The cause of the ground subsidence accident that occurred in July at an excavation site in Imun-dong, Dongdaemun-gu, Seoul, was found to be "poor construction." The Seoul Metropolitan Government said it asked the relevant departments (agencies) to impose administrative penalties for violations confirmed by the investigation, including suspension of business for the contractor (4 months) and suspension of duties for the supervisor (up to 2 years).
The Seoul Metropolitan Government said on the 25th that the Seoul Metropolitan Government Underground Accident Investigation Committee (hereinafter "the committee," Chairperson Lee Jong-seop of Korea University) announced the results of its investigation into the Imun-dong ground subsidence accident.
The accident occurred at about 7:33 p.m. on July 23 this year on the sidewalk adjacent to an excavation site on 28-gil, SiniMun-ro, Dongdaemun-gu, Seoul. Next to the spot where the sinkhole formed, construction was underway for an underground parking lot at the Imun 2-dong complex government building. The accident created a sinkhole measuring 13.5㎡ in area and 2.5 m deep. One man in his 70s was temporarily trapped inside a building because the entrance would not open as the building tilted. There were no injuries, but the building was demolished.
The committee said the main causes of the accident were poor "retaining wall" work to secure the stability of the excavation face in weak ground and poor "cutoff construction" to block groundwater inflow. Repeated leakage and loss of soil from the retaining wall created voids underground, and as the range of leakage expanded on the day of the accident, the ground subsided.
Specifically, during concrete pouring for the retaining wall (CIP), a tremie pipe specified by the construction standards was not used, causing material separation. As a result, under conditions of high groundwater flow, concrete loss was exacerbated and the foundation portion was constructed incompletely. The underground safety assessment report requires additional reinforcement if groundwater leaks, but this work was not sufficiently carried out.
To prevent similar accidents from recurring, the Seoul Metropolitan Government will revise the underground safety assessment manual to strengthen instrumentation management standards. It also plans to require immediate response regardless of thresholds when abnormal displacement is detected by multiple instruments.
It will establish a ground subsidence monitoring network by installing observation sensors down to a maximum depth of 20 m underground and expand ground-penetrating radar (GPR) surveys on roads around excavation sites. GPR surveys, currently conducted once a year, will be increased to once a month until within one year after completion.