Sensors show 'silent hazard' is swelling the ground beneath cities by up to 12mm

But there's good news. We could use this as an opportunity to capture an untapped energy source.
Sade Agard
Geological layers beneath the Chicago Loop
Geological layers beneath the Chicago Loop

Alessandro Rotta Loria/Northwestern University 

For the first time, scientists have quantified the effects of a silent hazard known as 'underground heat islands' beneath major global cities, according to new research published in Nature Communications Engineering on July 11. 

While the study highlights long-term damage to structures, it also sees an opportunity to capture underground waste heat— using geothermal technologies— as an untapped thermal energy resource.

What are underground heat islands?

The deformation resulting from temperature variations affects the normal operations of foundation systems and civil infrastructure, explained lead author Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern's McCormick School of Engineering, in a press release

"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," he highlighted. 

That said, subsurface heat islands are not limited to Chicago but are common to urban areas globally.

To understand the impact of underground climate change on civil infrastructure, Loria and his team deployed a wireless network of over 150 temperature sensors in the Chicago Loop. 

These sensors were placed above and below ground in buildings' basements, subway tunnels, underground parking garages, and streets. 

Data from the sensing network revealed that underground temperatures in the Loop are often 10 degrees warmer than those beneath Grant Park—a greenspace away from urban structures used for comparison. 

Sensors show 'silent hazard' is swelling the ground beneath cities by up to 12mm
Ground temperatures measured throughout the Chicago Loop

The team found that air temperatures in underground structures can be up to 25 degrees higher than the undisturbed ground temperature, exerting significant stress on materials that expand and contract with temperature changes.

Using the temperature data collected over three years, Rotta Loria developed a 3D computer model to simulate the evolution of ground temperatures from 1951 to the present. The model also predicts temperature changes until 2051. 

Ground swelling and sinking 

The simulations demonstrate how ground deformation occurs due to increasing temperatures, with some materials contracting and others expanding. 

Warmer temperatures can cause the ground to swell upward by up to 12 millimeters or sink downward by up to 8 millimeters, which poses challenges for building components and foundation systems.

Loria emphasizes that severe ground deformations caused by underground climate change can affect civil infrastructure performance, even though the impact may not be immediately noticeable. 

Over time, these deformations can lead to cracks and excessive foundation settlements

The study suggests that buildings designed before the emergence of underground climate change are less equipped to handle temperature variations than more modern structures.  

For instance, newer buildings in the United States may fare better; European cities with older buildings made of stone and bricks are more vulnerable to underground heat islands as they adhere to past design and construction practices sensitive to thermal perturbations.

To address this issue, the team suggests that future planning strategies incorporate geothermal technologies to harness waste heat for building heating. Additionally, thermal insulation can be installed on new and existing structures to minimize the amount of heat entering the ground. 

"What we don’t want is to use technologies to actively cool underground structures because that uses energy," he concluded. 

The complete study was published in Nature Communications Engineering on July 11 and can be found here.

Study abstract:

Urban areas increasingly suffer from subsurface heat islands: an underground climate change responsible for environmental, public health, and transportation issues. Soils, rocks, and construction materials deform under the influence of temperature variations, and excessive deformations can affect the performance of civil infrastructure. Here I explore if ground deformations caused by subsurface heat islands might affect civil infrastructure. The Chicago Loop district is used as a case study. A 3-D computer model informed by data collected via a network of temperature sensors is used to characterize the ground temperature variations, deformations, and displacements caused by underground climate change. These deformations and displacements are significant and, on a case-by-case basis, may be incompatible with the operational requirements of civil structures. Therefore, the impact of underground climate change on civil infrastructure should be considered in future urban planning strategies to avoid possible structural damage and malfunction. Overall, this work suggests that underground climate change can represent a silent hazard for civil infrastructure in the Chicago Loop and other urban areas worldwide, but also an opportunity to reutilize or minimize waste heat in the ground.

Add Interesting Engineering to your Google News feed.
Add Interesting Engineering to your Google News feed.
message circleSHOW COMMENT (1)chevron
Job Board