The investigation found that a fatal accident caused by a collapsed retaining wall on an overpass near Gajang Intersection in Osan, Gyeonggi, on July 16 last year was influenced by systemic deficiencies at every stage, including design, construction, and maintenance.
The Ministry of Land, Infrastructure and Transport and the Central Committee for Facility Accident Investigation on the Osan Retaining Wall Collapse (the committee) on the 26th announced the accident investigation results containing these findings and measures to prevent similar accidents at Government Sejong Complex.
The committee was composed of 10 civilian experts from various fields, including academia and industry. To identify the cause objectively, it conducted site investigations; reviewed related materials such as design documents; held hearings with parties related to the accident; and carried out ground investigations and quality tests through external services, holding a total of 21 committee meetings over seven months.
The committee determined that the collapse occurred because a large amount of rainwater that flowed into the mechanically stabilized earth (MSE) retaining wall did not drain properly, increasing the pressure (hydraulic pressure) acting on the wall. It said this resulted from combined deficiencies spanning all stages, including design, construction, and maintenance.
Committee Chairperson Kwon O-gyun said, "Rainwater continuously flowed into the MSE retaining wall through the drainage channel and cracks in the pavement on top of the wall, weakening the backfill, and as the L-shaped retaining wall installed at the top of the MSE wall settled, subsidence and cracking occurred in the pavement."
Just before the accident, heavy rain of 39.5 mm per hour increased the inflow of rainwater through cracks and subsided areas, and because this inflow did not drain properly, the pressure (hydraulic pressure) acting on the MSE retaining wall increased, leading to the collapse.
The committee concluded that the background to the collapse of a retaining wall built in September 2023, just two years later, was systemic deficiencies at every stage involving the design firm, the builder, the supervisor, and Osan City, the management authority.
First, the design firm should have conducted a thorough risk analysis of the composite structure in which an L-shaped retaining wall was installed on top of the MSE retaining wall, but its review was inadequate. It also should have established appropriate drainage measures to prevent hydraulic pressure in the MSE wall, but the drainage design was insufficient, and it did not clearly present quality standards for backfill materials, leading to poor construction.
Hyundai Engineering & Construction, the builder, inappropriately used soil with a high content of fine particles that do not drain well as backfill. Whether approval was granted for material (MSE block) changes and whether quality tests were conducted was unclear (no records), and construction quality issues were confirmed, including submitting the original design drawings, which did not reflect design changes, as the as-built drawings. In addition, the supervisor and inspector failed to properly manage and supervise such improper handling by the builder.
It also emerged that the collapsed retaining wall had been left out of the management system for a long period. Although the wall was completed in 2011, it was not handed over to the management authority until 2017, and it was not registered in the Facility Management System (FMS) until it opened in 2023, leaving legal obligations such as safety inspections unfulfilled for an extended period.
There had been two previous MSE retaining wall collapses in sections built by Hyundai Engineering & Construction, but reviews of the safety of MSE walls in the section and measures to prevent recurrence were insufficient. Even in the detailed safety inspection conducted in 2023, problems such as poor drainage and bulging were pointed out, but actions taken were inadequate.
In addition, from about 20 days before the accident until the day of the accident, multiple civil complaints were filed through the e-People system regarding pavement subsidence and concerns about collapse in the accident section, but the management authority did not take proactive measures such as cause analysis or safety reviews.
As measures to prevent recurrence, the committee proposed improving construction standards, strengthening maintenance systems, and conducting special inspections of MSE retaining walls. For composite structures where an L-shaped wall is installed on top of an MSE wall, as in the facility involved in this accident, it will specifically establish design and construction standards, including load application and construction methods, and significantly strengthen design standards for drainage facilities such as drainage channels and perforated pipes for MSE walls.
To ensure that finished facilities defined under the Facility Safety Act are not omitted from the FMS, authorities will periodically check FMS registration and the submission of design documents, and if unregistered facilities are identified, they will be required to register through a compliance order from the Ministry of Land, Infrastructure and Transport (MOLIT). To strengthen penalties for nonregistration and non-submission of design documents, the government will pursue amendments to the Enforcement Decree of the Facility Safety Act.
To prevent similar accidents, cases where bulging occurs in an MSE retaining wall or where there is concern about significant rainwater inflow through cracks will be designated as major defects under the Facility Safety Act and related regulations, so that repairs and reinforcement can be made in a timely manner. In addition, a full-scale survey will be conducted nationwide on composite-structure MSE walls and drainage designs, and special inspections will be carried out for facilities found lacking, with safety reviews and repair or reinforcement measures implemented as necessary.
Chairperson Kwon O-gyun said, "This accident is the result of systemic deficiencies that occurred throughout the construction process, including design, construction, and maintenance," adding, "Thorough implementation of countermeasures by relevant agencies is necessary to prevent similar accidents from being repeated."
The Ministry of Land, Infrastructure and Transport plans to revise related laws and standards to reflect the committee's findings, while taking steps to ensure that administrative dispositions and investigations proceed swiftly regarding the parties responsible for the accident.
Park Myeong-ju, acting director-general for Technology and Safety Policy at MOLIT, said, "Since there are overarching problems in design, construction, management and supervision, handover, and maintenance, after notifying the relevant agencies, if clear wrongdoing is revealed through the processes of administrative authorities and police investigations, we plan to implement appropriate administrative actions and dispositions."