Seoul's sinkholes point to deeper problems lurking underground

Social Affairs

Published: 02 Oct. 2023, 15:31 Updated: 02 Oct. 2023, 18:52

Seoul's sinkholes point to deeper problems lurking underground

Authorities conduct repair work after a suspected sinkhole near Exits 1 and 2 of Eonju Station in Gangnam District, southern Seoul, was reported earlier that morning on Sept. 19. [NEWS1]

Police received a report on Sept. 19 about a suspected sinkhole on a road near exit 8 of Eonju Station of Seoul Subway Line 9 in Gangnam District, southern Seoul.

Witnesses worried that the road had "gone soft."

The Seoul Gangnam Police Precinct immediately dispatched officers to the scene but found no sign of a sinkhole.

However, the road had turned a darker color, sparking concern that a water leakage or cavity lurked beneath the surface.

After an investigation, Seoul's city government confirmed that the road was sinking. Workers filled in the sunken area with compacted sand, a repair that took three hours.

This was not September's only sinkhole scare.

The incident near Eonju Station was just 20 meters (65.6 feet) from a sinkhole had appeared in the opposite lane just a week before.

A three-meter deep sinkhole is reported near Eonju Station in Gangnam District, southern Seoul, on Sept. 12. [NEWS1]

On the morning of Sept. 12, police received a report that a three-meter deep, one-meter-wide sinkhole had been spotted on a road near Eonju Station.

The sinkhole in the middle of Gangnam, a posh and heavily frequented neighborhood, raised a ruckus after a truck wheel got stuck in the hole.

Four lanes from Kyobo Tower to Cha Medical Center Gangnam, a notoriously high-traffic stretch of road, were closed as a result, much to the consternation of drivers.

Traffic was blocked for a total of eight hours while repairs were underway, resulting in infuriating traffic jams.

At the time, a water leak from damaged pipes was said to have weakened the ground.

Residents and commuters were further alarmed over another sunken spot so near the initial sinkhole.

"I pass by this area often, so I feel anxious after hearing news of sinkholes occurring every few days," a 40-year-old office worker who commutes to Gangnam said.

This is not the first time people have expressed concern over road subsidence in Seoul.

Public recognition of the sinkhole problem in the capital area came after a major incident at the entrance of Seokchon Underpass in Songpa District, southeastern Seoul, in 2014.

On Aug. 5 of that year, a 2.5-meter wide, five-meter-deep sinkhole was discovered near Seokchon Underpass. Seoul's city government determined that the sinkhole was due to shoddy construction of an underground tunnel for extending a metro line.

The city government said that the sinkhole occurred during the process of digging a horizontal tunnel by rotating a cylindrical shield to break soil and rocks during the third phase of the construction of Seoul Subway Line 9, operated by the Seoul Metropolitan Subway.

"The cause of the incident at the time was the construction of an underground tunnel, when soil was swept away from near the construction site, creating empty space underneath the ground," said Park Chang-geun, a civil engineering professor at Catholic Kwandong University, who served as head of the city investigation team at the time.

He said this is the "most frequent cause of road subsidence in the capital area" and that such cases can be resolved by installing better barriers to prevent water leakage.

"In Seoul, property damage can be significant if construction is done incorrectly, so construction workers already tend to conduct careful reviews," Park said.

Nevertheless, sinkholes continue to appear in the city.

According to the Seoul Metropolitan Government, there were 13 cases of land subsidence in 2019, 15 in 2020, 11 in 2021 and 20 cases last year. The city government classifies land subsidence when there is a collapse of an area wider than one square meter and more than one-meter deep, or when there are deaths, disappearances or injuries related to the sinkhole.

Though Gangnam is seen as a financial, educational and pop culture hub of Seoul today, the area was rural farmland in the 1960s before its massive transformation the following decade, and the hilly neighborhood is especially susceptible to flooding.

Immediately after the Seokchon Underpass sinkhole in 2014, Seoul's city government introduced ground penetrating radar (GPR) vehicles and began conducting regular road surveys. GPR equipment can detect an empty space within three meters of the surface.

The Seoul government allocated a budget of 4.335 billion won ($3.2 million) this year and 4.373 billion won for 2024 to prevent and respond to road subsidence. This budget also includes the cost of purchasing additional GPR vehicles.

The city surveyed a total of 17,292 kilometers of roads using GPR equipment between 2014 and August of this year. The surveys discovered 6,154 cases of road subsidence, which have been completely repaired, a Seoul Metropolitan Government official said.

Experts point out that a more fundamental solution is needed to address urban road subsidence.

"Sinkholes are formed when groundwater filling the space in the ground drains out," Han Moo-young, a professor at Seoul National University's Department of Civil and Environmental Engineering, said. "Tens of thousands of tons of leaked water are escaping from subway stations, but the ground surface is covered with concrete, preventing rainwater from entering, so empty cavities form underground."

He said that discovering empty spaces and replacing water pipes are only temporary fixes, adding that the city must devise "fundamental alternative measures to improve the underground space of the city and find ways to circulate water."

The Seoul Metropolitan Government began conducting an intensive joint investigation by deploying three GPR vehicles to survey the areas near Eonju Station from Sept. 21 to Oct. 13.

"We plan to mobilize all GPR vehicles in operation to survey the Eonju Station area, as well as nearby Bongeunsa Temple and Nonhyeon-ro," the city official said, referring to other stops on Line 9 in Gangnam District. "We will do our best to minimize public concerns."

BY SHIN HYE-YEON, SARAH KIM [kim.sarah@joongang.co.kr]