As the ground heats up, it also deforms. This phenomenon causes building foundations and the surrounding ground to move excessively (due to expansions and contractions) and even crack, which ultimately affects structures’ long-term operational performance and durability. The authors of the study also report that past building damage may have been caused by such rising temperatures and expect these issues to continue for years to come.
“Underground climate change is a silent hazard,” said Alessandro Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering who led the study. “The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations. 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.
By installing a wireless network of more than 150 temperature sensors in the basements of buildings, subway tunnels, underground parking garages and subsurface streets, the researchers were able to create a three-dimensional map of Chicago's underground.
“We used Chicago as a living laboratory, but underground climate change is common to nearly all dense urban areas worldwide,” Loria explains. “And all urban areas suffering from underground climate change are prone to have problems with infrastructure.”
The city of Chicago is the third-most populous city in the United States covering more than 600 square-kilometers. Located on the shore of Lake Michigan, the city stands on a limestone plateau buried beneath medium- to fine-grained sediments (sand and clay) deposited after the last ice-age. Clay sediments can pose risks to buildings as they are prone to deformations.
“Chicago clay can contract when heated, like many other fine-grained soils. As a result of temperature increases underground, many foundations downtown are undergoing unwanted settlement, slowly but continuously.”
A metropolitan area is a lot warmer than the rural areas surrounding it. The relatively barren landscape of a city, lack of shadow and large concrete and glass surfaces tend to absorb solar radiation, heating up the environment. A secondary source is waste heat generated by running machines, automobiles and the city's infrastructure.
“If you think about basements, parking garages, tunnels and trains, all of these facilities continuously emit heat,” so Loria.
Most temperature sensors were installed in Chicago's densely developed downtown district, simply known as the Loop. For comparison, the team also buried sensors in Grant Park, a greenspace located along Lake Michigan — away from buildings and underground transportation systems.
The data shows that underground soil temperatures beneath the Loop are often 10 degrees Celsius warmer than temperatures beneath Grant Park. Air temperatures in underground structures can be up to 25 degrees higher compared to undisturbed ground temperatures.
After collecting temperature data for three years, Loria built a computer model to simulate how ground temperatures evolved from 1951 (the year Chicago completed its subway tunnels) to today. He found values consistent to those measured in the field and used the simulation to predict how temperatures will evolve until the year 2051. As temperatures are rising on Earth's surface, so do they beneath, as more energy is conducted underground.
Loria modeled how ground deforms in response to increasing temperatures. Whereas some materials (like soft and stiff clay) contract when heated, other materials (like hard clay, sand and limestone) expand.
According to the simulations, warmer temperatures can cause the first 23 meters soil and rocks beneath ground to swell and expand upward by as much as 12 millimeters. They also can cause the ground to contract and sink downward — beneath the weight of a building — by as much as 8 millimeters. Although this seems subtle and is imperceptible to humans, the variation is more than many building components and foundation systems can handle without compromising their operational requirements. Older buildings made of stiff materials like stone and bricks are especially vulnerable to underground deformations.
“Based on our computer simulations, we have shown that ground deformations can be so severe that they lead to problems for the performance of civil infrastructure. It’s not like a building will suddenly collapse. Things are sinking very slowly. The consequences for serviceability of structures and infrastructures can be very bad, but it takes a long time to see them. It’s very likely that underground climate change has already caused cracks and excessive foundation settlements that we didn’t associate with this phenomenon because we weren’t aware of it.”
To prevent further damage, urban planners will need to minimize the amount of heat that enters the ground by planning ahead.
“The most effective and rational approach is to isolate underground structures in a way that the amount of wasted heat is minimal. If this cannot be done, then geothermal technologies offer the opportunity to efficiently absorb and reuse heat in buildings. What we don’t want is to use technologies to actively cool underground structures because that uses energy. Currently, there are a myriad of solutions that can be implemented,” Loria concludes.
The study "The silent impact of underground climate change on civil infrastructure" was published in the journal Communications Engineering (2023). Additional material and interviews provided by Northwestern University.
This hardhatNEWS article was first published on Forbes 
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