The Busan City Audit Committee on the 13th announced the results of its special investigation into the ground subsidence accident on the Busan urban railway Sasang–Hadan Line, saying it issued an institutional warning to Busan Transportation Corporation, called for light disciplinary action against three executives, and requested warnings and cautions for 41 employees. The committee also demanded seven administrative measures for Busan Transportation Corporation, including cautions and notifications.
The committee said the main cause of the subsidence was poor construction quality of the cutoff wall, and pointed to unauthorized construction differing from the contractor's design, neglect by the construction project management team serving as the supervision group, and Busan Transportation Corporation's failure to fulfill its guidance and inspection duties.
First, instead of using the SGR cutoff method that creates an underground cutoff wall by vertically injecting cement slurry at 60-centimeter intervals and then sequentially excavating and building a retaining wall, the contractor, without the supervision group's approval, excavated first, installed the retaining wall, and injected cement slurry horizontally (horizontal grouting).
When construction equipment snagged on underground utilities and other obstacles, making the SGR cutoff method difficult, the contractor carried out work in unauthorized deviation from the design.
The problem was that, unlike the SGR cutoff method, which builds a retaining wall after forming an underground cement wall, the contractor's method did not form a cutoff wall, greatly increasing the likelihood of underground voids due to groundwater or clay soil outflow.
In fact, the committee said that 8 out of 12 subsidence accidents in Zone 1 occurred at sites where the contractor ignored the design and carried out horizontal grouting.
Even though the contractor arbitrarily proceeded with temporary earth-retaining work using horizontal grouting without approval, the supervision group not only failed to order a work stoppage or rework, but also reported this to Busan Transportation Corporation only in February this year.
The committee said Busan Transportation Corporation neglected its duty to guide and inspect by merely reporting internally, without going through procedures to order design changes or grant approval, even after the supervision group's belated report.
The committee also flatly refuted Busan Transportation Corporation's explanation that heavy rain or water supply leaks were the cause of the subsidence accidents.
It said that while soil did enter pipelines and groundwater escaped due to damage to stormwater or sewage pipes, creating underground voids or ground settlement, this was not the fundamental cause of the subsidence accidents but only a factor that accelerated the subsidence.
The committee issued an institutional warning to Busan Transportation Corporation and, citing its failure to properly command and manage the supervision group and its complacent view that "old sewer lines are the cause" despite repeated subsidence accidents, demanded disciplinary action as well as warnings and cautions for failing to fulfill accident prevention duties.
Audit Committee Chair Yun Hee-yeon said, "I hope this audit will serve as an opportunity to correct Busan Transportation Corporation's complacent response to accidents and to set right the command and oversight responsibilities in risk management."