Here's a little-known fact about Chicago: The Midwest city has more miles of alleys—1900 in all—than any other city in the world. That's the equivalent surface area of five midsized airports.
That's also 3500 acres of impermeable surface in a city that's committed to going green.
But going green isn't the sole reason for the city's $900,000 alley-renovation pilot program, funded by general funds.
“One of residents' biggest complaints is flooding,” says Janet Attarian, project director of the city's streetscape and urban design program. Heavy rainfalls, or rain events that last for days, flood alleys. The water spills off into—and overwhelms—the city's combined sewer/stormwater system, causing basements to flood and the system to overflow into the Chicago River.
“Basements become short-term retaining ponds until the system clears enough for them to be emptied out,” says Attarian. “Plus, we're discharging contaminated water into the river because our system can't handle it.”
According to Attarian, the city's in-place solutions weren't good enough. For example, repaving a puddle-prone alley with asphalt worsens the ponding, causing excess water to run off into backyards instead of into the street where it's properly drained into the combined sewer system. And connecting sewer mains from the alleys to the city's sewer system is cost-prohibitive.
Other options were needed
The answer: Reconstruct alleys with permeable surfaces that allow water to pass through and infiltrate the soil below. Made of either porous asphalt and/or concrete or paving stones shaped to leave gaps at the corners, these pavements would reduce flooding, recharge groundwater, and save taxpayer dollars that would otherwise be spent treating stormwater or tearing up streets to install stormwater collection systems.
The Chicago DOT (CDOT) chose five alleys with maintenance problems and ideal soil conditions to test four anti-flood paving models, each incorporating high-albedo concrete and either recycled concrete or permeable
pavement (concrete, asphalt, or pavers) as a base material. Surfaces are pitched and graded to direct excess stormwater toward the center of the alley, where it drains into the existing street sewer system. Underneath is 1 foot of either gravel or crushed stone that traps water until it soaks into the ground. Naturally occurring iodes attack pollutants such as oils and antifreeze, helping to break them down before the water reaches aquifiers. Optional inlet structures or stormwater infiltration trenches also are installed to store and dispense water while the soil absorbs it.
The right ingredients
The CDOT turned to local materials consultant S.T.A.T.E. Testing LLC of East Dundee, Ill., to convert national ASTM standards to Illinois DOT specs, which are based on American Association of State Highway & Transportation Officials standards.
“We tried to make this as palatable as possible for contractors to bid,” says Cynthia Williams, quality assurance manager for CDOT. S.T.A.T.E. Testing experimented with mix designs to meet project requirements, which include reflectivity, a minimum permeability rate of 3 gallons per square foot per minute for all surface materials, and a compressive strength of at least 1800 psi for pervious concrete.