The heat shakes the ground beneath Chicago

(Chicago) Beneath Chicago’s downtown Art Deco skyscrapers, its busy freeways and its crowded subways, the ground is sinking. And it’s not just because of the crushing weight of all that steel and concrete.

Since the middle of the XX^e century, in Chicago, the ground between the surface and the bedrock has warmed by 3°C, on average, according to a recent study by Northwestern University. This heat, which comes from basements and other underground structures, has caused the layers of sand, clay and rock beneath some buildings to sag or swell by several millimeters over the decades, worsening cracks and cracks. other damage to walls and foundations.

“All around, there are heat sources,” points out study author Alessandro F. Rotta Loria at Chicago’s Millenium Station, a commuter rail terminal below the Loop neighborhood. “People don’t see them, so it’s like they don’t exist. »

“Underground climate change”

It’s not just happening in Chicago. The use of fossil fuels all over the world is warming the atmosphere. But underneath every major city on the planet, basements, parking lots, train tunnels, drains, sewers, and power cables also give off heat, which radiates out into the surrounding earth. Some scientists speak of “underground climate change”.

As the ground is warmer, the temperature of the metro tunnels increases: sometimes the rails overheat and users have the impression of being in a sauna. Over time, tiny ground shifts occur under buildings. The structural tensions thus caused remain imperceptible for a very long time.

“Today, you can’t see anything,” says Asal Bidarmaghz, professor of geotechnical engineering at the University of New South Wales, Australia. She studied underground heat in London, but was not involved in research in Chicago.

To measure underground climate change in Chicago, Dr. Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern University in Chicago, installed more than 150 thermometers at ground level and in the ground in the Loop neighborhood. After three years, he put his data into a computer model to simulate ground warming at different depths between 1951 and today, and how it will manifest itself by 2051.

The floor 15°C warmer

Near certain heat sources, Chicago’s subsoil has warmed by 15°C over the past 70 years. Under some buildings, the earth layers have expanded or contracted by about 1.2 cm.

Today, the ground is warming and deforming more slowly than in the XX^e century, he says, because there is less temperature difference between the surrounding earth and the basements and tunnels buried there. Increasingly, these structures will stay warm instead of dissipating heat into the ground around them.

Dr. Rotta Loria’s study was published last Tuesday in the journal Communications Engineering.

For building owners and tunnel operators, the best way to tackle the problem is to improve insulation to reduce heat leakage.

They could also recover it, says Mr. Rotta Loria, who is also technical director of Enerdrape. This Swiss start-up manufactures panels that absorb ambient heat from tunnels and parking lots and use it to heat the building above, which helps reduce electricity bills. Enerdrape installed 200 of its panels in the parking lot of a Lausanne supermarket as part of a pilot project.

Global warming

In his predictions of underground Chicago warming, Mr. Rotta Loria deliberately omitted one factor: global warming at the surface.

The hot weather is warming the upper layers of the ground, but Mr. Rotta Loria made his calculations as if Chicago’s temperature would remain at current averages until 2051. In other words, his estimates do not take into account future global warming. .

Mr. Rotta Loria justifies this omission by his desire to present conservative predictions of underground warming. No need to present the worst-case scenario, he said.

Chicago Mayor Brandon Johnson’s office did not respond to interview requests from the New York Times.

Recently, Mr. Rotta Loria showed the author of these lines his network of thermometers, which maps heat loss under the city.

He explains that the Chicago Transit Authority has not allowed him to install sensors in subway stations, fearing people will mistake them for bomb detonators. But he and his team were able to install sensors in many well-known and lesser-known places: on commuter train platforms and at service entrances behind skyscrapers in leafy Millennium Park.

Another collector, at the Blackstone Hotel, is in a room in the basement where folding chairs are stacked and bags of salt used in front of the entrance are stored to melt the ice during the winter. There is also one in the boiler room of the Union League Club in Chicago, where Mr. Rotta Loria noted temperatures reaching 35°C. A sensor in the Grant Park South parking lot recorded a temperature of 36°C in September 2021.

And all that heat spills out into the ground.

This article was first published in the New York Times.