Chicago's downtown buildings are slowly sinking. The culprit? Underground climate change

Two levels below ground at the Grant Park North Garage, the summer heat feels oppressive. As downtown commuters park and exit their cars, sweat quickly beads on their furrowed brows and foreheads.

Alessandro Rotta Loria zoomed in on a map on his phone, trying to locate a temperature sensor. Finally, he found the white, pocket-size instrument, inconspicuously zip-tied to a pipe next to a red column.

Using a wireless network of more than 150 such sensors above and below ground across the Loop, Rotta Loria, a Northwestern University assistant professor of civil and environmental engineering, discovered ground deformations in Chicago are causing buildings to sink and crack.

"Things are sinking very slowly," Rotta Loria said. "The good news is that people don't die."

Buildings are unlikely to crumble, he said, but damage from what he calls a "silent hazard" could lead to costly upkeep and retrofitting bills. And at the end of the day, he noted, sensible people tend to be uncomfortable with cracks in walls.

These deformations are occurring because of a phenomenon called underground climate change. Buildings, their garages and basements, as well as transportation systems like trains and tunnels, are constantly diffusing heat into the ground. The more of these are packed into a given area, the higher the temperatures will be below the surface, causing the ground to contract and expand, Rotta Loria explained.

"This exploratory study is, to the best of my knowledge, the first of its kind. So a lot needs to be done to know more," said Rotta Loria, whose study was published last month in the journal Communications Engineering. "This phenomenon is present in most, if not all, urban areas worldwide. It's important to understand which types of civil structures and infrastructures are particularly sensitive to underground climate change and can be prone to operational issues."

Subterranean temperatures in Chicago's downtown are probably not as high as in more densely urbanized areas with bigger underground transportation systems, such as London and New York City.

But the Millennium Parking Garage complex, touted as the largest downtown underground parking system in North America, presented a unique opportunity for research in Chicago's busiest and densest infrastructural area.

Part of Rotta Loria's research involved building a 3-D computer model to simulate the evolution of ground temperatures since 1951 — when Chicago completed its subway tunnels — and predict it until 2051. These simulations demonstrated that higher temperatures can cause soil like limestone and clay to either swell and expand upward up to 0.47 inches or to contract and sink downward up to 0.31 inches.

By comparing data collected over three years from the sensors near underground infrastructure with data from sensors buried in green spaces like Grant Park, the researchers also found that underground temperatures beneath the Loop are often 18 degrees Fahrenheit higher than temperatures below Grant Park.

As part of his job at an insurance firm, Chris Semlies keeps up to date on climate-related risks that affect property and construction projects, something he notes is intensifying. Semlies said he doesn't see an enhanced risk for underground structures such as parking garages.