# Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model
## 1 Introduction
Most researchers [13; 14] analyzed liquefaction in Pohang using one-dimensional soil column analysis, which used the equivalent linear method based on standard penetration test (SPT) or cone penetration test (CPT) data. In this method, total stress is used, and the generation of pore water pressure is omitted. Therefore, there is an argument over the resulting displacements obtained from this method due to its oversimplifications, and it is often performed to identify regions with high potential of liquefaction by computing safety factor for liquefaction (FS) or liquefaction potential index (LPI). Only a few researchers conducted analyses of the effective stress to assess liquefaction at Pohang. [PERSON] et al. [15] performed an effective stress nonlinear site responses analysis to estimate the generation of pore pressure and the significant reduction in effective stress, which are the two key criteria in evaluating liquefaction. [PERSON] et al. [11] carried out an effective stress analysis of the Pohang earthquake-induced quay wall and backfill displacement at Youngil-man port using a multiple simple shear mechanism [16; 17] to predict the nonlinear stress-strain behavior of soil under seismic loading. In this model, the requirements of some additional input parameters other than soil materials are needed to predict excess pore water during shaking, for example, four unrealistic parameters \(S_{1}\), \(w_{1}\), \(p_{1}\), and \(p_{2}\) relating to type of materials [16; 17]. [PERSON] et al. [12] also extended the analysis of [PERSON] et al. [10] in two drainage conditions (drained and undrained) during the shaking and post-shaking periods. In the analysis of [PERSON] et al. [15], [PERSON] et al. [11], and [PERSON] et al. [12], pore pressure changes were predicted by an independent pore pressure generator, and they did not relate to the volumetric strains produced by the soil skeletons. Hence, they can be classified as the partially coupled or loosely coupled effective stress analysis [2]. In addition, neither calibrations from experimental results nor parametric studies for the constitutive model prior to the application of the Youngil-man port case study were conducted in these studies. Therefore, it is essential to carry out a fully coupled effective stress analysis using a state-of-the-art approach, such as UBCSAND2, to more accurately model earthquake-induced liquefaction. In this study, the UBCSAND2 constitutive model was used to precisely predict the liquefaction characteristics and displacements due to dynamic loading. First, the model was calibrated with the ejected Pohang sand in loose and dense states under vertical effective stress of 100 and 200 kPa, respectively. Then, a two-dimensional effective stress analysis using the UBCSAND2 model was performed to investigate damages that occurred at the quay wall of Youngil-man port during the Pohang earthquake (\(M_{\rm w}=5.4\)) on 15 November 2017. The settlement and horizontal displacement at the top of the quay wall and ground settlement of reclaimed area from the numerical results were compared to the field observations after the Pohang earthquake.