# Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model
## 1 Introduction  
Earthquake-induced liquefaction is one of the primary causes of major damages to geotechnical structures under saturated soil conditions. Since the 1950s, very complex soil nonlinear models have been analytically developed to predict the phenomenon [1]. However, to date a reliable prediction of liquefaction-resulted displacements that developed in the plasticity formulation has still been challenging. [PERSON] and [PERSON] [2] pointed out these challenges should be considered when choosing and applying a constitutive model for a finite element analysis to predict liquefaction, which include: (i) adequate capture of cyclic soil element responses such as shear stress-strain curve, effective stress path, generation of pore pressure, (ii) best fit in the relationship between cyclic stress ratio (\(CSR=\tau_{\text{xy}}/\sigma^{\prime}y\)) and number of cycles to liquefaction (\(N_{\text{liq}}\)) obtained from laboratory test results and predictions, (iii) reasonable agreement of the calibrations to the empirical relationships (i.e., field test data) for liquefaction triggering, (iv) transparent input parameters based on the realistic properties of soils (e.g., relative density \(D_{\text{r}}\)), and (v) robust application to many elements analysis via back analysis of case histories. Following this, the fully coupled effective stress model is considered to be the most sophisticated class of continuum models of liquefaction [2]. Another one of the conventional models that has been widely used in earthquake engineering applications (e.g., dams, bridges, buildings, and ports) is the UBCSAND model. A major advantage of this model is its relative simplicity in predictingplasticity formulation. Additionally, it can accurately predict the onset of dynamic induced liquefaction in calibration from cyclic direct simple shear (CDSS) tests [3], verification from centrifuge tests [4; 5], or practical engineering applications [3; 6; 7]. Many versions of the UBCSAND model have been developed over two decades, and the UBCSAND v.904 aR model [3] is the most widely used version, as it includes important modifications to more accurately estimate the seismic behavior of sand. [PERSON] [5] indicated some limitations of the UBCSAND v.904 aR model, however, which include: (i) elastic unloading until shear stress reversal, (ii) omission of the effects of principal stress rotation, (iii) simplification of post-liquefaction volumetric strains due to the re-consolidation process, and (iv) single assessment of stress reversals on horizontal (or 45\({}^{\circ}\)) planes [5; 8]. Consequently, [PERSON] [8] proposed a two-mobilized planes model (i.e., UBCSAND2 model), which could significantly improve these drawbacks, except the limitation (iii). The model was calibrated by a series of CDSS tests, and validated by sloping-ground centrifuge tests on Fraser river sand, in which there was a good agreement between the numerical results obtained from the UBCSAND2 model and the experimental results. Therefore, the UBCSAND2 model is a promising tool to predict seismic-induced liquefaction. Korea had been recognized as safe from the threat of earthquakes until the recent Pohang earthquake, which hit the city of Pohang, Gyeongsangbuk-do with a magnitude of 5.4 on 2017 November 17. Near the epicenter, several damages induced by soil-structure interaction were observed, such as building collapses, sand blows, ground cracks, lateral spreading, and ground settlements were observed by solely visiting sites after the earthquake [9; 10]. In particular, there are two important reclaimed sites at Youngil-man port and Pohang port, which are mainly composed of backfill material and a caisson-type quay wall to prevent reclaimed area from the sea. These backfill materials are extremely sensitive to liquefaction caused by dynamic loading; however, liquefaction occurred only at Youngil-man port [11; 12] based on observation.