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The climate we usually understand refers to the multi-year average of changes in the atmosphere above the ground. But in fact, there is also climate change underground, and scientists call the phenomenon of urban underground climate warming an "underground heat island". Underground heat island will not only damage the health of urban residents, but also affect the economic development of the city.

Wen Jing | Wen Jing

Recently, scientists have for the first time assessed the impact of underground heat islands on underground soil layers in urban areas, the results were published in the academic journal Communication Engineering (Communications Engineering). New research shows that underground heat islands threaten the safety of buildings and all kinds of infrastructure.

The reasons for the formation of underground heat island affecting the life of urban residents can be divided into two categories. First, human factors, for example, in order to heat the interior of the building, the relevant infrastructure will constantly lose heat underground; underground traffic will also affect the original underground temperature field, train running, braking, and passengers will also generate heat; other underground pipes, high-voltage cables and so on will also lose heat. The other is meteorological factors, which can be considered as the cumulative effects of human factors in the past few centuries. for example, the urban heat island effect leads to the rise of surface temperature and further leads to the increase of underground environmental temperature.

The underground heat island effect is positively related to the urban population density and the total population, and is affected by the urban topography and hydrogeological conditions. Data show that due to the underground heat island effect, the underground ambient temperature of the world's major cities increases by about 0.1 to 2.5 ℃ every decade (100m below the ground). In some densely populated urban areas, there is even a temperature anomaly of 20 ℃. Scientists say that underground heat islands will bring many hidden dangers to urban areas that can not be ignored.

Underground heat island will lead to plant growth restriction, groundwater thermal pollution and so on, as well as a series of traffic problems and public health problems. For example, when the subway track is overheated, trains can only slow down or even stop running, and delays in public transport services will cause huge economic losses; underground extreme temperatures can also cause human discomfort and even cause disease. such as heat spasm, dehydration, high blood pressure, asthma and heatstroke.

What is worth paying attention to the "silent" underground heat island effect is that the harm of the underground heat island to urban areas seems to be silent. Underground heat island will lead to soil, rock, building materials deformation or loss of function, and some changes are irreversible, so that serious ground collapse, building deformation and other problems occur. However, the existing urban housing buildings and infrastructure do not take into account the underground heat island effect, so it is difficult to assess what problems will arise in the later stage of operation of these construction facilities.

In the new study, scientists used the Chicago Ring Road, the second most densely populated area in the United States, as the research area, using the digital model of the city and data from several wireless temperature sensors installed on the surface and underground in the area. the impact of underground heat islands on civil building facilities is evaluated for the first time.

The study shows that the underground environment of the Chicago Ring Road is close to thermal saturation, with an average annual warming of about 0.49 ℃ (100m below the ground) and now about 0.14℃. The data also show that with the passage of time, different types of soil layers will experience different temperature changes. This difference is affected by multiple factors, such as the existence of heat source, the location of heat source, the structure and density of soil, the distance from the surface, the thermophysical characteristics of the surface, the existence of groundwater and the magnitude of groundwater flow.

In this study, the researchers found that excluding factors such as the distance from the heat source and the surface, the temperature change of some limestone is obviously too large, indicating that the thermal saturation condition of this lithologic rock is higher than that of other rocks. This is something that should not be ignored when assessing the impact of underground heat islands on urban building facilities.

Collecting or isolating geothermal scientists have proposed two ways to alleviate subterranean heat islands.

One is to use geothermal technology to collect this extra heat. This also shows that although the underground heat island will bring a series of problems, it is also an opportunity for us to make rational use of the heat.

The second is to transform the underground environment, for example, by improving the original underground fence or building a new underground fence to isolate the heat. Usually, we take this kind of intervention only when the geothermal technology is not mature, or when the underground environment is really bad.

Scientists believe that in the research on underground heat islands, it is necessary to make comprehensive use of the technical means of physics and energy science, so as to provide suggestions for the revision of urban planning strategies of cities around the world from both theoretical and technical aspects.

References:

The silent impact of underground climate change on civil infrastructure | Communications Engineering (nature.com)

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