A groundbreaking study conducted by Northwestern University has revealed a concerning link between climate change and the shifting earth beneath urban areas. The research demonstrates that as the ground temperature rises, it undergoes deformation, resulting in increased ground movement and cracking due to expansions and contractions. These phenomena pose long-term risks to the durability and operational performance of structures.
The study suggests that rising temperatures may have already contributed to building deterioration in the past, and these problems are likely to persist for years to come.
While the implications of rising temperatures for infrastructure are undoubtedly concerning, the researchers also perceive it as a potential opportunity. They propose capturing the waste heat emitted underground from subterranean transportation systems, parking garages, and basement facilities as a means to mitigate the effects of underground climate change. This approach could also tap into an untapped thermal energy resource.
The study, set to be published in the Nature Portfolio journal Communications Engineering on July 11, marks the first comprehensive analysis of ground deformations caused by subsurface heat islands and their impact on civil infrastructure.
"Underground climate change is a silent hazard," stated Alessandro Rotta Loria, the study's lead researcher and an assistant professor of civil and environmental engineering at Northwestern's McCormick School of Engineering. "The ground is deforming due to temperature variations, and existing structures and infrastructure are not designed to withstand these changes. While this phenomenon may not directly endanger people's safety, it will affect the day-to-day operations and performance of foundation systems and civil infrastructure on a large scale."
Rotta Loria emphasized the effects of rising temperatures on foundations in downtown Chicago, where the clay soil contracts when heated. This phenomenon leads to unwanted settlement, slowly but continuously. He highlighted that living in a city experiencing sinking-like effects is not exclusive to Venice, as other cities face similar challenges driven by different causes.
The concept of underground climate change refers to the continuous diffusion of heat from buildings and underground transportation systems in urban areas, leading to alarming ground warming rates. Previous studies have indicated that the shallow subsurface beneath cities warms at a rate of 0.1 to 2.5 degrees Celsius per decade.
Termed as "subsurface heat islands," this phenomenon has already been linked to ecological and health issues such as contaminated groundwater, asthma, and heatstroke. However, its impact on civil infrastructure has remained largely unexplored until now.
Rotta Loria explained, "When you consider basements, parking garages, tunnels, and trains, all these facilities continuously emit heat. Cities are generally warmer than rural areas due to the periodic trapping and release of heat from human activity and solar radiation in construction materials. This process has been studied for decades. Now, we are examining its subsurface counterpart, primarily driven by anthropogenic activity."
After collecting temperature data over three years, Rotta Loria constructed a 3D computer model to simulate the evolution of ground temperatures in Chicago since the completion of its subway tunnels in 1951. The simulation aligned with field measurements and was used to predict temperature changes until 2051.
The research also involved modeling how the ground deforms in response to increasing temperatures. While some materials, such as soft and stiff clay, contract when heated, others like hard clay, sand, and limestone expand.
According to the simulations, warmer temperatures can cause the ground to swell and rise by up to 12 millimeters. Conversely, they can also lead to ground contraction and sinking, potentially compromising the stability of buildings, with a maximum subsidence of 8 millimeters. Although imperceptible to humans, these variations exceed the tolerances of many building components and foundation systems, risking their operational requirements.
Rotta Loria warned, "Based on our computer simulations, we have demonstrated that ground deformations can be severe enough to create problems for the performance of civil infrastructure. Buildings won't collapse suddenly, but they will experience slow sinking. The consequences for the functionality of structures and infrastructures can be significant, but they may take a long time to become evident. It is highly likely that underground climate change has already caused cracks and excessive foundation settlements that we haven't attributed to this phenomenon because we were unaware of it."
While modern buildings are relatively better equipped to handle temperature variations compared to older structures, which were not designed with the emergence of underground climate change in mind, Rotta Loria suggested that integrating geothermal technologies into future planning strategies could be a viable solution. By capturing and reusing waste heat in buildings, urban planners and architects can effectively address the challenges posed by underground climate change. Additionally, thermal insulation can be installed in both new and existing buildings to reduce the amount of heat that enters the ground.
"The most effective and rational approach is to isolate underground structures in a way that minimizes wasted heat," Rotta Loria explained. "If that is not possible, then geothermal technologies offer an opportunity to efficiently absorb and reuse heat in buildings. Active cooling of underground structures should be avoided, as it consumes energy. Currently, there are numerous solutions that can be implemented."
As cities worldwide grapple with the implications of climate change, this study serves as a crucial reminder to address the hidden risks beneath the surface. By acknowledging and incorporating the impact of underground climate change into urban planning and infrastructure design, cities can take proactive measures to safeguard their structures, reduce environmental impact, and tap into sustainable energy sources.