In this blog post, we’ll examine the causes of sinkholes, the impact of reckless urban development on the ground, and sinkhole prevention and response measures in other countries.
The Harmful Effects of Indiscriminate Development: Sinkholes
In the past, a hole approximately 100 meters deep opened up in the center of Guatemala City, causing buildings to be sucked in and resulting in three deaths. Additionally, unpredictable holes have been appearing one after another in various parts of downtown Seoul, causing anxiety among pedestrians and drivers. One of the issues that has recently gained significant attention is the sinkhole. A sinkhole, as the name implies, is a hole formed when the ground subsides. Sinkholes have occurred multiple times in Seoul alone. A survey conducted by the Gyeonggi Development Institute, which polled 1,000 adults in the Seoul metropolitan area regarding public perception of sinkholes, revealed that over 95% of respondents living in the region expressed anxiety about sinkholes. Of these, 53.5% responded that they were “very anxious.” The fact that it is impossible to predict where sinkholes will occur, how deep the holes will be, or when they will appear is causing even greater anxiety among citizens. However, current measures rely on makeshift solutions, such as filling the holes with compacted soil, whenever a sinkhole occurs. The Seoul Metropolitan Government must also devise groundbreaking countermeasures as soon as possible to prevent a situation like the one in Guatemala, where significant loss of life occurred. This article aims to explain the causes of sinkholes and the preventive measures implemented in other countries.
Causes
Simply put, the cause of sinkholes is “groundwater.” The principle is as follows: Beneath the ground, there is not only soil but also a significant flow of groundwater. Groundwater plays a role in supporting objects on the ground surface by withstanding the pressure from the overlying soil. Underground, the pressure increases by approximately 1 atmosphere for every 2.5 meters of depth. Currently, the pressure exerted on objects at the ground surface by atmospheric pressure is 1 atmosphere. Even just 250 meters underground, the pressure rises to about 100 atmospheres. Considering this, groundwater is bearing the immense pressure of the ground above. However, if the groundwater disappears, empty spaces form underground, and the force needed to support the overlying ground is lost. Eventually, the ground cannot withstand the pressure and collapses, creating a hole in the ground—this is a sinkhole.
Such sinkholes have always existed in nature. The Sarisarinama sinkhole in Venezuela is a prime example of a natural sinkhole. However, sinkholes that occur in urban areas are often caused by excessive human urban development rather than natural causes. When humans engage in urban development, they sometimes rely on groundwater to supply the water needed for construction. However, if too much groundwater is extracted, the water table drops, causing the ground that was supported by the groundwater to collapse, leading to subsidence. At that time, the number of groundwater development and utilization facilities was increasing by an average of 5.1% annually, and groundwater usage was rising by an average of 2.6% annually; if this trend continues, the likelihood and risk of sinkholes occurring in the future could continue to grow.
It is not necessarily the case that only areas where large amounts of groundwater are extracted experience subsidence. There are instances where the ground subsides even in areas far removed from those where groundwater is used. Groundwater flows from higher to lower elevations; if a large amount of water is used in a low-lying area, groundwater from higher elevations moves toward that low-lying area, creating empty spaces in the soil at the higher elevations and causing the ground there to subside.
Sinkholes can also form when the flow of groundwater is forcibly diverted elsewhere.
If a large volume of water flows through an area where there was previously no watercourse, the cohesion between soil particles decreases, weakening the ground and potentially causing it to collapse. Alternatively, if aging urban water and sewer pipes leak, water seeps into the surrounding soil, which can cause the ground to collapse. This can be considered a disaster resulting from indiscriminate human development and negligent management.
Another cause is ground erosion caused by groundwater. As groundwater flows, it carries away particles of various sizes, gradually eroding the path it follows and widening the gaps. If these gaps continue to grow until a large hole forms, the risk of a sinkhole occurring increases. This risk becomes even greater when typhoons or torrential rains cause excessive amounts of groundwater to flow. Furthermore, if the ground is composed of water-soluble substances such as limestone, salt, or carbonates, groundwater can dissolve the ground and create empty spaces, leading to sinkholes. Fortunately, unlike in other countries, South Korea’s ground is composed of hard rock such as granite, so sinkholes caused by groundwater dissolving bedrock are small in scale and cause relatively little damage. However, no matter how small the hole may be, a sinkhole that suddenly appears can pose a significant danger to pedestrians.
Sinkholes symbolize human folly—a failure to anticipate the future—resulting from excessive groundwater use, disruption of groundwater flow due to development projects, and the aging of water and sewer pipes. Currently, however, there is little information available regarding groundwater and the ground on which infrastructure is built. Additionally, a lack of data on groundwater location and flow, soil types, and soil cohesion means that sufficient alternatives for predicting and safely preventing sinkholes have not yet been developed.
Countermeasures
Under current circumstances, when a sinkhole occurs, a temporary solution is used: covering the hole with compacted soil. However, this method alone is insufficient to safely prevent sinkholes that may suddenly occur in the future. In the U.S. state of Florida, where the geology consists mainly of limestone and sinkholes occur frequently, sinkhole-related ordinances were once enforced. Homeowners in areas prone to sinkholes were required to purchase sinkhole insurance. Additionally, rating standards were established for building codes and construction methods that could influence sinkhole formation, and building insurance premiums were calculated accordingly. Furthermore, construction methods to prevent sinkholes were proposed, and guidelines were developed to help the general public recognize the signs of sinkhole formation, thereby increasing the likelihood of prevention.
So far, we have examined the causes of sinkholes and response measures based on international examples. The primary cause of sinkholes has been reckless urban development by humans. Human greed—which disregards nature and fails to consider the future—is now coming back to haunt us. We cannot undo the development that has already taken place, nor can we stop future development altogether. For this reason, the state of Florida in the United States is striving to prevent sinkholes by enforcing sinkhole-related ordinances and providing guidelines to recognize the signs of sinkholes, thereby preventing safety accidents. South Korea, too, needs to take a responsible approach to development—rather than rushing headlong toward urban expansion—and actively resolve the problems currently arising.
