Seoul City Designates Yeonhui-dong Area with Land Subsidence as 'Special Inspection' Zone... Comprehensive Survey of Underground Facilities

Seoul City recently designated the Seongsan-ro area in Yeonhui-dong, Seodaemun-gu, where a ground subsidence accident occurred, as a 'special inspection' zone. This measure aims to alleviate public anxiety and prevent additional accidents by conducting a full survey of underground installations in the area. In the mid to long term, maintenance work will also be carried out on 3,000 km of 'aged water and sewage pipelines,' the main cause of ground subsidence.

On the 4th, Seoul City announced that it will promote an 'Improvement Plan for the Prevention of Ground Subsidence' focusing on detailed maintenance plans to prevent recurrence of ground subsidence accidents and eliminate inspection blind spots.

[Image source=Yonhap News]

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Earlier, around 11 a.m. on the 29th of last month, ground subsidence occurred in the Seongsan-ro area, causing a passenger car to fall in and seriously injuring two people. Seoul City immediately began restoration work and conducted ground penetrating radar (GPR) surveys across all eight lanes, extending 500 meters to the left and right of the accident site. However, on the morning of the 30th, additional road subsidence was discovered near the accident site.

Seoul City analyzed that the cause of this ground subsidence accident was a combination of factors including terrain characteristics, weather impact, underground installations, and the influence of nearby construction sites. The accident site was judged to have relatively unstable ground due to its terrain features with strong groundwater flow and being composed of landfill layers. Additionally, weather factors also affected the road subsidence. The heavy rains and heatwaves in July and August caused rapid changes in groundwater levels, raising the possibility of underground soil erosion. Underground installations such as water and sewage pipes, gas, and communication lines may also have influenced the road subsidence.

The investigation is not yet complete. To identify the cause more clearly, Seoul City plans to install vibration meters and groundwater level gauges within the construction area and conduct additional investigations through ground drilling. The government is also conducting an investigation to determine the cause of the Seongsan-ro ground subsidence.

Accordingly, Seoul City will designate the accident area as a 'special inspection' target zone to alleviate the anxiety of citizens using Seongsan-ro and nearby residents. In particular, a full survey of underground installations along Seongsan-ro (Yeonhui IC to Sacheon Bridge) will be completed by September to reconfirm the safety of the area. The targets include sewage pipes and culverts (3 km in length), water supply pipes (2 km in length), urban gas and communication lines, and the investigation will be conducted jointly with related agencies.

A special inspection will also be promoted for the 'Sacheon Rainwater Pump Station' construction site nearby. GPR surveys will be conducted once a month on Seongsan-ro near the construction site, and construction site personnel will visually inspect the area twice a week. Additionally, vibration meters and groundwater level gauges will be newly installed around the construction site, and ground safety management will be strengthened through ground drilling injections.

Seoul City GPR Exploration Equipment.
Photo by Seoul City

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A mid to long-term management plan was also presented. Strengthening improvement work on 'aged water and sewage pipelines,' a major cause of ground subsidence, is a representative measure. Of the total 13,350 km of water supply pipes, 3,074 km that are over 30 years old will be maintained by 2040. This year, 62.5 km of water supply pipes will be maintained, and 64.6 km will be maintained next year.

Starting in September, all sewage pipes over 30 years old will be precisely inspected using endoscopic cameras equipped with closed-circuit TV. Plans will also be established annually to maintain sewage pipes reaching 30 years of use. This year, 441 km of sewage pipes are expected to reach 30 years of use.

Safety management around excavation construction sites (excavation depth of 10 m or more or tunnel construction) with a high risk of ground subsidence is also included. Previously, GPR surveys were conducted once initially for excavation sites and additional surveys were performed if necessary. Going forward, the target will be expanded to include construction sites completed within one year, with monthly GPR surveys conducted. As of this year, about 200 construction sites in Seoul are subject to this.

Previously, Seoul City established an 'Underground Safety Management Plan' and continuously promoted ground subsidence prevention activities such as GPR surveys. This year, up to August, 5,787 km were surveyed, and 559 voids (underground empty spaces) were discovered and repaired in advance.

Equipment upgrades and workforce expansion will also be promoted. To detect and address ground subsidence risks in advance, performance verification standards for GPR equipment will be strengthened, and new ground subsidence prevention technologies that can cover GPR survey blind spots will be introduced. Currently, GPR equipment can detect underground voids with over 80-90% accuracy up to 2 meters underground. The plan is to improve this accuracy and introduce new technologies to detect abnormal signs of ground subsidence occurring deeper than 2 meters underground, which are difficult to find with GPR equipment.

Workforce and equipment reinforcements will be made. Currently, Seoul City has seven GPR detection specialists, but one more specialist will be added in September, and two additional GPR survey vehicles will be secured to strengthen proactive inspections and measures in ground subsidence risk areas. Furthermore, the feasibility study for installing a 'Ground Subsidence Monitoring Network,' which can monitor ground changes from a more macroscopic perspective, will be completed promptly. This system can be used to manage ground stability not only on roads but also in parks, residential areas, and other various locations.

Yoo Chang-soo, Seoul City's Deputy Mayor for Administration 2, said, "Seoul City will thoroughly implement this improvement plan, which reviews and supplements the existing ground subsidence prevention measures, to create a safe and secure road environment where citizens can feel safe."

Example of ground subsidence safety map development.
[Photo by Seoul City]

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