Field investigation and analysis of ground sinking development in a metropolitan city, Seoul, Korea

Either geological features or some artificial factors, like sewer pipe leakage or incomplete construction work, cause ground sinking in urban areas. This study reports on a recent ground sinking, which occurred on account of complex factors related to geology and artificial excavation work. A cylindrical displacement of 14 m long in the ground, with a sinking of 3 m deep on the surface, was studied. This occurrence was situated adjacent to the excavation site, which was 38 m deep, during the construction of the high-rise building. Fine-grained soils were spilled out with groundwater, leaking through the bottom of the cutoff wall and before the ground sinking occurred completely. A scrutinized field survey was conducted through several means, such as observation through pit excavation, standard penetration test (SPT), and ground penetrating radar (GPR) exploration. Numerical groundwater flow analysis was also performed to evaluate the influence of the flow behavior from the surrounding area toward the excavated site using Visual MODFLOW. The study revealed that the geology of the site was comprised of multiple alluvial layers formed by many floods from the Han-river in Seoul, and it was very vulnerable to the piping near the excavation wall. The results of SPT displayed some void spaces of 20–30 cm thick under the sinking area, and the changes of soil properties are recognized before the excavation and after the ground sinking. The sequential displacement mechanism for ground sinking in this site is suggested from the analyses of the numerical groundwater flow model and the data obtained from field investigation.