Chang-Guk Sun

Development and Verification of a New Site Classification System and Site Coefficients for Regions of Shallow Bedrock in Korea

In this article, site response analyses for 124 sites collected in Korea were performed to evaluate earthquake ground motions in regions of shallow bedrock in Korea. Based on the results, a new two-parameter site classification system was developed for use in regions of shallow bedrock. The system incorporates depth to bedrock and mean VS of soil above bedrock as parameters for site classification instead of VS30. Soil sites were divided into seven site classes and the corresponding site coefficients were proposed for each site category. Some verification work demonstrates the superiority of the proposed system over the current seismic codes.

Geostatistical assessment for the regional zonation of seismic site effects in a coastal urban area using a GIS framework

Earthquake-induced hazards are profoundly affected by site effects related to the amplification of ground motions, which are strongly influenced by local geologic conditions such as soil thickness, bedrock depth, and soil stiffness. Seismic disasters are often more severe over soft soils than over stiff soils or rocks due to differences in local site effects. In this study, on the basis of a geotechnical information system (GTIS) framework, we developed an advanced geostatistical assessment for the regional zonation of seismic site effects. In particular, to reliably predict spatial geotechnical information, we developed a procedural methodology for building an advanced GTIS within a geographic information system framework and applied it to the Busan area in Korea. The systemized GTIS comprised four functional components: database, geostatistical analysis, geotechnical analysis, and visualization. First, to build the GTIS, we collected pre-existing geotechnical data in and around the study area, and then conducted a walk-over site survey to acquire surface geo-knowledge data. Second, we determined the optimum geostatistical estimation method using a cross-validation-based verification test, considering site conditions. The advanced GTIS was used in a practical application to estimate the site effects in the study area. We created seismic zoning maps of geotechnical earthquake parameters, such as the depth to bedrock and the site period, and present them as part of a regional synthetic strategy for earthquake risk assessment.

Determination of Mean Shear Wave Velocity to the Depth of 30m Based on Shallow Shear Wave Velocity Profile

The mean shear wave velocity to the depth of 30 m (Vs30) derived from the western Vs is the current site classification criterion for determining the design seismic ground motion taking into account the site amplification potential. In order to evaluate the Vs30 at a site, a shear wave velocity (Vs) Profile extending to at least 30 m in depth must be acquired from in-situ seismic test. In many cases, however, the resultant depth of the Vs profile may not extend to 30 m, owing to the unfavorable field condition and the limitation of adopted testing techniques. In this study, the Vs30 and the mean shear wave velocity to a depth shallower, than 30 m (VsDs) were computed from the Vs profiles more than 30 m in depth obtained by performing various seismic tests at total 72 sites in Korea, and a correlation between Vs30 and VsDs was drawn based on the computed mean Vs data. In addition, a method for extrapolating the Vs profile from shallow depth to 30 m was developed by building a shape curve based on the average data of all Vs profiles. For evaluating the Vs30 from the shallow Vs profiles, both the methods using VsDs and shape curve result in less bias than the simplest method of extending the lowermost Vs equally to 30 m in depth, and are usefully applicable particularly in the cases of the Vs profiles extending to at least 10 m in depth.미국 서부 지역을 근간으로 도출된 30m 심도까지의 평균 전단파속도(Vs30)는 부지 증폭 정도에 따른 설계 지진 지반 운동 결정을 위한 현행 지반 분류 기준이다. 부지의 Vs30을 산정하기 위해서는 현장탄성파 시험으로부터 적어도 30m 심도까지의 전단파 속도(Vs) 분포를 획득해야 한다. 그러나 많은 경우에서 현장의 불리...

Geospatial Big Data-Based Geostatistical Zonation of Seismic Site Effects in Seoul Metropolitan Area

Seismic site effects are influenced mainly by geospatial uncertainties corresponding to geological or geotechnical spatial variance. Therefore, the development of a geospatial database is essential to characterize site-specific geotechnical information in multiscale areas and to optimize geospatial zonation methods with potentially high degrees of spatial variability based on trial-and-error geostatistical assessments. In this study, a multi-source geospatial information framework, which included the construction of a big data platform, estimation of geostatistical density, optimization of the geostatistical interpolation method, assessment of seismic site effects, and determination of geospatial zonation for decision making, was established. Then, this framework was applied to the Seoul metropolitan area, South Korea. The GIS-based framework was established to develop the geospatial zonation of site-specific seismic site effects before considering the local characteristics of site effects dependent on topographic or geological conditions, based on a geospatial big-data platform in Seoul. The zonal conditions were composed of geo-layers, site effect parameters, and other multi-source geospatial maps for each administrative area, and infrastructure was determined based on the integration of the optimized geoprocessing framework.

GIS-based regional assessment of seismic site effects considering the spatial uncertainty of site-specific geotechnical characteristics in coastal and inland urban areas

ABSTRACT Earthquake-induced hazards are profoundly affected by site effects related to the amplification of ground motions, which are strongly influenced by site-specific geologic conditions such as soil thickness, bedrock depth and soil stiffness. Seismic disasters are often more severe in coastal or riverside locations than over stiff soils or rocks due to differences in local site effects. In this study, a recently developed geographic information system-based framework was applied in coastal and inland urban areas in Korea, and its applicability for regional assessments was evaluated using appropriate geostatistical zonation of site-specific seismic site effects. The proposed framework was composed of four functional components: multivariable statistical clustering, geostatistical optimization, geotechnical analysis, and local visualization. The framework was applied in the Seoul and Busan areas of Korea for consideration of site effects in inland and coastal urban areas. Such zones of thick soil, or with a deep depth to bedrock, are susceptible to ground motion amplification due to site effects during earthquakes. The earthquake losses associated with possible building damage can be estimated based on spatial zoning maps considering geological and topographical characteristics and by a comparison of the spatial correlations of seismic site classes between inland and coastal areas of Korea.

Deduction of Correlations between Shear Wave Velocity and Geotechnical In-situ Penetration Test Data

다양한 탄성파 시험으로부터 획득할 수 있는 전단파속도(Vs)는 주로 지진공학 분야에서의 내진 설계 및 내진 성능 평가를 위한 대표적 지반 통적 특성으로 강조되어 왔다. 일반적인 지반공학적 부지 조사 기법의 지반지진공학적 활용을 목적으로, 표준관입 시험(SPT)과 피에조콘관입시험(CPTu)을 국내 여러 부지들을 대상으로 다양한 시추공 탄성파시험과 함께 수행하였다. 본 연구에서는 현장 시험 자료들의 통계학적 모델링을 통해 전단파속도와 표준관입시험의 타격수(N 값) 및 선단저항력(q t ), 주면마찰력(f s )과 간극수압계수(B q )로 구성되는 피에조콘관입 자료 간의 상관관계를 도출하고 전단파속도 결정을 위한 경험적 방법으로 제안하였다. 비록 일반적인 지반공학적 관입시험과 시추공 탄성파시혐의 대상 변형률 수준이 상이하다 할지라도, 본 연구에서 제안된 상관관계들은 국내 토사 지층의 예비적 전단파속도 산정에 활용될 수 있을 것으로 보인다.

Evaluation of Site-Specific Seismic Amplification Characteristics in Plains of Seoul Metropolitan Area

Total 350 borehole profiles were selected from the database of borehole logs in Seoul, for the site-specific seismic evaluation at two 4km4km plain areas. Equivalent-linear site response analyses for the selected 350 sites were conducted based on shear wave velocity (Vs) Profiles, which were determined from the N-Vs correlation established using borehole seismic testing results in the inland areas of Korea. Most sites were categorized as site classes C and D based on the mean Vs to 30 m in depth (Vs30) ranging from 250 to 550 m/s. The she periods of the plains in Seoul ranging between 0.1 and 0.4 sec were significantly lower than those of the western US, from which the site coefficients in Korea were derived. For plains in Seoul, the site coefficients, Fa`s and Fv`s specified in the Korean seismic design guide, underestimate the ground motion in short-period (0.1-0.5 sec) band and overestimate the ground motion in mid-period (0.4-2.0 sec) band, respectively, because ol the differences in the geotechnical conditions between Seoul and the western US, although the Fa`s in several sites overestimate the motion due to the base Isolation effect resulted from the soft layer in soil deposit.

Visible Assessment of Earthquake-induced Geotechnical Hazards by Adopting Integrated Geospatial Database in Coastal Facility Areas

Earthquake event keeps increasing every year, and the recent cases of earthquake hazards invoke the necessity of seismic study in Korea, as geotechnical earthquake hazards, such as strong ground motion, liquefaction and landslides, are a significant threat to structures in industrial hub areas including coastal facilities. In this study, systemized framework of integrated assessment of earthquake-induced geotechnical hazard was established using advanced geospatial database. And a visible simulation of the framework was specifically conducted at two coastal facility areas in Incheon. First, the geospatial-grid information in the 3D domain were constructed with geostatistical interpolation method composed of multiple geospatial coverage mapping and 3D integration of geo-layer construction considering spatial outliers and geotechnical uncertainty. Second, the behavior of site-specific seismic responses were assessed by incorporating the depth to bedrock, mean shear wave velocity of the upper 30 m, and characteristic site period based on the geospatial-grid. Third, the normalized correlations between rock-outcrop accelerations and the maximum accelerations of each grid were determined considering the site-specific seismic response characteristics. Fourth, the potential damage due to liquefaction was estimated by combining the geospatial-grid and accelerations correlation grid based on the simplified liquefaction potential index evaluation method.

Reliable Evaluation of Dynamic Ground Properties from Cross-hole Seismic Test using Spying-loaded Lateral Impact Source

Soil and rock dynamic properties such as shear wave velocity , compressional wave velocity and corresponding Poisson`s ratio (v) are very important geotechnical parameters in predicting deformational behavior of structures as well as practicing seismic design and performance evaluation. In an effort to measure the parameter efficiently and accurately, various bore-hole seismic testing techniques have been, thus, developed and used during past several decades. In this study, cross-hole seismic testing technique which is known as the most reliable seismic method was adopted for obtaining geotechnical dynamic properties. To perform successfully the cross-hole test for rock as well as soil layers regardless of the ground water level, spring-loaded source which impact laterally a subsurface ground in vertical bore-hole was developed and applied at three study areas, which contain four sites composed of two existing port sites and two new LNG storage facility sites. The geotechnical dynamic properties such as and v with depth from the soil surface to the engineering and seismic bedrock were efficiently determined from the laterally impacted cross-hole seismic tests at study sites, and were provided as the fundamental parameters for the seismic performance evaluation of the existing ports and the seismic design of the LNG storage facilities.

Geospatial Assessment of the Post-Earthquake Hazard of the 2017 Pohang Earthquake Considering Seismic Site Effects

The 2017 Pohang earthquake (moment magnitude scale: 5.4) was South Korea’s second strongest earthquake in decades, and caused the maximum amount of damage in terms of infrastructure and human injuries. As the epicenters were located in regions with Quaternary sediments, which involve distributions of thick fill and alluvial geo-layers, the induced damages were more severe owing to seismic amplification and liquefaction. Thus, to identify the influence of site-specific seismic effects, a post-earthquake survey framework for rapid earthquake damage estimation, correlated with seismic site effects, was proposed and applied in the region of the Pohang earthquake epicenter. Seismic zones were determined on the basis of ground motion by classifying sites using the multivariate site classification system. Low-rise structures with slight and moderate earthquake damage were noted to be concentrated in softer sites owing to the low focal depth of the site, topographical effects, and high frequency range of the mainshocks.