The concept of underground climate change and its impacts have relatively recently begun being studied. Finding that the change of ground temperature affects building foundations, a new study, published in the Communications Engineering journal, calls it a “silent hazard” lurking underneath our major global cities.
1. Underground climate change
In many urban areas around the globe, heat continuously diffuses from buildings and underground transportation, causing the ground to warm at an alarming rate. Previous researchers have found that the shallow subsurface beneath cities warms by 0.1 to 2.5 degrees Celsius per decade.
Known as “underground climate change” or “subsurface heat islands”, this phenomenon has been known to cause ecological issues (such as contaminated ground water) and health issues (including asthma and heatstroke). But the new Northwestern University study is the among the first to look into the effect of underground climate change on civil infrastructure.
“If you think about basements, parking garages, tunnels and trains, all of these facilities continuously emit heat”, explained lead researcher Alessandro Rotta Loria. “In general, cities are warmer than rural areas because construction materials periodically trap heat derived from human activity and solar radiation and then release it into the atmosphere. That process has been studied for decades. Now, we are looking at its subsurface counterpart, which is mostly driven by anthropogenic activity.”
2. Infrastructure problems
The researchers found that, as the ground heats up, it also deforms. This phenomenon causes building foundations and the surrounding ground to move excessively (due to expansions and contractions) and even crack, which ultimately affects structures’ long-term operational performance and durability.
Rotta Loria and his team installed a wireless network of more than 150 temperature sensors across the Chicago Loop neighbourhood, both above and below ground. This included placing sensors in the basements of buildings, subway tunnels, underground parking garages and subsurface streets like. For comparison, the team also buried sensors away from buildings and underground transportation systems, in Grant Park, a greenspace located along Lake Michigan.
And all urban areas suffering from underground climate change are prone to have problems with infrastructure.
Data sensors indicated that underground temperatures beneath the Loop are often 10˚C higher than temperatures beneath Grant Park, while air temperatures in underground structures can be up to 25˚C higher compared to the undisturbed ground temperature.
When the heat diffuses toward the ground, it puts significant stress on materials that expand and contract with changing temperatures. Whereas some materials (soft and stiff clay) contract when heated, other materials (hard clay, sand and limestone) expand.
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 and used the simulation to predict how temperatures will evolve until the year 2051. According to the simulations, warmer temperatures can cause the ground to swell and expand upward by as much as 12 millimetres. They also can cause the ground to contract and sink downward, beneath the weight of a building, by as much as 8 millimetres. Although this seems subtle and is imperceptible to humans, the variation is more than many building components and foundation systems can handle without compromising their operational requirements.
Moreover, the study points out that while most modern buildings were designed before underground climate change emerged and are thus unable to tolerate the temperature variations we experience today, the risks are even higher for older constructions. “European cities with very old buildings will be more susceptible to subsurface climate change”, said Rotta Loria. “Buildings made of stone and bricks that resort to past design and construction practices are generally in a very delicate equilibrium with the perturbations associated with the current operations of cities. The thermal perturbations linked to subsurface heat islands can have detrimental impacts for such constructions.”
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.Alessandro Rotta Loria, Researcher at Northwestern University
“It’s not like a building will suddenly collapse. Things are sinking very slowly”, Rotta Loria explained. “The consequences for serviceability of structures and infrastructures can be very bad, but it takes a long time to see them. It’s very likely that underground climate change has already caused cracks and excessive foundation settlements that we didn’t associate with this phenomenon because we weren’t aware of it.”
3. Harvesting waste heat
Although rising temperatures do pose a threat to our infrastructure, the researchers also view it as a potential opportunity. By capturing the waste heat emitted underground from subterranean transportation systems, parking garages and basement facilities, urban planners could mitigate the effects of underground climate change as well as reuse the heat into an untapped thermal energy resource.
Going forward, Rotta Loria said future planning strategies should integrate geothermal technologies to harvest waste heat and deliver it to buildings for space heating. Planners also can install thermal insulation on new and existing buildings to minimize the amount of heat that enters the ground.