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Busan Metropolitan City Announces Results of Sinkhole Accident Investigation on Saebyeok-ro in Sasang-gu – Full Efforts to Prevent Recurrence

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Busan Metropolitan City (Mayor Park Heong-joon) and the Underground Accident Investigation Committee (Chairperson Lim Jong-cheol) announced the results of the investigation into the circumstances and causes of the sinkhole accidents that occurred on Saebyeok-ro in Sasang-gu on April 13 and 14.

Status of Sinkhole Accidents on Saebyeok-ro, Sasang-gu (April 2025)

No.	Date of Accident	Location	Width (m)	Length (m)	Depth (m)	Damage
1	April 13, 2025 (Sun)	99 Saebyeok-ro, Sasang-gu (under Dongseo Overpass)	3.0	4.5	5.0	None
2	April 14, 2025 (Mon)	140 Saebyeok-ro, Sasang-gu (Kocom Intersection)	0.8	0.8	0.5	First detection

On April 13 and 14, two ground subsidence accidents occurred at 99 and 140 Saebyeok-ro in Sasang-gu. Accordingly, from April 18, the city organized and operated the Underground Accident Investigation Committee (hereinafter “the committee”), composed of eight experts in geology and geotechnical fields.

The committee identified the causes of the accidents through thorough field investigations and data analyses, including: ▲ ground and geological characteristics ▲ fluctuations in groundwater levels ▲ conditions and impacts of underground facility installations ▲ status of retaining structures. It also formulated measures to prevent recurrence.

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The committee determined that the main cause was the inadequate construction quality of the cutoff wall during the “Sasang–Hadan Line Urban Railway (Section 1)” project adjacent to the accident site, which failed to properly block groundwater leakage.

As the first direct cause, the committee pointed to careless cutoff work during temporary retaining wall construction, which allowed groundwater and soil particles to escape and form small underground cavities prior to the accident.

Initially, considering that the ground at the accident site was sedimentary (silty sand) and weak, the retaining wall had been designed as “C.I.P overlapped bored piles,” which do not require additional cutoff walls. However, due to potential interference with underground pipelines such as water and sewer pipes and concerns over traffic complaints, the design was changed to “H-Pile + Earth Retaining Wall Concrete,” with an additional “SGR cutoff method.”

During construction, grout material (cutoff chemical solution) was not sufficiently injected into ground voids, and some sections around underground facilities were not properly treated. This resulted in groundwater and soil particle leakage.

Compared with the geotechnical survey conducted in 2015, the groundwater level had dropped significantly (by 2–3.6m). The fact that multiple horizontal grouting works (to reinforce leaks in cutoff walls) were performed suggests frequent groundwater leakage during excavation, which may have worsened during grouting and led to soil particle loss.

Thus, leakage in the cutoff wall during excavation lowered the groundwater level, soil particles escaped, and small underground cavities formed before the sinkhole occurred.

Furthermore, accumulated rainfall and leakage from sewer facilities caused the cutoff wall to lose its sealing function, leading to a large outflow of groundwater and soil particles, the expansion of underground cavities, and ultimately, sinkhole formation.

At the Dongseo Overpass accident site (April 13), clusters of abandoned pipelines penetrating the side wall of a stormwater box remained cut but not removed, causing continuous leakage. At the Kocom Intersection site (April 14), a sewer pipe subsided and was damaged due to a small sinkhole, leading to leakage.

At both locations, the sewer system damage and accumulated rainfall (April 12–13) locally raised groundwater levels. This, combined with the head difference from the excavation zone, caused partial loss of cutoff function. Subsequently, large amounts of soil particles escaped along with water leakage, leading to the final sinkhole events.

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The committee recommended multiple measures to prevent recurrence, including: ▲ reinforcement of cutoff walls ▲ repair of damaged underground pipelines ▲ establishment of an automated monitoring system based on real-time measurements ▲ risk assessment of ground subsidence.

Because high groundwater levels are present, greater excavation depth may result in significant leakage and settlement from large water level differences. Therefore, vertical high-pressure jet grouting should be applied (instead of horizontal methods) to strengthen cutoff functionality.

The committee further recommended: ▲ repairing damaged underground facilities ▲ establishing an automated real-time monitoring system ▲ selecting appropriate monitoring points and conducting comprehensive analysis of measurement validity ▲ carrying out ground subsidence risk assessments across sections where construction methods were changed ▲ establishing follow-up response plans and construction progress under expert technical consultation.

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To respond to ground subsidence accidents, since April the city has been operating a permanent special task force (TF) on “road ground subsidence (sinkholes),” led by the Vice Mayor for Administrative Affairs.

This TF, consisting of five city departments (Citizen Safety Office, Urban Innovation & Balance Office, Urban Spatial Planning Bureau, Environmental Water Policy Office, Busan Water Authority), Sasang-gu Office, and Busan Transportation Corporation, has convened 17 meetings in the past six months to share all information closely and prevent further accidents.

Efforts include: ▲ concentrated ground-penetrating radar (GPR) surveys of the Saebyeok-ro area (twice or more monthly) ▲ repair and reinforcement of the Gamjeon 1 Stream sewer box ▲ real-time leak monitoring of water pipelines (56 locations, ongoing) ▲ reinforcement of groundwater level monitoring (expanded from 61 locations twice a week to 108 locations three times a week) ▲ ground subsidence risk assessment ▲ maintenance of old sewer facilities (side gutters) in the Saebyeok-ro area.

The full accident investigation report is available on the city’s website under “Field Information > Safety > Real-time Safety Information > Ground Subsidence” (www.busan.go.kr/depart/GPR).

Min Soon-ki, Director General of the Urban Spatial Planning Bureau, stated, “The purpose of the investigation is to identify the causes and provide direction to prevent similar accidents in the future. We will ensure that the committee’s recommended measures are promptly implemented by relevant agencies such as Busan Transportation Corporation, and we will make every effort to prevent recurrence through the permanent road ground subsidence (sinkhole) special task force.”