The team at Northwestern University was reportedly the first to measure the effects of subsurface climate change on civil infrastructure, which they call a "silent hazard".
As the ground heats up, it also deforms. This phenomenon causes building foundations and the surrounding ground to expand and contract, which could lead to cracks and undermine the longevity of urban structures.
The researchers also reported that past building damage may have been caused by such rising temperatures and expect these issues to continue for years to come.
"Underground climate change is a silent hazard," said Alessandro Rotta Loria, who led the study. "The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations. Although this phenomenon is not dangerous for people's safety necessarily, it will affect the normal day-to-day operations of foundation systems and civil infrastructure at large.
"Chicago clay can contract when heated, like many other fine-grained soils. As a result of temperature increases underground, many foundations downtown are undergoing unwanted settlement, slowly but continuously.
"In other words, you don't need to live in Venice to live in a city that is sinking – even if the causes for such phenomena are completely different."
Geological layers beneath the Chicago Loop/ Alessandro Rotta Loria/Northwestern University
Image credit: Alessandro Rotta Loria/Northwestern University
In many urban areas around the globe, heat continuously diffuses from buildings and underground transport systems, causing the ground to warm at an alarming rate.
Known as "underground climate change" or "subsurface heat islands," this phenomenon has been known to cause ecological issues such as contaminated groundwater and health issues including asthma and heatstroke. But, until now, the effect of underground climate change on civil infrastructure has been poorly understood.
In order to address this, Rotta Loria and his team installed a wireless network of more than 150 temperature sensors across the Chicago Loop district – 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 green space located along Lake Michigan, away from buildings and underground transportation systems.
The data from the sensors showed that underground temperatures beneath the Loop are often 10 degrees warmer than temperatures beneath Grant Park.
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. The team then used the data to simulate how temperatures will evolve until the year 2051.
The results of the simulation showed that warmer temperatures can cause the ground to swell and expand upward by as much as 12 millimetres, or to sink – beneath the weight of a building – by as much as 8 millimetres.
Although small, this 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."
However, this heat could also be leveraged for good, the researchers said. Going forward, Rotta Loria said future planning strategies should integrate geothermal technologies to harvest waste heat and deliver it to buildings for space heating.
"The most effective and rational approach is to isolate underground structures in a way that the amount of wasted heat is minimal," Rotta Loria said. "If this cannot be done, then geothermal technologies offer the opportunity to efficiently absorb and reuse heat in buildings. What we don't want is to use technologies to actively cool underground structures because that uses energy. Currently, there are a myriad of solutions that can be implemented."
The study has been published in the journal Communications Engineering.
