Heat is a silent hazard sitting beneath cities, threatening to shift infrastructure. A study published this week in Communications Engineering, outlines how heat could be changing major cities, but urban areas throughout the U.S. may not be prepared.
Study researchers set up more than 150 sensors above and below ground throughout the Chicago Loop. They gathered data from 2019 into the end of 2022. These sensors were placed in subway tunnels, subsurface streets, building basements, and underground parking garages. Researchers also buried several sensors in Grant Park near Lake Michigan.
The sensors showed that temperatures under the Loop in Chicago could be up to 10 degrees Celsius hotter than temperatures underneath the park. Data from the sensors also showed that air temperatures in underground structures can rise up to 25 degrees Celsius higher than undisturbed ground temperatures. The heat can stress infrastructure, leading to warping and, over time, it can also cause cracks.
“Urban areas increasingly suffer from subsurface heat islands: an underground climate change responsible for environmental, public health, and transportation issues,” the study authors wrote. “Soils, rocks, and construction materials deform under the influence of temperature variations and excessive deformations can affect the performance of civil infrastructure.”
Researchers also created a 3D model to simulate how underground temperatures changed from 1951, which is when the subway tunnels were completed, to present day. They used that to model how the ground deformed over time with the changing temperatures. Soft materials under cities, like limestone and clay, contract when they become heated. Other materials like sand, limestone, or hard clay will expand with the heat.
“This underground climate change might have contributed to at least some of the operational issues that have affected buildings,” Rotta Loria told Earther. “Only time and further research will really allow us to understand [what] are the most sensitive foundation systems.”
The ground beneath Chicago is filled with clay, so the city’s ground has contracted over time. Researchers suggested that some of the foundational shifting has already happened, but has not been attributed to underground heat islands because officials have been unaware. “I really want to stress that this is on a case by case basis, so it cannot be generalized,” Rotta Loria told Earther.
Rotta Loria also emphasized that climate change does play a role. Some of the heat found underground is connected to the heat found above ground in Chicago. The study called this phenomenon “underground climate change.” Like many urban centers, Chicago also has neighborhoods that struggle with the heat urban island effect, which describes how some parts of a city absorb more heat. Lower income and lower resourced communities are often in or near these hotter areas. The study suggested gathering more data on underground climate change, because it is less studied and understood than the heat urban island effect.
“The impact of underground climate change on civil infrastructure should be considered in future urban planning strategies to avoid possible structural damage and malfunction,” study authors wrote.
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