Showing papers by "Dong-Soo Kim published in 2004"

Evaluation of Various Downhole Data Reduction Methods for Obtaining Reliable V~S Profiles

Abstract: The downhole method has been widely used to measure in situ shear wave velocity profiles for the seismic response analysis of geotechnical sites. To analyze the downhole data, the direct and interval methods are mostly used in practice. In this study, the modified interval method based on a straight ray path and the inversion method based on Snell's Law ray path were introduced to improve the quality of the wave velocity profiles evaluated by the downhole seismic method. Various synthesized wave velocity profile models were developed to perform the parametric study and the arrival times were determined by a forward modeling scheme based on Snell's Law. By comparing the velocity profiles obtained by four different data reduction methods with actual velocity profiles, the accuracy and limitation of various data reduction methods were assessed. The direct method was difficult for evaluating the detailed velocity profiles, and the interval method was found to provide severe errors, particularly when a stiff layer is located beneath the soft layer. The modified interval method provides reliable results, except when a strong soft-to-stiff contrast exists. Snell's Law ray path method provides the most reliable velocity profiles. Finally, in situ downhole seismic tests were performed at three sites, and the importance of considering the ray path in the data reduction was emphasized by comparing the reduced shear wave velocity profiles with crosshole and standard penetration test results.

Time-Frequency Analysis for Impact Echo-SASW (IE-SASW) Method

Abstract: IE-SASW method which combines two nondestructive testing techniques can be applied to detect the flaws and the thickness of concrete structures. In this method, IE method is used for the detailed nondestructive evaluation of concrete structure and Spectral Analysis of Surface Waves (SASW) method is employed for the measurement of the average P-wave velocity of whole thickness. The signal received by the transducer in the IE test consists in a sum of the direct surface wave and side echoes, the bottom or defect echo (target echo), and the background noise. In the FFT based frequency domain analysis, sometimes, it is difficult and not straightforward to extract the information on the target echo. The time-frequency analysis can provide the temporal variation in frequency components of the signal and thereby has advantages in the analysis of IE signal. In this paper, time-frequency analysis based on short-time Fourier transform is presented to improve the interpretation of IE signal. Experimental studies were performed in the concrete model slab in which poor concrete was included at known location. Test results obtained by time-frequency analysis are compared to the conventional IE results and the advantages of using the proposed method were verified. Introduction Non-destructive testing (NDT) of concrete structures is becoming increasingly important due to the aging and deterioration of infrastructures, e.g. bridges, roadways, water and sewer systems, ports, harbors, airports and buildings. Various NDT techniques have been adopted, and further developed to assess concrete structures. Impact echo (IE) method have been widely used for determining thickness or internal defects of concrete structures [1,2,3]. Spectral analysis of surface waves (SASW) method has been used for nondestrucrtive evaluation of concrete quality as well as for the site characterization [4,5]. In this study, IE-SASW method is applied to the NDT of concrete structures; IE method is used for the detailed nondestructive evaluation of concrete structure and SASW method is employed for the measurement of the average P-wave velocity of whole thickness. Time domain IE analysis works well for the long and slender structures like drilled shaft and for the thin structures like slab, resonance frequencies of signals reflected from defects or opposite side of structure can be obtained in frequency domain using FFT for the estimation of internal information of structure. In practical application of IE test, it is sometimes hard to obtain resonance frequency clearly in the frequency domain analysis [6]. Vibration transducer on the surface perceives the impactgenerated direct surface wave, reflected wave from sides of structure, and ambient noise as well as the target echoes which is the multiple reflections of P-wave in vertical direction of structure [7]. If these unexpected waves have large amplitude and are periodic, these noise signals will be dominant in the frequency domain result and it would be hard to determine the true target echoes. In this paper, time-frequency analysis is introduced as an effort to overcome the limitation in the frequency domain analysis. The variation in frequency characteristics with time can be obtained in the time-frequency analysis, and the target echoes can be separated from the undesirable noise signals. The theoretical backgrounds of IE-SASW test and the time-frequency analysis were briefly Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1529-1534 doi:10.4028/www.scientific.net/KEM.270-273.1529 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:38:08) Title of Publication (to be inserted by the publisher) discussed. IE-SASW tests were performed on model slabs including defects or boundaries at known locations. Test results obtained by the conventional FFT and by the proposed time-frequency analysis are compared and the advantages of using time-frequency analysis were verified. Theoretical Background Impact Echo Method. The stress pulse generated by an impact on the surface propagates back and forth between the internal defects or bottom and top surfaces of a test object, as shown in Fig. 1. The arrival of these reflected waves at the surface is recorded by a receiving transducer, and the received signal is periodic. Both time and frequency domain analyses are used to identify the transient resonance conditions occurred by the multiple reflections of waves between impact surface, defects, and/or other external surfaces which can be used to determine the locations of defects or member thickness. The period of the reflected waves is equal to the travel path, 2T, divided by the compressive wave speed, VP. Since the frequency is the inverse of the period, the resonance frequency, f , is given by: T V f p 2 = (1) This is the fundamental equation of impact echo response for solid structures. If VP is predetermined, the thickness of structure and the location of internal defects can be evaluated. Concrete Vertically Propagating Reflected Stress Wave T Fig. 1. Impact-Echo test configuration SASW Method. The general configuration of receivers, source, and recording equipment in the SASW test is shown in Fig. 2. Two vertical receivers were placed on the surface of structure at an equal distance from a fixed centerline. Impulsive load was used to apply vertical excitation in the line with the two receivers at a distance, d, away from the near receiver. A FFT signal analyzer was used to record the receiver signals and then to transform them into a frequency domain. Phase information of the cross-power spectrum, which represents the phase difference between two receiver signals as a function of frequency, was obtained. From the cross power spectrum, the surface wave phase velocity is then calculated [4]. The variation in surface wave velocity (VR) with the wavelength (experimental dispersion curve) is determined in SASW method. The wavelength can be correlated with the thickness of a concrete structure. In the layered system, the wave velocity profile is obtained through an iterative inversion process [4,5], but in one layer system such as concrete slab, it is possible to obtain a representative VR from the experimental dispersion curve without inversion. P-wave velocity (Vp) can be converted from VR with the Poisson’s ratio(ν) of the concrete that exists in narrow band of between 0.15 and 0.22. R p V V ) 2 1 ( ) 1 ( 2 12 . 1 87 . 0 1 ν ν ν ν − − + + = (2) The possible error in determining P-wave velocity with the assumed Poisson’s ratio of 0.2 is about 3%, which is minimal, and Eq. (2) reduces to R p V V 79 . 1 = (3) Therefore, the representative P-wave velocity can be determined from SASW method easily at the surface without coring where IE test is performed. 1530 Advances in Nondestructive Evaluation Title of Publication (to be inserted by the publisher) d d d CL DYNAMIC SIGNAL ANALYZER SOURCE (FORWARD) RECEIVER SOURCE (BACKWARD) Fig. 2. SASW test configuration IE-SASW Method. In the conventional IE method, Vp is determined by measuring the direct travel time of Por R-waves on the concrete surface, or by performing IE test on a structure of known thickness or on cores taken from the structure [8]. Sometimes, the geometry of test structure is unknown, coring is prohibited, and, the measured P-wave velocity on the concrete surface may not be representative for the whole thickness of structure. In IE-SASW method which combines two NDT methods, the average P-wave velocity for whole thickness of structure is measured by SASW method and the detailed nondestructive evaluation of concrete structure is performed by IE method. Based on numerical and experimental studies, Kim et al. [9] showed the feasibility and applicability of IE-SASW method. Time-Frequency Analysis. The main goal of the analysis in the IE test is to obtain the information about the material integrity. The signal received by the transducer consists in a sum of components, i) direct surface wave, i) surface echo, i) target echo which is the bottom or defect echo, and i) ambient noise. In the analysis of IE method, it is extremely important to extract information on the target echo. In that sense, a Fourier transform-based analysis could be difficult to perform due to a possible shifting and/or a over-shadowing of the meaningful signal frequency components because it would neglect the temporal distribution of frequency contents for nonstationary signals, whose frequency content changes with time. In an effort to correct this deficiency, Dennis Gabor (1946) adopted the Fourier transform to analyze only a small section of the signal at a time – a technique called windowing the signal [10]. Gabor’s adaptation, called the Short-time Fourier transform (STFT), maps a signal into a twodimensional function of time and frequency. It can be shown as following equation. ∫ − − = τ τ τ τ π d e t g x f t STFT f j 2 ) ( ) ( ) , ( (4) where, ) (t x is a time signal and ) (t g is a window function. When using STFT on IE signals, it is hard to see the frequency characteristics of latter part of signals because the amplitude of latter part is relatively small compared to the early part where the direct surface wave energy is dominant. Therefore, in this paper, a modified form of STFT is used in the analysis. STFT coefficients at each time step are normalized by the maximum value at that time in order to easily distinguish the resonance frequency peaks with time variation on the timefrequency map. The variation of frequency characteristics with time can be clearly noticed in the modified time-frequency map as shown in Fig. 3. Finally adding normalized values at each frequency, the sum of normalized density is obtained and the advanta

Development of Seismic Site Characterization Method using HWAW (Harmonic Wavelet Analysis of Wave) Method (I) - Determination of Dispersion Curve -

Abstract: In general, in situ, seismic site characterization method based on dispersive characteristic of wave consists of three steps: 1 ) field test, 2) evaluation of dispersion curve, and 3) determination of shear modulus profile by inversion process. In this paper, the new seismic site characterization method is proposed using the HWAW(Harmonic Wavelet Analysis of Wave) method which is for evaluation of phase and group velocities of wave. The HWAW method mainly uses the signal portion of the maximum signal/noise ratio to evaluate the phase velocity and it can minimize the effects of noise. So, this method can determine the dispersion curve of whole depth from one experimental setup. To estimate the applicability of HWAW method in determination of dispersion curve, the numerical simulation at various layered soil and pavement profiles were performed and the dispersion curves determined by the HWAW method match nicely with theoretical velocities showing good potential of applying the HWAW method in determination of dispersion curve.

Development of Seismic Site Characterization Method based on HWAW (Harmonic Wavelet Analysis of Wave) Method(II) - Experimental Setup and Inversion Process -

Abstract: In general, in situ, seismic site characterization method based on dispersive characteristic of wave consists of three steps: 1) field test, 2) evaluation of dispersion curve, and 3) determination of shear modulus profile by inversion process. In this paper, the experimental setup and inversion process for the seismic site characterization method using the HWAW method are proposed. The experimental setup for this method consist of one pair of receivers, so the field test of this method is relatively simple and fast. This method use single array inversion. To estimate the applicability of HWAW method, numerical simulations at various layered soil and pavement profiles were performed and the results of numerical simulations show the reliability of the proposed experimental setup and inversion method.

Measurement of Gmax of Sands Using Bender Element in Resonant Column and Torsional Shear Equipment

Abstract: The bender element method is an experimental technique to determine very small strain (<10 -3 %), elastic shear modulus of a soil, Gmax by measuring the velocity of shear wave propagation through a sample. Bender elements have been applied as versatile transducers to measure small strain modulus of wet or dry soils in various laboratory apparatus. In this paper, bender element (BE), resonant column (RC) and torsional shear (TS) tests were performed on Toyoura sand at various testing conditions using the modified Stokoe type RC/TS testing equipment capable of performing BE test. Based on the results, applicabilities of the testing method using bender element were evaluated by comparing the values of Gmax obtained from RC/TS and BE testing methods. For more dependable evaluation, the loading frequency of each testing method was considered for the results obtained for samples in saturated condition by adapting Biot's theory.

Non-Destructive Testing and Evaluation of Civil Infrastructures Using Stress Wave Propagation

Abstract: The nondestructive testing and evaluation is commonly used in civil engineering for the evaluation of existing structures and for the quality control in new construction. Various NDT methods have been developed for civil infrastructures. In this paper, NDT methods based on the stress wave propagation such as impact echo, SASW and IE-SASW are introduced. The methods have advantage because the measured wave velocity is directly related with the engineering property of structure such as stiffness. The testing setup and data reduction techniques of various methods are discussed. The factors influencing test result and the advantages/limitations of each method are also reviewed. Time domain, frequency domain, and emerging time-frequency domain analysis techniques are utilized for the better interpretation of test results. The models of slab type structures and deep foundations including various defects and boundary conditions at known locations were made and the applicability of various methods was investigated. Finally, field applications to the concrete member in nuclear power plant containment structure and deep foundation were performed. Introduction Non-destructive testing (NDT) of concrete structures is becoming increasingly important due to the aging and deterioration of infrastructures, e.g. bridges, roadways, water and sewer systems, ports, harbors, airports and buildings. Various NDT techniques have been adopted, and further developed to assess concrete structures. In recent years, numerous studies have been reported, giving application examples of NDT techniques in detecting and locating anomalies in concrete [1]. In this paper, NDT methods based on the stress wave propagation are reviewed. These methods including impact echo and spectral analysis of surface waves (SASW) enable inspection well below the surface of the structure and offer direct information concerning the effective dynamic elastic constants and/or the presence of flaws [2]. In addition, cross-hole sonic logging (CSL) and parallel seismic techniques that are used to assess the integrity of piles are also introduced. IE-SASW technique is presented to improve conventional impact echo method, and time-frequency analysis is introduced to overcome the limitation of frequency domain analysis. It is found that NDT methods using stress wave propagation can be applied to civil infrastructures reliably. NDT Methods using Stress Wave Propagation Methods for Slab-Type Structures Impact echo. Impact echo (IE) is a method for nondestructive evaluation of concrete and masonry based on the use of impact-generated stress waves that propagate through the structure and are reflected by internal flaws and external surfaces. IE method can be used to measure the thickness of concrete slabs, and to determine the location and extent of flaws such as cracks, delaminations, voids, and debonding in concrete structures [3]. For the slab-type structures, the frequency domain analysis based on fast Fourier transforms (FFT) is performed to determine the dominant frequencies which Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1616-1621 doi:10.4028/www.scientific.net/KEM.270-273.1616 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:38:10) Title of Publication (to be inserted by the publisher) related to the P-wave echoes between the test surface and the reflection interfaces of concern [3]. To determine the thickness of slab and/or the location of flaws, the P-wave velocity should be pre-determined. SASW. This method based on the dispersive characteristics of surface waves is capable of determining material stiffness in the layered media [4]. Two vertical receivers are placed on the surface of the structure at an equal distance and impulsive load is used to apply vertical excitation. From the cross-power spectrum between two receiver signals, the phase difference is obtained and then the surface wave phase velocity is calculated. The variation in surface wave velocity (VR) with the wavelength (experimental dispersion curve) is determined in SASW method. The wavelength can be correlated with the thickness of a concrete structure. In the layered system, the wave velocity profile is obtained through an iterative inversion process [4], but in one layer system such as concrete slab, it is possible to obtain a representative VR from the experimental dispersion curve without inversion. P-wave velocity (Vp) can be converted from VR with the Poisson’s ratio(ν) of the concrete that exists in narrow band of between 0.15 and 0.22. R p V V ) 2 1 ( ) 1 ( 2 12 . 1 87 . 0 1 ν ν ν ν

Evaluation of Earthquake Ground Motion Considering Dynamic Site Characteristics in Korea

Abstract: The local geologic and dynamic site characteristics, which include soil profiles, shear wave velocity profiles and depths to the bed rock were gathered from 148 sites all over the Korean peninsula and those values are compared to those in the western USA. Site response analyses were performed based on equivalent linear scheme using design rock-outcrop acceleration of 0.154g which corresponds to the collapse level of earthquake for seismic category I structure. The results show that the amplification factor based on Korean seismic design guideline underestimates the motion in short-period range and overestimates the motion in mid-period range. It is suggested that the existing Korean seismic guideline based on UBC is required to be modified considering dynamic site characteristics in Korea for the reliable estimation of site amplification.