The earth beneath our major cities is warming and changing in such a way that buildings, bridges and transport systems can be damaged.
Just ask a sweltering passenger on the London Underground or The New York City Subwayand they’ll sweat when they tell you how underground transportation systems spew out heat.
As that heat spreads into the ground, ground temperatures rise, which, according to new research, has shifted the soil beneath a U.S. city slightly — but more than most buildings can withstand.
Using the Chicago Loop district as a case study and three years’ worth of data from a network of wireless temperature sensors, civil engineer Alessandro Rotta Loria of Northwestern University in Illinois built a 3D computer model to simulate how rising temperatures have affected the subsurface environment.
His simulations span a century, from 1951 (the year Chicago completed its subway tunnels) to 2051, revealing “a quiet but potentially problematic impact of subsurface urban heat islands on the performance of civil structures and infrastructures,” Rotta Loria writes in his peer-reviewed paper.
Only recently have we learned how New York City could sink under the weight of its skyscrapers. Add heat to the mix and the soil beneath cities can slowly shift, settle and sag as the soil dries out and compacts.
Aside from crowded subway tunnels, that heat comes from underground pipelines and power lines that traverse our cities; the ground is littered with building foundations and parking garages that also leak heat.
While all built environments absorb heat from the sun, fine-grained clay sediments such as those under Chicago are particularly susceptible to shrinking or swelling from heat and water.
Buildings are unlikely to collapse due to slow-moving, heat-related deformations, Rotta Loria say. But subtle changes in the subsurface of just a few millimeters can stress or mobilize foundations and affect the durability or performance of building materials over time.
“The ground [in Chicago] deforms due to temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations,” explains Rotta Loria who found that ground temperatures in Chicago are currently warming at about 0.14°C per year.
“Although this phenomenon is not necessarily dangerous for people’s safety, it will affect the normal day-to-day operation of foundation systems and civil infrastructure in general.”
Scientists have known about subsurface climate change (or subsurface heat islands) for several decades, where soil and groundwater hotspots under cities such as Amsterdam, Istanbul, NankingAnd Berlin.
In Rotta Loria’s study, he found greater ground temperature variations in the northern, more built-up portion of Chicago’s Loop district compared to the thinner, southern end.
On average across the district, temperatures within different soil layers varied by about 1-5 °C (1.8-9 °F). Depending on the soil type, warmer temperatures caused displacements of 8-12 millimeters under different buildings, the modeling showed.
While a few millimeters may sound small and buildings are designed to tolerate some flexing, older buildings and other infrastructure are not built to withstand today’s temperature swings, says Rotta Loria.
“It’s very likely that subsurface climate change has already caused cracks and excessive foundation settlement that we didn’t associate with this phenomenon because we weren’t aware of it,” he said. say.
While cutting emissions to lower global temperatures would certainly ease the pressure, some cities are experimenting with this using residual heat from transport systems such as the Paris Metro to the heating of apartment buildings and hot water systems.
Called heat recycling, scientists say it’s a feasible idea that may become increasingly necessary as the world warms and our cities get bigger.
“Ongoing subsurface climate change must be mitigated to prevent undesired impacts on civil structures and infrastructures in the future,” Rotta Loria concludes in the newspaper.
The study is published in Communication technology.