Frequently Asked Questions:

Permeable interlocking concrete pavement (PICP) consists of manufactured concrete units that reduce stormwater runoff volume, rate, and pollutants. The impervious units are designed with small openings that create permeable joints. The openings typically comprise 5% to 15% of the paver surface area and are filled with highly permeable, small-sized aggregates. The joints allow stormwater to enter a crushed stone aggregate bedding layer and base that supports the pavers while providing storage and runoff treatment. PICPs are highly attractive, durable, easily repaired, require low maintenance, and can withstand heavy vehicle loads. The picture shows installed pavers in a Seattle, Washington residential neighborhood. PICP significantly reduces the total amount of impervious surface and runoff. Unit Paving has access to free design software for Permeable Projects. Contact us today to learn how to access this software!

The openings in the surface of PICPs will require periodic removal of detritus and sediment trapped by the small sized crushed stone. Dirt is typically removed by a vacuum-sweeping street cleaning machine. Cleaning is done when the pavement surface and detritus are dry and can be loosened by sweeping and vacuuming. The frequency of cleaning will vary with the use of the pavement and position of sediment, leaves, etc. from adjacent areas.

A North Carolina State University study has shown that the initial surface infiltration rate of PICPs can be as high as 2,000 inches of rainfall per hour. For reference a 100 year storm may produce 4-8 inches per hour with 43 inches over 24 hours holding the record for the greatest one-day rainfall in the US. Per ASTM standards, a permeable project Unit Paving installed 3 years ago still has an average infiltration rate of 175 in./hr. That is 30 times greater than the 4-8 in./hr a 100 year storm may produce. We expect the system to maintain greater than 100 in./hr for many years to come.

Other research has shown that near initial surface infiltration rates can easily be restored through cleaning and replacement of the initial 3/4 to 1 in. (18 to 25 mm) depth of small stones in the openings of PICPs. For highly clogged pavement openings, the stones can be removed with vacuuming and replaced with clean material. This is a distinct maintenance advantage over pervious concrete and porous asphalt pavements.

Yes. If soil infiltration is slow (generally under 0.5 in./hour or 4 x 10-6 m/sec), perforated plastic pipe drains at the bottom of the base can remove excess water while still allowing some of the water to infiltrate into the soil. Digging trenches up to 4ft deep into the soil often allows access to soils with infiltration rates up to 100 times greater than those found at the surface. The drainage rate for the water contained in the base is typically no greater than 24 hours. Over practically impervious soils or high bedrock, an impervious pond liner can be used to detain, filter and release the water through drain pipes. Regardless of the rate of soil infiltration, the filtering action of the open-graded base can reduce water pollutants.