No specific case study was performed in the state of Washington. Some issues of concern regarding the use of permeable pavements were raised, however, by the King County Road Service Division from the State of Washington. It may be useful for other states to know or learn this information when considering a permeable pavement road application. Thus, the content of relevant information regarding these issues that was presented in a technical memo is
summarized below.
The Memo was written on June 13, 2011 in response to a Washington State Department of Ecology proposal requiring the use of permeable pavement when a new or replaced roadway surface is created by projects that are subject to drainage review. The memo expresses many concerns about the widespread use of permeable pavements and communicates specific design requirements and/or situations in which permeable pavement should not be used, in the opinion of the King County Road Services Division. Unfortunately, the negative views are mostly speculative and not backed by fact and proper references related to design, monitoring or maintenance experience gained from studies performed in Washington or elsewhere. A summary of the concerns, opinions, and recommendations indicated in the King County Road Service Division memo are as follows:
1. Porous asphalt does not perform well under medium to heavy traffic but studies have indicated it may perform well in applications related to pedestrian walkways, driveways, parking lots, and low volume roads. It is suggested that locations in Kings County where permeable pavement might be successful include dead-end cul-de-sacs that service no more than 16 lots if soil conditions are conducive to drainage. The memo, however, added numerous situations in which porous asphalt would not be feasible in cul-de-sacs. Those conditions are summarized below:
a. The area is a landslide hazard area
b. Geotechnical evaluation recommends avoiding infiltration
c. The site is within 100 feet of a contaminated site or abandoned landfill d. The site is within 100 feet of a drinking water well or spring
e. The site is within 10 feet of a small, on-site sewage disposal drainfield f. The site cannot be designed with a pavement slope less than 5%
g. For pollution generating sites, the native soils do not meet treatment criteria h. The site will likely have long-term sediment deposition even after construction i. The site is down slope of areas likely to contribute sediment
j. There is a risk of concentrated pollutant spills
k. Seasonal high groundwater creates prolonged saturated conditions at or near the surface
l. Fill soils that become unstable when saturated are used m. Sand is applied to road surfaces for winter road safety
n. The porous asphalt would compromise nearby non-porous asphalt pavements or threaten nearby basements
o. Underground utilities or underground storage tanks would be threatened. 2. The majority of existing permeable pavements studies are related to parking lots and
roads with no traffic or very low speed traffic with low traffic volumes.
3. Granular capping and subbase layers must be strong enough to provide an adequate construction platform for the overlying pavement layers.
4. The necessary air voids in the asphalt layer and the infiltration of water into the underlying soil reduces the pavements strength and ability to resist traffic loads. This will also create maintenance and safety issues.
5. Widespread use of porous asphalt will increase maintenance needs and costs associated with maintenance and pavement repair.
6. Porous asphalt is more susceptible to damage and wear from studded tires.
7. Sanding will decrease infiltration capability (according to the Washington DOT, some data indicates a 96% decrease in hydraulic conductivity on pavements treated with sand and salt). Added note: most industry and published work recommend no application of sand on permeable pavement.
8. Porous asphalt will crack more quickly than dense-graded asphalt. Note: This was not found to be true in the MnROAD study, section 9.1.1.
9. Maintenance will include patching with dense-graded asphalt and this will create a "patch work" road surface.
10. Porous asphalt roads would have to be overlayed in a shorter time frame to prevent overall failure but, due to limited funds, this may not occur as needed. The result would be failed and unsafe roads.
11. Due to durability issues, porous asphalt has the risk of total replacement at the end of its life.
12. Porous asphalt must be inspected and cleaned regularly using specialized equipment. This will increase costs due to equipment, personnel time, and other resources (e.g. gas). 13. Porous asphalt and associated infiltration of stormwater may negatively affect
underground utilities. The risk of associated liabilities may be too steep for agencies to assume.
14. Porous asphalt is more open to the air and therefore more susceptible to oxidation damage. This will cause the porous asphalt to crack and ravel more quickly.
15. The structural number (used in structural design) of porous asphalt is about one-half of that of conventional asphalt. Thus, porous asphalt pavements must be thicker and will be more expensive.
16. Porous asphalt pavements will have shorter life-cycles and more maintenance needs than conventional asphalt pavements.
17. A designed water conveyance system may still be required and this will increase the cost of porous asphalt systems even more.
18. There are no standards to test the quality of the porous asphalt.
19. Porous asphalt requires special asphalt oil that may be difficult to obtain.
The King county memo acknowledged that more investigation and research into porous asphalt is needed before widespread usage in the state of Washington. As noted by the experiences gained in Minnesota, New Hampshire, and Colorado, and additional experience gained from Ontario, Canada (see below), some of issues raised in the King County Road Services Division memo have been addressed while some still require further investigation.
9.2.4 Ontario, Canada, Experience