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Busan Metropolitan City Audit Committee Announces Results of Special Investigation into the “Sasang–Hadan Line Ground Subsidence Accident”

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Busan Metropolitan City’s Audit Committee (Chairperson Yoon Hee-yeon) announced today (13th) the results of the special investigation conducted over 31 days from April 28 to June 13 regarding the “Busan Metro Sasang–Hadan Line Ground Subsidence Accident.”

○ This special investigation was undertaken following the findings of the “2024 Audit of the Sasang–Hadan Line Construction Project,” which indicated that the causes of ground subsidence in Section 2 were not limited to heavy rainfall and deteriorated sewer pipes, but were also related to the metro construction work. It was launched to address public concerns and distrust regarding the causes of the twelve ground subsidence accidents that occurred in Construction Section 1 (Saebyeok-ro).

The Audit Committee called for ▲seven administrative actions (four cautions and three notifications) and ▲forty-five personnel actions (one institutional warning, three disciplinary measures, thirty-one warnings, and ten cautions).

○ Additionally, the Committee notified the Busan Transportation Corporation to impose administrative penalties, such as point deductions, in accordance with relevant regulations for issues identified in relation to the contractor and the construction management consultant.

A total of twelve ground subsidence accidents occurred in Construction Section 1 (Saebyeok-ro) between January 2023 and April 2025.

○ Of the twelve accidents, ten occurred at intersections where the “H-pile + concrete lagging wall + low-pressure cutoff SGR method” was applied. Eight of these cases (Nos. 4–9, 13, and 14) were found to involve horizontal grouting executed after excavation without cutoff construction.

○ Subsidence at locations where the “CIP overlapping bored pile method” was applied (Nos. 2 and 12) was found to be limited to several tens of centimeters, caused by poor backfilling and compaction.

The main cause of ground subsidence near intersections was determined to be the prolonged outflow of groundwater and fine soil into the worksite, resulting from horizontal grouting carried out after excavation and retaining wall installation without prior cutoff construction.

○ The Busan Transportation Corporation approved the design change from the “CIP overlapping bored pile method” to the “H-pile + concrete lagging wall + low-pressure cutoff SGR method” to address traffic congestion complaints and utility conflicts at intersections, without following the required review procedures for construction method changes.

○ When the contractor found that the equipment required for the low-pressure cutoff SGR method interfered with underground utilities and could not be used, it proceeded with excavation and horizontal grouting without approval from the construction management consultant, deviating from the design documents and ultimately causing the ground subsidence.

The construction management consultant failed to take measures such as suspending the work or ordering rework, even though the contractor carried out horizontal grouting without approval during temporary retaining structure construction. The consultant only submitted an actual work report on the horizontal grouting to the Busan Transportation Corporation in February 2025, revealing inadequate supervision and oversight.

○ Despite receiving the report on the horizontal grouting, the Busan Transportation Corporation merely processed it internally without issuing instructions for design changes or going through approval procedures, neglecting its duty of oversight and inspection.

As a result of the special investigation, the Audit Committee pointed out ▲insufficient review of design changes and construction management for temporary retaining structures; ▲inadequate review and verification of construction management reports; ▲insufficient coordination and maintenance of underground utilities; and ▲improper drainage handling such as failure to report temporary use of public sewer systems. Based on these findings, the Committee issued an “institutional warning” to the Busan Transportation Corporation and requested disciplinary measures, warnings, or cautions for responsible personnel.

○ In particular, the head of the Facilities Construction Division was subjected to disciplinary action above a light penalty for failing to properly direct and manage the construction management team’s supervision responsibilities. The director of the Facilities Construction Department was also disciplined above a light penalty for repeatedly responding with the complacent assumption that “old sewer pipes are the cause,” thereby failing to fulfill accident-prevention responsibilities despite recurring incidents.

○ The construction management consultant neglected its supervisory responsibilities, taking no action even though it submitted inadequate construction management reports lacking essential items such as earthwork and temporary structure construction photos and the list of personnel involved.

○ The consultant also failed to properly coordinate with relevant agencies and to obtain necessary permits and approvals regarding the relocation and protection of underground utilities (sewage and stormwater).

○ Furthermore, the contractor failed to properly treat discharge water (groundwater mixed with fine soil particles and inflow of stormwater) generated during excavation, thereby disrupting the function (flow) of the public sewer system.

Yoon Hee-yeon, Chairperson of the Audit Committee, stated, “This special investigation has resolved citizens’ fundamental concerns and safety anxieties regarding why ground subsidence has repeatedly occurred exclusively in the Saebyeok-ro section since the start of subway construction.” He added, “It has also provided an opportunity to improve structural issues in the Busan Transportation Corporation’s accident-response practices and reinforce accountability in the command and oversight of risk management.”