It was found that last year's collapse of the Sinansan Line tunnel in Gwangmyeong, which left two casualties, was caused by human error. From design mistakes to improper construction and supervision, there were deficiencies throughout every phase of the project. The government plans to pursue business suspension against the builders, designers, and supervisors for this project, including POSCO E&C, based on these findings.
The Construction Accident Investigation Committee for the collapse in section 5-2 of the Sinansan Line in Gwangmyeong said on Apr. 2 at the Government Complex Sejong in Eojin-dong, Sejong, "Due to a load calculation error at the design stage, the central pillar, a key component of the twin-arch tunnel, lacked structural stability, and improper construction management, including failure to identify a fault zone in the ground of the accident section and noncompliance with the safety management plan, led to the collapse of the central pillar and the tunnel."
On Apr. 11 last year, a twin-arch tunnel under construction in section 5-2 of the Sinansan Line collapsed underground near Yangji Intersection in Iljik-dong, Gwangmyeong, and Ori-ro, the road above, caved in. One person died and one was injured in the accident. A twin-arch tunnel is a tunnel excavated by first driving a central tunnel and installing a central pillar, then widening to the left and right.
This accident revealed deficiencies across all phases of the project, from design to construction and supervision. The designer underestimated by a factor of 2.5 the load acting on the central pillar during the design process. In fact, the pillar length should have been more than 4 m, but the design input listed it as 0.335 m, among other design errors that were found.
However, even at the design-supervision stage, which oversees construction projects during design, these errors were not screened out. The builder and the construction supervisor also reviewed the design documents before breaking ground but failed to catch these errors, allowing the unsafe design to proceed to construction without correction.
At the construction management stage, POSCO E&C, the builder, was found to have fallen short in complying with its safety management plan. POSCO E&C failed to properly observe the tunnel face (the end of the tunnel excavation surface) during ground surveys and tunnel excavation, and did not identify changes in the fault zone. Face observation is conducted to compare the actual ground conditions with those anticipated at the design stage and, if needed, to implement on-site responses such as additional surveys or design changes.
However, during tunnel excavation, POSCO E&C's ground engineers were required to directly observe the face every 1 m, but some observations were replaced with photographs. POSCO E&C's own safety management plan required a senior engineer with at least five years of practical experience to observe the face, but it was found that an underqualified engineer conducted the observations. Ultimately, due to inadequate face observation, the builder failed to identify in advance that the fault zone would weaken ground strength and impose excessive additional loads on the central pillar.
The committee's investigation also found that POSCO E&C did not carry out its own daily safety inspections by work type or the required periodic safety inspections of the tunnel. The builder did not conduct crack management, such as preparing a crack management ledger for the central pillar, and by wrapping the central pillar with nonwoven fabric, failed to detect precursor signs of failure in the central pillar, such as concrete cracking and deformation.
There were also issues with construction quality. POSCO E&C changed the tunnel construction sequence specified in the design documents and, after obtaining only the approval of the chief construction supervisor, did not verify structural safety. In addition, while the design documents required maintaining the depth difference between the left and right tunnels within 20 m when excavating them on both sides of the central tunnel, the actual construction saw differences of up to 36 m.
Construction supervision also failed to report factual information to the client (project implementer) after determining there were quality and safety issues.
As measures to prevent recurrences of similar accidents, the committee proposed ▲ strengthening ground investigations during design and construction ▲ strengthening standards and procedures for the safety management of the central pillar. The Ministry of Land, Infrastructure and Transport (MOLIT) plans to pursue institutional improvements to prevent similar accidents based on this investigation.
Committee Chairperson Son Murak (Daegu University professor) noted, "It can be seen as the result of multiple factors: errors at the design stage were not filtered out during verification, unexpected ground conditions of a fault zone emerged during construction, and on-site management was insufficient." Son added, "We plan to compile the investigation results and submit the final report to the Ministry of Land, Infrastructure and Transport this month," and "We hope institutional improvements reflecting the committee's proposals will be promptly implemented to strengthen safety in tunnel construction and other works."
MOLIT plans to push for business suspensions and other penalties against the designer, builder, and supervisor for design negligence and deficiencies in construction and supervision. It also plans to share the full investigation findings with investigative agencies such as the police and the Ministry of Labor to enable strict action on criminal charges, including occupational manslaughter and violations of industrial safety laws.
Park Myeong-ju, acting director for technology and safety policy at the Ministry of Land, Infrastructure and Transport, said, "We plan to impose the sanctions as quickly as possible, but we need to conduct hearing procedures and investigations to determine negligence or intent, so we expect the process to proceed until the first half of next year."