In Seoul, Korea, there have been large and small ground subsidence since 2014, which have become a social issue. Most ground subsidence occurred in the pavement of urban areas, and the main causes are supposed to be the improper construction of water supply pipes, sewers, gas pipes, and subways under the pavement.

The analysis of the location of the roadway cavity that occurred in the past five years in Seoul revealed that 81% of cavities occurred at a depth less than 0.8 m, 17% of cavities occurred at a depth between 0.8 m and 1.5 m, and 2% of cavities occurred at a depth more than 1.5 m from the pavement surface.

Therefore, 98% of the cavities were distributed within 1.5 m depth, the average depth of underground utilities, and the cavity scale was small and mainly occurred in the shallow layer. Such cavities are likely to expand over time and lead to ground subsidence [1].

Ground subsidence due to cavity can bring about various problems, such as casualties, decrease of the safety of the structures, and indirect social costs. Therefore, ground subsidence should be prevented through the exploration and recovery of the cavity under the pavements [2, 3].

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