Chicago Is Literally Sinking Because of an Insidious Underground Threat
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Cities around the world are sinking for a variety of reasons, but a new threat lies beneath our feet.
Scientists from Northwestern University discovered that increased temperatures underground are making Chicago’s clay soil contract, which is causing the Windy City to slowly sink.
Installing heat insulators, as well as other geothermal technologies, could help mitigate the effects of “underground climate change.”
The world’s cities are in trouble. Because buildings, roads, and other bits of infrastructure absorb and re-emit heat more than the natural world, metropolises around the globe are becoming sweltering heat islands where daytime temperatures can climb to as much 7 degrees Fahrenheit higher than nearby suburbs and exurbs. But this is only half of the heat equation, because some cities are also boiling below the surface.
In a study published last week in the journal of Communications Engineering, Northwestern assistant professor of Civil and Environmental Engineering and lead author Alessandro Rotta Loira argues that heat escaping from basements, parking garages, sewers, tunnels, and other underground infrastructure is shifting the landscape and causing Chicago to slowly—but definitively—sink. Referring to it as a “silent hazard,” Rotta Loria calls this man-made phenomenon “underground climate change.”
“The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations,” Rotta Loira said in a press statement last week. “Although this phenomenon is not dangerous for people’s safety necessarily, it will affect the normal day-to-day operations of foundation systems and civil infrastructure at large.”
Much of Chicago was built on a glacial lakebed, which means very clayey soils. This type of earth—along with other fine-grained soils—can swell when heated, causing buildings to prematurely settle at faster rates than normal. However, hard clay soils and sand will contract when heated, causing even more instability.
To discern if Chicago’s underground is in fact warming, and to what degree, Rotta Loira placed more than 150 sensors around the city—on commuter rail platforms, high-rise service entrances, Lower Wacker Drive, and the famous (or infamous, depending on your point of view) downtown tunnel. After gathering three years of readings with 20,000 records per day, according to The New York Times, Rotta Loira combined the data along with models of the city’s basements and tunnels to discern how underground temperatures changed and are continuing to change. The data showed changes from 1951 (the year Chicago finished its subway tunnels) to present, and predicted changes from present to 2051. Near some heat sources, underground temperatures have risen some 27 degrees Fahrenheit since 1951.
The study also discovered that the ground could swell some 12 millimeters, while structures could sink 8 millimeters. Although imperceptible to humans, those miniscule distances can be devastating for buildings that aren’t designed for that kind of movement.
Thankfully, one piece of good news is that U.S. buildings are relatively new compared to centuries-old European structures, and thus more adept and handling these stresses. But these simulations don’t take into account increased surface temperature due to climate change or increased A/C use, which will likely pump more heat in the ground. In other words, this is the best case scenario.
Like many of the human-induced causes of climate change, there are potential ways to engineer ourselves out of this mess. Geothermal technologies, for example, can actually take waste heat destined for the ground and use it for a building’s own heating. Engineers can also install thermal insulation to limit the amount of heat escaping into the ground.
Yes, the world’s cities are sinking—but that doesn’t mean we have to let them drown.
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