The Earth's surface is constantly changing due to natural and human-induced processes, such as erosion, weathering, tectonics, volcanism, and land use.
These processes can cause the ground to deform, or change its shape and position, over different spatial and temporal scales.
Ground deformation can have various impacts on the environment and society, such as triggering landslides, earthquakes, floods, or subsidence.
It can also affect the stability and performance of civil structures and infrastructures, such as buildings, bridges, roads, pipelines, or dams.
However, not all ground deformation is caused by natural forces.
A new study has revealed that there is a "silent hazard" lurking underneath our major global cities, caused by underground climate change.
This phenomenon is affecting the ground beneath urban areas in ways that our buildings were not designed to handle.
What is underground climate change?
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Underground climate change is a term that refers to the changes in the temperature and moisture conditions of the shallow subsurface (the upper few meters of the Earth's crust) due to human activities, as per ScienceDaily.
In many urban areas around the world, heat continuously diffuses from buildings and underground transportation systems, such as subways, tunnels, or parking garages.
This heat can be generated by various sources, such as air conditioning, lighting, ventilation, or friction.
It can also be amplified by the urban heat island effect, which is the phenomenon of higher air temperatures in cities compared to rural areas due to human-made structures and materials.
As a result of this heat diffusion, the shallow subsurface beneath cities can warm up by 0.1 to 2.5 degrees Celsius per decade, according to previous studies.
This warming can have various consequences for the subsurface environment and its interactions with the surface.
For example, it can alter the hydrological cycle, affecting groundwater flow and recharge. It can also affect the biological activity and diversity of soil organisms.
Moreover, it can induce thermal expansion and contraction of soil particles and pore fluids, leading to ground deformation.
How does underground climate change affect buildings?
Ground deformation caused by underground climate change can have negative effects on buildings and their foundations, as per Newswise.
As the ground heats up, it also expands or contracts depending on its material properties and moisture content.
This can cause the ground to move vertically (upward or downward) or horizontally (sideways) relative to its initial position.
These movements can be transmitted to the building foundations and structures above them, causing them to deform as well.
Depending on the magnitude and direction of these movements, building deformation can manifest in different ways.
For example, it can cause cracks in walls or floors, tilting or settlement of foundations or columns, misalignment of doors or windows, or damage to pipes or cables.
These effects can compromise the structural integrity, functionality, durability, and safety of buildings. They can also increase the maintenance costs and reduce the service life of buildings.
The study by Northwestern University researchers is the first to quantify ground deformations caused by underground climate change and their effect on civil infrastructure.
The researchers used numerical models to simulate how different types of soils respond to temperature changes in terms of thermal expansion and contraction.
They also considered how different types of foundations (such as shallow or deep foundations) react to ground movements in terms of displacement and stress.
The researchers found that underground climate change can cause significant ground deformations that exceed the allowable limits for most building codes and standards.
They also found that these deformations are not uniform across urban areas but vary depending on the local soil conditions and foundation types.
For example, they found that clay soils tend to contract more than sandy soils when heated up and that shallow foundations tend to experience more displacement than deep foundations when subjected to ground movements.
The researchers also conducted a case study for Chicago, where they applied their models to real soil data and building information from downtown areas.
They found that many buildings in Chicago are undergoing unwanted settlements due to underground climate change, which could affect their normal operation and performance over time.
What can be done to mitigate underground climate change?
Underground climate change poses a serious threat to our infrastructure, but it also offers a potential opportunity for urban planning and engineering.
By capturing the waste heat emitted underground from subterranean facilities, urban planners could mitigate the effects of underground climate change as well as reuse the heat into an untapped thermal energy resource.
The researchers suggest that waste heat recovery systems could be installed in subways, tunnels, parking garages, or basement facilities, to collect the excess heat from these sources and transfer it to other uses, such as heating or cooling buildings, generating electricity, or producing hot water.
This could reduce the heat diffusion into the subsurface, lowering its temperature and minimizing its deformation.
It could also reduce greenhouse gas emissions, improve energy efficiency, and create economic benefits.
The researchers also recommend that building codes and standards should be updated to account for underground climate change and its effects on civil infrastructure.
They suggested that building designers and engineers should consider the thermal properties and behavior of soils and foundations when designing and constructing buildings in urban areas.
Lastly, they also suggested that building owners and managers should monitor and inspect their buildings regularly for signs of deformation or damage due to underground climate change.
