Permeable Pavers and Porous Pavement

By Ashley Harbaugh, LC/dbm

The installation of permeable interlocking pavers differs from that of traditional interlocking pavers in that coarser, open-graded base and sub-base materials are used, as well as coarser bedding and joint fill materials. The volume and thickness of bases is determined by both traffic loading and amount of water coming into the system.

With the movement toward creating spaces that are more environmentally friendly, permeable pavers and porous pavement are becoming an increasingly popular hardscape option.

With land restrictions, stormwater management and other environmental regulations, land developers are seeking hardscape solutions that conform to these new standards. Porous pavement and permeable pavers provide attractive surfaces that address these environmental concerns.

 

One of the most common misconceptions when designing or approving permeable interlocking concrete pavement is the assumption that the percentage of open surface area of the pavement is equal to the percentage of perviousness. Actually, the permeability and amount of infiltration are dependent on the infiltration rates of the aggregates used for the joint and drainage openings, the bedding, base, and subbase, and ultimately, the subgrade.

Permeable interlocking concrete pavers are placed on a permeable, open-graded crushed stone bedding layer (typically No. 8 stone), and this layer is placed over an open graded-base (typically No. 57 stone) and sub-base (typically No. 2 stone). Other aggregate materials also may be suitable depending on specific project parameters. The pavers and bedding layer are placed over an open-graded crushed stone base with exfiltration to the soil subgrade. Photos: Uni-Group U.S.A.

Permeable Interlocking Concrete Pavers (PICP)

Permeable interlocking concrete pavers (PICP) are similar to traditional pavers that contractors have installed for decades. Interlocking shapes, notched corners, or enlarged joints between PICPs allow for water infiltration. These void spaces can provide enough permeability to significantly reduce stormwater runoff. Manufacturers of these pavers provide a wide variety of colors.

Uni-Group U.S.A., a major manufacturer of permeable interlocking concrete pavers, provides PICPs that can be installed either manually or mechanically.

Permeable interlocking concrete pavers can be used for a wide variety of residential, commercial, municipal and industrial applications. They can be used for parking lots, driveways, overflow parking, emergency lanes, boat ramps, walkways, low-speed roadways, and storage facilities. Permeable or porous pavements should not exceed 5% slope for maximum infiltration.

Pavers are placed on a one inch bed of sand over a compacted aggregate base. The thickness of the base will vary depending on its application. Paving stones are restrained using edge restraints. Commonly used edge restraints include concrete curbing, plastic edge restraints, and aluminum edge restraints. Once the pavers are placed and restrained, they are compacted into place. Fine masonry sand is swept into the joints, and the pavement compacted for 3 or 4 passes until full interlock is achieved. Upon completion of this compaction process, the area is ready for immediate use. Unlike asphalt and concrete, no curing or setting time is required for interlocking concrete paving stones.

Gravel porous pavement is composed of grid pavers made from either recycled plastic or concrete, with the grid pattern either honeycombed or lattice-shaped. The voids collect water, which then slowly drains into the soil below. Flexible gravel porous pavers can have any number of colorful aggregates that add an attractive appeal to paths and driveways.

Porous Pavement

Grass and gravel flexible porous pavements are made up of plastic cylinders connected on a grid system to provide flexible and structural, load-bearing surfaces. A grass porous paver, as the name suggests, has a full grass-covered surface and looks no different than a normal turf area or lawn.

A gravel porous paver has a filter fabric backing attached to the grid and can be filled with a variety of aggregates to integrate with other landscaping. These pavers are extremely strong and have a large amount of void space for almost no stormwater runoff. Unsupported grass areas will compact and rut under the pressure of an automobile. Unsupported gravel drives and roads do not remain porous when driven upon and eventually cause runoff similar to asphalt or concrete. Flexible porous pavers and permeable interlocking concrete pavers transfer the automobile load to the base.

Pervious pavements to be installed in low permeability (clay) soils require the base materials to act as temporary storage for storm events and subsequent drainage through the soil. The base should be composed of clean, washed stone with 25-35% voids. Capacity of the base material storage should be incorporated into design criteria and related to the desired performance of the system during storm events. Typically, design should accommodate runoff for the 6 month-24 month duration storm event. The system should fully drain after a storm event within a minimum of 12 hours, and a maximum of 72 hours (the recommended time is 24 hours).

A grass porous pavement system requires mowing, fertilization and water, but it does not need to be aerated as the grid rings and subbase sand will prevent compaction and allow for air infiltration.

Limitations

There are certain circumstances when permeable pavers and porous pavement should not be used. Any site classified as a stormwater hotspot (anywhere there is risk that stormwater could infiltrate and contaminate groundwater) is not a candidate for permeable pavements. This might include salvage and recycling yards; fueling, maintenance, and cleaning stations; industrial facilities that store or generate hazardous materials; storage areas with contents that could damage groundwater and soil; and land uses that drain pesticides and/or fertilizers into permeable pavements. In addition, permeable pavements may not be feasible when the land surrounding and draining into the pavement exceeds a 20% slope, or the pavement is on a downslope from buildings where the foundations have piped drainage at the footers.

The aesthetic, performance, and environmental benefits of using permeable pavers and porous pavements make this type of hardscape a viable alternative to the usual concrete and asphalt.