A sinkhole incident occurred last year in the Myeongil-dong area of Gangdong-gu, Seoul, during the construction of the first section of the fourth phase of the Seoul Subway Line 9. It has been revealed that a different ground reinforcement method was used than what was proposed in the underground safety impact assessment. Seoul City is facing criticism for approving the design without addressing the differences in methods and whether adequate safety reviews were conducted.
According to materials obtained by Rep. Bae Seung-chan of the Democratic Party, a member of the National Assembly's Land, Infrastructure and Transport Committee, the contractor for the first section of the fourth phase of the Seoul Subway Line 9 incorporated a 'simultaneous injection method' for ground reinforcement in the design before tunnel excavation.
The simultaneous injection method allows for the simultaneous injection of reinforcement materials across multiple sections, which can shorten construction time. However, it has limitations in closely monitoring and supplementing the injection status for each section. This raises concerns that the ground reinforcement performance may not be sufficiently secured in weak ground areas like Myeongil-dong.
In contrast, the underground safety impact assessment for the project proposed a 'multi-stage injection method.' This method involves injecting reinforcement materials in stages and sections, allowing for verification of the injection status at each stage and additional reinforcement where needed, making it relatively advantageous in weak or heterogeneous ground conditions.
However, the contractor's submitted design reflected the simultaneous injection method, and Seoul City awarded it 84 points, deeming it qualified. It appears that there were no specific comments regarding the differences between the two methods or the ground reinforcement performance during the design qualification review process.
Photos from the accident site show some steel pipes that have broken. Images obtained by Rep. Bae's office depict pipes that failed to withstand soil pressure. In contrast, other accident sites presented by the office showed some steel pipes that remained bent under soil pressure.
Another point of contention is whether Seoul City, as the contracting authority, adequately reviewed the safety of the design. There are calls to clarify the circumstances under which the method proposed in the underground safety impact assessment was changed during the design phase and whether Seoul City properly verified the ground reinforcement performance and safety impacts resulting from this change.
Rep. Bae stated, "If Seoul City, as the contracting authority, failed to properly verify the differences in safety due to the method change, it would be a clear case of inadequate management and oversight. If they approved it knowingly, then the sinkhole incident in Myeongil-dong would be a man-made disaster caused by Seoul City."
* This article has been translated by AI.
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