Numerical prediction of settlement due to the Pohang earthquake

On 15 November 2017, an earthquake with a magnitude of 5.4 caused geotechnical damage over a wide area of Pohang, South Korea. In this study, the UBCSAND constitutive effective stress model is calibrated by cyclic direct simple shear (CDSS) tests conducted on a type of sand from liquefied paddy fields at Pohang under confining stresses of 100 and 200 kPa. The calibrated UBCSAND model is then used to assess the liquefaction and calculate the ground settlement based on standard penetration test (SPT) data near the epicenter of the Pohang earthquake. The liquefaction characteristics from the UBCSAND model matched those from laboratory tests. According to the numerical results, liquefaction mostly occurred within the uppermost 10 m, and ground surface settlement from excess pore water pressure dissipation varied from 3.86 to 10.95 cm. These settlements were higher than the surface displacement estimated by satellite image analysis with interferometric synthetic aperture radar (InSAR) and lower than the ground subsidence observed solely by visiting the sites.