Practice Makes Permeable

Technology helps shape what hardscapes can be.

By Haniya Rae

The pavers are installed as a pathway at Sasaki’s Boston location. Image courtesy Matthew Arielly.

Courtney Goode was working on a project in Houston when Hurricane Harvey hit. Buffalo Bayou, one of the slow-moving rivers that Houston relies upon to hold stormwater, flooded, and the waters would end up spilling out over the city’s aging infrastructure and impermeable surfaces, exacerbating the problem.

“My heart was in my throat,” Goode says. “We had been working on these super-detailed axonometric drawings of all angles of the city—we knew the city like the back of our eyelids. It was a total shock to see the bayous obliterated and murky, debris-filled water covering the walkways, roads, and even ground floors of the buildings near the bayou. The flood just engulfed everything we had been designing.”

For Goode, a landscape designer in Sasaki’s Urban Studio and a Fabrication Studio coordinator, the disaster afforded her a very real account of how the city managed stormwater and led her to think more about how low-impact development can divert stormwater from streets during flooding. She describes a scenario in which a city like Houston could divert some of the excess water by excavating 40-foot-deep gravel dry wells (the size of a typical four-story parking garage) topped off with permeable pavers that could hold excess rainwater until it’s able to seep back into the ground.

“Houston didn’t need water to soak into the ground and be redirected to the already-flooded bayous,” Goode says. “It needed for the water to be held in place right where it fell.”

The batch of pavers was installed on top of a gravel reservoir, in compliance with the Americans with Disabilities Act’s guidelines. Image courtesy Matthew Arielly.

Permeable pavers are not an obvious choice, given the scale of the urban flooding problem in Houston. Many cities must consider problems of maintenance cost when it comes to laying down anything other than blanket applications of asphalt or concrete. They’re also only as useful as the setting bed on which they’re placed—and the dry well below the paving stones must be correctly sized to adequately contain excess floodwater. Furthermore, there is less attention to what pavers are made of and what they’re going to look like on a project—which means there’s ample opportunity to design new pavers that serve multiple functions and still look good.

Taking advantage of a competitive grant program at Sasaki that allows employees to work on research projects as part of their billable work hours, Goode proposed a project to use Sasaki’s fabrication lab to create a paver prototype that could help with urban flooding. The lab, which includes 3-D printers, vinyl and laser cutters, a CNC mill, and a wood shop, would be key to finishing the project in-house without needing to take it to an outside shop, which would certainly be an extra cost. She also wanted to enlist colleagues who were interested in learning fabrication methods to aid in research for their own projects, but didn’t have much experience working in a fabrication lab. She estimated a budget for the project of around $10,000, which covered the raw material costs of concrete and the molds for the pavers (but did not include billable hours).

Initial foam prototypes with a more porous surface proved difficult to remove from the mold. Image courtesy Matthew Arielly.

As part of the grant process, she took her proposal to Sasaki principal Zachary Chrisco, who would oversee her efforts and provide feedback when necessary. She also enlisted the critical advice of the landscape architect and then Sasaki principal Gina Ford, FASLA, as well as fellow landscape designer Briana Outlaw, Associate ASLA, who helped her research and organize the construction of the pavers.

Given that these pavers were designed for urban areas, concrete was chosen as the paver material because it’s widely available and would be familiar to most design professionals and installers at a relatively inexpensive price, should Sasaki choose to market them. “We wanted the pavers to be economical and responsible in terms of material use, which helped keep costs down, but for us this was a best practice design measure to not use excessive amounts of concrete if we don’t need it for strength,” Goode says.

She borrowed the design methods put forward by Tim Brown of IDEO in the book Change by Design. “At the core of IDEO’s design thinking tool kit is iterative prototyping and the ethos that you should aim to ‘fail early and fail fast’ so that you are able to improve your design equally as fast,” Goode says. Encouraged by Brown’s writing, Goode started to sketch a number of options for potential pavers. Since concrete itself can’t be made permeable, Goode decided to enlarge the holes within her designs so that water could easily fall through to a reservoir below.

Courtney Goode, left, and Briana Outlaw, Associate ASLA, pour concrete into the paver’s polyurethane mold in Sasaki’s fabrication lab. Image courtesy Victoria Fisher.

As she sketched, she shared her ideas with Ford and Chrisco, who critiqued the sketches and helped Goode hash out the scope for the project. Through this process, Goode figured she wanted a simple shape that could be tiled, more interesting than a square or rectangle, but not a complex polygon. Goode thought a pentagon could work, and once she had an idea of the size she needed, she took her ideas to Ken Goulding, a principal in the strategies department of Sasaki, which specializes in coding, mathematics, and technology.

“While it might seem like there are endless ways you can arrange tiles, geometric constraints impose limits on the options available,” Goulding says. “The mathematical study of tilings for a flat plane, or tessellation, is very well documented, so we simply needed to unearth a tiling solution that suited the goals of the project.”

Goulding looked at the range of known pentagons that could create patterns, especially those that can tile a plane as a single shape. There are only 15 such pentagons that exist, the most recent being pentagon 15, which was discovered in 2015. Interestingly, pentagon 15’s angles are all round numbers—150 degrees, 60 degrees, 135 degrees, 105 degrees, 90 degrees—which would make it easier to measure and fabricate. Goulding explains that unlike other pentagonal tilings, its angles can’t be altered or it would ruin the tiling pattern.

As part of a research grant at her firm, Courtney Goode prototyped permeable pavers with a 3-D printer. Image courtesy Matthew Arielly.

“We were drawn toward pentagon 15 for a number of reasons,” Goulding says. “It’s a single repeating geometry, so that means fewer molds would be needed. It features no sharp angles that would chip or crack in concrete, as the sharpest angle is only 60 degrees on this pentagon.” Any less than 60 degrees on an angle and the integrity of the concrete paver would be challenged.

Goode began to build the prototype of the pentagon 15 paver in Rhino. Because the final product would be made of concrete, she had to consider its size and weight as she modeled the form.

“One cubic foot of concrete weighs 150 pounds—which is kind of mind-blowing,” Goode says. “A block of concrete the size of a toaster weighs more than I do.” Goode needed to make sure that the final pavers would be easy for manufacturers and installers to move around and put in place by hand, so she kept the final weight to slightly less than 20 pounds per paver.

In total, Goode modeled about 40 different forms with pentagon 15. For some of the forms, she wrote a Grasshopper Voronoi script to generate spongelike holes within the pavers. She also wrote a second script to generate a gradient tiling design tool, a tool that can quickly create a repeating pattern with her paver design using the pentagon 15 shape, so that she could make renderings for projects.

The pentagonal form didn’t come without its drawbacks, though. Convex pentagons, Goode explains, only tile through mirroring—depending on what the interior of the paver design looks like, there needs to be an “A” unit, then the mirror of the “A” unit placed next to it for the design to express itself properly. As well, no fewer than two and no more than four pavers would need to come together at a corner—if the angles aren’t exact when the pavers are molded, there might be issues with the fit.

From the 40 shapes created in Rhino, Goode took those designs and looked over them again with Ford. Then, she took 36 of the 40 to be milled on a CNC machine with cheap, white foam. Within two days’ time, Goode was able to see the shapes in real life.

The foam paver prototypes were put on display, and Sasaki members were able to vote on their favorite iteration. Two designs were selected from this group. Goode was able to make a working polyurethane mold for one of the designs—a paver with a single hole, an aperture, through the center. The second design ended up being too complex with too many holes, and got stuck when trying to release it from the mold.

At this stage, Goode enlisted Outlaw’s help to set up and manage a fabrication demonstration in Sasaki’s lab for the staff. More than 30 colleagues from different disciplines at Sasaki, many of whom had never worked in a fabrication shop before, showed up to learn how to mix concrete for the pavers. “One of the things that surprised me in a good way was the potential for it to build a community here at Sasaki,” Goode says. In total, more than 36 of her colleagues donated a collective 330 hours of labor to help create the pavers and to learn more about the design iteration process Goode used.

“Often we create models that aren’t at human scale to get a sense of quality of design,” Outlaw says. “But this project allowed us to make things quickly at human scale to understand how it’s going to work or what doesn’t work. We can see that it’s possible to do this for other projects in the future now.”

Once the concrete had cured, 240 finished pavers were installed at the Sasaki office in Boston so that clients and other visitors can get a sense of how pavers can add to the design of a landscape and also offer a way to divert excess rainwater.

Goode is working with Anuja Kamat, an assistant professor of civil engineering and technology at Wentworth Institute of Technology, and her students to chemically test and develop the pavers further for compression and tensile strength. The batches Goode and her colleagues made had a one-day cure time, though additives were used for such a fast-setting concrete. Kamat and her students will eventually compare seven-day and 28-day cure times to see which might be best for the pavers, though the final outcome of these tests isn’t available quite yet. Once she has a firm idea of the concrete mixture that would provide the best result at the best price, Goode plans to create the ultimate paver. “I wanted to create these super pavers that could withstand the weight of a fire truck and city plumbing and be ADA compliant,” Goode says. “I want to have a fail-safe product.”

Haniya Rae is a freelance design and technology journalist who lives in Brooklyn.

2 thoughts on “Practice Makes Permeable”

  1. Excellent commitment to capturing rain where it falls instead of contributing to storm water runoff leading to flooding and erosion of bayou banks. Kudos.

    I suggest a Permeable Interlocking Concrete Pavement (PICP) infiltration rate testing on the installation at Saskai’s Boston office to give more credence to how many inches of rainfall this PICP system can handle.

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