## 2 Methodology
### Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model
#### 2.3.2 Numerical Modeling  
Figure 15 presents the representative cross section of Youngil-man port, which is in the north-south direction and was created using the FLAC2D program [30]. It was mostly built from gravel as a backfill material near the quay wall, and silty sandy soil for the reclaimed area. These layers are extremely sensitive to liquefaction under earthquake motion. [PERSON] et al. [32] and [PERSON] et al. [9] indicated that the bedrock of the Pohang basin, including the observed area, is approximately 500 m deep, and the mudstone is primarily distributed below a depth of 20 m up to the bedrock. In the case of Youngil-man port, the foundation soils include a thin thickness of sedimentary layer mainly from silty-like soil, and weathered rock is below the sedimentary layer [12]. In addition, a row of steel pipe piles, with a diameter of 600 mm, was installed at the depth of the sedimentary layers in order to reduce displacements if a failure occurs at the quay wall. Nodes N1, N2, N3, and N4 in Figure 15 depict the positions of the output nodes for the displacement predictions of the caisson, landfill ground behind the quay wall, and wave transmission checking. Similarly, elements E1, E2, E3, and E4 are used for the extraction of pore pressure generation and reduction in effective stress when liquefaction occurs. Material properties: Table 3 shows the input parameters implemented in the numerical analysis. In this study, two silty sandy layers in the reclaimed region were considered as the liquefiable soil layers,  
Figure 14: Information of Pohang earthquake’s ground motion at Youngil-man port extracted and compiled from the equivalent analysis (PROSHAKE) at 25 m: (**a**) acceleration-time history, (**b**) velocity–time history, (**c**) displacement–time history, and (**d**) power spectra of acceleration. Figure 15: Youngil-man port cross section for FLAC modeling and analysis (dimension unit in meters). in which a reasonable \(N_{1(60)}\) value of 15 was determined to be the best input into the numerical analysis using the UBCSAND2 model. All of the generic parameters required in the UBCSAND2 model in Table 1 can be calculated directly via the single \(N_{1(60)}\) value. The perfectly plastic Mohr-Coulomb model was assigned for non-liquefiable soil layers such as sediment, gravel, weather rock, and riprap. The elastic materials of concrete and steel were selected for the caisson-type quay wall and the steel pipe pile, respectively. Element size: The mesh size of the analysis was selected to be \(\Delta l=1\) m in order to ensure accurate wave transmission, and is based on the material properties in Table 3. Silty sand 1,2 has the lowest shear-wave speed \(C_{s}\), for an elastic shear modulus \(k_{G}e\) and a density \(\rho\), which can be calculated as follows:  
\[C_{s}\ =\ \sqrt{\frac{k_{G}^{c}}{\rho}}\ =\ \sqrt{\frac{21.7\times 20\times N_{1(60 )}^{0.333}\times P_{a}\times(\sigma^{\prime}{}_{m}/P_{a})}{\gamma_{d}/10}}\ =\ 237.24\ (m/s) \tag{17}\]  
Then, the maximum frequency transmitted in the largest zone in FLAC must be higher than the maximum frequency of the input motion [30]. \[f\ =\ \frac{C_{s}}{10\cdot\Delta l}\ =\ 23.72\ (Hz)>20\ (Hz) \tag{18}\]  
From Equation (18), it can be seen that the selected mesh size of \(1\times 1\) m can transfer the seismic wave in an accurate way when the earthquake motion is applied at the base of the numerical analysis. Boundary conditions for static analysis: During the static analysis, both the horizontal and vertical directions were fixed against the movement at the base of the model. Two sides of the model allowed for vertical movements, but were laterally fixed to prevent horizontal movements. At the bottom, a free pore pressure condition was assigned to allow for changes in pore pressure, but it restricted flow across the boundary. The left and right sides applied pore pressure below the level of \(-3.00\) m relative to the ground surface, which was the maximum sea level rise.