Ecological Permeable Paver System

    FAQ

Questions & Answers

Question: What is the load capacity of these systems?

Answer: The same methodology for flexible pavement per AASHTO designs – the structural coefficient for asphalt and concrete pavers are the same. Port applications with 80,000 lb/axle loading (e.g. Port of Oakland) are in use.

Question: How much underlayment is required to support the pavers and is this system applicable for a street that probably has utilities and vaults located in its substrate?

Answer: Design for detention is usually performed first, then compared with structural design and evaluated for long-term performance taking into consideration soils and design life factors. In this scenario, I don’t believe the detention requirements in the City of Chicago would require a design to accommodate a 100 year storm, but a 24” sub-base would meet a 24-hour, 100 year storm. A factor that the city may be interested in is that the pavers could be removed and replaced in the event access is required for these utilities and vaults.

Question: How much stormwater can these systems absorb?

Answer: The coefficient of runoff for permeable pavements at dead flat has been proven to be .15, where asphalt is .98 using the rational method. We would recommend a minimum 1% grade and then the c value becomes .25. We would suggest that the design value be at .40 to allow for 50% clogging in the first five years. After that time period, the bacteria system is on a 100% breakdown of first-flush pollutants, which takes 5 years to complete. At this point, the system will continue to absorb a minimum 2.5” storm event. The void ratio in the sub-base aggregate is 40%. There is still another 9” of system (Paver depth, setting bed and base depth) left to store runoff from rainstorms; however, it is not part of the design… I just wanted to point out that we do have a degree of error designed onto this system.

Question: Are there any streets where this type of pavement has been used?

Answer: Not in Chicago; however, there is a permeable paver street in Havre de Grace, MD, Westmoreland Street in Portland, OR and also a townehome project that has been a case study in Connecticut called Jordan’s Cove. Unit pavers have been used in streets in America for over 100 years, but the material was clay. Concrete pavers have been in place for approximately 30 years in the U.S. The use of permeable pavers in England, Germany, Australia and Japan is well documented for the past 20 years. There are successful permeable unit paver applications in the U.S.; however, these are primarily parking lot applications. I would also point out the Howland Hook terminal at the Port of New York/New Jersey has permeable pavers being utilized in a container yard.

Question: What about handicapped accessibility – how does this system address those city needs?

Answer: Unit pavers are acceptable for handicapped movement as proven by the University of Pittsburgh study. The use of permeable pavers, assuming the openings do not exceed 13mm, will not conflict with ADA standards. There are also pavers that have truncated domes that could be used at curb/ramp areas.

Question: Is this system applicable to a street or would you recommend it for a city sidewalk or merely parking areas?

Answer: Any area that requires a pavement surface could be designed with permeable pavers. The issue is to understand the needs of the client, site and users of the pavement. Sites that are stormwater hotspots and have high exposure to toxic materials ( e.g. scrap yards, fuel stations, etc.) would not be a good application for a permeable pavement system. Areas that pose concern regarding the possible migration of pollutants into the groundwater could use an impermeable membrane to contain these fluids and be sent to a secondary system for treatment. These membrane systems could also be utilized for capture of grey water and then used for irrigation purposes.