A sinkhole that appeared on Monday and killed a motorcyclist is seen in Seoul's Gangdong District, Friday. Yonhap

The sinkhole site in Seoul where a motorcyclist was killed on Monday was designated as an "area of caution" in a city government report written two years ago, a document revealed Friday.

The report, titled Underground Safety Impact Assessment Service for the Phase 4 Construction of Seoul Metro 9, was obtained by Rep. Park Yong-kab of the main opposition Democratic Party of Korea from the Seoul Metropolitan Government. It had warned that the ground near the accident site was weak and prone to significant subsidence.

Commissioned by the city government in 2021 and completed in 2023, the report assessed the risk of ground subsidence in the area due to the extension of Seoul Metro Line 9.

"The fault zone near the area (Station 939) has a relatively high amount of subsidence, requiring careful monitoring of measurement results during excavation work or the installation and removal of temporary structures to ensure safe construction," the report pointed out.

It further noted that the area is a "fault fracture zone," where rock has deformed and lost continuity.

The report classified the area as a fourth-grade risk in terms of ground sinking or breaking, the second highest rank in the city government's five-tier system, citing that water pipes and underground water flow in the area could weaken the ground.

The 20-meter-wide and 20-meter-deep sinkhole appeared at an intersection near Daemyeong Elementary School in Gangdong District, Seoul, on Monday at 6:29 p.m., killing a motorcyclist in his 30s and injuring the driver of another vehicle.

The point adjacent to the underground tunnel in the Gangdong section of the Seoul Sejong Expressway was also classified as an area of concern.

"The construction plan should be thoroughly observed so that the next stage of construction can be carried out after [the] sufficient concrete curing period is secured to fully retain strength when constructing the structure," the report said.

Rescue authorities inspect a sinkhole on an intersection in Seoul, Monday. Yonhap

There have also been multiple warnings about the risk of ground subsidence surrounding the area.

In April 2021, the Korean Tunneling and Underground Space Association, a group of urban underground development experts, sent an official letter to the city, citing fears of ground subsidence due to the subway extension in the area.

The city government also internally classified the area as Grade 5, indicating the highest risk of sinkholes.

The city government created a ground subsidence safety map that predicts the sinking and breaking of ground surfaces, and manages dangerous areas by classifying them into five-tier safety grades. However, such information is not disclosed publicly as it could affect real estate prices, a city official told Korean news media on Thursday.

Despite a series of safety warnings, the construction omitted proper GPR (ground penetrating radar) exploration to check the risk of ground subsidence.

Following the sinkhole accident in Seodaemun District last year, the city government decided to conduct monthly GPR checkups in areas with a high risk of sinkholes. However, these checkups were never carried out for the Gangdong District sinkhole due to administrative issues.

"We need to thoroughly analyze the cause to prevent such accidents from reoccurring and improve underground safety evaluation," Park said.