Structural safety rated 'A'; city warns reinforcement start may slip to next year

Seoul City Hall [Herald DB]
Seoul City Hall [Herald DB]

The Seoul Metropolitan Government on Wednesday formally asked the Ministry of Land, Infrastructure and Transport to quickly decide on a reinforcement method for columns found to be missing rebar in Package 3 of the Yeongdong Boulevard underground complex development, warning that delays will make the work increasingly difficult.

According to the city, it sent the Ministry of Land, Infrastructure and Transport and Korea National Railway the interim results of an emergency structural safety inspection, along with findings from an external expert advisory session held Sept. 10, via official correspondence on Sept. 11.

The interim results of the emergency precision safety inspection conducted by the city gave the currently completed structures an overall "A" rating for both condition and safety. A structural load calculation based on construction completed up to the second basement-floor slab also met the allowable threshold.

Cracks found in the ceiling slab of the fifth basement floor were mostly less than 0.3 millimeters wide and did not penetrate through the slab. The city said the cracks are likely the result of concrete shrinkage during drying and are not directly linked to the missing rebar in the columns.

However, experts said the longer reinforcement is delayed, the less effective the same steel plate will be. Steel plates attached to a column begin sharing the load only from the point of installation onward — they cannot absorb loads the column was already carrying before they were put in place. This means that the further construction progresses before reinforcement begins, the thicker the plates must be to achieve the same level of strengthening.

Expert analysis found that using a 22-millimeter steel plate, the post-reinforcement strength would reach 123 percent of the original design strength if work is done immediately after completing the fifth basement floor, but would fall to 118 percent after the fourth basement mezzanine floor is finished, 113 percent after the third basement floor, and 105 percent after the second basement floor.

To meet the 120 percent reinforcement efficiency level set by the Ministry of Land, Infrastructure and Transport, the plates would need to be made progressively thicker as construction advances — reaching up to 58 millimeters if reinforcement begins only after the second basement floor is complete. The city noted that thicker plates add weight and make welding and installation significantly more difficult.

Construction on the third basement floor is currently under way at Package 3. The city said further delays in deciding on a reinforcement method could extend the reinforcement work period and affect the overall schedule for the complex development project.

The Ministry of Land, Infrastructure and Transport, however, said additional review is needed before a method can be chosen. A study commissioned by Korea National Railway to assess the adequacy of column reinforcement options is scheduled to run through Dec. 31, and the government's joint inspection period has been extended by four months, from July 20 to Nov. 20.

"If the method is decided only after all studies and reviews are complete, the start of reinforcement could slip into next year," the city said, adding that it would proceed with whatever method the ministry ultimately selects — including filling the gap between the column and steel plate with high-strength mortar instead of epoxy.

The city also said that delays in finalizing the reinforcement method and resuming comprehensive test operations have pushed back both the reinforcement work and the resumption of GTX-A trains stopping at Samsung station. It said responsibility for any additional operating-loss compensation arising from those delays rests with the ministry for holding up the decision.

The city recently expressed regret over remarks made by Land Minister Hong Ji-seon when he was still a ministerial nominee, in which he said he would seek to recover operating losses stemming from the missing rebar from Seoul. The city called it "highly inappropriate" to unilaterally treat Seoul's liability as a foregone conclusion and to announce plans to seek compensation before any objective verification of the cause and responsibility for the losses had been completed.


lucky@heraldcorp.com