Evidence of liquefaction during the 2017 Pohang earthquake has highlighted the urgent need to evaluate the current seismic design standard for liquefaction in Korea, particularly the liquefaction triggering standard. With the simplified method, which is the most popular method for evaluating liquefaction triggering, the factor of safety for liquefaction triggering is calculated via the cyclic stress ratio (CSR) and the cyclic resistance ratio (CRR). The parameters in the CSR and CRR have undergone changes over time based on new research findings and lessons learned from liquefaction case-histories. Hence, the current design standard for liquefaction triggering evaluation in Korea should also reflect these changes to achieve seismic safety during future earthquakes. In this study, liquefaction susceptibility criteria were discussed initially and this was followed by a review of the current liquefaction triggering codes/guidelines in other countries and Korea. Next, the parameters associated with the CSR such as the maximum ground acceleration, stress reduction factor, magnitude scaling factor, and overburden correction factor were discussed in detail. Then, the evaluation of the CRR using the SPT N-value and CPT qc-value was elaborated along with overburden and clean-sand correction factors. Based on this review of liquefaction triggering evaluation standards, recommendations are made for improving the current seismic design standard related to liquefaction triggering in Korea.

In the companion papers (I, II), site-specific response analyses were performed at more than 300 domestic sites and a new site classification system and design response spectra (DRS) were proposed using the results of the site-specific response analyses. In this paper, the proposed site classification system and the design response spectra are compared with those in other seismic codes and verified by different methods. Firstly, the design response spectra are compared with the design response spectra in Eurocode 8, KBC 2016 and MOCT 1997 to estimate quantitative differences and general trends. Secondly, site-specific response analyses are carried out using VS-profiles obtained using field seismic tests and the results are compared with the proposed DRS in order to reduce the uncertainty in using the SPT-N value in site-specific response analyses in the companion paper (I). In addition, site coefficients from real earthquake records measured in Korean peninsula are used to compare with the proposed site coefficients. Finally, dynamic centrifuge tests are also performed to simulate the representative Korean site conditions, such as shallow depth to bedrock and short-period amplification characteristics. The overall results showed that the proposed site classification system and design response spectra reasonably represented the site amplification characteristic of shallow bedrock condition in Korea.

In the companion paper (I – Database and Site Response Analyses), site-specific response analyses were performed at more than 300 domestic sites. In this study, a new site classification system and design response spectra are proposed using results of the site-specific response analyses. Depth to bedrock (H) and average shear wave velocity of soil above the bedrock (VS,Soil) were adopted as parameters to classify the sites into sub-categories because these two factors mostly affect site amplification, especially for shallow bedrock region. The 20 m of depth to bedrock was selected as the initial parameter for site classification based on the trend of site coefficients obtained from the site-specific response analyses. The sites having less than 20 m of depth to bedrock (H1 sites) are sub-divided into two site classes using 260 m/s of VS,Soil while the sites having greater than 20 m of depth to bedrock (H2 sites) are sub-divided into two site classes at VS,Soil equal to 180 m/s. The integration interval of 0.4 ~ 1.5 sec period range was adopted to calculate the long-period site coefficients (Fv) for reflecting the amplification characteristics of Korean geological condition. In addition, the frequency distribution of depth to bedrock reported for Korean sites was also considered in calculating the site coefficients for H2 sites to incorporate sites having greater than 30 m of depth to bedrock. The relationships between the site coefficients and rock shaking intensity were proposed and then subsequently compared with the site coefficients of similar site classes suggested in other codes.

Korea is part of a region of low to moderate seismicity located inside the Eurasian plate with bedrock located at depths less than 30 m. However, the spectral acceleration obtained from site response analyses based on the geologic conditions of inland areas of the Korean peninsula are significantly different from the current Korean seismic code. Therefore, suitable site classification scheme and design response spectra based on local site conditions in the Korean peninsula are required to produce reliable estimates of earthquake ground motion. In this study, site-specific response analyses were performed at more than 300 sites with at least 100 sites at each site categories of SC, SD, and SE as defined in the current seismic code in Korea. The process of creating a huge database of input parameters - such as shear wave velocity profiles, normalized shear modulus reduction curves, damping curves, and input earthquake motions - for site response analyses were described. The response spectra and site coefficients obtained from site response analyses were compared with those proposed for the site categories in the current code. Problems with the current seismic design code were subsequently discussed, and the development and verifications of new site classification system and corresponding design response spectra are detailed in companion papers (II-development of new site categories and design response spectra and III-Verifications)