Intensive human activity and global heating are warming the ground beneath downtown Chicago, a thermal shift that is causing tiny underground deformations that can undermine the integrity of buildings, say researchers.
The average ground temperature underneath the city’s central business district, the Chicago Loop, has increased significantly over the past 70 years, finds a study published in the journal Nature Communications Engineering.
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“In the past, an annual average ground warming of 0.49 °C per year has characterized the Loop down to 100 metres of depth,” the study states. Currently, the ground is warming at an annual rate of 0.14 °C per year, as the soil approaches thermal saturation.
“This implies that the worst [underground heating] has already happened,” study author Alessandro Rotta Loria, an architectural engineer at Northwestern University’s Department of Civil and Environmental Engineering, told the Washington Post.
Scientists are calling this subsurface heating “underground climate change,” the Post writes. “This subterranean warming is much more intense than above the surface, especially in densely built cities.”
It owes to the heat produced from basements, subways, sewer systems, and underground electrical cables in the built environment, as well as to the heat absorbed by buildings due to the heat island effect, which is then transferred to the ground.
In fact, “subsurface urban heat islands can partly be considered as the underground thermal imprint of meteorological urban heat islands,” writes Rotta Loria.
Since natural subsurface materials like soil and human-made ones like concrete deform as temperatures change, his team set out to discover what impacts such deformations might have on the infrastructure of a densely-built city, writes the Post.
After three years measuring above- and below-ground temperatures at a number of locations in the Loop, they found that subsurface temperatures beneath buildings were often 10°C warmer than the soil beneath green spaces like Chicago’s iconic Grant Park. And air temperatures in underground structures were up to 25°C warmer than undisturbed ground temperatures.
The team fed this temperature data into a 3D computer simulation to study the trajectory of subsurface temperatures since 1951, and to predict the pattern of heating out to 2051. They also looked at how subsurface heating has affected ground movement.
The model showed that the ground beneath the Loop “could rise as much as 12 millimetres and sink as much as eight millimetres,” tiny shifts that could still do some damage.
“Even these relatively small displacements can lead to problems,” like cracked foundations and subsequent water damage, Rotta Loria said. But he clarified that since the displacements are small, it’s unlikely that they pose an imminent danger to building integrity or public safety.
As for what to do about the “silent hazard” of underground climate change seen in many of the world’s cities, the team pointed out that these islands “can be considered a resource because they provide the opportunity to harness large quantities of waste heat that would otherwise be dispersed in the ground.”
At the same time, it would make sense to minimize such heat loss in the first place, “via adequate retrofit interventions in buildings and infrastructures,” they said.
