On March 31 at 10 a.m. at the intersection near Daemyung Elementary School in Myeongil-dong, Gangdong-gu, Seoul, people wearing safety helmets were bustling about. This is the site where a large sinkhole, measuring 20 meters in diameter and depth, occurred a week earlier, on March 24.
The sinkhole observed from the outside was so massive that it swallowed five lanes of a six-lane road. The remaining lane was barely held together by asphalt and concrete. The depth was difficult to gauge with the naked eye, but if translated to the height of an apartment, it exceeds seven stories.
Twenty-one personnel, including officials from the Seoul Arisu Headquarters and Gangdong District Office, formed a line. Dressed in safety helmets, the employees climbed the steps of a flower shop right in front of the sinkhole to take pictures of the sinkhole and its surroundings.
In the afternoon, 12 Commissioners from the Ministry of Land, Infrastructure and Transport came out for a site investigation. They examined not only the outside but also entered the sinkhole to inspect the interior. Drones were also deployed. The drones recorded the site, flying in and out of the sinkhole based on the pilot's input.
An official from the Commission noted, "We examined the external damage, the internal situation of the sinkhole, and the ongoing construction to confirm whether there were any additional risks and said, 'We will investigate to resolve all suspicions.'
◇ Citizens living in fear... Gas station owner said, "I take sleeping pills every day to rest."
At the accident site, police lines were drawn, and the area was cordoned off. Passersby expressed their concern, inquiring about the status of the sinkhole and the recovery efforts.
Residents asked the gas station owner, who could not operate and was only painting, 'When will you open?' and 'Will we receive compensation?' The gas station owner sighed, saying, 'I don't know how long we won't be able to operate. If the government provides compensation, how much will it be?'
Oh Doo-il (60), a resident passing by the sinkhole, stopped and stared at it for a long time. Oh said, 'This intersection is a frequent site for sinkholes,' and added, 'I didn't even imagine such a large sinkhole would form.' Resident Kim Myung-guk (64), who was a regular at the gas station, said, 'I now feel uneasy even passing this area.'
A week has passed since the Ministry of Land, Infrastructure and Transport began its accident investigation, but the exact cause of the accident has not yet been identified. Experts and local residents speculate that construction on the 4th phase extension of Seoul Subway Line 9, conducted underground at the site, is a key cause.
It is also suggested that the combination of subway construction and water leaks from the sewage system may have triggered the accident. Indeed, evidence of damage to a water main and leakage has been detected below the accident site. Both Seoul City and the Ministry of Land, Infrastructure and Transport are focusing on complex causes and plan to clarify the exact reasons through in-depth investigations.
◇ Ground subsidence incidents occur successively in Seoul and Busan.
Before the official announcement of the cause of the Myeongil-dong sinkhole accident, another sinkhole incident occurred.
On the morning of the 13th, a sinkhole measuring 5 meters in width, 3 meters in length, and 5 meters in depth occurred near a city subway construction site in Hakhjang-dong, Sasang District, Busan. Fortunately, there were no casualties.
In that area, six ground subsidence incidents occurred, including one in September of last year. In September, a large sinkhole formed, causing two trucks to fall 8 meters below the surface.
On the same day in the morning, a sinkhole also appeared near Aeogae Station in Mapo District, Seoul. Although it was small, measuring 40 cm in diameter, its depth reached 1.3 meters. When the point directly below the sinkhole was excavated, a water pipe about 60 cm in diameter was found to be cracked.
Local authorities suspect the cause of this incident is a cavity formed due to a crack in the sewage pipe. Water leaking from the sewage pipe weakened the deposition layer, resulting in ground subsidence.
◇ Investigating the causes of sinkholes, including leaks from aging pipes and subway construction.
Over the past five years, the ground has collapsed once every two days. According to data submitted by Democratic Party of Korea lawmaker Hwang Hee from the Korea Safety Management Agency, a total of 957 sinkholes occurred across the country from 2019 to 2023. By year, there were 193 in 2019, 284 in 2020, 142 in 2021, 177 in 2022, and 161 in 2023.
By region, Gyeonggi Province had the highest number with 197 cases, followed by Gwangju (122), Busan (85), Seoul (81), Jeonbuk (70), Gangwon (68), and Daejeon (66).
The most common cause of accidents was damage to sewage pipes, accounting for 446 cases (46.6%). Poor compaction (backfilling) was cited in 171 cases (17.9%), deficient excavation work in 82 cases (8.6%), other buried object damage in 64 cases (6.7%), and damage to water mains in 39 cases (4.1%).
"Ground subsidence" occurs due to the aging of underground facilities, poor compaction after construction, and the impacts of surrounding construction. Insufficient management of groundwater and soil that arises during excavation, along with construction defects and failures in tunnel face management, can also be causes.
The recent Myeongil-dong incident is presumed to be a complex accident caused by a combination of ground weakening due to the 4th phase extension of Subway Line 9 and water leakage. In particular, this incident is likened to the large sinkhole that occurred in the Seokchon underpass in Songpa District, Seoul, in August 2014, where construction on Subway Line 9 was directly pointed out as the cause.
Professor Park Chang-kun, who led the accident investigation team at Catholic Kwandong University, said, "Seoul City habitually attributes sinkhole causes to aging water and sewage pipes, but damage to sewage pipes generally occurs at a depth of 2-3 meters underground and is small-scale. A large sinkhole occurring at a depth equivalent to seven stories of an apartment, as in this incident, is likely primarily due to deficiencies in the construction and supervision of major underground works."
Professor Park emphasized, "It is crucial to trace whether the ground reinforcement methods that should have been applied during the underground construction process were implemented properly." He added, "When tunnels are dug without reinforcement to firmly secure the surrounding soil and sand layers, shear failure can occur, resulting in ceiling collapse. It appears that construction was carried out aggressively without proper reinforcement based on the nature of the ground."
It is suggested that leakage may have caused the accident. Professor Gong Ha-sung from Woosuk University emphasized, "When water and sewage systems age, water leaks cause the soil within the ground to erode. When the road surface is continuously subjected to vehicular loads, it ultimately leads to the road collapsing."
Professor Gong highlighted the importance of proactive improvements to aging water and sewage systems. He stated, "To prevent sinkhole incidents, it is necessary to establish the lifespan of aging underground pipes and conduct regular inspections to proactively replace them."
An official from the Ministry of Land, Infrastructure and Transport said, "We plan to form an accident investigation committee with experts not related to the construction project of Subway Line 9 to conduct an independent and objective investigation. We will thoroughly identify the technical causes while keeping various possibilities open, including subway construction and damage to sewage pipes, to ensure that the suspicions are resolved through a multifaceted approach."