On the 24th, a large sinkhole measuring 20 meters in diameter and depth occurred in the Gangdong District of Seoul. During the accident, a motorcycle driver in their 30s passed by the site and fell into the hole, and was found in cardiac arrest the following day, the 25th. The sudden incident turned an ordinary day into a tragedy.
Sudden sinkholes are not actually rare occurrences. According to the Ministry of Land, Infrastructure and Transport, sinkholes occurred an average of once every 1.9 days in South Korea from 2018 to 2024. In 2018 alone, a staggering 333 cases were reported, and this year, there have already been 9 cases.
To prevent sinkhole damage, it is necessary to find empty spaces underground in advance or detect minute movements of the ground. For this, various technologies are being deployed, from ground-penetrating radar to satellite imagery. A new method that reinforces aging sewer pipes is also being explored to completely block sinkholes.
The causes of sinkholes lie beneath the urban ground. Of the 1,346 sinkholes that occurred in the country, approximately 53%, or 716 cases, were due to damage to water pipes. Faulty backfilling, which is the process of refilling soil after excavation, accounted for 244 cases, and poor construction accounted for 162 cases. Collapse occurs when soil is eroded by water leaking from water pipes or when empty spaces are created due to substandard construction, leading to ground failure.
The representative sinkhole detection technology is ground-penetrating radar (GPR). GPR is a non-destructive method that identifies voids underground using electromagnetic waves, and is widely used in urban areas where direct excavation is not possible. However, this technology also has limitations; if the void is deep, it cannot be detected.
Ryu Dong-woo, a principal researcher at the Korea Institute of Geoscience and Mineral Resources, noted, "GPR is the most commercialized technology for detecting voids, but it has limitations in detecting deeper than 2 meters." In fact, the sinkhole that occurred in Yeonhui-dong, Seodaemun District last August was from a location where no problems were detected during a GPR survey conducted three months prior, in May.
Technologies are also being developed to address these limitations. Professor Yu Chung-sik from Sungkyunkwan University is researching an interferometric synthetic aperture radar (InSAR) technique that utilizes images captured by satellites. The method involves satellites capturing the same location multiple times with a time difference, analyzing minute changes in between to detect subsidence on the millimeter scale.
In fact, applying the InSAR method to the 4th stage construction section of Incheon International Airport revealed an annual subsidence of 15 millimeters, confirming a maximum accumulated subsidence of 130 millimeters.
Technologies are also being developed to eliminate causes before ground subsidence occurs. The Korea Institute of Civil Engineering and Building Technology developed a 'flexible pipe reinforcement lining method' in January of this year that can reinforce old and weakened sewer pipes without excavation. This method involves adding a thin and flexible protective layer inside existing sewer pipes. Since damage to water pipes accounts for more than half of the causes of sinkholes, the actual utility of the developed technology is expected to be high.
There were previously methods to line pipes with protective layers, but they were only effective for rigid concrete pipes. They did not adhere well to flexible pipes such as polyethylene or polyvinyl chloride, making it easy for gaps to form. The newly developed technology uses acrylic fiber reinforcement that bends well, does not tear easily, and reduces thickness by half while increasing adhesion.
The acrylic fiber protective layer prevents soil from escaping from sewer pipes, thus blocking the formation of spaces beneath the ground. Yoo Seong-soo, the chief researcher at the Korea Institute of Civil Engineering and Building Technology, explained, "The sinkholes that frequently occur in our country mostly happen above sewer pipes and this method can help prevent sinkhole accidents that could arise due to aging sewer pipes."
References
Journal of the Korean Geotechnical Society (2022), DOI: https://doi.org/10.12814/jkgss.2022.21.4.055