Stormwater | Project Profile

Cooling Chicago's "Fire Alley"

With the predominance of dark rooftops and pavement, which absorb and radiate heat, most cities experience summer temperatures that are 6-10ºF higher than rural temperatures. This phenomenon, known as an urban heat island, results in higher energy use for cooling buildings, causing atmospheric pollution to create ground-level ozone.

5500 N. Clark alley before construction. The concrete-base asphalt alley contributed to the urban heat island effect and frequent flooding.

Aggregate base course: C6 stone

The Chicago Department of Environment's (DOE) Urban Heat Island Initiative strives to reduce this problem by using alternative paving, constructing light-colored roofs, using alternative energy sources, increasing greenspace, and installing rooftop gardens, the last of which offers its most famous example atop city hall.

In fall 2001, Chicago DOE, in cooperation with Alderman Mary Ann Smith (48th Ward) and the city's Department of Transportation, replaced a 630-ft.-long, 16-ft.-wide asphalt alley located at 5500 North Clark with one using Invisible Structures's porous gravelpave2 system. Along with reducing the heat island effect, the new alley surface eliminated chronic flooding.

"The Gravelpave2 alley created a dual effect," reports Brendan Daley, project coordinator for Chicago DOE. "It helped reduce neighborhood temperatures while also offering stormwater management. There's no sewer system in the alley.

"From one end to the other, the alley drops in elevation about 8 feet. Every time it rained, backyard areas, which are lower than the alley, flooded," he explains. "The ward knew it was a problem. The rainwater didn't easily migrate to the street sewers, nor did it percolate through the soil."

Chicago DOE considered other solutions. "We'd also considered going back to the brick pavers that alleys once had, or maybe a coating on the alley, to cut down on the heat, but it was decided that Gravelpave2 would be the best idea to solve both problems," Daley relates.

C6 stone, newly constructed concrete garage aprons

Fine gravel poured into 1-in. rings

Gravel being raked into rings prior to compacting

The porous surface eliminates flooding, adds no new drain or sewer system, and reduces the urban heat island effect.

Installing stormwater drains in the alley wasn't considered an option. That not only would have further stressed Chicago's combined sewer system, but it also was cost-prohibitive. "We had considered it," Daley explains, "But once we talked to the sewer department, they talked us out of it. They don't need anymore water in their system. In fact, there's a program underway to disconnect homeowners' rain gutters from the sewer system."

Another benefit of the porous system: "The project took only about six weeks, which included tearing out 3 to 4 inches of asphalt and 8 to 10 inches of concrete. We also removed topsoil but left the sand base before installing the Gravelpave2," Daley describes.

For such a project, citizen cooperation was needed–and received. "We had strong resident input. About 99% of the alley's residents were onboard with the project. As we had to keep them out of their garages, the block club organized parking in nearby commercial and school lots," he says.

The system includes 100% recycled plastic rings that have been molded onto nonwoven geotextile filter fabric. This structure is then filled with gravel, producing a porous surface that offers heavy-load-bearing support and unlimited traffic volume and parking duration. For this project, the Gravelpave2 was laid atop a 10-in. compacted aggregate base course (two-thirds stone mixed with one-third sand). The new alley can handle up to a 3-in./hr. rainfall, allowing rainwater to soak into the ground and thereby reduce polluted runoff and flooding.

"The alley will bear the load of a full garbage truck–that's 7 or 8 tons–with no problem," Daley points out. "This is an ‘I' alley; the two ends of the alley are still asphalt because Invisible Structures thought perhaps heavy-truck turning would wear down the Gravelpave2. As the grade of the alley is a little below grade of the garages, we also had to add curbs and garage aprons."

The 5500 North Clark alley is a pilot project for both eliminating heat and excessive runoff. "We might do this again, on a case-by-case basis," he notes. "We can't do Gravelpave2 on clay soil unless French drains are first placed underneath. In this alley, the substructure was sand."

Rain isn't the only drainage concern; Chicago usually experiences heavy winter snowfalls, and although the city's Department of Transportation has dedicated alley maintenance crews, alleys are not plowed. "The top surface itself actually froze this winter with snowmelt," Daley recalls. "When we got warmer temperatures, the snow melted through. However, we had less snowfall than usual this winter. We will study the alley for at least a year before we decide whether to install another system."

Although the alley is still on "probation," it has already won an award. "The Consulting Engineers Council of Illinois gave it a 2002 Engineering Excellence Merit award," Daley proudly reports.

For more information on the 5500 North Clark alley, point your Web browser to www.cityofchicago.org/environment/AirToxPollution/UrbanHeatIsland.

SW - November/December 2002

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