Developments of site classification system for seismic code provision and settlement prediction of footings using shear wave velocity

Abstract

The shear wave velocity ($V_S$) has been used as an essential input parameter for dynamic studies, such as geotechnical earthquake engineering, but recently its application has widened to an engineering property to solve static deformational problems. In this doctoral dissertation the application of $V_S$ to geotechnical problems are discussed separately in two parts including three cases: (1-1) Evaluation of $K_O$ and OCR values using $V_S$ in centrifuge model, (1-2) Settlement prediction of shallow footings using $V_S$ , (2) Evaluation of earthquake ground motions in Korea. In the first part of this doctoral dissertation, in-flight stress states of centrifuge model grounds such as $K_O$ and OCR values are evaluated based on $V_S$ measurement, since the VS represents the stress states of ground. A centrifuge test is conducted with centrifuge acceleration variation to cause stress history on centrifuge model ground and the $V_S$ and vertical effective stress were measured at each level of acceleration. For the evaluation of $K_O$ , a $V_S-K_O$ relationship, which requires one horizontally propagating $V_S$ and vertical effective stress shows feasibility. The $K_O$ values determined using the $V_S-K_O$ relationship vary with respect to location in the model. $K_O$ at the center is greater than that near the boundary for unloading stages because the OCR is higher at the center as a results of insignificant effects from the arching effect. A relationship between $G_{max}$ and OCR based on $V_S$ measurement is also established to identify the stress history of centrifuge model. By establishing the relationship, in-flight stress states of centrifuge model ground could be investigated in terms of OCR. The OCR evaluation according to the established $G_{max}$ relationship accomplishes that the OCR value of centrifuge model could be estimated based on $V_S$ measurements irrespective of NC or OC loading conditions even for cohesionless soil. Finally, it is expected that the causes of stress history on centrifuge model such as compaction, g-level variation and past overburden load can be analyzed quantitatively. New settlement prediction method based on $V_S$ measurement and nonlinearity of soil was developed and verified by centrifuge tests. The development was derived from the conceptual framework of Schmertmann’s method (1978), which is one of the commonly used methods in engineering practice. The procedures for obtaining confinement and strain dependent modulus values from $V_S-profile$ considering pressure distribution under the footing were developed. To verify the developed method, load-settlement curves for footings having different length to breadth ratio, together with VS-profiles, were obtained by centrifuge modeling, and then compared with the predictions from the developed method. The developed method was refined considering stress history of ground based on OCR evaluation, which was proposed in this study, to incorporate plastic deformation of soils on settlement calculation. A coefficient $(f)$ was adopted to consider the plastic deformation over elastic region, empirically. Based on the evaluated OCR, the empirical coefficient f was determined, and adopted in the refined settlement prediction method. The refined method based on empirical coefficient f was validated by comparison of the predictions with centrifuge testing results even for NC loadings. The second part of this doctoral dissertation is related to earthquake ground motions in Korea. 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, generally. 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 (DRS) 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 are performed at more than 300 sites with at least 100 sites at each site categories of $S_C$ , $S_D$ , and $S_E$ 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 are described. The response spectra and site coefficients obtained from site response analyses are compared with those proposed for the site categories in the current code. Problems with the current seismic design code are subsequently discussed. In order to develop a new site classification system and DRS using results of the site-specific response analyses, bedrock depth (H) and average $V_S$ of soil above the bedrock ( $V_{S,Soil}$ ) are 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 are 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 $V_{S,Soil}$ 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 is adopted to calculate the long-period site coefficients ( $F_v$ ) for reflecting the amplification characteristics of Korean geological condition. In addition, the frequency distribution of depth to bedrock reported for Korean sites is 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 are proposed and then subsequently compared with the site coefficients of similar site classes suggested in other codes. The proposed site classification system and the DRS are compared with those in other seismic codes and verified by different methods. Firstly, the DRS are compared with the DRS in Eurocode 8, KBC 2016 and MOCT 1997 to estimate quantitative differences and general trends. In addition, site coefficients from real earthquake records measured in Korean peninsula are used to compare with the proposed site coefficients. In addition, 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 test results are compared with the proposed DRS. Finally, site coefficients from real earthquake records measured in Korean peninsula are used to compare with the proposed site coefficients. A series of Gyeongju earthquakes occurred in September 2016 are also adopted for the verification. The overall results showed that the proposed site classification system and DRS reasonably represented the site amplification characteristic of shallow bedrock condition in Korea.