Sinkhole site deemed 'safe' despite plummeting groundwater levels

Groundwater levels plummeted over the last two years near the site of the sinkhole that occurred in Yeonhui-dong on Aug. 29.

First responders and police officials control and examine the site where a sinkhole appeared in Yeonhui-dong, western Seoul, on Aug. 29. [NEWS1]

Groundwater levels plummeted over the last two years near the site of the sinkhole that occurred in Yeonhui-dong on Aug. 29.

A rapid change in the groundwater level, which refers to the distance from the ground's surface to the groundwater below, typically precedes a sinkhole.

According to the drilling column map — a cross-section map showing the order, thickness and type of strata — of Seongsan-ro posted on the Geotechnical Information Database System of the Ministry of Land, Infrastructure and Transport on Sunday, the groundwater level near the sinkhole in Yeonhui-dong had dropped by up to 7 meters (7.6 yards) between 2017 and 2022.

The drilling column map was one of the ground survey data ordered by the Seoul Metropolitan Government for the construction of a pipeline to bring rainwater to Sacheon Pumping Station, which is about 700 meters away from the sinkhole.

According to the construction drawings, the pipeline extends from near Yeonhui Intersection to Sacheon Bridge Intersection and passes about 12 meters underground on Seongsan-ro, where the sinkhole occurred. Construction began in 2020 and is currently about 70 to 80 percent complete.

Looking specifically at the 2017 and 2022 drilling column maps, the groundwater level in the ‘NBH-02’ area, about 80 meters from the sinkhole, dropped significantly from 3.5 meters in 2017 to 10.5 meters in 2022. The groundwater level in ‘NBH-03,’ about 40 meters from the sinkhole, reached 6.6 meters in 2022.

This is in contrast to the groundwater levels in ‘NBH-01’ and ‘NBH-07,’ which are relatively far from the sinkhole, which dropped by about 1.9 to 3 meters during the same period.

Experts estimate that the construction of the rainwater pumping station pipeline in the Seongsan-ro area may have affected the groundwater level. If a cavity is created deep underground during a large-scale underground construction process, the groundwater passing over it will flow downward, and the soil in the supper layer will fill in a chain reaction, causing the groundwater surface to collapse.

“The rapid decline in groundwater levels is one of the most typical causes of sinkholes,” said Lee Su-gon, a retired professor of civil engineering at the University of Seoul. “It is presumed that groundwater and soil flowed along a steep slope around the accident site, as shown in the drilling column map, creating a cavity. It must have developed into a sinkhole over a long period.”

“The groundwater level can vary by a meter or two meters depending on season factors such as monsoons, but 7 meters is an unusual amount,” said Park Chang-keun, a professor of civil engineering at Catholic Kwandong University. “Considering the scale of the recent sinkhole — 6 meters wide, 4 meters long and 2.5 meters deep — it is difficult to see it as a problem with old sewage pipes.”

The Seoul Metropolitan Government announced last Wednesday that it was checking the amount of soil discharge due to the inflow pipe construction, pointing out that this could be one of the causes of the sinkhole. However, it was revealed that the accident area was classified and managed as a grade B area, which is considered a manageable level on the city government's five-tier risk scale.

Only about 26 percent of the sidewalks and roads managed by the city government are classified as D to E — insufficient to bad — which are considered sinkhole risks.

The occurrence of a sinkhole in a section previously assessed as low-risk has sparked safety concerns. In particular, on the day of the sinkhole accident in Yeonhui-dong, the Seoul Metropolitan Government explained that “no cavities were found during the ground-penetrating radar (GPR) survey in May,” further raising public concerns. GPR surveys can only confirm cavities 2 meters deep from the ground surface.

“We are investigating the cause of the accident by considering various possibilities,” said a Seoul Metropolitan Government official. ”The lowered groundwater level could be a sign of a sinkhole, but other factors could have also caused it, so we need to investigate the exact cause. We took measures to prevent groundwater from entering during the construction of the inflow pipe.”

BY LEE CHAN-KYU, LIM JEONG-WON [lim.jeongwon@joongang.co.kr]