Seoul City said on the 26th that it reinforced the ground at the Phase 4 construction site of Line No. 9 in Myeongil-dong, Gangdong District, where a ground subsidence (sinkhole) accident occurred in Mar. last year, and will resume work starting on the 31st of this month.
The city investigated the cause for about nine months with the Central Underground Accident Investigation Committee of the Ministry of Land, Infrastructure and Transport. Regarding the cause, the city said, "A discontinuity in the deep weathered zone underground, which was not identified in the design and construction stages, weakened due to a drop in the groundwater level and a sewage pipe leak and then slipped, and an external force exceeding the design load acted on the tunnel, leading to tunnel collapse and ground subsidence."
Over the past three months, the city conducted ▲ inspections for abnormal leaks around the excavation section ▲ inspections of the condition of sewer facilities (sewer lines, manholes, and catch basins) around the cut-and-cover section ▲ electrical resistivity surveys, and strengthened ground and tunnel stability by reflecting technical advice and the investigation findings.
It also completed a supplemental design applying a reinforcement method that can sufficiently withstand loads during tunnel excavation. The method is "steel pipe reinforcement grouting," which involves installing perforated steel pipes in the ground around the tunnel and injecting a solidifying agent inside to harden the soil and enhance ground stability.
To monitor conditions during construction, the city will deploy an additional tunnel construction expert with more than 10 years of experience in geotechnical and geological fields, and will actively use digital mapping technology, which stores and analyzes the condition of the tunnel face as digital data, to secure safety.
To more closely manage construction safety and quality, closed-circuit (CC)TV will continuously record the work process, and the video footage will be maintained.