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One of the most effective approaches in modern stormwater man- agement is the installation of hardscapes that allow the water to per- colate into the ground instead of running into storm drains, sewer systems, and drainage ditches. Different types of materials and systems are used in this approach, including permeable interlocking concrete pavements or PICPs. Though they are very familiar to landscape contractors, their BMPs are not necessarily so. First of all, PICP systems are not suitable on sites where stormwa- ter infiltration could contaminate groundwater. But other than that, "The benefits of permeable paving stones are economic, ecological and performance, making them superior to poured-in place concrete and asphalt," says Burt Plett, product manager for Willow Creek Paving Stones. When properly installed by hand or machine, a PICP system should provide 15 to 20 years of service before major maintenance is needed. And with regular inspection to identify and solve problems, a PICP system can function indefinitely. According to Plett, "Performance begins with the contractor cor- rectly building the open-graded subbase and base." Framework of a Functioning Foundation The Interlocking Concrete Pavement Institute recommends ASTM No. 2 stone for the subbase, No. 57 stone for the base and No. 8 stone for the bedding layer. These numbers and gradations are found in ASTM D448. Stone materials should be crushed for the highest rate of interlock and stability during construction and load-spreading capacity during service. Granite and other igneous materials of similar hardness are recommended to ensure consistent, long-term performance under heavy loads. It's important to keep sediment away from the stone and the open excavation. Simple practices such as restricting muddy construction equipment, installing silt fences, staging excavation, and diverting runoff with temporary drainage swales will make a difference in in - filtration performance. The paver surface should not receive runoff until the contributing drainage area is stabilized with vegetation. Erosion control blankets and sediment wattles can stabilize soil while grass or other vegetation starts to grow. Equipment passing over the soil subgrade will cause some compac- tion. If the soil is repeatedly compacted by equipment during con- struction, there will be a substantial loss of infiltration. Operators should avoid rapid acceleration, hard braking, and sharp turning. Tracked equipment is recommended. Geotextiles are used in some permeable pavement applications and are optional when using a No. 2 aggregate subbase, which acts as a filter layer while providing additional stability. 18 LC DBM Best Practices in Permeable Paver Installations By Willow Creek Paving Stones Above: The aggregate-filled voids between permeable pavers allow rainwater and snowmelt to naturally infiltrate into the ground or to where it can be harvested for irrigation or other uses. At Fish Lake Regional Park in Maple Grove, Minn., Glacial Ridge Inc. from Willmar, Minn., installed 19,000 square feet of Willow Creek Brickstone permeable paving stones to rehabilitate the parking lots and to compensate for poor soil conditions. Left: This is a cross section of a permeable paver system for a residential, pedestrian or vehicular application. (Willow Creek Paving Stones)