In the alley-center infiltration approach, permeable material (asphalt, concrete, or pavers) is placed in the center of the alley and high-albedo concrete is placed along the outer edges. This way, water soaks through the center of the alley, but truck tires roll on top of the stronger pavement.
Each of Chicago's four pervious-pavement designs can be tweaked to fit just about any alley in the city. This is an example of full-alley infiltration using permeable pavement. Source: Chicago DOT
“It's a little bit of a bump in price because of material development,” says David Leopold, project manager with the city's streetscape and urban design program. “But as we keep building these alleys, the costs will come down as more people get on board.”
Construction began in summer 2006 and continued into fall. All five alleys are finished, and an older, gravel alley has been retrofitted. Plans are in the works to reconstruct more alleys, and there are plenty of choice locations. Twenty percent of Chicago's 13,000 alleys are “unimproved,” meaning they're dirt or gravel, and another 20% of improved alleys need repair.
Meanwhile, the performance of the four pilot alleys is being monitored to see what materials and techniques work, and what doesn't work. A survey also will be sent to residents to gauge user feedback.
So far, the results are good. Residents are happy to get new alleys. Chicago-area developers and other organizations have expressed interest in CDOT's newly developed materials. Aldermen are requesting green alleys for their neighborhoods. Local contractors that worked on the project, like ready-mix concrete provider Prairie Materials of Bridgeview, Ill., are hosting certification courses for working with pervious materials.
And the alleys have undergone one season of freeze-thaw cycles without a hitch.
“It's been a pretty severe winter, and they're holding up so far,” says Behnke.
Monitoring the alleys will help CDOT and the city's streetscape and urban design program develop “green alley” best management practices. The alleys also may help answer environmental and stormwater management questions.
“We don't have all the answers,” says Attarian. “But this is the first step in finding them.”Not just about the water
“Green alleys” are more than flood-management devices.
Managing stormwater runoff is a big issue in Chicago and the county (Cook) in which the city resides. But David Leopold, project manager for the city's streetscape and urban design program, says flooding was just one reason for Chicago's “green alley” pilot project.