Permeable Bed Prep 101

Compiled by Matthew Doyle

Permeable installations offer unique challenges, and the Cliff Cottage Project at Furman University in Greenville, S.C. was no exception. The site, built to LEED certification standards with hardscapes by local general contracting company Triangle Construction, was completed in May of 2008. Pine Hall Brick donated the StormPave pavers, with 1/4-inch permeability joints. All images courtesy Pine Hall Brick

Staying ahead of the curve is absolutely essential to run a profitable business. In light of this consideration, LCN has asked what bed preparation techniques will eventually be required of the hardscapes contractor. Among the likely answers is competence in permeable paver bed preparation.

The Cliff Cottage model home at Furman University in Greenville, S.C. was completed in May of 2008 to Leadership in Energy Efficiency and Design (LEED) certification standards. Permeable walkways are among the features able to meet this specification. Triangle Construction was responsible for the installation, and William Trammell was the project manager.

 

This #89 open-graded aggregate (ASTM-D 448) was used for a one-inch setting layer. The Pave Tech restraints were spiked in next. The pavers were then laid with more #89 swept in the joints before a final compaction. Note that regular bedding sand (ASTM C-33) is inappropriate for this application, as the fine material will clog permeable void space.

The process began with stringing the perimeter to set the finished grade before the soil was excavated with a New Holland unit to nine inches. Since plastic edging with landscape spikes was specified, six inch deep by 12 inch wide bands of road base (ASTM-D 2940) were then installed along the perimeter. The next step involved laying the six-inch base of #57 stone (ASTM-D 448), which was then compacted. Pave Edge Rigid and Flex restraints were placed next before the one-inch setting bed of #89 (ASTM-D 448) aggregate went in and was screeded level. More #89 was swept in the cracks and the surface compacted to finish the job.

When asked, Trammell estimated the approach selected resulted in two to three extra steps and about 15 percent additional labor time. He also believed 60 percent of the hardscape emplacement time consisted of bed preparation, with curves increasing the figure.

The pedestrian walkway required this #57 aggregate (ASTM-D 448) in a six-inch base. To assure permeability it?EUR??,,????'?????<

Industry Standards

LCN also spoke with Pine Hall?EUR??,,????'?????<

First off, the contributor was very careful to point out that the soil hydrology and individual site conditions make every job substantially different. In particular, the rate at which the ground can absorb precipitation is key in determining capacity and its associated design parameters. Also, the thickness of the cumulative layers really determines how large a precipitation event the system can handle. After all, Corvey claims that RainPave and StormPave products allow for over 50 inches per hour of infiltration when initially placed, but the base actually holds that flow while soil absorbs it. These factors and many others must be considered in a quality design. As a result, he feels a qualified design professional is necessary to handle this phase of these installations.

The #57 base layer and the dense graded aggregate perimeter beams are compacted to level the surface and minimize settling. Since void space of the #57 is between 35 percent and 40 percent, the aggregate ?EUR??,,????'?????<

The first step alone can be a complex matter. If a full exfiltration hardscape is planned, which totally relies on the water permeating vertically through the soil to the water table, the sub grade should not be compacted. However, if a clay soil is present, the procedure may be required, but cannot exceed 92 percent density. A stabilizing geo-textile may also be needed.

The sub-base is the next step, and for a one or two year, 24-hour storm dropping four to five inches of precipitation about 12 inches is needed. Bear in mind that, according to Corvey, most events drop one inch of rain. He also claimed most pollutants contaminate only the first inch or ?EUR??,,????'?????<

The #89 aggregate setting layer was conventionally screeded before the Pine Hall pavers were installed. The Pave Tech plastic edge restraints were selected for their flexibility to follow the many curves of the hardscape and for frost heave lips that meet climate concerns. The setting bed was built to 1" to accommodate this decision, whereas in a vehicular permeable pavement, a 2" setting bed is more common.

The base goes in next, and primarily serves as a transition between the fine particles the bricks sit on and course ones in the sub-base. It typically consists of four inches of #57 stone (ASTM-D 448) and is to be compacted. A 1 to 2 inch setting bed, typically of #8 particles (ASTM-D 448), is then laid and screeded level. Appropriate edging can be put down next or after the following step. The pavers themselves then go in and a finer chipstone, like #89, is swept into the joints (#8 can also be used). Compaction of the pavers is performed to complete the process.

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NPDES

Another topic of discussion was the National Pollution Discharge Elimination System (NPDES), a federal permit program to limit water pollution. Corvey noted that Phase II now requires small and medium municipalities to have a storm water management plan. Some localities have taken a storm water management fee approach determined by the percentage of permeable surface area on a property. Permeable pavements can reduce this bill or make otherwise impossible additions and re-builds feasible. Residential install opportunities result.

After attending the ASLA convention in Philadelphia, Corvey was surprised at the number of landscape architects working on residential permeable projects. An increasing number of universities and commercial entities have also been interested in renovating walkways with the unique design. Combined with the ever-increasing popularity of sustainable products and services, he sees installation of these designs as an emerging industry.

As such, Corvey feels that now is the time for contractors to educate themselves, claiming that the earliest entrants in emerging fields generally experience the best profit margins. He thinks an ideal approach would be to connect with a local civil engineer or landscape architect that specializes in the design of such systems once basic knowledge and experience is gained.

A caution he was emphatic about, however, is that far more than one class is necessary to be an expert. Furthermore, a plethora of poorly designed or built sites would ultimately ruin the profit potential for all.

Both Hardscape North America www.hardscapenorthamerica.com/woh/contentatt_ektid106.aspx#FRISESS12 and the School for Advanced Segmental Paving and Walls www.paverschool.com/Classes/permeable_systems.htm offer demo classes and seminars in the subject.

Building Blocks

.25: Inches. During excavation and base preparation, contractors need to determine the depth of the excavation by adding together the recommended depth of the base layers and the thickness of the product you have chosen. For paving stone installations and vehicular applications, the surface of the installed product should be 0.25" above grade.

6: Inches. After excavation, base material should be spread uniformly throughout the excavated area with a hard toothed rake in layers of no more than six inches.

Source: Hanson Hardscape Products