• Smart Structures and Systems
• /
• v.33 no.1
• /
• pp.41-53
• /
• 2024

Grouting sleeves are an essential connecting component of prefabricated components, and the quality of grouting has a significant influence on structural integrity and seismic performance. The embedded grouting sleeve (EGS)'s grouting defects are highly undetectable and random, and no effective monitoring method exists. This paper proposes an ultrasonic guided wave method and provides a set of guidelines for selecting the optimal frequency and suitable period for the EGS. The optimal frequency was determined by considering the group velocity, wave structure, and wave attenuation of the selected mode. Guided waves are prone to multi-modality, modal conversion, energy leakage, and dispersion in the EGS, which is a multi-layer structure. Therefore, a time-reversal (TR)-based multi-mode focusing and dispersion automatic compensation technology is introduced to eliminate the multi-mode phase difference in the EGS. First, the influence of defects on guided waves is analyzed according to the TR coefficient. Second, two major types of damage indicators, namely, the time domain and the wavelet packet energy, are constructed according to the influence method. The constructed wavelet packet energy indicator is more sensitive to the changes of defecting than the conventional time-domain similarity indicator. Both numerical and experimental results show that the proposed method is feasible and beneficial for the detection and quantitative estimation of the grouting defects of the EGS.

• Journal of Korean Tunnelling and Underground Space Association
• /
• v.14 no.5
• /
• pp.503-516
• /
• 2012

Rock mass rating (RMR) is the key factor when designing the appropriate support pattern of tunnel projects. Borehole drilling is usually performed along the tunnel route in order to determine the rock mass rating to be used for tunnel design. The rock mass rating at the non-boring region between boreholes is usually assessed through geophysical surveys such as electrical prospecting, seismic prospecting, etc. Many studies were carried out to find out the correlation between electrical resistivity and rock mass rating. However, most researches were aimed at obtaining the relationship between the two parameters utilizing experimental results obtained from laboratory tests or electrical prospectings. In this paper, efforts were made to analyze and obtain relationships between the electrical resistivity obtained from in-situ electrical resistivity logging data and the rock mass rating. Correlation studies using field data showed that the electrical resistivity is highly correlated with the rock mass rating with the determination coefficient more than 90%. The correlation analysis was also carried out between RMR classification parameters and the electrical resistivity. It was shown that the correlation between the condition of discontinuities and the electrical resistivity was very high with the determination coefficient more than 80%; that between the groundwater condition and the electrical resistivity was very low with the determination coefficient less than 57%.

• Journal of the Mineralogical Society of Korea
• /
• v.32 no.3
• /
• pp.201-211
• /
• 2019

The particle size and crystallinity of fault gouge generally decreases with slip. Phyllosilicates including talc are known to be present in fault gouge and play an important role in fault weakening. In particular, the coefficient of friction varies depending on the presence of a water molecule on the surface of mineral. The purpose of this study is to investigate the effect of talc on fault weakening by changing the water content and dehydration behavior of talc before and after grinding, which systematically varied particle size and crystallinity using high energy ball mill. Infrared spectroscopy and thermal analysis show that the as-received talc is hydrophobic before grinding and the water molecule is rarely present. After grinding up to 720 minutes, the particle size decreased to around 100 ~300 nm, and in talc, where amorphization proceeded, the water content increased by about 8 wt.% and water molecule would be attached on the surface of talc. As a result, the amount of vaporized water by heating increased after grinding. The dihydroxylation temperature also decreased by ${\sim}750^{\circ}C$ after 720 minutes of grinding at ${\sim}950^{\circ}C$ before grinding due to the decrease of particle size and crystallinity. These results indicate that the hydrophobicity of talc is changed to hydrophilic by grinding, and water molecules attached on the surface, which is thought to lower the coefficient of friction of phyllosilicates. The repeated slip throughout the seismic cycle would consistently lower the coefficient of friction of talc present in fault gouge, which could provide the clue to the weakening of matured fault.

• Journal of the Korean earth science society
• /
• v.25 no.8
• /
• pp.774-783
• /
• 2004

The S wave velocity and Q$s^{-1}$ structure of the uppermost part of the soil in Nakdong Delta area have been obtained to determine the characteristics of the forementioned soil. The phase and attenuation coefficients of multichannel seismic records were inverted to obtain the S wave velocity and Q$s^{-1}$ structure of the soil. The inversion results have been compared with the borehole measurements of the area. The seismic signal of the nearest geophone from a seismic source was used as the source signal to obtain the attenuation coefficients. Amplitude ratios of the signal at each geophone to the source signal wave plotted as a function of distance for the frequency range between 10 Hz and 45 Hz. The slope of a linear regression line which fits amplitude ratio-distance relationship best for a given frequency was used as the attenuation coefficients for the frequency. The dispersion curve of Rayleigh waves and the attenuation coefficients were inverted to obtain the S-wave velocity and Q$s^{-1}$, respectively, in the uppermost 8 meter of soil layer. The borehole measurements of the area show that are two distinct layers; the upper 4 meter of silty-sand and the lower 4 meter of silty-clay. The inversion results indicate that the shear wave velocity of the upper layer is 80 m/sec and 40m/sec in the lower silty-clay layer. The spacial resolution of the shear wave velocity structure is very good down to a depth of 8 meter. The Q$s^{-1}$ in the upper silty-sand layer is 0.02 and increase to 0.03 in the lower silty-sand layer. The spacial resolution of quality factor is relatively good down to a depth of 5 meter, but very poor below the depth. In this study, the S-wave velocity is higher in the silty-clay and the Q$s^{-1}$ is smaller silty-sand than in the silty-clay. However, much more data should be analyzed and accumulated before making any generalization on the shear wave velocity and Q$s^{-1}$ of the sediments.

• The Journal of Engineering Geology
• /
• v.23 no.4
• /
• pp.457-465
• /
• 2013

We integrated and correlated datasets from surface and subsurface geophysics, drilling cores, and engineering geology to identify geological interfaces and characterize the joints and fracture zones within the rock mass. The regional geometry of a geologically weak zone was investigated via a fence projection of electrical resistivity data and a borehole image-processing system. Subsurface discontinuities and intensive fracture zones within the rock mass are delineated by cross-hole seismic tomography and analyses of dip directions in rose diagrams. The dynamic elastic modulus is studied in terms of the P-wave velocity and Poisson's ratio. Subsurface discontinuities, which are conventionally identified using the N value and from core samples, can now be identified from anomalous reflection coefficients (i.e., acoustic impedance contrast) calculated using a pair of well logs, comprising seismic velocity from suspension-PS logging and density from logging. Intensive fracture zones identified in the synthetic seismogram are matched to core loss zones in the drilling core data and to a high concentration of joints in the borehole imaging system. The upper boundaries of fracture zones are correlated to strongly negative amplitude in the synthetic trace, which is constructed by convolution of the optimal Ricker wavelet with a reflection coefficient. The standard deviations of dynamic elastic moduli are higher for fracture zones than for acompact rock mass, due to the wide range of velocities resulting from the large numbers of joints and fractures within the zone.

• Journal of the Earthquake Engineering Society of Korea
• /
• v.10 no.2 s.48
• /
• pp.51-62
• /
• 2006

In the companion paper (I-Problem Statements of the Current Seismic Design Code), the current Korean seismic design code is required to be modified considering site characteristics in Korea for the reliable estimation of site amplification. In this paper, three site classification methods based on the mean shear wave velocity of the top 30m $V_{S30}$, fundamental site periods $(T_G)$ and bedrock depth were investigated and compared with each other to determine the best classification system. Not enough of a difference in the standard deviation of site coefficients $(F_a\;and\;F_v)$ to determine the best system, and neither is the difference between the average spectral accelerations and the design response spectrum of each system. However, the amplification range of RRS values based on $T_G$ were definitely concentrated on a narrow band than other classification system. It means that sites which have a similar behavior during earthquake will be classified as the same site category at the site classification system based on $T_G$. The regression curves between site coefficients and $T_G$ described the effect of soil non linearity well as the rock shaking intensity increases than the current method based on $V_{S30}$. Furthermore, it is unambiguous to determine sue category based on $T_G$ when the site investigation is performed to shallower depth less than 30m, whereas the current $V_{S30}$ is usually calculated fallaciously by extrapolating the $V_s$ of bedrock to 30m. From the results of this study, new site classification system based on $T_G$ was recommended for legions of shallow bedrock depth in Korea.

• Geophysics and Geophysical Exploration
• /
• v.11 no.2
• /
• pp.148-152
• /
• 2008

Multichannel seismic profiles reveal a strong bottom simulating reflector (BSR) occurring below the seafloor in the plain of the Ulleung Basin, East Sea (Japan Sea). The essential characteristics of the BSR include its cross-cutting relationship to strata, strong amplitude, and reverse polarity with respect to the seafloor reflection, representing the base of the gas hydrate stability zone (BHSZ). The BSR reflection coefficient ranging from -0.23 to -0.26 is 1.5${\sim}$1.7 times that of the seafloor reflection and interval velocities decrease to less than 700 m/s below the BSR. These features indicate the existence of free gas beneath the GHSZ. Heat flow, estimated from the BSR depth as $95{\sim}98mW/m^2$, is in good agreement with measured values. Therefore, the BSR can be efficiently used to estimate regional distribution of heat flow in the Ulleung Basin.

• Land and Housing Review
• /
• v.5 no.2
• /
• pp.115-120
• /
• 2014

Site coefficient and amplification factor of current domestic Seismic Design Code (KBC-2009) have no consideration for the domestic ground condition in which the base rock is normally placed within 30m form the surface. Accordingly, in this study dynamic centrifugal test and analysis for pile foundation into clay were achieved. and the response spectrums of free surface and basement were compared with each other. Within the period 1sec., the measured spectral acceleration of free surface and basement was bigger than the design spectral acceleration of SC and SD site. However the measured spectral acceleration of free surface and basement for the period over 1.5sec. was smaller than the design spectral acceleration of SC site. There was no severe difference of spectral acceleration according to the upper structure, embedded depth of foundation and free surface conditions. Consequently, normal domestic apartment housing for the period range over 1.5sec. could be design more economically applying these test result.

• Geophysics and Geophysical Exploration
• /
• v.16 no.4
• /
• pp.211-216
• /
• 2013

In general, smoothing filters regularize functions by reducing differences between adjacent values. The smoothing filters, therefore, can regularize inverse solutions and produce more accurate subsurface structure when we apply it to full waveform inversion. If we apply a smoothing filter with a constant coefficient to subsurface image or velocity model, it will make layer interfaces and fault structures vague because it does not consider any information of geologic structures and variations of velocity. In this study, we develop a selective smoothing regularization technique, which adapts smoothing coefficients according to inversion iteration, to solve the weakness of smoothing regularization with a constant coefficient. First, we determine appropriate frequencies and analyze the corresponding wavenumber coverage. Then, we define effective maximum wavenumber as 99 percentile of wavenumber spectrum in order to choose smoothing coefficients which can effectively limit the wavenumber coverage. By adapting the chosen smoothing coefficients according to the iteration, we can implement multi-scale full waveform inversion while inverting multi-frequency components simultaneously. Through the successful inversion example on a salt model with high-contrast velocity structures, we can note that our method effectively regularizes the inverse solution. We also verify that our scheme is applicable to field data through the numerical example to the synthetic data containing random noise.

• Journal of the Korean Geotechnical Society
• /
• v.29 no.8
• /
• pp.5-14
• /
• 2013

Coarse granular geomaterials containing large gravels are broadly used for construction of large geotechnical systems such as dams, levees, railways and backfills. It is necessary to evaluate deformation characteristics of these materials for dynamic analysis, e.g. seismic design. This study presents evaluation of dynamic deformation characteristics of coarse materials using large scale resonant column testing apparatus, which uses specimens with 200 mm in diameter and 400 mm in height, and the effects of gradation characteristics on maximum shear modulus, shear modulus reduction curve and damping characteristics were investigated. From experimental study using rock-fill materials for a dam, we could see that the largest or mean particle size affects the shape of shear modulus reduction curve. When the specimens are prepared under the same conditions for maximum particle size, the coefficient of uniformity affects the confining stress exponent of maximum shear modulus. It could be concluded that the maximum particle size is an factor which affects shear modulus reduction curve, and that the coefficient of uniformity is for small strain shear modulus, especially for the sensitivity to confining stress.