Using alleys to fight heat, water runoff
Chicago boasts 1,900 miles of public alleys, the largest system in the world and a prime city asset.
Alleys are fine places to walk a dog, shoot hoops, and stash garbage cans so they don't clutter the sidewalks.
But alleys pose a problem in how they handle water. Alleys are designed high on the sides and low in the center, so water flows to the street and into catch basins. But over time, alleys lose their grade, and water can pool up during storms. Instead of running into the street, water runs into yards and basements.
The alley can be regraded, or rebuilt with a sewer, but both solutions add water to a sewer system already burdened during heavy rains.
Alleys also create "heat islands," places that absorb heat, and increase city temperatures.
About four years ago, the Chicago Department of Transportation sought ways to "expand its toolkit" for alleys, said Janet Attarian, director of the department's streetscape and sustainable design program. The CDOT wanted permeable alleys, so that water goes into the ground, and it wanted alleys that reflect rather than retain heat. And it wanted alleys that are long-lasting.
The solution, called the "Green Alley Program," uses permeable asphalt and concrete, developed by engineers who work for the city and outside engineers.
The innovative Green Alley program earned the City of Chicago a 2007 Chicago Innovation Award.
"It was a really great, interesting project," said Attarian, an architect by training and the program instigator. "Usually, with impermeable asphalt, water is the enemy. In this way, water is your friend."
The asphalt recycles tire rubber in its mix, and the concrete uses "slag" -- a byproduct of metal processing -- to make it reflective.
The city now has laid nearly 40 green alleys, attracting interest from private developers and other towns in Illinois and Canada.
"We've been able to offer solutions that are the most effective from a cost point of view and a functioning point of view," Attarian said.
The city team hired State Testing Co., a consulting firm, to help develop and test mixtures.
One puzzle centered on making asphalt permeable, but still strong. Permeable asphalt mixtures rely on fibers for strength, but the fibers Green Alleys needed were hard to buy. They were costly and, with the O'Hare Airport expansion roaring ahead, hard to acquire.
City engineers eventually found ground automobile-tire rubber with its fiber-rich content made an excellent substitute. About 600 tires can be recycled into a 600-square-foot alley.
The tires turn into a "black juice," replacing some of the oil that ordinarily goes into asphalt, and the juice is added to gravel and other material. Bigane Paving of Chicago produced the material on site, where it was adjusted as it was laid down in the pilot alleys.
The permeable materials are laid over rock beds that contain 40 percent "voids," or empty spaces, out of the total volume, to store water so it takes longer to get to the sewer. The area under the alley can thus act as a detention pond -- instead of nice, carpeted basements playing that role.
Attarian said the project saves money, both because the cost of materials is going down and because CDOT doesn't always have to entirely rebuild an alley.
"Maybe there's a section that has a low spot. Instead of having to regrade all of it, we can put in a permeable center trench and solve the problem," Attarian said.
About five of the new alleys use lighting that casts light down and out, rather than up, to reduce light pollution.
Attarian doesn't know if all alleys can use permeable materials.
"The original alleys we put on good, permeable soils," Attarian said. "We want to work on places where the soils are not permeable. That's something we want to do homework on."