The land is deformed, and the buildings are not ready

There are “silent dangers” lurking beneath our global megacities, and our buildings are not designed to handle them.

A new Northwestern University study has, for the first time, linked subterranean climate change to shifting soil beneath urban areas. As the ground heats up, it also changes shape. This phenomenon causes the building foundation and surrounding soil to move excessively (due to expansion and contraction) and even crack, which ultimately affects the long-term operational performance and durability of the structure. The researchers also reported that past damage to buildings may have been caused by rising temperatures and it is expected that this problem will continue in the coming years.

Even though rising temperatures pose a threat to our infrastructure, researchers also see it as a potential opportunity. By capturing waste heat emitted underground from underground transportation systems, parking garages and basement facilities, city planners can mitigate the effects of underground climate change as well as repurpose heat into untapped sources of thermal energy.

The study will be published on July 11, at Communication Engineering, journal Nature Portfolio. This marks the first study to measure ground deformation caused by subsurface heat islands and their effect on civil infrastructure.

“Subterranean climate change is a silent danger,” said Northwestern’s Alessandro Rotta Loria, who led the study. “The soil deforms as a result of temperature variations, and no civil structure or infrastructure is designed to withstand these variations. Although this phenomenon is not dangerous to human safety, it will affect the normal daily operation of the foundation system and civil infrastructure in general.

“Chicago clay shrinks when heated, like many fine-grained soils. Due to rising temperatures underground, many downtown foundations are experiencing unwanted settlements, slowly but steadily. In other words, you don’t need to live in Venice to live in a sinking city — although the causes for the phenomenon are entirely different.

Rotta Loria is an assistant professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering.

What is underground climate change?

In many urban areas around the world, heat continues to diffuse away from buildings and underground transportation, causing the ground to warm at an alarming rate. Previous researchers have found that the shallow surface beneath cities warms by 0.1 to 2.5 degrees Celsius per decade.

Known as “subterranean climate change” or “subsurface heat islands”, this phenomenon is known to cause ecological problems (such as contaminated groundwater) and health problems (including asthma and heat stroke). However, until now, the effects of underground climate change on civilian infrastructure remain unstudied and little understood.

“If you think about basements, parking garages, tunnels and railroads, all of these facilities are constantly giving off heat,” says Rotta Loria. “In general, cities are warmer than rural areas because building materials periodically trap heat from human activities and solar radiation and release it into the atmosphere. The process has been studied for decades. Now, we’re looking at its subsurface counterpart, which is largely driven by anthropogenic activity.”

Chicago as a living laboratory

In recent years, Rotta Loria and his team installed a wireless network of more than 150 temperature sensors in the Chicago Loop — both above and below ground. This includes placing sensors in building basements, subway tunnels, underground parking garages, and subsurface streets such as Lower Wacker Drive. For comparison, the team also buried the sensor in Grant Park, a green space located along Lake Michigan — away from buildings and underground transportation systems.

Data from wireless sensing networks shows that underground temperatures under the Loop are often 10 degrees warmer than those under Grant Park. Air temperatures in underground structures can be up to 25 degrees higher than those in undisturbed soil. When heat diffuses into the ground it exerts significant pressure on the material which expands and contracts with changes in temperature.

“We use Chicago as a living laboratory, but underground climate change is common in nearly all dense urban areas around the world,” says Rotta Loria. “And all urban areas that suffer from underground climate change tend to have problems with infrastructure.”

Slowly sinking

After collecting three years of temperature data, Rotta Loria created a 3D computer model to simulate how ground temperature evolved from 1951 (the year Chicago completed its subway tunnels) to today. He found values ​​consistent with those measured in the field and used simulations to predict how temperatures would develop up to 2051.

Rotta Loria also models how soil changes shape in response to increasing temperature. Whereas some materials (soft and stiff clay) contract when heated, other materials (hard clay, sand, and limestone) expand.

According to simulations, warmer temperatures can cause soil to swell and expand by up to 12 millimeters. They can also cause the ground to contract and sink downwards – under the weight of the building – by as much as 8 millimeters. While this appears subtle and imperceptible to humans, the 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 deformation can be so severe as to pose problems for the performance of civil infrastructure,” said Rotta Loria. “It’s not like a building will suddenly collapse. Things sink very slowly. The consequences for the serviceability of structures and infrastructure can be dire, but take a long time to see. It is very likely that underground climate change has caused cracking and over-settlement of foundations which we did not attribute to this phenomenon because we were not aware of it.”

Harvest heat

Because urban planners and architects designed most modern buildings before subterranean climate change emerged, they didn’t design structures to tolerate the temperature variations we experience today. Even so, modern buildings will be better than buildings from earlier time periods, such as the Middle Ages.

“In the United States, all the buildings are relatively new,” says Rotta Loria. “European cities with very old buildings will be more vulnerable to subsurface climate change. Buildings made of stone and brick using past designs and construction practices are generally in a very delicate balance with the disturbances associated with the city’s current operations. The thermal disturbances associated with subsurface heat islands can be disastrous for such constructions.”

Looking ahead, Rotta Loria said future planning strategies should integrate geothermal technology to harvest waste heat and send it to buildings for space heating. Planners can also install thermal insulation in new and existing buildings to minimize the amount of heat entering the ground.

“The most effective and rational approach is to insulate the underground structure in such a way that the amount of wasted heat is minimal,” says Rotta Loria. “If this cannot be done, then geothermal technology offers the opportunity to efficiently absorb and reuse heat within buildings. What we don’t want is to use technology to actively cool underground structures because that uses energy. Right now, there are a myriad of workable solutions.”

The study, “The silent impact of underground climate change on civil infrastructure,” was supported by the National Science Foundation (grant number 2046586). The wireless sensing network at the base of this work, which also serves as a living laboratory for a course taught by Rotta Loria, was supported in part by the Murphy Society and Northwestern University Alumni.