Underground climate change threatens urban infrastructure

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A new Northwestern University study has, for the first time, linked underground climate change to the shifting ground beneath urban areas. As the ground heats up, it also deforms. This phenomenon causes building foundations and the surrounding ground to move excessively (because of expansions and contractions) and even crack, which ultimately affects structures’ long-term operational performance and durability. Additionally, researchers claim that such rising temperatures may have contributed to past building damage, and they anticipate that these problems will persist for years to come.

Although rising temperatures do pose a threat to our infrastructure, the researchers also view them as a potential opportunity. By capturing the waste heat emitted underground from subterranean transportation systems, parking garages, and basement facilities, urban planners could mitigate the effects of underground climate change as well as reuse the heat as an untapped thermal energy resource.

The study was published in Communications Engineering. It marks the first study to quantify ground deformations caused by subsurface heat islands and their effect on civil infrastructure.

"Underground climate change is a silent hazard," said Northwestern’s 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 necessarily dangerous for people’s safety, it will affect the normal day-to-day operations of foundation systems and civil infrastructure at large.

"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. In other words, you don’t need to live in Venice to live in a city that is sinking – even if the causes for such phenomena are completely different." 

What is underground climate change?

In many urban areas around the globe, heat continuously diffuses from buildings and underground transportation, causing the ground to warm at an alarming rate. Previous researchers have found that the shallow subsurface beneath cities warms by 0.1 to 2.5 degrees Celsius per decade.

Known as ‘underground climate change’ or ‘subsurface heat islands,’ this phenomenon has been known to cause ecological issues (such as contaminated ground water) and health issues (including asthma and heatstroke). But, until now, the effect of underground climate change on civil infrastructure has remained unstudied and poorly understood.

"If you think about basements, parking garages, tunnels, and trains, all of these facilities continuously emit heat," Rotta Loria said. "In general, cities are warmer than rural areas because construction materials periodically trap heat derived from human activity and solar radiation and then release it into the atmosphere. That process has been studied for decades. Now, we are looking at its subsurface counterpart, which is mostly driven by anthropogenic activity."

In recent years, Rotta Loria and his team have installed a wireless network of more than 150 temperature sensors across the Chicago Loop, both above and below ground. This included placing sensors in the basements of buildings, subway tunnels, underground parking garages, and subsurface streets like Lower Wacker Drive. For comparison, the team also buried sensors in Grant Park, a greenspace located along Lake Michigan, away from buildings and underground transportation systems.

Data from the wireless sensing network indicated that underground temperatures beneath the Loop are often 10 degrees Celsius warmer than those beneath Grant Park. Air temperatures in underground structures can be up to 25 degrees Celsius higher compared to the undisturbed ground temperature. When the heat diffuses towards the ground, it puts significant stress on materials that expand and contract with changing temperatures.

"We used Chicago as a living laboratory, but underground climate change is common to nearly all dense urban areas worldwide," Rotta Loria said. "And all urban areas suffering from underground climate change are prone to having problems with infrastructure."