The Ground Is Deforming, and Buildings Aren’t Ready

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 little 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.” 

Chicago as a Living Laboratory
In recent years, Rotta Loria and his team 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 temperatures 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 toward 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 have problems with infrastructure.”

‘Things Are Sinking Very Slowly’
After collecting temperature data for three years, Rotta Loria built a 3D 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. 

Rotta Loria also modeled how ground deforms in response to increasing temperatures. Whereas some materials (soft and stiff clay) contract when heated, other materials (hard clay, sand and limestone) expand.

According to the simulations, warmer temperatures can cause the 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. 

“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,” Rotta Loria said. “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.”

Amanda Morris is Senior Science and Engineering Editor at Northwestern University. The article was originally posted on the website of Northwestern University.