In this blog post, we will analyse the causes of sinkholes from a geological perspective and examine whether they are natural disasters or man-made disasters.
One of the unfamiliar terms that has been appearing frequently in the news lately is ‘sinkhole.’ A sinkhole is a phenomenon in which the ground collapses due to a change in the state of the earth’s crust, weakening the force that supports the ground above it. It is easy to understand if you think of the old Korean expression ‘the ground is sinking.’ However, the term ‘sinkhole’ is not commonly used in everyday language, so it may be unfamiliar to many people, and even those who know its meaning are rarely able to explain its causes. Therefore, in this article, I will identify the causes of sinkholes based on my expertise in geology and, using the example of an incident that occurred in Songpa-gu, Seoul, South Korea, I will explain the rock mechanics characteristics related to the destruction of underground spaces.
First, let us look at the causes of sinkholes. The occurrence of sinkholes is greatly influenced by groundwater distributed in the earth’s crust. The influence of groundwater can be divided into two main types: sinkholes caused by large flows of groundwater due to crustal movements, and sinkholes caused by human development of groundwater. In other words, sinkholes can be divided into natural and artificial causes. The former are large in scale and can create pits as deep as a 20-storey building, while the latter are relatively small. It is a misconception that sinkholes are caused solely by humans. Sinkholes that occur during subway construction, such as in Songpa-gu, Seoul, South Korea, are considered to be man-made, and almost all sinkholes in Korea are man-made. Now, let us explain why from a geological perspective.
In order for water to be stored inside the earth’s crust, there must be a lot of empty space underground. At this time, sedimentary rocks, which are relatively weaker than metamorphic and igneous rocks, are more likely to crack. Therefore, the more the ground is composed of sedimentary rocks, the more vulnerable it is to sinkholes. Fortunately, however, most of the ground in Korea is composed of granite, a type of igneous rock, and gneiss, a type of metamorphic rock. Therefore, even if sinkholes occur, they do not collapse deep into the ground and are relatively small in scale. The 20-storey sinkhole mentioned above occurred in Guatemala, Central America, where the bedrock is composed of sedimentary rock.
In summary, the sinkhole in Songpa-gu, Seoul, South Korea, collapsed because the holes created underground by construction work could not withstand the force from above, despite the solid granite bedrock. Now, let’s look at the factors that determine the force that underground cavities, or holes, can withstand from a rock mechanics perspective. Underground cavities are subject to the force of gravity. At this point, it is important to consider how much force they can withstand, and the limit varies depending on the type and condition of the bedrock and the shape of the underground cavity.
First, the type of bedrock that makes up the crust has an influence. The harder the bedrock, the more weight it can withstand. For example, concrete can withstand much heavier weights than tofu. Second, the strength of the rock varies depending on its condition. There are many cracks in the bedrock caused by crustal movements, and the size and shape of these cracks vary. The cracks are filled with groundwater, which directly supports the empty space in the bedrock and increases its viscosity, thereby increasing its strength. This is easy to understand if you remember how the toad houses you built as a child were stronger when made with wet soil. Finally, when cavities are artificially created underground for purposes such as subway construction, the shape of the cavity also has an important influence. When the cavity is in the form of a typical tunnel (semicircular cylinder), the amount of force it can withstand varies depending on the ratio of its width to its height. When the width is greater than the height, it is more stable because the force acting on the hole is distributed over a larger area. This is the same principle as when you press an egg with your palm, it is easier to break it when it is lying down than when it is standing up.
In addition to the sinkhole in Songpa-gu, Seoul, South Korea, which was mentioned as an example, underground space development is currently underway in city centres, and it is unclear when a similar accident will occur again. What is more serious is that sinkholes are disasters that the general public cannot prepare for in advance. Therefore, experts argue that periodic ground surveys should be conducted and various factors should be considered when designing underground spaces to ensure safety.
