• 한국지반공학회:학술대회논문집
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• 한국지반공학회 1995년도 가을 학술발표회 논문집
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• pp.15.2-22
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• 1995

Evaluating stiffness of near-surface materials has been one of the critically important tasks in many civil engineering works. It is the main goal of geotechnical characterization. The so-called deflection-response method evaluates the stiffness by measuring stress-strain behavior of the materials caused by static or dynamic load. This method, however, evaluates the overall stiffness and the stiffness variation with depth cannot be obtained. Furthermore, evaluation of a large-area geotechnical site by this method can be time-consuming, expensive, and damaging to many surface points of the site. Wave-propagation method, on the other hand, measures seismic velocities at different depths and stiffness profile (stiffness change with depth) can be obtained from the measured velocity data. The stiffness profile is often expressed by shear-wave (S-wave) velocity change with depth because S-wave velocity is proportional to the shear modulus. that is a direct indicator of stiffiiess. The crosshole and downhole method measures the seismic velocity by placing sources and receivers (geophones) at different depths in a borehole. Requirement of borehole installation makes this method also time-consuming, expensive, and damaging to the sites. Spectral-Analysis-of-Surface-Waves (SASW) method places both source and receivers at the surface, and records horizontally-propagating surface waves. Based upon the theory of surfacewave dispersion, the seismic velocities at different depths are calculated by analyzing the recorded surface-wave data. This method can be nondestructive to the sites. However, because only two receivers are used, the method requires multiple measurements with different field setups and, therefore, the method often becomes time-consuming and labor-intensive. Furthermore. the inclusion of noise wavefields cannot be handled properly, and this may cause the results by this method inaccurate. When multi-channel recording method is employed during the measurement of surface-waves, there are several benefits. First, usually single measurement is enough because multiple number (twelve or more) of receivers are used. Second, noise inclusion can be detected by coherency checking on the multi-channel data and handled properly so that it does not decrease the accuracy of the result. Third, various kinds of multi-channel processing techniques can be applied to f1lter unwanted noise wavefields and also to analyze the surface-wavefields more accurately and efficiently. In this way, the accuracy of the result by the method can be significantly improved. Fourth, the entire system of source, receivers, and recording-processing device can be tied into one unit, and the unit can be pulled by a small vehicle, making the survey speed very fast. In all these senses, multi-channel recording of surface waves is best suited for a routine method for geotechnical characterization in most of civil engineering works.

• 한국지반공학회논문집
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• 제24권10호
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• pp.33-43
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• 2008

본 연구에서는 현재 운영 중인 경상북도 영천시에 위치한 Y댐에 대하여 댐 축조재료 중 코어죤의 최대전단탄성계수 산정을 위한 합리적인 전단파속도를 산정하기 위하여 시추공을 이용한 크로스홀시험과 다운홀시험을 수행하였고, 지표탐사인 MASW(Multi-channel Analysis of Surface Wave, 다중수의 수진기를 이용한 주파수영역 표면파해석)와 반사법 탄성파탐사를 수행하였다. 현장시험으로부터 코어죤의 심도별 전단파속도를 산정하고 그결과들을 비교 분석하였다. 또한, 현장시험 결과와 Sawada의 경험적 제안식을 비교 분석하였다. 본 연구의 목적은 댐 코어죤의 심도별 전단파속도를 산정하기 위해 수행된 4종류의 현장시험 결과를 비교 분석하고, 시험결과와 경험적 제안식을 비교하여 경험식의 적정성을 판단하고, 기존댐 코어죤의 심도별 전단파속도를 합리적으로 산정하는 방법을 제안하는 데 있다. 시험결과, 측정심도 내(18m 이내)에서 다운홀시험, MASW, 반사법탐사에 의한 코어죤의 심도별 전단파속도 산출 결과는 유사한 결과를 보였으며, MASW와 반사법탐사의 경우에는 심도 30m까지 그 산출 결과가 거의 같은 것으로 나타났다. 기존 댐 로어죤의 심도별 전단파속도를 산출하고자 할 때 운영 중인 댐 코어죤에 시추공을 형성하는 것은 현실적으로 매우 어려운 사안임을 감안한다면 MASW나 반사법탄성파탐사와 같은 지표탐사에 의한 전단파속도 산정이 가장 현실적인 방법일 것으로 판단된다. 현장조사가 여의치 않거나 예비조사의 경우에는 기존의 방법과 같이 Sawada의 경험식을 이용할 수 있는데, 경험식을 이용할 경우 Sawada의 제안식 중 하한값을 적용하는 것이 합리적인 방법일 것으로 판단되었다.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2006년도 추계 학술발표회
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• pp.434-478
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• 2006

Stress waves have been used for geophysical and geotechnical applications for more than 50 years. The early-stage applications were simply based on travel-time measurements of stress waves and limited to site characterization. Currently stress-wave techniques are expanded to monitoring processes for grouting of damaged geotechnical structures, compaction of embankment, and deformational analyses for static geotechnical problems. Seismic techniques used to be good enough for rough estimators of engineering properties. Nowadays, the sophisticated modeling theory of stress-wave propagation substantially improved reliability and accuracy of the seismic techniques. In this paper, difficulties involved in currently available seismic techniques are discussed and analyzed. Herein some recently-developed non-intrusive seismic techniques, which make optimal use of stress waves for further improvement of reliability and accuracy, are also presented.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2000년도 추계 학술발표회 논문집 Proceedings of EESK Conference-Fall 2000
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• pp.81-88
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• 2000

The elastic wave equation is solved using the finite-difference method in 3D space to simulate the seismic wave propagation. It is based on the velocity-stress formulation of the equation of motion on a staggered grid. The nonreflecting boundary conditions are used to attenuate the wave field close to the numerical boundary. To satisfy the stress-free conditions at the free-surface boundary, a new formulation combining the zero-stress formalism with the vacuum one is applied. The effective media parameters are employed to satisfy the traction continuity condition across the media interface. With use of the moment-tensor components, the wide range of source mechanism parameters can be specified. The numerical experiments are carried out in order to test the applicability and accuracy of this scheme and to understand the fundamental features of the wave propagation under the generalized elastic media structure. Computational results show that the scheme is sufficiently accurate for modeling wave propagation in 3D elastic media and generates all the possible phases appropriately in under the given heterogeneous velocity structure. Also the characteristics of the ground motion in an sedimentary basin such as the amplification, trapping, and focusing of the elastic wave energy are well represented. These results demonstrate the use of this simulation method will be helpful for modeling the ground motion of seismological and engineering purpose like earthquake hazard assessment, seismic design, city planning, and etc..

• 한국지구물리탐사학회:학술대회논문집
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• 한국지구물리탐사학회 2005년도 공동학술대회 논문집
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• pp.165-180
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• 2005

충격탄성파 검사법은 콘크리트 구조물 내부의 결함과 외부 표면 사이에서 반사되어 전파되는 응력파를 이용한 비파괴 시험 방법이다. 본 연구에서 토목 구조물의 안전진단을 위하여 충격탄성파 검사법을 이용한 비파괴 시험을 수행하였다. 이를 위해서 토모그래피 방식을 이용한 투과법, 크로스 홀 방식을 이용한 투과법 그리고 일반적으로 터널라이닝과 같은 1차원 면에서 사용되는 반사법을 적용해 그 결과를 비교${\cdot}$분석하였다.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2004년도 추계학술대회 논문집
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• pp.868-873
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• 2004

Recently, The surface-wave method has widely been used for the site investigation due to the economic advantage and the improved reliability. The typical surface-wave methods currently available are SASW method, MASW method and CSW method. The CSW method has a potential of high-quality measurement, but its inherent problems limited its use to the special cases such as the compaction-quality control. The CSW method uses the steady-state harmonic vibration for the seismic source as in the steady-state Rayleigh-wave method, which is superior to the impact source used for other methods. This study proposed a new procedure to solve the inherent problems of the CSW method and to improve the reliability of the CSW measurements. To verify the validity of the proposed in this study, the SASW results were compared with the CSW results for the numerical simulation of the CSW testing. Also, the feasibility of the proposed method was verified using the field measurements at a geotechnical site.

• 한국지반환경공학회 논문집
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• 제10권7호
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• pp.135-141
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• 2009

지진원 및 구조물과 지반상호간의 동적 특성을 보다 신뢰성 있게 도출하기 위해 지반의 증폭특성은 반드시 고려되어야하는 요소이다. 지반증폭 특성을 분석할 때 여러 가지 방법이 제시되어 있으나 본 연구에서는 Nakamura(1989)에 의해 제시된 방법을 적용하였다. 본 방법은 얕은 지반의 상시미동의 표면파 특성을 이해하기 위해 제시되어 한계점이 존재하나 근래에 와서 S파 등에 적용되어 지반의 동적인 증폭 특성연구에 많이 이용되고 있다. 일반적으로 S파가 이용되고 있으나 본 연구에서는 지진파 에너지인 Coda 파에 새로이 적용하였다. 최근 국내에 설치된 지진 관측기에 관측된 5개 지진(규모 3.6-5.1)의 약 60 여개의 관측자료를 이용하여 지진관측소 각각 지반의 동적인 증폭 특성을 분석하였다. 관측소마다 저진동수, 고진동수 및 우월주파수가 서로 다른 증폭특성을 보여주었다. 본 연구 결과 관측지반진동에서 지반 고유의 증폭특성을 제거하면 지진원 변수를 보다 신뢰성 있게 도출할 수 있다.

• Earthquakes and Structures
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• 제17권6호
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• pp.557-566
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• 2019

The evaluation of the seismic hazard for a given site is to estimate the seismic ground motion at the surface. This is the result of the combination of the action of the seismic source, which generates seismic waves, the propagation of these waves between the source and the site, and site local conditions. The aim of this work is to evaluate the sensitivity of dynamic response of extended structures to spatial variable ground motions (SVGM). All factors of spatial variability of ground motion are considered, especially local site effect. In this paper, a method is presented to simulate spatially varying earthquake ground motions. The scheme for generating spatially varying ground motions is established for spatial locations on the ground surface with varying site conditions. In this proposed method, two steps are necessary. Firstly, the base rock motions are assumed to have the same intensity and are modelled with a filtered Tajimi-Kanai power spectral density function. An empirical coherency loss model is used to define spatial variable seismic ground motions at the base rock. In the second step, power spectral density function of ground motion on surface is derived by considering site amplification effect based on the one dimensional seismic wave propagation theory. Several dynamics analysis of a curved viaduct to various cases of spatially varying seismic ground motions are performed. For comparison, responses to uniform ground motion, to spatial ground motions without considering local site effect, to spatial ground motions with considering coherency loss, phase delay and local site effects are also calculated. The results showed that the generated seismic signals are strongly conditioned by the local site effect. In the same sense, the dynamic response of the viaduct is very sensitive of the variation of local geological conditions of the site. The effect of neglecting local site effect in dynamic analysis gives rise to a significant underestimation of the seismic demand of the structure.

• 지구물리와물리탐사
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• 제12권1호
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• pp.99-104
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• 2009

본 연구에서는 천부 토양층의 2차원 불균질 S파 단면을 결정하기 위해 천부 굴절법 탐사로부터 얻은 SH파 자료에 대한 파형역산 기법을 제안한다. 2차원 매질에서 SH파의 전파를 모사하기 위해 2.5차원 파동 방정식을 사용한다. 합성탄성파를 계산하기 위해 공간축으로 4차, 시간축으로 2차 근사한 staggered grid 유한차분법을 사용하여 파동 방정식을 푼다. 계산된 파형과 측정 파형의 잔여오차로 정의되는 목적함수를 하이브리드 발견적 탐색기법에 의해 최소화한다. 2차원 지하구조 모형은 각기 다른 심도 경계면을 갖는 블록과 블록 내부의 S파 속도에 의해 매개화한다. 수치실험은 암영층파 불규칙한 경계를 갖는 모델에 대해 백색잡음을 추가한 합성 SH파 자료를 이용하여 수행하였다. 지표 굴절법 자료로부터 암영층을 포함한 구조를 적절한 계산시간 내에 영상화할 수 있었다

• Earthquakes and Structures
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• 제5권2호
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• pp.161-205
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• 2013

We study seismically induced, anti-plane strain wave motion in a non-homogeneous geological region containing tunnels. Two different scenarios are considered: (a) The first models two tunnels in a finite geological region embedded within a laterally inhomogeneous, layered geological profile containing a seismic source. For this case, labelled as the first boundary-value problem (BVP 1), an efficient hybrid technique comprising the finite difference method (FDM) and the boundary element method (BEM) is developed and applied. Since the later method is based on the frequency-dependent fundamental solution of elastodynamics, the hybrid technique is defined in the frequency domain. Then, an inverse fast Fourier transformation (FFT) is used to recover time histories; (b) The second models a finite region with two tunnels, is embedded in a homogeneous half-plane, and is subjected to incident, time-harmonic SH-waves. This case, labelled as the second boundary-value problem (BVP 2), considers complex soil properties such as anisotropy, continuous inhomogeneity and poroelasticity. The computational approach is now the BEM alone, since solution of the surrounding half plane by the FDM is unnecessary. In sum, the hybrid FDM-BEM technique is able to quantify dependence of the signals that develop at the free surface to the following key parameters: seismic source properties and heterogeneous structure of the wave path (the FDM component) and near-surface geological deposits containing discontinuities in the form of tunnels (the BEM component). Finally, the hybrid technique is used for evaluating the seismic wave field that develops within a key geological cross-section of the Metro construction project in Thessaloniki, Greece, which includes the important Roman-era historical monument of Rotunda dating from the 3rd century A.D.