In this blog post, we’ll examine the causes of sinkholes—which occur when hollow spaces underground collapse—the differences between naturally formed sinkholes and ground subsidence in urban areas, and ways to prevent them.
Imagine if the home you worked so hard to make your dream of homeownership a reality were suddenly swallowed up by a massive hole that appeared out of nowhere in the ground. It would likely be a terrifying experience. In fact, there was an incident in Guatemala City where a massive ground collapse caused several homes to collapse and resulted in three deaths. It was reported that damage to underground sewer pipes and rainwater contributed to the ground collapse. In South Korea as well, these “holes” and ground subsidence are drawing public attention. In particular, in the area around Seokchon Lake in Seoul, the causes of ground subsidence and safety issues came under intense scrutiny after underground voids and road collapses were discovered one after another during subway construction and large-scale development projects in the surrounding area. At the time, the Seoul Metropolitan Government investigated the relationship between subway tunnel construction and the surrounding ground and implemented ground reinforcement and safety measures. Although this phenomenon of the ground sinking is commonly referred to as a “sinkhole,” South Korean laws and regulations use the term “ground subsidence.” The current “Special Act on Underground Safety Management” also defines ground subsidence as the sinking of the surrounding ground during underground development or the use and management of underground facilities. So, why exactly do sinkholes occur, and what measures can be taken to prevent them?
Simply put, sinkholes occur when a void forms underground and the ground supporting that void collapses. Beneath the ground, there may be spaces such as cracks in rock, gaps between strata, or naturally formed caves, and these voids can grow as water dissolves rock or erodes the ground. In particular, in areas where water-soluble rocks such as limestone are widely distributed, cavities can form due to the chemical and physical actions of groundwater, leading to sinkholes in karst terrain where the ground surface suddenly subsides. Therefore, not all sinkholes are simply caused by the drainage of groundwater. Natural sinkholes are formed through the combined effects of various factors, including rock dissolution, groundwater flow, and geological structures.
A prime example is the “Swallow Cave” in the state of San Luis Potosí, Mexico. The name “Swallow Cave” derives from the birds that inhabit the area around and inside the cave. Unlike typical urban ground subsidence, this is a massive vertical cave formed over a long period of time by geological processes in a limestone region. The entrance is reported to have a diameter of approximately 60 meters and a depth exceeding 376 meters. Currently, the site is designated as a nature reserve and is being preserved due to its unique geological environment and ecological value. Compared to natural vertical caves like the “Swallow Cave,” the ground subsidence occurring in South Korea’s urban centers differs significantly in scale and formation process. In major urban areas of South Korea, a significant portion of the bedrock consists of igneous and metamorphic rocks, resulting in a geological environment different from that of typical karst regions. However, this does not mean that ground subsidence does not occur in South Korea. In urban areas, factors related to human activity—such as underground excavation, damage to underground infrastructure, soil erosion, and changes in groundwater levels—can be major causes, rather than natural geological processes.
Unlike “Swallow Caves” formed by nature, ground subsidence in cities is closely linked to human activity. As the population grows, water consumption increases; when excessive groundwater extraction lowers the water table, the effective stress on the ground rises, potentially causing the ground to consolidate or subside. Furthermore, changes in groundwater flow can cause fine soil particles within the ground to shift or be washed away, creating underground cavities. However, since the role of groundwater also varies depending on local geological and soil conditions, not all cases of urban ground subsidence can be explained solely by groundwater depletion. The current Underground Safety Act treats groundwater and underground development as critical factors in ground safety, defining underground development to include not only excavation and burial but also pumping.
Groundwater is a key factor influencing the physical state and stress conditions of the ground. As explained in the original text, the pressure exerted on the surrounding ground increases with depth; however, it is not accurate to simply state that “pressure increases by 1 atmosphere for every 2.5 meters of depth.” While water pressure increases by approximately 1 atmosphere for every 10.3 meters of depth underwater, the actual stress within the ground varies depending on soil type, rock, groundwater level, and the density and structure of the strata. Therefore, rather than simply explaining that “the empty space collapses because it cannot withstand the pressure” when groundwater disappears, it is more accurate to understand that subsidence can occur due to an increase in effective stress caused by changes in the groundwater level or changes in the ground’s bearing conditions.
There are various other causes of ground subsidence in urban areas. When conducting large-scale civil engineering projects or constructing large buildings, the ground is excavated deeply for foundation work. Such excavation work alters the stress state of the surrounding soil and the flow of groundwater; if proper soil retention, waterproofing, and ground reinforcement are not implemented, it can cause the surrounding soil to deform or lead to soil loss. Furthermore, artificially diverting surface water or changing drainage conditions can alter the moisture content and strength of the soil, potentially weakening the ground. Even when water leaks due to damage to urban water and sewer pipes, continuous loss of surrounding soil or the formation of voids can lead to ground subsidence. Excessive underground development is also a factor that can compromise ground stability. In particular, the Seoul Metropolitan Government has recently been investigating the link between aging sewer pipes and ground subsidence, and views defects and leaks in aging sewer pipes as key targets that must be managed to prevent ground subsidence.
So, what measures can be taken to prevent such ground subsidence in urban areas? The most fundamental approach is to thoroughly investigate the ground conditions, topographical changes, groundwater flow, and the locations of nearby underground facilities in the relevant area from the planning stage of any construction project. Currently, South Korea operates a system requiring underground safety assessments for projects above a certain scale to prevent ground subsidence caused by underground development. In particular, projects involving large-scale excavation or those likely to affect underground safety must investigate the impact on ground conditions, groundwater, and surrounding facilities in advance and establish necessary safety measures. In areas such as limestone regions, where sinkholes are likely to occur naturally, detailed surveys that take geological characteristics into account and ongoing management are essential. Additionally, inspecting aging water and sewer pipes and replacing or repairing them at the appropriate time is a crucial preventive measure. The Seoul Metropolitan Government is also conducting detailed surveys of aging sewer pipelines and managing them by identifying pipeline defects and prioritizing repairs in sections with a high risk of ground subsidence.
In addition, underground geophysical survey technologies, such as ground-penetrating radar (GPR), are being used to detect cavities beneath the ground surface in advance. Cavities are empty spaces that can lead to ground subsidence if they grow large enough, even if the ground surface has not yet subsided. Therefore, when a void is discovered underground, the condition of underground infrastructure, the presence of leaks, and the characteristics of the surrounding ground must be investigated together to determine the cause and carry out appropriate reinforcement or restoration. The Seoul Metropolitan Government is also conducting void surveys using ground-penetrating radar to prevent ground subsidence and has recently been expanding its monitoring system, which utilizes sensors that observe ground changes in real time along with artificial intelligence and information and communication technologies. As such, today’s approach to preventing ground subsidence is evolving beyond simply filling in the ground after an incident occurs; it now focuses on proactively surveying underground spaces and continuously monitoring for signs of danger.
While not all sinkholes and instances of ground subsidence are related to “water” underground, there are many cases where water flow and changes in the ground are closely linked. Naturally occurring sinkholes are often formed by natural processes such as rock dissolution, groundwater flow, and geological structures. In contrast, ground subsidence in urban areas is frequently closely linked to human activities, such as leaks from aging sewer pipes, underground excavation, changes in the groundwater table, soil erosion, and damage to underground infrastructure. Furthermore, the outcomes of sinkholes and ground subsidence differ significantly. Giant caves and sinkholes formed naturally create unique geological landscapes, provide habitats for diverse species, and offer humans majestic natural scenery. However, ground subsidence in urban areas can damage roads, buildings, and underground infrastructure, potentially causing significant loss of life and property. Thus, sinkholes possess two distinct “faces”: one that showcases the wonders of nature, and another that threatens urban safety. In this context, our society’s management and responsibility in developing and utilizing underground spaces play a crucial role in reducing the risks of ground subsidence in urban areas.
