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.

 

This is a cross section of a permeable paver system for a residential, pedestrian or vehicular application. (Willow Creek Paving Stones)

One of the most effective approaches in modern stormwater management is the installation of hardscapes that allow the water to percolate 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 stormwater 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 correctly 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 infiltration 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 compaction. If the soil is repeatedly compacted by equipment during construction, 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.

In Farmville, Va., an impervious asphalt parking lot with an existing drainage problem was torn out and Eagle Bay Commercial installed 19,300 square feet of Filterra???(R)???AE???? BioPave• pavers that cover the entire parking lot, and three bioretention units, which together now treat water from 1.22 acres or 53,143 square feet. The work took about 11 days total.

Permeable pavers can be integrated with other stormwater BMPs as well. The parking lot at Fish Lake sloped toward the lake, so asphalt driving aisles and curb cuts were located between each parking pad to allow sheeted runoff to flow to each pad where infiltration can occur again before eventually reaching a retention basin. Also, a drain system with perforated pipe was installed under the pervious pavement. Sedona was the color of choice for the parking pads and slate for the soldier course.

Piecing It Together
No. 2 subbase material should be spread in minimum 6-inch-thick lifts and compacted with a static roller. Compacting is easier when all stone surfaces are moist. At least four passes should be made. The roller is often in vibratory mode for the first few passes and then static mode for the final passes.

This compaction method applies to the No. 57 layer as well, which can be spread and compacted as one 4-inch-lift. If the base surfaces are disturbed, they should be re-leveled and re-compacted. The objective is to have no visible movement in the stone during the last compactor pass and no crushing of the stone.

The surface should then be covered with a 2-inch-thick layer of moist No. 8 crushed stone. This layer of finer crushed stone is screeded and leveled over the No. 57 base. No. 8 stone should not be compacted at this point. The surface tolerance of the screeded No. 8 material should be ±3/8 inches over 10 feet.

Concrete pavers should be placed immediately after the No. 8 base bedding is placed and screeded. After placement, the paver joints are filled with No. 8 stone and compacted with a minimum 5,000-lbf plate compactor, which pushes the No. 8 stone into the upper portion of the No. 57 stone base.

Finishing Touches
A vacuum sweeper can be used to remove encrusted sediment, leaves, grass clippings, and clogged void materials. Vacuum and sweeper settings may require adjustments to prevent uptake of aggregate from the voids and joints. Additional void materials can be added by mechanically or manually sweeping but it is not recommended to use pressure washers on open-jointed systems.

The Americans with Disabilities Act requires that surfaces in pedestrian access routes be firm, stable and slip-resistant. Openings should readily accommodate persons traveling with wheeled mobility devices. When designed properly, PICP installations can meet ADA requirements and can be painted to indicate pedestrian routes.

For more information:

• ASTM D448 Standard Classification for Sizes of Aggregate for Road & Bridge Restoration
• ASTM C936 Solid Concrete Interlocking Paving Units
• ASTM C1645 Freeze Thaw and De-icing Salt Durability of Solid Interlocking Paving Units
• Guide specifications for Willow Creek permeable pavers are available at www.willowcreekpavingstones.com/permeable-systems.