Precaution is the best medicineThe Seoul city government has discovered more urban sinkholes in the path of ongoing construction for a tunnel being built underneath the Seokchon Underpass in Songpa District, southern Seoul, to extend the section of Metro Line No. 9.
Citizens were concerned over safety in the area after seven air pockets were found underneath a stretch of ground many cars pass over. The Seoul city government has concluded that the tunnel construction could have weakened the ground after washed-out soil damaged the sediment.
The Seokchon underground is historically rich with cultural assets and also weak with soft sands and stratum, requiring a more complex and careful building process than typical tunneling techniques can provide.
Builders have applied a circular shield model that is more sensitive to the soil and other environmental conditions. The circular excavation method is widely used worldwide as a safe underground tunnel construction method. But holes can be made if the backfill grouting process is neglected during the tunnel boring process.
If the builders on the subway project are sloppy in key grouting work, the safety of thousands who use the subway and underground facilities could be at risk. The shield mechanism has been applied to four sections on the No. 9 subway line. Three are in the Seokchon area, where the sinkholes were discovered, and the fourth was found at Yeouido, western Seoul, where the subway line ends. Each section was built by different companies. The city government said that it did not see flaws in other sections.
But the problem is that we cannot trust the city government’s words. Two areas in Yeouido where subway construction was completed gave way twice in June and July. Seoul city officials said that mud from old sewage could have caused the collapse. The Yeouido section is similar to the Seokchon area in terms of geological conditions. It is extremely scary to think that a giant sinkhole lies underground in Yeouido, which hosts a number of skyscrapers and the National Assembly building.
The city administration said that it will re-examine the Yeouido section of the subway line. But that may not be enough to ease public fears. The media has been reporting that grouting has been neglected in most shield-based tunnel areas. The city government must examine all areas in Seoul’s underground that were constructed using the shield-tunneling process. The government must also conduct separate investigations for holes around several subways and underground areas across the nation. Needless to say, precaution is always the safest way to prevent accidents.
JoongAng Ilbo, Aug. 21, Page 30