over the 4-year study period, but at 123 in/hr, it remained well above the 10 in/hr threshold required for the site to be effective as a BMP (2). Based on the infiltration rates measured, the site was projected to infiltrate at a rate greater than the threshold for 12 to 15 years at a cost that was significantly less than the costs projected for other BMP options originally considered for the lot. Performance findings have been similar for a pervious concrete VDOT parking facility constructed near Salem. After several years of use, the site is still performing well, with monthly vacuuming (see figure 1).

Completed Porous Asphalt Park & Ride Facility.

Still Room for Improvement in Performance

Similar to VDOT’s projects, there are numerous studies documenting the performance of PPSs. Still, these systems have room for improvement. Work continues on methods to improve binder performance by incorporating additives such as nanosilica and fibers (3, 4). In addition, advances are being made with respect to maintenance options, optimal timing of maintenance based on site-specific parameters, and even complete rehabilitation options such as partial depth milling (5, 6). It is anticipated that all of these efforts will result in PPSs, and porous asphalt specifically, becoming a viable option for even more locations.

The combination of increased awareness of the need to generate less runoff by increasing infiltration of precipitation on-site, additional experience with PPSs, and the continual improvements in mix design and maintenance practices will almost certainly result in these systems being used more frequently as a stormwater BMP. The bigger question is this: Can and will the use of PPSs expand beyond conservative applications such as parking lots, bike lanes and driveways to more heavily used pavements where resiliency is a concern because of flooding? As pavement engineers determine how to design pavements that will withstand saturated base conditions and stormwater engineers face the challenges of increased water volumes, how these uniquely designed pavement systems are viewed will need to change to address both of these resiliency obstacles.


References

  1. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 7, Version 1.8. Richmond, 2011.
  2. Wisconsin Department of Natural Resources. Conservation Practice Standard: Permeable Pavement (1008). Madison, 2016.
  3. Gupta, A., Rodriguez-Hernandez, J. and Castro-Fresno, D. Incorporation of Additives and Fibers in Porous Asphalt Mixtures: A Review. Materials, Vol. 12, 2019.
  4. Tang, G., Gao, L., Ji, T. and Xie, J. Study on the Resistance of Raveling for Porous Asphalt Pavement. 2017 International Conference on Transportation Infrastructure and Materials. DEStech Transactions on Materials Science and Engineering, May 2017.
  5. Radfar, A. and Rockaway, TD. Clogging Prediction of Permeable Pavement. Journal of Irrigation and Drainage Engineering, Vol. 142, 2016.
  6. Winston, R.J., Al-Rubaei, A.M., Blecken, G.T., Viklander, M. and Hunt, W.F. Maintenance Measures for Preservation and Recovery of Permeable Pavement Surface Infiltration Rate: The Effects of Street Sweeping, Vacuum Cleaning, High Pressure Washing and Milling. Journal of Environmental Management, Vol. 169, 2016, pp. 132–144.
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