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Frequent sinkholes reflect deepening risks in Korea’s underground development

Published: 17 Apr. 2025, 00:03

Lee Su-gon

The author is a former professor at the University of Seoul

As underground development accelerates across Korea, sinkholes — both large and small — are occurring with increasing frequency in urban areas. While some are relatively minor, many have begun to raise alarm bells regarding safety, exposing vulnerabilities in the way Korea manages subterranean construction. Experts have pointed to aging water and sewage infrastructure, as well as inadequate soil compaction during the final phases of construction, as contributing factors.

Busan officials investigate the sinkhole site in Gamjeon-dong, Sasang District, Busan on April 14. [SASANG DISTRICT OFFICE]

On April 14, a small sinkhole was discovered near Aeogae Station on Seoul Metro Line 5. Measuring 40 centimeters (16 inches) in diameter and 1.3 meters (1.4 yards) deep, it was found to have been caused by a ruptured sewer pipe. The incident was resolved quickly, and no injuries occurred. But while this case was relatively minor, it served as a reminder that urban infrastructure across the country is under stress.

Of greater concern are large sinkholes, which pose a more serious threat to public safety. These typically emerge in connection with underground excavation and construction projects. In recent months, sinkholes in Seoul, Gyeonggi and Busan have all been linked to tunneling or open-cut work. The scale of these incidents and their proximity to major infrastructure have raised urgent questions about how such projects are managed and monitored.

In one fatal case, a massive sinkhole opened last month in eastern Seoul’s Myeongil-dong neighborhood, directly above a tunnel being bored 11 meters underground as part of the extension of Seoul Metro Line 9. The sinkhole measured 20 meters in diameter and 18 meters deep, swallowing a passing motorcycle. One person was killed, and another suffered injuries.

In Busan’s Sasang District, 14 sinkholes of varying sizes have been recorded in the past two years near the construction site of the Sasang-Hadan subway line. The tunnel there is being built using open-cut excavation, reaching depths of up to 20 meters below the surface. Repeated incidents in this area warrant deeper investigation.

Despite government efforts to assess subsurface risks using ground-penetrating radar, current technology can only detect voids up to about two meters beneath the surface. This makes it extremely difficult to detect cavities or structural weaknesses associated with deeper construction. Large sinkholes, often triggered by tunneling, require more comprehensive diagnostic tools and, more important, a reassessment of how subsurface projects are designed and supervised.

Preventing such accidents depends on understanding where and why sinkholes occur and addressing the structural vulnerabilities that make them more likely. There are three fundamental areas that require attention.

First, engineering projects must be matched to the geological conditions of each site.

This means thoroughly understanding the distribution of rock types and geotechnical features in an area before beginning construction. In Seoul, approximately 64 percent of the subsurface is composed of metamorphic rock, while the remaining 36 percent consists of igneous rock. Metamorphic rock, formed under high pressure and temperature, is typically more complex and often includes fault zones where the rock has been fractured. These zones are especially vulnerable to water infiltration and chemical weathering.

A sinkhole in Gangdong District, eastern Seoul, on March 24 [NEWS1]

Such features create irregular underground environments where tunnel collapse is more likely. Fault zones, in particular, contain clay layers that become slippery when wet and may shift unpredictably. These risks must be carefully considered from the survey stage through to design and construction.

From 1991 to 1992, five sinkholes occurred during subway construction in Seoul. All were located in areas with metamorphic bedrock, and most were associated with fault zones. More recent sinkholes in Yeonhui-dong, western Seoul; Myeongil-dong, eastern Seoul; and Gwangmyeong, Gyeonggi, share similar geological characteristics. In each case, highly weathered soil and decomposed rock appear to have developed within localized fault zones, raising suspicion that poor rock quality and irregular distribution contributed to the collapses.

Busan poses a different challenge. In Sasang District, the area’s topography combines mountainous terrain with coastal features. The district lies on an alluvial fan formed by sediment flowing down valleys from the surrounding hills. Layers of sand and clay deposited by the Nakdong River create a complex and unstable underground environment. In such conditions, careful attention must be paid to drainage, water-blocking and reinforcement techniques. Without these, open-cut excavation is likely to trigger frequent sinkholes.

First responders oversee a site where a sudden road sinkhole swallowed a Tivoli SUV in Seodaemun District, western Seoul on August, 2024. [YONHAP]

Second, tunnel design must reflect both geological conditions and construction realities.

Typically, borehole drilling is conducted every 100 meters along a proposed tunnel route. However, the areas between boreholes are extrapolated based on assumptions. To compensate, geological experts should be stationed full-time at tunnel sites to evaluate conditions at the tunnel face and advise on reinforcement in real time. This is especially important in areas with metamorphic rock. Yet in practice, geological inspections are often outsourced to monitoring firms, where technicians handle both instrumentation and geological analysis. This approach must be reconsidered and improved.

Third, structural reforms are needed to ensure sinkhole prevention is built into the process.
Korean civil engineering is world-class in many respects, but geological assessments often receive less attention than they deserve. Cost constraints and tight deadlines frequently prevent engineers from customizing designs to suit local ground conditions. In this environment, even experienced professionals may be unable to fully apply their skills. To change this, engineers must be given the autonomy and time to implement the proper measures.

Ultimately, preventing large sinkholes will require more than patchwork fixes or new detection tools. What is needed is an institutional shift in how underground construction is approached — one that integrates deep geological understanding with sustained professional oversight.

Translated from the JoongAng Ilbo using generative AI and edited by Korea JoongAng Daily staff.