Seoul finds 41 underground cavities

The Seoul Metropolitan Government announced on Wednesday that 41 underground cavities had been detected in four areas of Seoul, the result of a five-day inspection that started on Nov. 30.

The assessment, which was conducted in collaboration with Geo Search, Japan’s biggest geological prospecting company, uncovered 18 cavities in the area around Jongno 3-ga Station; five near Yeouido Station; and 18 in the area around Seoul National University of Education Station.

No underground cavities were found around Seokchon Underpass in the most recent survey following repair work that was completed in September to address six underground air pockets found the previous month, the city government said.

The cause of the cavities has been traced back to deteriorating water pipes. Water leaking from the pipes affected the soil nearby, ultimately causing the depressions, said Kwon Wan-taeg, the team leader of the city’s road management department.

The city government also found in its inspection that the cavities had been caused by long-term subsidence from road restoration work that was conducted before the subway opened - 30 years ago, in the case of Jongno 3-ga Station.

Most of the cavities developed over a long period and the possibility of sinking is very low at this point, the city government said.

The city government said it had graded each cavity as either A, B or C depending on the priority for repairs. Cavities less than 30 centimeters (12 inches) deep were placed in category A; those more than 30 centimeters deep with a minimum width of 50 centimeters were labeled B. The rest were placed in category C.

The city government will get down to repairing the 18 A-rated cavities once the weather gets warmer. Repair work will likely start sometime at the end of February, Kwon said.

Seoul will set up measures to deal with the remaining cavities - 11 ranked B, and 12 labeled C - following additional inspection and continued observation.

The city government also presented a plan to inspect areas around Seoul that are geologically weak and where old subway lines and deteriorating water pipes are present every three years. “We will do our best to prevent accidents related to the road subsidence,” said Lee Taek-geun, the chief of the road management department.

BY PARK YUNA [ypc3c@joongang.co.kr]