Beneath Chicago, heat takes toll
Land is sinking amid warming
CHICAGO — Underneath downtown Chicago’s soaring art deco towers, its multilevel roadways, and its busy subway and rail lines, the land is sinking, and not only for the reasons you might expect.
Since the mid-20th century, the ground between the city surface and the bedrock has warmed by 5.6 degrees Fahrenheit on average, according to a new study out of Northwestern University. All that heat, which comes mostly from basements and other underground structures, has caused the layers of sand, clay, and rock beneath some buildings to swell by several millimeters over the decades, enough to worsen cracks and defects in walls and foundations.
“All around you, you have heat sources,” said the study’s author, Alessandro F. Rotta Loria, walking with a backpack through Millennium Station, a commuter rail terminal underneath the city’s Loop district. “These are things that people don’t see, so it’s like they don’t exist.”
It isn’t just Chicago. In big cities worldwide, humans’ burning of fossil fuels is raising the mercury at the surface. But heat is also pouring out of basements, parking garages, train tunnels, pipes, sewers, and electrical cables and into the surrounding earth, a phenomenon that scientists have taken to calling “underground climate change.”
Rising underground temperatures lead to warmer subway tunnels, which can cause overheated tracks and steam-bath conditions for commuters. And, over time, they cause tiny shifts in the ground beneath buildings, which can induce structural strain, whose effects aren’t noticeable for a long time until suddenly they are.
“Today, you’re not seeing that problem,” said Asal Bi-darmaghz, a senior lecturer in geotechnical engineering at the University of New South Wales in Australia. “But in the next 100 years, there is a problem. And if we just sit for the next 100 years and wait 100 years to solve it, then that would be a massive problem.”
Bidarmaghz has studied subterranean heat in London but wasn’t involved in the research in Chicago.
Near some heat sources, the ground beneath Chicagoans’ feet has warmed by 27 degrees over the past seven decades, Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern, found. This has caused the earthen layers to expand or contract by as much as half an inch under some buildings.
The warming and ground deformation are now happening more slowly than in the 20th century, he found, simply because the earth is closer to being just as warm as the basements and tunnels buried within it. More and more, those structures will stay warm rather than dissipating heat into the ground around them.
Rotta Loria’s findings were published Tuesday in the journal Communications Engineering.
The most effective way for building owners and tunnel operators to address the issue, he said, would be to improve insulation so less heat leaks into the earth. They could also put the heat to work.