Construction? Climate change? As sinkholes open in Seoul, causes remain a mystery.

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Construction? Climate change? As sinkholes open in Seoul, causes remain a mystery.

Published: 05 Sep. 2024, 16:59 Updated: 05 Sep. 2024, 17:00

Excavator fills up a sinkhole that appeared in Yeonhui-dong in Seodaemun District, western Seoul, last Thursday. [JOONGANG ILBO]

The shocking appearance last Thursday of a sinkhole that swallowed an SUV in Seodaemun District, western Seoul, has prompted Seoul to renew investigations into road subsidence.

The 80-something driver of the SUV and a female passenger in her 70s were transported to a nearby hospital. The woman initially suffered cardiac arrest, but her pulse was eventually restored.

The 24-square-meter (258 square feet) wide, 2.5-meter (8.2 feet) deep sinkhole on Seongsan-ro, Yeonhui-dong, was soon followed by road subsidence 3 centimeters (1.18 inch) deep some 30 meters away from the sinkhole the next day. The Seoul Metropolitan Government, Seodaemun District Office and other related institutions joined hands to probe the cause of sinkholes.

According to the Seoul Metropolitan Government, 11 sinkholes over 1 meter deep or 1 square meter wide appeared within the city limits this year, bringing the total number of sinkholes since 2014 to 227.

According to the JoongAng Ilbo, over 100 sinkholes have appeared annually across Korea since 2015, with some years seeing over 200 occurrences. The number of sinkholes this year has yet to surpass last year's total, but the causes of many of the fissures remain a mystery.

Typically, investigation results emerge quickly when underground infrastructure such as water and sewage pipes is the cause. However, the causes of four of this year's 11 sinkholes in Seoul remain unknown, having been classified as "other."

Experts point to climate change as one of the reasons for sinkholes in urban areas. Sinkholes usually happen when the underlying residual soil is eroded due to groundwater, leaving an empty space behind. This process — called cavitation — is caused by extreme weather conditions, such as strong rains and droughts from heat waves, resulting in a sinkhole.

Destabilizing construction and weather

“Climate change immediately affects sinkholes,” Cho Won-cheol, professor of Civil and Environmental Engineering at Yonsei University, told the JoongAng Ilbo. “When there is a lot of rainfall, the issue arises because there is a greater likelihood of soil being washed away."

First responders oversee a site where a sudden road sinkhole swallowed a Tivoli SUV in Seodaemun District, western Seoul, last Thursday. [YONHAP]

Underground water could also dry up due to prolonged heat after the rain. “Cavitation underground can also happen when there isn’t much rain,” said Cho.

The sinkhole that appeared in Seodaemun District could have been caused by extreme rain and heat, construction experts say. In July, there was a heavy downpour of 64 millimeters per hour in Seodaemun District, the first time in approximately a decade that the district had seen rainfall of over 60 millimeters per hour.

The Seoul Metropolitan Government probed the area three months beforehand but found no problems.

Road authorities control traffic in Yeonhui-dong in Seodaemun District, western Seoul, on Friday morning after an additional road subsidence was discovered near a sinkhole that appeared a day earlier. [YONHAP]

"Heavy and intense rain can be a direct or indirect cause of sinkholes,” said Professor Kong Ha-sung of Woosuk University's Department of Fire Prevention and Safety. “Depending on the conditions underground, soil layers can be swept away rapidly, or they may erode slowly over many years without being replenished.”

Seoul's city government is investigating whether rain pump construction near Seongsan-ro has affected the groundwater path below. The pump was being installed to combat flooding.

“In general, the deeper the residual soil layer, the more frequent the underground construction in the area and the heavier the rainfall, the higher the likelihood of sinkholes,” said Park Eui-seob, head of the Geological Research Division of the Korea Institute of Geoscience and Mineral Resources.

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

Sinkhole-proofing vulnerable areas

Experts say an intricate underground map is necessary to prepare for sinkholes. While Seoul has comparatively stable granite bedrock, more frequent extreme weather has made guaranteeing safety impossible.

There are spots where sinkholes occur more frequently in Seoul. In the past five years, sinkholes have occurred most frequently in Gangnam District and Songpa District, both located in southern Seoul. Each district has seen 12 sinkholes over 1 meter deep or 1 square meter wide.

"There are places in Seoul with thick residual soil layers, which have already been mapped out," said Park. "As climate change progresses, areas with thick residual soil layers may become more vulnerable, so preparing based on the ground conditions is necessary."

BY JEONG EUN-HYE [kim.minyoung5@joongang.co.kr]