Downtown Chicago’s building foundations are sinking.
That revelation was one of many discovered by researchers at Northwestern’s Subsurface Opportunities and Innovations Laboratory in a study published last year. The sinking comes from excess heat from basements and tunnels that deform the ground supporting the Loop’s towering skyscrapers,
Since its creation in 2019, the lab has made breakthroughs in understanding the changing underground. The group’s emphasis on innovation sets it apart from a traditional laboratory, said SOIL Director and McCormick Prof. Alessandro Rotta Loria.
“We are one of the very few groups who work on subsurface heat islands worldwide,” he said. “We have these pieces of equipment which are custom-made. They’re unique, so we can also create unique experiments ourselves.”
The lab, buried in a remote corner of NU’s Technological Institute, is an engineer’s playground.
Two giant testing devices for thermal and electrical experiments sit on each side of the room. Countless bottles, beakers and buckets occupy the shelves next to more testing gadgets.
At the far side of the room, a complex network of wires and tubes poke through four tubs of water, each attached to a sensor. Third-year postdoctoral researcher Nishu Devi uses this network to make sand-like particles, known as aggregates.
“These will act as sand particles to use in concrete mix,” Devi said. “In practical application, we will be using CO2 from the industry and sequestering carbon in the form of the aggregates.”
This process, Rotta Loria says, helps counter climate change by taking carbon dioxide out of the atmosphere. Aggregates can also prevent coastal erosion from rising sea levels, he added.
In the field, researchers have turned Chicago’s Loop into what Rotta Loria calls a “living lab,” deploying over 150 temperature sensors in commuter tunnels and parking garages across the district.
They then use the data to compare what underground temperatures in Chicago would have been in 1951 to what they predicted in 2051.
The results, published in a 2023 study, revealed a subsurface urban heat island caused by civil infrastructure underneath Chicago’s streets.
In one map of underground temperatures in the Loop, the parking garage under Millennium Park is about 18 degrees Fahrenheit warmer than the soil surrounding it.
“We have unveiled that the ground beneath cities is deforming due to these temperature variations, caused by the heat that is projected in the ground by building basements, parking garages, tunnels, all the civil infrastructure you can think of,” Rotta Loria said. “These deformations can be large enough that they can affect the performance of civil infrastructure.”
The lab’s study found that these deformations are shifting Chicago’s foundations by millimeters. Foundations could crack, be exposed to excessive sediment and have other unwanted problems, according to Rotta Loria.
This phenomenon has been going on for decades without notice because it was “out of sight and out of mind,” he explained, and it likely applies to all cities worldwide.
Rotta Loria estimates that on average, heat islands underneath urban areas are warming by 2.5 degrees Celsius per decade globally.
Though he initially wanted to become an architect, Rotta Loria said he became fascinated with geological materials while studying engineering in Italy.
“Rather than focus on the buildings above, I focus on what supports them below,” he said.
When he arrived at NU in 2019, he said the lab sprouted in an empty room. After painting the walls, he designed and ordered his own equipment before the COVID-19 pandemic delayed everything.
It was a “tricky period,” he said, “but exciting.”
Most recently, the lab also built a machine learning algorithm capable of accurately simulating subsurface temperature changes across the entire Loop on data from just 0.9% of the area, allowing researchers to run simulations over 100 times faster.
The project’s leader, third-year Ph.D. student Zhonghao Chu, said computer modeling is the future of civil engineering. Building the algorithm was an interdisciplinary project combining computer science and engineering, Chu said.
Chu added that one of SOIL’s goals is to let more people know about underground climate change because it affects everyone’s daily life.
“It will influence everyone, so everyone should know something about this, and everyone should take action,” he said. “This is what I call making a positive social contribution.”
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