The polar regions are well aware of the problem that rising temperatures cause on ground stability. Due to warming, the permafrost (permanently frozen soil layer) loses strength or disappears and the foundations of the houses are left unprotected, along with other consequences such as the alteration of ecosystems or the release of greenhouse gases hitherto retained in the subsoil. .
But rising ground temperatures are not just affecting the Nordic countries, a study led by experts from Northwestern University (United States) now links ground-scale climate change to altered conditions for the first time. stability of many urban areas. As the ground heats up, some of these layers suffer hitherto unknown deformations, affecting the safety of buildings and structures, the study authors say.
We are facing a “silent danger” at the base of many of our cities and we must recognize that most of the buildings are not prepared to face this new situation, summarize the authors of this research whose results appear in a scientific article published this week by the journal Communications Engineering, from the publishing group Nature.
Increased temperatures cause building foundations and the surrounding soil to move excessively (due to expansions and contractions) and even crack, ultimately affecting the operational performance and long-term durability of structures. The researchers also report that damage to buildings in the past may have been caused by such high temperatures and expect these problems to continue for years to come.
Although 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, urban planners could mitigate the effects of climate change underground and reuse the heat as an untapped thermal energy resource.
The study marks for the first time the first study to quantify ground deformations caused by subsurface heat islands and their effect on civil infrastructure.
Alessandro Rotta Loria, from Northwestern University and leader of the new study, explains that “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 necessarily dangerous for people’s safety, it will affect the normal day-to-day operations of foundation systems and civil infrastructure in general.”
One studied example is that of Chicago clay, which can shrink when heated, like many other fine-grained soils. “As a result of rising temperatures underground, many foundations of the city center are experiencing unwanted settlement, slowly but continuously. In other words, you don’t have to live in Venice to live in a sinking city, even if the causes of such phenomena are completely different”, explains the main author of the study.
In many urban areas of the world, heat continually diffuses from buildings and transportation underground, causing the ground to heat up at an alarming rate. Previous researchers have found that the shallow subsoil beneath cities warms by between 0.1 and 2.5 degrees Celsius per decade.
Known as “underground 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). But, until now, the effect of underground climate change on civil infrastructure has remained unstudied and little understood.
“If you think about basements, parking garages, tunnels and trains, all of these facilities emit heat continuously,” Rotta Loria said. “In general, cities are warmer than rural areas because building materials periodically trap heat from human activity and solar radiation and then release it into the atmosphere. That process has been studied for decades. Now, we’re looking at its subsurface counterpart, which is driven primarily by anthropogenic activity.”
In recent years, Rotta Loria and his team have installed a wireless network of more than 150 temperature sensors in the Chicago Loop, both above and below ground. This included placing sensors in building basements, underground tunnels, underground parking garages, and underground streets such as Lower Wacker Drive. For comparison, the team also buried sensors in Grant Park, a green space located along Lake Michigan, far from underground buildings and transportation systems.
Data from the wireless sensing network indicated that underground temperatures below the Loop are often 10 degrees warmer than temperatures below Grant Park. Air temperatures in underground structures can be up to 25 degrees higher than undisturbed ground temperatures. When heat diffuses into the ground, it puts significant pressure on materials that expand and contract with changes in temperature.
“We use Chicago as a living laboratory, but underground climate change is common in almost every dense urban area in the world,” Rotta Loria said. “And all urban areas that suffer from underground climate change are prone to problems with infrastructure.”
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 underground tunnels) to the present day. He found values ??consistent with those measured in the field and used the simulation to predict how temperatures will evolve up to the year 2051.
Rotta Loria also modeled how soil deforms in response to rising temperatures. While some materials (soft and hard clay) contract when heated, other materials (hard clay, sand, and limestone) expand.
According to simulations, warmer temperatures can cause the soil to swell and expand up to 12 millimeters. They can also cause the ground to contract and sag downward, under the weight of a building, up to 8 millimeters. Although this seems 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 deformations can be so severe that they cause problems for the performance of civil infrastructure,” said Rotta Loria. “It’s not like a building suddenly collapsed. Things are sinking very slowly. The consequences for the serviceability of structures and infrastructure can be very bad, but it takes a long time to see them. It is very likely that subterranean climate change has already caused excessive foundation cracking and settlement that we did not associate with this phenomenon because we were not aware of it.”
harvesting heat
Because urban planners and architects designed most modern buildings before underground climate change arose, they did not design structures to tolerate the temperature variations we experience today. Still, modern buildings will fare better than structures from earlier periods, such as the Middle Ages.
“In the United States, the buildings are all relatively new,” Rotta Loria said. “European cities with very old buildings will be more susceptible to underground climate change. Buildings made of stone and brick that draw on design and construction practices of the past are often in a very delicate balance with the disturbances associated with the current operations of cities. Thermal disturbances linked to subsurface heat islands can have detrimental impacts on such buildings.”
Moving forward, Rotta Loria said future planning strategies should integrate geothermal technologies to collect waste heat and deliver 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 underground structures so that the amount of wasted heat is minimal,” Rotta Loria said. “If this cannot be done, 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 wide variety of solutions that can be implemented.”
