Permeable Pavements Go Mainstream
Permeable materials are becoming more common for both new installations and retrofits.
Permeable products have gone mainstream as cities struggle to manage stormwater and create aesthetically pleasing spaces. As more project designers and owners understand the value of permeable products, the industry has responded by providing a variety of types and designs. The products are available in a mixture of colors and sizes, so there will likely be one that fits a specific project.
Permeable products are being included in new construction as well as retrofits of older spaces. The projects in this article range from refurbishing an older schoolyard, to rebuilding a community space for exercise or shopping, to new construction in a space-constrained environment.
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Space Saving in Portland
On many projects, space for stormwater systems is very limited. When an apartment building in Portland, ME, was built on a small lot, the parking lot and driveway had to be constructed in an extremely tight area. Another complication was the location of the property at the headwaters of the Capisic Brook, an urban impaired stream. The City of Portland is actively working to clean up this stream to improve overall water quality in the watershed. The adjacent wetlands meant that the site could not have untreated stormwater rushing off the parking lot and driveway.
“Urban infill development begs for space-efficient innovative solutions,” says Robert Woodman, senior environmental engineer and green infrastructure specialist with ACF Environmental. “When you look for function, features, and meeting rigorous state stormwater standards, innovative and creative design, paired with quality installation, are essential.” ACF supplied the R-Tank modules and the PaveDrain blocks used in this project, as well as design support to the engineer during the permitting process and construction support and oversight.
Ransom Consulting and Mitchell & Associates were tasked to plan for stormwater management on this site. Peters Construction was the site contractor for the 72 Bishop Street project, and Great Falls Construction was the general contractor.
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The south side of the area has a large block retaining wall, which had to be protected so that it would not be undermined by runoff. It provides the grade change needed from the parking lot to the wetlands below. A 30-mil PVC liner was wrapped around the wall to protect its integrity and stability.
An R-Tank subsurface storage system was placed underground at the location. It provides 4,600 cubic feet of storage, enough capacity to capture runoff from a major storm, and allows controlled release of the water through an outlet control structure. This meets the channel protection requirements to ensure no impact on the nearby brook. The R-Tank system was chosen because it has 95% void space and a reduced footprint compared to other systems. The tank has enough capacity to handle two-year, 10-year, and 25-year storms.
An 18-inch sand filter course was placed between the R-Tank and the PaveDrain blocks, ensuring water-quality treatment to meet Maine Department of Environmental Protection standards.
PaveDrain is an articulated concrete block paving system. The blocks are not permeable in themselves. However, each weighs almost 50 pounds, so once the blocks are in place they are stable and the joints remain open to allow water infiltration.
Each block interlocks with six others and an arched space on the block bottom provides room to store water so that it can be released gradually. About an inch of rainfall can be captured this way. The joints are open so that water will drain evenly from the paved surface. Approximately 2,400 square feet of the PaveDrain system was installed at 72 Bishop Street, curving around the outside edge of the property.
The installation had to be closely coordinated to allow the correct placement of each layer: base stone to tank, sand filter to stone, stone to pavers. A variety of fabric types, 30-mil liner, and geogrid were placed. Small areas of soft soils were found, leading to a need to over-excavate and place granular material in those spots.
Maureen McGlone of Ransom Consulting notes that although the pavers used are more costly than asphalt paving, other factors must be considered. With the space limitations and the stormwater runoff requirements both for quality and quantity, the elements of design needed to save space and money. “There were other ‘soft costs’ associated with the less-expensive options, which compromise the overall design—maybe that means eliminating parking, or reducing the footprint or number of units in the structure, which impacts the development viability on a larger scale,” she says.
Several other stormwater management options were included in the project. Roofline drip edge treatment and internal cartridge filters for roof drains helped ensure that stormwater was treated effectively onsite.
Maintenance will include periodic vacuuming to remove accumulated sediment from the open joint system. In winter, the surface can be plowed, lightly sanded, and salted as needed. The blocks were sprayed with a sealer after installation to repel salt damage on the block surface.
McGlone said that the project was so successful that her company has designed others in the Portland area using similar methods, especially in areas that are already heavily developed.
Credit: Paul MacInnis
Parrish Medical Center in Titusville, FL
The City of Kirkland, WA, needed to upgrade a two-block long retail area. Park Lane had tree roots breaking through sidewalks, and a failing water main was causing water to flow into Lake Washington. The city decided to create a woonerf; the name comes from a Dutch term meaning “living street.” In this design, pedestrians and vehicles share the right of way.
The project was taken on by Cascade Design Collaborative, landscape architects, and Perteet Engineers for utility and engineering design. Grants were obtained from the Washington State Department of Ecology and the Transportation Alternatives Program. With added money from the city, the project totaled $3 million.
After the water main was replaced, pavers were installed. The products chosen were Eco-Priora and Hollandstone, both manufactured by Unilock. The blocks were supplied by Mutual Materials of Bellevue, WA.
Eco-Priora pavers are installed with a 7-millimeter gap between the pavers. Spacer bars help installers maintain a uniform gap, which is filled with fine stone chip that allows water to percolate into the sub-base and soil. The pavers are available in a variety of colors and sizes.
Hollandstone is an interlocking concrete paver. For this project, 19,000 square feet of Hollandstone and 7,000 square feet of Eco-Priora were installed. Both types of pavers were 8 centimeters thick, giving a smooth finish to the surface. Using the two kinds of pavers, project designers were able to create an interesting color design, as well as incorporate stormwater management.
The 60-foot right of way has no curbs. It weaves along for the two blocks. Parking bays are included, as well as street furniture, bicycle parking, and art plinths for art displays. The non-permeable interlocking concrete Hollandstone pavers were used for the streets and sidewalks and Eco-Priora pavers were installed for the angled parking.
The plaza-style design slows traffic and allows easy interaction between vehicles and pedestrians. Bollards can be used to block the street for special occasions, permitting only pedestrian access. Bioretention planters and trees provide vegetation and additional stormwater management. They also give the public a chance to see water infiltration in action.
Eastern States Paving Inc. in Portland, OR, was the masonry contractor for the project. Dave Carlton of Eastern States Paving says a six-person crew installed the pavers. Access to existing businesses had to remain open throughout the construction. The company has worked on similar projects throughout the US.
Park Lane has 23 shops and cafes and overall is an inviting community space. Events are planned for this renovated space. In October 2017, the Park Lane project was awarded the Great Streets Award by the American Planning Association.
Education and Environment
A renovation project in West Philadelphia illustrates the need for better stormwater management. Many older cities, including Philadelphia, have combined sewer systems, mixing stormwater runoff with septic sewer flows, all of it passing through wastewater treatment plants. As the cities expand, more impervious surface is added, leading to increased runoff. More people in the municipality also produce more sewage. The increases can overload the treatment plants and cause combined sewer overflows, in which untreated effluent is released into nearby waterways.
Lea Elementary School in Philadelphia is more than 100 years old. The area is highly urbanized, so the school is surrounded by pavement, apartments, and commercial buildings. The site was covered in asphalt that needed to be replaced, and the community chose to use a durable, permeable surface that would have the added advantage of decreasing stormwater runoff.
The final plan was drawn up by designer Sara Schuh of SALT Design Studio in Bala Cynwyd, PA. Thomas Johnston, landscape architect and principal of ThinkGreen LLC, also worked on the planning and construction.
The project was fairly low budget but received a grant from Philadelphia Water Department, as the city is trying to foster these types of projects. Schools and the community are raising money for similar projects.
The goal was to make the area around the school accessible and interesting to the students and for it to handle runoff not only from the schoolyard but also from the street. Products from Pine Hall Brick Company of Winston-Salem, NC, were chosen.
Pine Hall Brick Company manufactures permeable clay pavers, as well as other types of pavers; it is the largest manufacturer of clay pavers in the US. The pavers used for this project are rated for pedestrian and light vehicular traffic. They are slip resistant and skid resistant, even in wet weather.
The installation started with a storage basin sized to handle runoff from the site and the street. Geotextile fabric was added over the subgrade to keep sediment out of the rock layer. Construction continued with a sub-based course of large crushed stone, 2 1/2- to 3-inch size. On top of that, a base course of small- to medium-sized crush stone, 1/2 to 1 inch in diameter, was placed, followed by a bedding course of open-graded washed aggregate. The pavers were set on top of this layer.
The permeable clay brick pavers have spacers, or nubs, on the sides to ensure a gap of 1/4 inch between the pavers. Open-graded aggregate was used to fill the joints, allowing stormwater to infiltrate into the lower courses through the joints. The water is stored in the voids between the stones and then passes on down. The pavers were placed in a crosshatch pattern. The pavers chosen were a rosy red color, an attractive contrast to the dark brick of the school building.
The planners of the Lea School project did not stop with permeable pavers. They added rain gardens with native plants, including cool- and warm-weather grasses, flowering perennials, and edible shrubs and fruit-bearing plants. A variety of switchgrass was planted in some areas because it has thick biomass that does an excellent job of capturing sediment and pollutants. Schuh says one goal of the project is educating students at the school, giving them a chance to observe and learn about plants native to the area. For example, many of them thought that the grasses died when they went dormant. Students have also learned about pollinators and why they are important. The Philadelphia Orchard Project cooperated with the planting and volunteers from the school also joined in.
Johnston says one of the project’s challenges involved scheduling. School officials had hoped to complete the project in the summer when school was not in session. However, a number of things happened to push the timing back, so construction was carried out while students were at school. The top priority was making sure that everyone was safe, so extra fencing was used and the crews were reminded to be extra vigilant while they were working.
The school will handle maintenance of the new pavers and the rain gardens. The permeable paving should not have sand or rock salt applied. Since water drains into the joints, icing should be minimal. The area can be shoveled with plastic or rubber blades, or a snowblower with plastic implements can be used. The joints should be vacuumed periodically.
Schuh notes that it is extremely important in designing a site to know how it will be used. She is using the knowledge gained from this project on the design of a second school project, incorporating input from teachers and administrators as well as observations about how students use the space.
Credit: Paul MacInnis
Parrish Medical Center in Titusville, FL
The CEO of the new Parrish Medical Center in Titusville, FL, wanted to encourage employees and the community to exercise and be active to improve health. Next door to the new building was the location of the original hospital, which had been torn down once the new facility was built. A park was created on the 5-acre lot with a trail and exercise stations. In the center is a small stage of concrete pavers, used for hospital celebrations and as a platform for the hospital’s 30-foot Christmas tree each year. Park vegetation includes a small area of low-maintenance lawn, as well as live oak and citrus trees. Existing oak trees on the site were preserved.
Site runoff drains to an existing stormwater pond on the northeast side, and the trail also drains to a large Brevard County stormwater park to the northeast. The system drains into Indian River Lagoon, which has been impaired by pollutants in the past and is now being cleaned up. This project fosters that goal.
The designer, Kathleen Burson, ASLA, owner of Indian River Garden Company LLC, explains that the medical center wanted the park trails to look as natural as possible, with full accessibility for a future nearby rehabilitation facility. The trail needed a non-slip surface that would provide good wheelchair access.
Gravelpave2, a porous gravel paver material manufactured by Invisible Structures Inc., was used for the trail. Gravelpave2 consists of a ring-and-grid structure with a geotextile fabric backing; once the system is in place, the rings are filled with aggregate. Gravelpave2 comes in rolls and is installed and anchored over a base course of gravel up to 12 inches deep, depending on the load that the site will carry.
Once the Gravelpave2 grid and aggregate are in place, the aggregate is smoothed with power broom, blades, or shovels. The gravel can be compacted with a small roller. For the trail, borders were constructed of brick, accenting the gravel path.
Burson designed the project and oversaw the construction. The contractor was Rush Construction Inc. of Titusville. Burson says exercise stations were clustered, rather than spread along the trail. Many park users have commented that the gravel is easy on jogging feet. The area remains a focal point for employees of the hospital and the surrounding medical buildings, as well as the community.
The hospital CEO is pleased with how the project turned out and how it is holding up in the few years since its construction. Even with heavy jogging and walking traffic, the trail is in good condition, and no weeds have sprung up.