Investigation reveals combined failures behind the fatal Seoul sinkhole collapse

A government investigation has concluded that the March 2025 sinkhole in Myeongil-dong, which resulted in one fatality, was caused by weakened ground conditions driven by human-related factors. The Central Underground Accident Investigation Committee under the Ministry of Land, Infrastructure and Transport found that long-term groundwater decline, combined with leakage from deteriorated sewage pipes, critically reduced the stability of a deep weathered zone beneath the roadway.

The collapse occurred above an active excavation site for the Seoul Subway Line 9 extension. Investigators determined that a discontinuity surface within the weathered rock mass had not been identified during earlier design stages. Over time, the drop in groundwater levels, linked to past tunnel construction for the Sejong–Pocheon Expressway, altered stress distributions and reduced effective support and in turn shear strength within the subsurface. Measurements showed groundwater depths decreasing from 3.1–6.9 metres in 2017 to as much as 25.5 metres by 2022.

Additional weakening occurred due to continuous leakage from aging sewage pipes. Although a condition survey in 2022 identified defects, no structural repairs were carried out. This allowed progressive soil softening and loss of confinement around the tunnel crown. When the discontinuity slipped, the resulting load exceeded the tunnel’s design capacity, causing structural failure and rapid ground subsidence.

The committee used field drilling, laboratory testing, drone-based 3D geological reconstruction and numerical modelling to validate the collapse mechanism. A wedge-shaped block bounded by multiple discontinuities was confirmed as the critical failure element.

To prevent similar incidents, the investigation recommends reducing ground investigation spacing, improving groundwater management procedures, and adopting non-drainage tunnelling methods such as TBM in deep weathered zones. It also calls for accelerated replacement of aging underground utilities and enhanced real-time monitoring during urban excavation works. On the other end of the spectrum, a civic group has criticised the investigation, stating that design and construction faults were overlooked and that the response was significantly delayed. They argue that the late formation of the committee and inadequate analysis weakened public confidence. The group demands restructuring of the committee and insists that victims and local residents participate directly in future investigations.

Following, our recent article, discussing the ground stability concerns for Seoul's underground infrastructure.