# Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model
## 2 Methodology
### UBCSAND2 Calibration for Undrained CDSS Tests on Pohang Sand  
After the Pohang earthquake, many sand boils occurred in the paddy fields near the epicenter. The ejected sands from these sand boils were collected and tested by a series of undrained CDSS tests [24]. These results were used to calibrate the element response of the UBCSAND2 model. The samples were reconstituted with an air-pluviation method into loose (\(D_{\mathrm{r}}\) = 40%) and dense (\(D_{\mathrm{r}}\) = 80%) states under vertical effective stresses of 100 and 200 kPa, respectively. Symmetrical and non-symmetrical loading (i.e., \(\alpha=\tau_{\mathrm{sta}}/\sigma^{\prime}{}_{\mathrm{y}0}=0\), and \(\alpha>0\)) were applied in the CDSS tests to investigate the effects of initial shear stress on the cyclic resistance of sand. Figure 4 shows the experimental results for the relationship of cyclic stress ratio (\(CSR\)) and number of cycles to liquefaction (\(N_{\mathrm{I}\mathrm{I}\mathrm{q}}\)) of Pohang sand in both loose and dense states under a vertical effective  
stress (\(\sigma^{\prime}{}_{\rm v0}\)) of 100 kPa. At \(N_{\rm liq}\) = 15, if the relative density of sand increased from 40% to 80%, then the resistance of liquefaction increased by approximately 28.6% (from 0.14 to 0.18). Based on the SPT base clean-sand curve from a workshop of the National Center for Earthquake Research (NCEER) summarized by [PERSON] et al. (2015), the corrected SPT values \(N_{1(60)}\) of 12.5 and 16.5 were inferred from a _CSR_ of 0.14 and 0.18 at \(\sigma^{\prime}{}_{\rm v0}\) = 100 kPa, respectively. From Figure 4, two corrected SPT blow counts \(N_{1(60)}\) = 12.5 for loose sand and \(N_{1(60)}\) = 16.5 for dense sand were used to calibrate the UBCSAND2 model. The generic input parameters of the UBCSAND2 model are the same as those that were used in the commercial version v.904 aR ([PERSON] et al., 2017), which is shown in Table 1. We note that the UBCSAND2 model has an additional input parameter corresponding to the cyclic hardening parameter \(C_{\rm h}\), which is a function of relative density \(D_{\rm r}\) based on the experiment results of Fraser river sand. A detailed description of this parameter was previously presented. The calibration parameters were calculated identically to the generic parameters for both loose and dense sand, to match the laboratory data. It is interesting that providing only corrected SPT blow counts \(N_{1(60)}\) as the input parameters of UBCSAND2, and the bulk modulus of water, a predictive model of liquefaction can be achieved. Figures 5 and 6 show the comparison results between the laboratory tests and the calibrated UBCSAND2 model for loose and dense Pohang sand with \(\sigma^{\prime}{}_{\rm v0}\) = 100 kPa, \(CSR\) = 0.14, and \(\alpha\) = 0 (i.e., without initial static shear stress), respectively. The predictions are generally in good agreement with the measurements in terms of effective stress path (i.e., vertical effective stress versus shear stress), shear stress-strain curve, pore water pressure generation, and shear strain versus number of cycles. In the effective stress path, there are two distinct phases: (i) rapid effective drop for the initial three cycles of loose sand and five cycles of dense sand, and (ii) slow effective stress drop for 10 cycles of loose sand and 32 cycles of dense sand. This leads to a rapid rise in pore pressure of up to 35-45% the initial vertical effective stress during three cycles and five cycles for loose and dense sand, respectively. Herein, the effect of PSR is accounted for in the model, in which both the maximum shear stress plane and horizontal plane participate in the generation of plastic shear strains. This effect gradually reduces as the direction of the principal stress rotates, and it vanishes when \(\sigma^{\prime}{}_{\rm x}=\sigma^{\prime}{}_{\rm y}\) (i.e., at three cycles for loose sand and four cycles for dense sand).