Scientists have been warning about the risks of climate change for years. Extreme temperatures, almost uncontrollable droughts and forest fires, rising sea levels and devastating weather events are some of the well-known consequences of global warming. Architects and engineers have found another, a “silent danger” that they call underground climate change: “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 Alessandro Rotta. Loria, professor of civil and environmental engineering at Northwestern University (Illinois, United States), author of the first study linking ground disturbances in cities to heat generated by subways, garages, and basements.
The surface of urban areas is increasing in temperature at a dizzying rate. It is estimated that between 0.1 and 2.5 degrees Celsius per decade. This heat is affecting the foundations of the buildings. “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 the foundation systems and civil infrastructure in general,” explains the expert in an article published this Tuesday in ‘Communications Engineering’a magazine specializing in engineering from the prestigious ‘Nature’.
Eugenia del Río, architect and general secretary of the Madrid Official College of Architects, confirms that temperature can affect the structure of a building. «There are three factors to take into account when constructing a building. They are allowable stress – how much load the building can withstand per square meter; the phreatic level -where the underground water is-, and the composition of the terrain. It would affect it because that higher temperature alters the original plan, “she explains. But she also rejects any kind of alarmism. «Modern constructions are erected with a safety factor, a margin for the unforeseen. All the elements are designed to withstand the loads and changes they may undergo. And we must also take into account that the metro, for example, has been in cities for many years and the buildings have been built taking it into account », she emphasizes.
chicago as a model
The study has focused on the city of Chicago, although it is applicable to almost any modern city. “We use Chicago as a living laboratory, but underground climate change is common in almost all dense urban areas in the world,” explains the professor. For three years, temperatures were measured both above and below the ground in two very different areas of the Illinois state capital. One was the so-called Chicago Loop, the financial district; the other, Grant Park, a green space without buildings and suburban transportation systems. The differences are very striking. Up to ten degrees more temperature was recorded in the subsoil of the Chicago Loop.
According to their simulations, warmer temperatures can cause the soil to swell and expand upward by as much as 12 millimeters. They can also cause the ground to contract and sag downward, under the weight of a building, up to 8 millimeters. Imperceptible to the naked eye but important. “We have shown that ground deformations can be so severe that they cause problems in buildings. It’s not like a building suddenly collapsed. The consequences for the infrastructures 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 cracking and settlement in foundations that we do not associate with this phenomenon because we were not aware of it,” she stresses.
The consequences of this “silent danger” will be worse in Europe. Although modern buildings are not designed to withstand these temperature variations, they have more capacity than old ones, such as those from the Middle Ages, emphasizes Rotta Loria. “In the United States, the buildings are all relatively new. European cities with very old buildings will be more susceptible to underground climate change. Thermal disturbances to subsoil heat can have detrimental impacts on such constructions.” Eugenia del Río qualifies that it depends on the construction in question, because “the old foundations used to be massive, they did not focus so much on the aspect of efficiency.”
Reuse heat from the ground
This problem could carry with it a possible solution. Reuse with geothermal technology all that heat generated to, for example, heat homes during the winter. This is what he proposed in a paper published in July of last year in the magazine ‘Nature Communications’. The idea was to channel the superheated groundwater to pumps on the surface to bring it in turn to the buildings. According to their study, 43% of the areas they studied, mostly close to densely populated areas, would have accumulated enough heat in the first 20 meters underground to meet heating demand for a year.
Chicago already faced this problem more than a century ago
It is not the first time that Chicago has faced a problem in its underground. According to Pedro Torrijos, an architect and popularizer, the city had to rise four meters in the middle of the 19th century. The reason was the outbreak in 1854 of a cholera epidemic that killed almost 4,000 people, 5% of its population. The fundamental cause was the unhealthiness of a city that had skyrocketed its population in recent decades but lacked a sewage system. As it is built on a plain and one meter above the level of Lake Michigan, there was no choice but to literally raise it. The buildings were mounted on platforms and raised using hydraulic jacks. Once this was done, the engineers laid the sewer system and filled in the roads and sidewalks up to the new level.
