Permeable Pavement Study


The main parking lot (approximately 100 parking spaces) at WSU Puyallup was removed and replaced with permeable asphalt and concrete. While the entire lot provides stormwater management benefits, nine separate asphalt and six concrete cells divided by concrete curbs are dedicated to research. The asphalt cells have two parking stalls and associated travel lanes (3 m x 18 m or 10 x 60 ft) and the concrete  cells have four parking stalls and travel lanes (6 m x 18 m or 20 x 60 ft). The asphalt is three inches thick and the concrete eight inches. Eighteen inches of clean crushed aggregate provide a structural base and stormwater storage for the entire parking lot.

The measured infiltration rate for the permeable parking lot subgrade is 0.0076 cm/hr (0.003 in/hr).

The porous asphalt cells are lined to prevent infilration to the subgrade and include three treatment replicated three times. Three of the asphalt cells are conventional impervious asphalt, three are pervious and will be maintained with high efficiency street sweeping equipment and three permeable cells will not be maintained. The asphalt cells are equipped with surface collection, drains just under the pavement and under-drains at the bottom of the pavement section. Tipping bucket flow gauges (Hydrological Services TBL1) measure flow continuously at the surface- and under-drains.

The concrete cells are not lined and are equipped with monitoring wells continuously recording ground water levels and sampling ports to access and measure long-term pollutant concentrations in the aggregate base and subgrade.

A large cistern (45,424 liters or 12,000 gals) collects water from an adjacent building roof and synthetic stormwater will be mixed and applied to the permeable pavement with rainfall simulators in future experiments. The flexibility built into the collection and delivery system will also allow for the application of various pollutants of concern as scientists and agencies in the region determine research needs.

Some of the specific questions assessed in the permeable pavement study include:

  • How well are typical road and parking pollutants filtered at various levels under the pavement?
  • What is the long-term pollutant accumulation in soils beneath permeable pavement?
  • How do infiltration rates compare between maintained and un-maintained sections?
  • What is the stormwater volume reduction on soils with poor infiltration rates?