The cause of the large sinkhole that spanned more than four lanes in the middle of a road in Myeongil-dong, Gangdong District, Seoul, was found to be the sliding of discontinuities in a deep weathered zone weakened by sewage pipe leakage and other factors. A deep weathered zone refers to a section deep underground where the ground has weakened after long-term physical and chemical weathering.
The Myeongil-dong sinkhole incident was a case in which ground subsidence occurred on the road at the Daemyung Elementary School intersection in Myeongil-dong, Gangdong District, Seoul, on Mar. 24, creating a large sinkhole measuring 18 meters long, 20 meters wide, and 30 meters deep. One motorcycle rider who fell into the sinkhole died, and one female driver who passed through the road just before the collapse was taken to a hospital. The Ministry of Land, Infrastructure and Transport (MOLIT) formed the Central Underground Accident Investigation Committee (Sajo Group) four days after the accident to determine the cause. The initial activity period was until September, but it was extended once to the end of November.
The Ministry of Land, Infrastructure and Transport on the morning of the 3rd at the Sejong Government Complex announced the Sajo Group's investigation results and measures to prevent recurrence regarding the Myeongil-dong sinkhole in Seoul. The Sajo Group is composed of 12 private experts by field with no conflicts of interest with the ordering authority and contractor for Seoul Subway Line 9 construction.
To determine the cause of the accident, the Sajo Group held 26 meetings, including on-site investigations (collecting samples from the sinkhole section, selecting drilling locations, and surveying underground facilities), quality tests (shotcrete thickness and initial strength tests, test construction of steel pipe reinforcement grouting), hearings of related parties, and cross-review of findings among investigators.
To ensure the accuracy and reliability of the findings, the committee commissioned external specialized agencies to conduct 3D modeling based on drone footage (reproduction of geological structures) and detailed tunnel stability reviews through numerical analysis, analyzing various collapse scenarios.
Based on the investigation to date, the Sajo Group determined that a discontinuity in a deep weathered zone that was not identified at the design and construction stages weakened and slid due to a drop in groundwater level and leakage from sewage pipes. As a result, external forces exceeding the design load acted on the tunnel, causing the tunnel to collapse and the ground to sink.
In particular, the Sajo Group found multiple discontinuities through on-site investigations near the accident site and analysis of drone footage. A wedge-shaped block formed at the intersection of three of these discontinuities was identified as the decisive cause of the sinkhole.
In addition, at the accident site, the groundwater level fell due to the Section 13 tunnel construction of the Sejong–Pocheon Expressway in the past, changing the stress distribution in the ground. As of January 2017, the design groundwater level for Section 13 of the Sejong–Pocheon Expressway was G.L(-)3.1–6.9 meters, but as of January 2022, the design groundwater level for Phase 4 of Urban Railway Line 9 was G.L(-)18.9–25.5 meters, a drop of about 18.6 meters.
The Sajo Group also determined that inadequate maintenance of aging sewage pipes near the accident site led to continuous leakage that weakened the ground, contributing to the sinkhole. The underground facility manager conducted a survey of the sewage pipe in 2022, but no repairs were made for cracks and joint offsets. The Sajo Group also identified one instance of noncompliance with the obligation to prepare a side development map of the excavation face during construction and one instance of inadequate preparation of specifications for ground reinforcement grouting works.
Chairperson Park In-jun of the Sajo Group said, "This accident is not a sinkhole typically caused by subsidence in a limestone terrain," adding, "If the wedge-shaped block is a natural factor, the drop in groundwater level and leakage from sewage pipes can be seen as a man-made disaster."
As measures to prevent recurrence, the Sajo Group proposed: strengthening design and construction management by reducing the spacing of ground investigations and recommending that daily excavation speed and volume be reflected in construction plans; strengthening groundwater management by improving procedures related to groundwater level declines and recommending non-drainage tunnel construction (such as TBM) in deep weathered zones in urban areas; strengthening underground facility management by enhancing ground surveys, replacing aging sewage pipes near excavation works, investigating groundwater components inside tunnels, and forming a consultative body with related agencies; and strengthening tunnel stability by applying triple-row overlapping steel pipe reinforcement grouting methods in deep weathered zones in urban areas and reinforcing the evaluation system for excavation faces.
TBM construction is a method that uses equipment to seal the excavation face with earth and water pressure to prevent the outflow of groundwater or soil, completing the tunnel as the ground is excavated.
Chairperson Park In-jun said, "TBM construction costs about 1.5 to 2 times more than the existing method, so this was an unusually strong request," adding, "We plan to organize and supplement the investigation findings and submit the final report to the Ministry of Land, Infrastructure and Transport in December." Park added, "To prevent a recurrence of similar accidents, we expect swift institutional improvements and follow-up measures by related agencies, including the Ministry of Land, Infrastructure and Transport."