On the 11th, a collapse occurred at the construction site of the Shinansan Line in Iljik-dong, Gwangmyeong City. The underground tunnel construction site being worked on and the upper road collapsed. Among the 18 workers present at the site, 2 became trapped and missing. Approximately 13 hours after the accident, one missing person was rescued, but the other was found dead on the 16th, five days after the incident.
As of the 21st, the exact cause of the collapse at the Gwangmyeong Shinansan Line underground tunnel construction site has not been precisely identified. However, past environmental impact assessments in the area raised the possibility of 'ground subsidence due to large-scale groundwater drainage,' leading to speculation about a potential connection to this accident.
According to the 'Shinansan Line double-track electric railway private investment project environmental impact assessment (2019 report)' released by the office of Democratic Party of Korea member Park Yong-gap, the Ministry of Environment pointed out concerns regarding 'structural stability issues and the impact on nearby groundwater facilities due to large-scale groundwater drainage.'
◇ Ground subsidence incidents concentrated in summer… emerging as a 'flood control issue'
The collapse of the Shinansan Line tunnel construction incident is included in ground subsidence incidents. Ground subsidence incidents refer to the phenomenon where surrounding ground sinks during underground development or the use and management of underground facilities. The phenomenon commonly referred to as 'sinkholes' is also a type of ground subsidence.
According to the '2024 underground safety statistical annual report' published by the Korea Land and Geospatial Informatrix Corporation in September of last year, there were 957 cases of ground subsidence incidents in South Korea over five years from 2019. Monthly analysis shows that August had the most with 241 cases, followed by June (138 cases) and July (130 cases). More than 53.2% of all ground subsidence incidents occurred in the summer months of June to August.
Experts analyze that the high frequency of ground subsidence incidents in summer is due to rainfall effects. Rainwater on the surface penetrates into the ground, weakening the soil's stability, or flows through cracked drainage pipes, leading to soil erosion.
Therefore, academia points out the need for a new 'flood control' management that transcends the issues of river flooding and inundation caused by heavy rain. The Korea Land and Geospatial Informatrix Corporation stated, 'It is analyzed that the damage to drainage pipes during heavy rain periods and insufficient compaction after excavation lead to ground subsidence' and emphasized the necessity for focused inspections such as visual surveys and joint inspections of the ground surrounding underground facilities in preparation for the rainy season.
The government also reported that urgent repairs to aging sewer pipes are needed and has allocated 125.9 billion won in the recently announced supplementary budget to prevent sinkholes.
The government allocated 55.6 billion won for the replacement of aging sewer pipes at 65 locations, including 27 in Seoul and 38 in regional areas. This budget is intended to subsidize the replacement projects for old pipes promoted by local governments. The government also reflected 70.3 billion won in the budget to comprehensively repair 500 kilometers of aging paved roads and to double the sinkhole exploration areas.
Local governments have faced criticism for being passive in the management and repair of waterworks. According to the Seoul Financial Portal, the execution rate of Seoul's waterworks and water quality budget reached around 50% in 2016, but after dropping to the 6% range in 2018, it has remained in the 2% range since 2021. The city of Seoul has also been unable to explain why the related budget execution rate is so low.
A Seoul city official stated, 'I don’t know how the execution rate was calculated' and mentioned, 'It seems that the execution rates of some projects did not link properly, resulting in the low recorded execution rate.' This indicates the city's indifference to the execution and management of waterworks and water quality budgets.
◇ GPR exploration also has limitations… precise geological surveys are needed to create a 'geological map'
Seoul City analyzed the risk of ground collapse and created a 'ground subsidence safety map (priority maintenance area map)' dividing the entire city into five levels, but has not made it public. The city reportedly created this map to enhance the efficiency of ground-penetrating radar (GPR) exploration.
GPR is a measurement method that uses electromagnetic waves to detect underground voids or soil imbalances without excavating the ground. It is currently a non-destructive technology that can confirm ground stability, but there are also opinions that its preventive effect against sinkholes is limited due to its measurement depth being restricted to 2 meters underground.
In fact, the site of the 2.5-meter deep sinkhole incident that occurred in Yeonhui-dong, Seoul, last August had undergone GPR exploration three months prior, but no significant abnormal signs were detected. Geologist Lee Soo-gon, a former professor in the Department of Civil Engineering at the University of Seoul, noted, 'GPR, which estimates the density of the ground, can only detect down to 2 meters underground. Therefore, while it can assist in road maintenance, it cannot detect large sinkholes.'
Ultimately, the consensus among experts is that precise investigations of the geology and faults where sinkhole phenomena occur must precede any actions. Lee added, 'Countries like the UK and Hong Kong have invested significant time and resources to create geological maps of the underground. We lack such underground information,' and emphasized, 'The Seoul area features metamorphic rock terrain, leading to severe weathering and alteration underground. Developing this area without any information is akin to driving without a navigation system.'
Professor Lee further mentioned, 'In our country, there tends to be negligence in geological surveys during design and construction phases of large-scale civil engineering projects.' He pointed out that the irregular underground water distribution in metamorphic rock areas can lead to collapse incidents during tunnel construction. Considering this, it is necessary to conduct geological surveys regularly during the investigation, design, and construction phases and apply reinforcement techniques accordingly.