Busan, South Korea – Two ground subsidence incidents in Busan have highlighted significant challenges in the city’s urban infrastructure, prompting safety concerns and urgent responses from local authorities. The accidents, which occurred on August 20 and 21, involved sinkholes forming in two key areas, disrupting daily life and exposing vulnerabilities in construction practices.
On August 20, a sinkhole measuring 7 meters in width, 5 meters in length, and 3.5 meters in depth appeared at Saebyeok-ro 122 in Gamjeon-dong. Just a day later, another smaller sinkhole, roughly 1 meter in all dimensions, was reported beneath the Dongseo Elevated Highway in Hakjang-dong. Although no injuries were reported, the second incident caused minor damage to a vehicle.
The Busan Metropolitan Government, in collaboration with its Geotechnical Accident Investigation Committee, has conducted detailed investigations into the causes of these incidents and proposed a series of preventive measures to ensure public safety.
Root Causes: Rainfall, Design Changes, and Structural Weaknesses
The investigation revealed that the first incident was the result of design alterations in the retaining walls during construction. Originally designed with Continuous Flight Auger (CFA) piles for water-tightness, the retaining walls were later modified to use H-piles and Soil Grouting Reinforcement (SGR) due to underground utility constraints. However, this alternative approach proved insufficient for the silty sand layers in the area, particularly under high groundwater pressure.
Heavy rainfall on the day of the incident further exacerbated the situation, causing significant backflow in sewage pipelines. The difference in water levels on either side of the retaining wall led to soil particles being washed out through weak points in the grouting, ultimately resulting in the collapse.
The second incident, in contrast, stemmed from structural failures at the junction of old and newly installed stormwater boxes. The damage caused water to erode the soil beneath the boxes, which, combined with continuous vehicle traffic, led to the ground caving in.
Continuous Flight Auger (CFA) piles involve using a continuous flight auger drill bit to excavate soil while simultaneously injecting concrete into the drilled shaft. This method prevents soil collapse during excavation and efficiently forms a pile in one continuous operation.
Soil Grouting Reinforcement (SGR) involves injecting grout (a mixture of cement and water) into the soil to enhance its stability and water-tightness. This method is often employed in areas where the soil is weak or water permeable, ensuring the structural integrity of retaining walls and reducing groundwater movement.
These incidents are part of a series of ground subsidence events reported along the Sasang–Hadan Line construction route. Over the past six months, eight such incidents have occurred, with the most severe on September 21 in Hakjang-dong, Sasang-gu. In this instance, a massive sinkhole measuring 10 meters wide, 5 meters long, and 8 meters deep caused a fire truck and another vehicle to fall into the opening.
In response to these incidents, the Ministry of Land, Infrastructure, and Transport conducted a special inspection of the affected areas, emphasizing the need for enhanced safety measures. The ministry highlighted the challenges posed by Busan’s soft ground conditions, urging contractors to adopt tailored engineering solutions to mitigate risks.
The Busan Metropolitan Government has unveiled a comprehensive action plan to address the findings of the recent investigations and enhance urban safety. A central focus of the plan is conducting detailed ground subsidence risk evaluations in all areas where design changes were made during construction. This initiative aims to ensure full compliance with the Underground Safety Act while verifying the stability of the affected subsurface areas.
To bolster structural defenses, the city plans to employ high-pressure jet grouting techniques, particularly in zones with high groundwater pressure. This method is expected to significantly improve the water-tightness of retaining structures and prevent further incidents. Additionally, soil-cement filling behind retaining walls will undergo thorough inspections. Any identified deficiencies will be promptly rectified, and accumulated sediment or leaked water will be removed to restore and maintain the structural integrity of the walls.
The action plan also includes rigorous inspections of all temporary retaining structures. Where necessary, vertical reinforcement grouting will be applied to provide added stability and reduce the likelihood of future incidents. Furthermore, the city will reassess the placement and calibration of monitoring instruments used to detect subsurface changes. Enhancing these systems is a priority to ensure that potential risks are identified early, allowing for timely interventions.
The incidents have sparked widespread concern among Busan residents, many of whom are questioning the safety of ongoing construction projects. The recurring nature of these subsidence events underscores the challenges of urban development in a city with complex geological conditions.
Mayor Park Heong-joon has vowed swift action, stating, “The safety of our citizens is our highest priority. We will implement all recommended measures and work toward comprehensive policy improvements to prevent future incidents.”
The Busan Metropolitan Government has also pledged to complete its ongoing investigation into the September 21 sinkhole and incorporate its findings into a long-term strategy for subterranean safety. “These incidents are a wake-up call,” said an official from the city’s Geotechnical Accident Investigation Committee. “We must adopt a proactive approach to safeguard Busan’s infrastructure and ensure the well-being of its citizens.”
As the city moves to address these challenges, restoring public trust remains a critical goal. The incidents have not only disrupted daily life but also raised questions about the oversight and accountability of construction practices in Busan. By taking decisive action and maintaining transparency, the city aims to reassure its residents and set a new standard for urban safety.
While the immediate focus is on rectifying the issues along the Sasang–Hadan Line, these efforts could serve as a blueprint for preventing similar problems in other rapidly developing urban areas. For a city like Busan, where modern infrastructure must coexist with challenging natural conditions, ensuring safety is not just a priority—it’s a necessity.
