Fall 2012

Wouldn't that be a green alley?

Paving the Way



To maintain its original character, one of only two remaining segmental wood-paved areas in Chicago saw restoration of its original construction. Dating from 1909 and located in the city’s Gold Coast neighborhood along Lake Michigan, the alley is enshrined on the National Register of Historic Places and designated a City of Chicago Historic Preservation Landmark. While not officially one of Chicago Department of Transportation’s 100-plus green alleys designed to reduce stormwater runoff, the alley has some sustainable aspects without official certification from the Forest Stewardship Council.

It took 10 years for the CDOT and the community to undertake the venture, says Michael Lev, vice president of TranSystems, the major consultant for the project. “The traditional thing to do would be to cut everything out and put in a concrete alley, but the community felt strongly that concrete was not the right solution,” Lev says. Instead, CDOT decided to replace the original wood pavers with new ones. The construction team vetted various wood species for the new blocks, first considering used railroad ties but finally deciding on black locust wood due to its durability, density, resistance to insects and rotting, visual appeal and lack of odor.

The base of the alley was another issue. “We thought it was pretty unusual to have poured a concrete base under a wood paver street at that time,” Lev says. Although using the existing base reduced costs and construction waste, he says, it also presented drainage complications. “We actually had to create a permeable paver design, but there was this solid concrete base underneath,” Lev says. “We ended up detailing a pattern of weep holes, which the contractor drilled into the base, to allow water to percolate through the bedding sand and the sand in between the pavers, through some filter fabric, and then through the weep holes,” he says. Obviously, open-graded, highly permeable jointing and bedding aggregates used in today’s permeable interlocking concrete pavements aren’t compatible with wood pavers. While sand isn’t as permeable as open-graded aggregates, the passage of any water through sand joints and bedding certainly contributes a bit to stormwater runoff reduction.

Most importantly, the renovated alley retained its cultural value. The renovation of the century-old alley further confirms Chicago as the birthplace of green alleys before naming them as such. As a testament to the alley’s history and durability, a small portion of its west end remains paved with the original 1909 paver blocks. The project won the 2012 American Public Works Association Project of the Year Historical Preservation category award for projects less than $5 million. A glance back to the past suggests that permeable interlocking concrete pavement assumed a role begun by permeable interlocking wood pavement alleys.