• Journal of the Korean Geosynthetics Society
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• v.15 no.4
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• pp.9-16
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• 2016

During the 2004 Niigata-ken Chuetsu, Japan, earthquake, more than 1,450 underground structures, known as sewer manhole, were uplifted up to 1.5m in Nagaoka and Ojiya city. The uplift damage can be a serious matter because they not only hinder the flow of wastewater systems, as a part of lifeline systems, but also disturb traffic flows. For restoration works, an open-cut investigation of damaged wastewater system was conducted by the Nagaoka city government. The results from the investigation compiled valuable data sets for buried pipeline damage due to earthquakes. In the present study, the factors affecting the uplift amount of the underground structure is investigated by using the data sets which include locations of damaged sections and inclination of pipeline before and after the earthquake and the SPT borehole logs in the affected area. Correlation analysis between the underground structure uplift and the geological settings in the affected area revealed that ground water depth and original subsoil, including thickness of clay layer, SPT N-value and fill thickness are the key parameters for the uplift phenomenon.

• Proceedings of the Korean Geotechical Society Conference
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• 2002.03a
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• pp.237-244
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• 2002

Televiewer is a logging tool capable of scanning the borehole wall. The tool uses a rotating acoustic beam generator that acts as both a transmitter and receiver. The beams are sent toward the wall. The amplitude of a returning signal from the wall has nearly a linear relationship with the reflection coefficient R of the borehole wall, when the wall is smooth. As R depends only on rock impedance for fixed water impedance, the amplitude is directly associated with mass density and seismic velocity of rock. Meanwhile, the amplitude can be further reduced by wall roughness that may be caused by drilling procedures, differences in rock hardness, because the rough surface can easily scatter the acoustic energy and sometimes the hole becomes elongated in all directions according to the degree of weathering. In this sense, the amplitude is related to the hardness of rocks. For convenience of analysis, the measured amplitude image(2-D data(azimuth ${\times}$ depth)) is converted, with an appropriate algorithm, to the 1-D data(depth), where the amplitude image values along a predetermined fracture signature(sinusoid) are summed up and averaged. The resulting values are subsequently scaled simply by a scalar factor that is possibly consistent with a known strength. This scaled Televiewer reflectivity is named, as a matter of convenience,“Televiewer rock strength”. This paper shows, based on abundant representative case studies from about 8 years of Televiewer surveys, that Televiewer rock strength might be regarded, on a continuous basis with depth, as a quitely robust indicator of rock classification and in most cases as an approximate uniaxial strength that is comparable to the rebound value from Schmidt hammer test.

• Journal of Soil and Groundwater Environment
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• v.27 no.spc
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• pp.19-33
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• 2022

When subsurface is polluted, contaminants tend to migrate through groundwater flow path. The groundwater flow path is highly dependent upon underground geological structures in the contaminated area. Geophysical survey is an useful tool to identify subsurface geological structure. In addition, geophysical logging in a borehole precisely provides detailed information about geological characteristics in vicinity of the borehole, including fractures, lithology, and groundwater level. In this work, surface seismic refraction and electrical resistivity surveys were conducted in a test site located in Namyangju city, South Korea, along with well logging tests in five boreholes installed in the site. Geophysical data and well logging data were collected and processed to construct an 3D geological map in the site.

• Tunnel and Underground Space
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• v.32 no.6
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• pp.586-597
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• 2022

In this study, a model was developed for the estimating the locational thickness information of the upper clay layer to be used for the consolidation vulnerability evaluation in the Nakdong river estuary. To estimate ground layer thickness information, we developed four spatial estimation models using machine learning algorithms, which are RF (Random Forest), SVR (Support Vector Regression) and GPR (Gaussian Process Regression), and geostatistical technique such as Ordinary Kriging. Among the 4,712 borehole data in the study area collected for model development, 2,948 borehole data with an upper clay layer were used, and Pearson correlation coefficient and mean squared error were used to quantitatively evaluate the performance of the developed models. In addition, for qualitative evaluation, each model was used throughout the study area to estimate the information of the upper clay layer, and the thickness distribution characteristics of it were compared with each other.

• Geomechanics and Engineering
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• v.32 no.1
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• pp.49-68
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• 2023

The three-dimensional failure criterion is essential for maintaining wellbore stability and sand production problem. The convenient factor for a stable wellbore is mud weight and borehole orientation, i.e., mud window design and selection of borehole trajectory. This study proposes a new three-dimensional failure criterion with linear relation of three in-situ principal stresses. The number of failure criteria executed to understand the phenomenon of rock failure under in-situ stresses is the Mohr-Coulomb criterion, Hoek-Brown criterion, Mogi-Coulomb criterion, and many more. A new failure criterion is the extended Mohr-Coulomb failure criterion with the influence of intermediate principal stress (σ₂). The influence of intermediate principal stress is considered as a weighting of (σ₂) on the mean effective stress. The triaxial compression test data for eleven rock types are taken from the literature for calibration of material constant and validation of failure prediction. The predictions on rock samples using new criteria are the best fit with the triaxial compression test data points. Here, Drucker-Prager and the Mogi-Coulomb criterion are also implemented to predict the failure for eleven different rock types. It has been observed that the Drucker-Prager criterion gave over prediction of rock failure. On the contrary, the Mogi-Coulomb criterion gave an equally good prediction of rock failure as our proposed new 3D failure criterion. Based on the yield surface of a new 3D linear criterion it gave the safest prediction for the failure of the rock. A new linear failure criterion is recommended for the unique solution as a linear relation of the principal stresses rather than the dual solution by the Mogi-Coulomb criterion.

• Tunnel and Underground Space
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• v.34 no.4
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• pp.301-311
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• 2024

In South Korea, following the 2016 Gyeongju and 2017 Pohang earthquakes, the need for earthquake disaster prevention has been increasing. Reliable techniques for probabilistic seismic hazard analysis and ground motion models are required for quantifying earthquake damage. Recently, there has been growing demand for deep underground facilities, necessitating accurate quantification techniques for earthquake damage in deep underground. In this study, ground motion models within rock were proposed using ground motion data measured at borehole seismic stations. A regression analysis, a type of empirical technique, was applied to 17 periods selected in a range from 0.01 to 10 s of spectral accelerations to develop the ground motion models. Residual analysis was performed to evaluate and improve the prediction performance of the ground motion model, with correction factors added to the model equation. When applying the proposed model, the group means of residuals approached zero, and the standard deviation of total residuals, similar to existing models proposed in other countries, confirmed the reliability of the proposed model.

• Journal of the Korean GEO-environmental Society
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• v.25 no.11
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• pp.5-12
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• 2024

In this study, the liquefaction potential index was assessed using actual borehole data and seismic waves, and a predictive model was developed based on machine learning methods. A total of 10 features were selected including factors reflecting the characteristics of the seismic waves. To identify candidate methods, a preliminary test was conducted using commonly used machine learning methods for regression, followed by Bayesian optimization to optimize the hyperparameters for these candidate methods. Among artificial neural networks, Gaussian process regression, and random forest, it was found that the random forest effectively predicted the liquefaction potential index, as indicated by a low root mean square error, a high coefficient of determination, and considerations regarding overfitting. However, it was noted that the model tends to underestimate the liquefaction potential index when the index was 5 or higher.

• Geophysics and Geophysical Exploration
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• v.12 no.3
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• pp.255-262
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• 2009

Conventionally, vertical-seismic-profiling (VSP) survey that provides high-resolution information has mainly performed to obtain the exact depth of the gas hydrate-bearing sediment, which is one of the key factors in the development of the gas hydrate. In this study, we extracted interval velocities and created corridor stacks from the first domestic zero-offset VSP data, which were acquired with three component receivers at UBGH09 borehole in Ulleung Basin where gas hydrate exists. Then we compared the corridor stacks with a CMP stacked section from surface seismic data. First of all, we converted the signals recorded with three component receivers to true vertical and horizontal components by phase rotation, and divided the data into direct waves and reflected waves by wavefield separation processing. The trend of the interval velocity extracted from the zero-offset VSP was similar to that of the sonic log obtained at the same borehole. Because the interval velocity of the gas hydrate-bearing sediment above the BSR was high, and it decreased suddenly through the BSR, we could infer that free gas is accumulated below the BSR. The results of comparing the corridor stacks to the CMP stacked section of the surface seismic data showed that most reflection events agreed well with those in the surface CMP stacked section and that the phase-rotated VSP data corresponded better with the surface seismic data than the VSP data without phase rotation. In addition, by comparing a corridor stack produced from the transverse component with the CMP stacked section of the surface seismic data, we could identify PS mode-converted reflections in the CMP stacked section.

• 한국신재생에너지학회:학술대회논문집
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• 2007.11a
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• pp.511-514
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• 2007

This study was performed to construct a geothermal data base about thermal conductivity of ground heat exchanger and thermal properties of grouting material which used to refill the borehole. We have acquired geothermal data sets from 39 sites over wide area of South Korea except to Jeju island. From data analysis, the range of thermal conductivity is $1.5{\sim}4.0$ W/mK. It means that thermal conductivity varies with grouting material as well as regional geology and ground water system.

• Proceedings of the SAREK Conference
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• 2007.11a
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• pp.423-428
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• 2007

This study was performed to construct a geothermal data base about thermal conductivity of ground heat exchanger and thermal properties of grouting material which used to refill the borehole. We have acquired geothermal data sets from 39 sites over wide area of South Korea except to Jeju island. From data analysis, the range of thermal conductivity is 1.5$\sim$4.0 W/mK. It means that thermal conductivity varies with grouting material as well as regional geology and ground water system.