Underground climate change could crack foundations and warp subway tracks

The urban heat island effect goes deeper than the sidewalk: Pockets of heat in cities exist below ground as well, in what some scientists refer to as underground climate change.

Yet scientists are just beginning to understand how these subsurface heat islands alter the ground itself and the surrounding structures. A new study looking at the phenomenon in Chicago's business district, or the Loop, suggests temperature fluctuations could cause the ground to expand and contract, threatening the stability of buildings and urban infrastructure.  If excessive, this "can lead to issues such as cracking and unwanted deflections," said Alessandro Rotta Loria, a civil and environmental engineering researcher at Northwestern University.

Findings from his study, published Tuesday in the journal Communications Engineering, do not suggest that the safety and integrity of structures will be compromised. But cities and businesses could contend with a buildup of operational and maintenance issues over time, including the deformation of underground railway tracks or leaky walls from water seeping in through cracks in building foundations.

"Underground climate change represents a silent hazard and a potential problem for civil infrastructure that should be assessed on a case-by-case basis," he said.

The phenomenon has a few causes. Asphalt, steel and bricks are sponges for sunlight, causing warmer temperatures in cities, in what is known as the urban heat island effect. (This is also compounded by climate change.) Hot air tends to sink to cooler areas below the surface, warming the earth beneath our feet. The largest driver are the buildings, transit networks and other elements of the built environment that continuously pump heat into the surrounding ground.

Research suggests temperatures in the shallow ground below various cities across the globe rise at an average rate of between 0.1 and 2.5 degrees Celsius (up to 3.6 degrees Fahrenheit) per year, depending on factors like density and layout of the built environment. Subway commuters have felt the effects: When summer is in full swing, subterranean stations often provide little respite from the oppressive heat outside, causing not only a public health hazard but also service disruptions from speed restrictions and outages.

To study the impacts on the ground itself, in 2019 Rotta Loria and his students installed a network of 150 sensors throughout the Chicago Loop to monitor temperatures at and below the surface, and within the layers of clay that support the city. Temperatures inside underground structures such as building basements, parking garages and train tunnels reached as high as 36C (96.8F) between 2020 and 2022, according to their data.

The geological layers beneath the Chicago Loop, shown here, have absorbed heat from above and below the surface. Photographer: Alessandro Rotta Loria/Northwestern University

In the clay itself, temperatures near those types of structures measured roughly 21.5C (70F) — about 10 degrees hotter than areas not heavily affected by the diffusion of heat, such as underneath parks. Some of the warmest areas were near basements, said Rotta Loria.

The team then used that data to create a digital twin of the Loop, including underground structures like the Millennium Garages — the largest underground parking system in North America, comprising four facilities that span 3.8 million square feet — to simulate ground temperature rise and its impact on ground deformation from the year 1951, when the area's subway tunnels were completed, to 2051.

According to the model, ground temperatures increased at an estimated average rate of 0.49C (less than 1F) for the first 50 years of that period.  They are currently warming, and are projected to continue to warm, at an annual average rate of 0.14C (about 0.25F). The decline in the rate of warming seems to be linked to the science of thermal conductivity — the ground can absorb only so much heat before temperatures begin to level off. Yet these fluctuations have caused soil layers in the ground to settle and heave, sometimes by more than 10 millimeters.

As part of his study of underground heat islands, Rotta Loria measured ground temperatures in the layers of clay beneath the Chicago Loop. Photographer: Alessandro Rotta Loria/Northwestern University.

That figure may not seem like much, but "construction materials, soil, and rocks are very sensitive to temperature variations," said Rotta Loria. "And in civil engineering, displacements on the order of inches or tens of millimeters can be problematic for the performance of city infrastructure." 

In an extreme case of how ground movement can damage surface structures, consider the Auditorium Building, one of many edifices built atop Chicago's swampy soil in the late 19th century. Despite efforts by its engineers, the theater venue sunk more than 18 inches in its first year. Different parts of the buildings settled to different levels, "leaving it with uneven floors that can make visitors feel drunk as they navigate," as one magazine described it. Cracks also formed along its concrete floors.

The story of the Auditorium Building isn't linked to underground climate change, but the study makes clear that warming the ground can cause deformations. Rotta Loria said even small cracks in building foundations and retaining structures can affect their function. Older cities are more likely to be affected, he added, as they tend to have older buildings with poorer insulation and more outdated materials and construction. 

Rotta Loria plans to continue exploring the issue, looking at which kinds of infrastructure are particularly susceptible to rising ground temperatures. Eventually he hopes to create a tool that other cities can use to monitor their own underground heat island effects. 

The good news, he said, is that there are ways to mitigate the effect, including better insulation for underground structures. Another way is to harness and recycle the heat waste into geothermal energy to power the very structures pumping heat out.   

"It's a big opportunity from the perspective of urban planning, especially right now with all the attention on energy efficiency in buildings and renewable energy harvesting," he said. "We can turn a potential problem into an opportunity."

Disclaimer: This article first appeared on Bloomberg, and is published by special syndication arrangement.