• Geophysics and Geophysical Exploration
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• v.19 no.1
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• pp.20-28
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• 2016

Recently many sinkholes have appeared in urban areas of Korea, threatening public safety. To predict the occurrence of sinkholes, it is necessary to investigate the existence of cavity under urban roads. Ground-penetrating radar (GPR) has been recognized as an effective means for detecting underground cavity in urban areas. In order to improve the understanding of the governing physical processes associated with GPR wave propagation, and interpret underground cavity effectively, a theoretical approach using numerical modeling is required. We have developed an algorithm employing a three-dimensional (3D) staggered-grid finite-difference time-domain (FDTD) method. This approach allows us to model the full electromagnetic wavefield associated with GPR surveys. We examined the GPR response for a simple cavity model, and the modeling results showed that our 3D FDTD modeling algorithm is useful to assess the underground cavity under urban roads.

• Geophysics and Geophysical Exploration
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• v.25 no.4
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• pp.189-200
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• 2022

Ground subsidence on urban roads is a social issue that can lead to human and property damages. Therefore, it is crucial to detect underground cavities in advance and repair them. Underground cavity detection is mainly performed using ground penetrating radar (GPR) surveys. This process is time-consuming, as a massive amount of GPR data needs to be interpreted, and the results vary depending on the skills and subjectivity of experts. To address these problems, researchers have studied automation and quantification techniques for GPR data interpretation, and recent studies have focused on deep learning-based interpretation techniques. In this study, we described a hyperbolic event detection process based on deep learning for GPR data interpretation. To demonstrate this process, we implemented a series of algorithms introduced in the preexisting research step by step. First, a deep learning-based YOLOv3 object detection model was applied to automatically detect hyperbolic signals. Subsequently, only hyperbolic signals were extracted using the column-connection clustering (C3) algorithm. Finally, the horizontal locations of the underground cavities were determined using regression analysis. The hyperbolic event detection using the YOLOv3 object detection technique achieved 84% precision and a recall score of 92% based on AP50. The predicted horizontal locations of the four underground cavities were approximately 0.12 ~ 0.36 m away from their actual locations. Thus, we confirmed that the existing deep learning-based interpretation technique is reliable with regard to detecting the hyperbolic patterns indicating underground cavities.

• Journal of Korean Tunnelling and Underground Space Association
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• v.10 no.2
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• pp.193-205
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• 2008

The construction of underground structures such as oil and food storage caverns are recently increasing in our country. The stability of those underground caverns are greatly influenced by their shape and size. In this study therefore, the effect that the shape of an underground cavern have on its stability were analyzed in terms of safety factor. To this end, caverns with 5 different shapes were investigated and sensitivity analyses were performed based on rock class, overburden, and lateral earth pressure coefficient. The proper amount of shotcrete and rockbolt as supports of a cavern was also assumed based on the shape and site of the cavern and rock conditions. This study is expected to be helpful in designing and evaluating the stability of caverns in future.

• Journal of the Korean Geosynthetics Society
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• v.17 no.1
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• pp.45-54
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• 2018

In this study, numerical analysis is performed to determine highly influential factors that increase the possibility of asphalt road collapse due to cavity underneath the road. The considered influence factors on road collapse due to underground cavity were the asphalt layer thickness, the cover depth, the cavity width, and the cavity height. The concentrated load and uniform distributed pressure were applied on the top surface of asphalt pavement layers with different shape of cavity and asphalt thickness. For each analysis case of given cavity and asphalt thickness, failure load was analyzed under displacement controlled condition. Based on the analyzed failure loads, the applicability of the cavity management system developed by Seoul city was evaluated. As a result of the analysis, the effect of cavity height on road collapse was not significant while the other factors considerably influenced road collapse. Consequently, degree of road collapse susceptibility should be classified by failure load rather than by the condition of existing cavity.

• Journal of Korea Spatial Information System Society
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• v.7 no.2 s.14
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• pp.29-38
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• 2005

Even though underground conduits have became important city-infra structures which should be exhaustively and efficiently managed, there is few systems which supports the well-defined facility management standards. Due to the lack of the supporting systems, experts must visit underground conduits scattered several cities over the country to see and check the status of the underground conduits including built-in facilities. This type of management gives us a little bit delayed status information at the end of so much time and money costs. In this paper, to solve this problem and manage the conduit synthetically, we developed a web-based facility management system for underground conduits by using information technologies. The developed management system has a simplified map drawing interface to depict the overall architectures and locations of underground conduits and their built-in facilities into sketch maps. And, the system uses the 3D panorama image technology with zooming functions in addition to still images and video images to give the feeling of a spot inspection. Moreover, since the system accumulates the data of repair/reinforcement, occasional inspections and safety diagnosis, conduit managers can synthetically and effectively manage the facilities within underground conduits and themselves.

• Tunnel and Underground Space
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• v.25 no.3
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• pp.275-283
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• 2015

Excavation of underground space in soft ground implicate to the structure, such as subsidence. As a result, it has been acting as a serious risk to the stability of the roads and facilities. Therefore, in order to stabilize the soil stabilization and reinforcement of the structure, we have been using a number of methods and injecting material. In this study, we compared and analyzed the amount of subsidence regarding the ground reinforcement during underground excavation in soft ground by performing model test. And three-dimensional numerical analysis was performed using FLAC 3D. The subsidence was simulated numerically according to the tunnel excavation. The subsidence results of the model tests and numerical analyzes were relatively consistent. Thus comparing the ground subsidence by varying the reinforcement area on the numerical analysis was analyzed. As a results, three-dimensional numerical simulation could be regarded to simulate better on the ground subsidence by various kinds of underground excavation and it can be used as a material of subsidence prevention methods.

• Tunnel and Underground Space
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• v.21 no.4
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• pp.297-306
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• 2011

In this paper, we performed thermodynamic energy balance analysis of the underground lined rock cavern for compressed air energy storage (CAES) using the results of multi-phase heat flow analysis to simulate complex groundwater-compressed air flow around the cavern as well as heat transfer to concrete linings and surrounding rock mass. Our energy balance analysis demonstrated that the energy loss for a daily compression and decompression cycle predominantly depends on the energy loss by heat conduction to the concrete linings and surrounding rock mass for a sufficiently air-tight system with low permeability of the concrete linings. Overall energy efficiency of the underground lined rock caverns for CAES was sensitive to air injection temperature, and the energy loss by heat conduction can be minimized by keeping the air injection temperature closer to the ambient temperature of the surroundings. In such a case, almost all the heat loss during compression phase was gained back in a subsequent decompression phase. Meanwhile, the influence of heat conductivity of the concrete linings to energy efficiency was negligible.

• Journal of the Society of Disaster Information
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• v.19 no.3
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• pp.737-746
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• 2023

Purpose: Ground subsidence due to cavity can bring about various problems, such as casualties, decrease of the safety of the structures, and indirect social costs. Therefore, ground subsidence should be prevented through the exploration and recovery of the cavity under the pavements. Method: In this study, GPR exploration method was carried out on both actual roadway and mock-up site to compensate for the problems caused by excavation and restoration process. Result: This study compared the cavity scales obtained from GPR exploration results and the direct excavation of the identified cavity. It was confirmed that the predicted soil depth by GPR exploration was similar to the identified soil depth, but the predicted cavity scale by GPR exploration overestimated the longitudinal and cross-sectional widths compared to the identified cavity scale. Conclusion: Based on the correlation between the predicted cavity scales by GPR exploration, it is possible to qualitatively estimate the cavity scales using the empirical formula proposed in this study.

• Geophysics and Geophysical Exploration
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• v.5 no.2
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• pp.78-83
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• 2002

The finite element method combined with the sensitivity method is adopted for 2-dimensionl Fourier transform inversion. To improve the efficiency of inversion calculation, multi-resolution wavelet method is proposed., Theoretical data which is obtained from above method is shown to examine the proposed method. Theoretical model assumes that underground cavity is located in limestone area. In theoretical model, 16 current and potential electrodes are located to get theoretical data. It is shown that the about inversion method is very exact and useful calculation method, in case the larger model is very small such as under ground cavity.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.5
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• pp.621-640
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• 2019

Quantitative stability assessment of underground cavities can be presented as a factor of safety based on the Shear Strength Reduction Method (SSRM). Also, SSRM is one of the stability evaluation methods commonly used in slope stability analysis. However, there is a lack of research that considers the relationship between the probability of occurrence of cavities in the ground and the potential failure surface of the slope at the same time. In this study, the effect of small underground cavities on the failure behavior of the slope was analyzed by using SSRM. Considering some of the glaciology studies, there is a case that suggests that there is a cavity effect inside the glacier in the condition that the glacier slides. In this study, the stability evaluation of underground cavities and slope stability analysis, where SSRM is used in geotechnical engineering field, was carried out considering simultaneous conditions. The slope stability analysis according to the shape and position change of underground cavities which are likely to occur in the lower part of a mountain road was analyzed by using SSRM in FLAC3D software and the influence of underground cavities on the slope factor of safety was confirmed. If there are underground cavities near slope potential failure surface, it will affect the calculation of a factor of safety. The results of this study are expected to be basic data on slope stability analysis with small underground cavities.