Government officials visit a road subsidence site in Mapo District, western Seoul, and oversee restoration efforts on April 14. [NEWS1]

The Seoul city government will double down on its surveillance of sinkholes at construction sites after a series of sinkholes occurred in the capital city this year.

It will also increase the number of inspections from once a year to once a month to more thoroughly assess ground subsidence risks and the stability of retaining walls, especially ahead of the rainy season and thaw periods when the ground becomes more vulnerable.

The city will strengthen on-site management at construction sites, as on-site supervisors and lead engineers will be required to patrol the surrounding area at least once a day and report inspection results.

Crews will install CCTV cameras at excavation sites to monitor work around the clock and review footage daily. If workers detect subsidence or cracking near a site, they must immediately halt construction.

Police will then block off the road, and crews will conduct ground-penetrating radar (GPR) scans and drilling surveys for a detailed investigation.

The Seoul government said it will shift its focus from post-incident recovery to proactive prevention. The announcement was part of the city’s new "Underground Space Management Innovation Plan," which it unveiled Tuesday.

The plan targets what the city calls the three major causes of sinkholes: aging water and sewage pipes, excavation work and tunnel construction.

A large sinkhole in Gangdong District, eastern Seoul, on March 24 [YONHAP]

City data show that of the 92 sinkhole incidents reported in Seoul over the past five years, 51 cases, or 55.4 percent, resulted from damaged water and sewage pipes. Excavation work caused 12 cases, or 13.2 percent.

To tackle aging infrastructure, Seoul will invest 2.5 trillion won ($1.7 billion) by 2040 to replace 3,074 kilometers (1,910 miles) of old water pipes within the capital city.

In the first phase, from 2020 to 2027, the city plans to spend 1.05 trillion won. This year, the city will replace 89.2 kilometers of aging pipes, marking a 43 percent increase compared to last year.

The city will also double its efforts to repair old sewage pipes. Instead of investing 200 billion won a year to renovate 100 kilometers, Seoul will now spend 400 billion won annually to upgrade 200 kilometers.

Seoul Mayor Oh Se-hoon, center, inspects safety facilities and alarm systems at the underground complex development construction site along Yeongdong-daero in Gangnam District, southern Seoul, on April 23. [YONHAP]

Seoul will introduce a new "ground subsidence monitoring system" to overcome the limitations of existing GPR technology, which only detects risks up to two meters underground.

Workers will embed pillar-shaped sensors up to 20 meters deep to monitor underground soil changes. Seoul will be the first local government in Korea to adopt this system.

The city will install the monitoring system next month at a construction site near the location of a ground subsidence incident in Myeongil-dong, Gangdong District, part of Section 1 of Line 9 Phase 4.

The city will also expand its GPR capabilities by investing 1.5 billion won to purchase three additional vehicle-mounted GPR units, bringing the total to seven.

A small sinkhole appears in Gangdong District, eastern Seoul, on April 2. [YONHAP]

Seoul plans to disclose safety inspection results more actively. It will release the GPR inspection outcomes for five major rail construction sites, including the underground complex development along Yeongdong-daero, covering 49.3 kilometers.

District offices will select an additional 50 locations covering 45 kilometers for public disclosure.

Officials will also create and publish a "geotechnical property map" focused on large excavation project areas.

To oversee the expanded measures, the city will establish a new Underground Safety Division.

"We will thoroughly check to ensure there are no blind spots in our management of sinkholes, aiming for 100 percent predictability and control over incidents near major excavation sites," Seoul Mayor Oh Se-hoon said. "In addition to replacing old water and sewage pipes, we will use advanced digital twin technologies to more meticulously prevent sinkhole accidents."

Translated from the JoongAng Ilbo using generative AI and edited by Korea JoongAng Daily staff.