• Tunnel and Underground Space
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• v.29 no.2
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• pp.75-88
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• 2019

With Kori nuclear power plant unit 1 as a beginning in April 1978, 24 nuclear power plants have been operated in Korea and two more plants are under construction. As the nuclear power plants being operated, radioactive wastes from the plants have been accumulated so that various methods for disposing them have been proposed. In Korea, researches have been conducted, being focused on DGD (Deep Geological Disposal), however, DBD (Deep Borehole Disposal) method needs considering as an alternative. In this technical note, element technologies for DBD were analyzed by compiling previous researches and their applicability on domestic cases were investigated. Conceptual studies regarding relevant designs were conducted and finally, technical challenges for actual disposal were described.

• Tunnel and Underground Space
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• v.29 no.1
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• pp.30-37
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• 2019

Some leading countries in mining have a very quantitative guideline for underground mine opening design which is useful to minimize mine hazards such as rockfall and collapse. Those hazards sometimes can cause a huge damage on human life and property in the mines. Construction guidelines of underground mines in Korea consist of qualitative and general expressions although the workers' safety rules and guides are well provided. Recently, mining operations in Korea are going underground due to the environmental regulations and resource depletion at shallow depth, and therefore there is a growing demand on a specialized and systematic guideline for mine opening design securing the underground stability. In this paper, current status of mining industry, research trends, and mining guidelines in Korea and overseas have been reviewed to give an insight into developing a new Korean guideline for underground mine design.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.4
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• pp.265-272
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• 2019

In this paper, a vision-based deep learning algorithm and image processing method are proposed to detect bolt-loosening in steel connections. To achieve this objective, the following approaches are implemented. First, a bolt-loosening detection method that includes regional convolutional neural network(RCNN)-based deep learning algorithm and Hough line transform(HLT)-based image processing algorithm are designed. The RCNN-based deep learning algorithm is developed to identify and crop bolts in a connection image. The HLT-based image processing algorithm is designed to estimate the bolt angles from the cropped bolt images. Then, the proposed vision-based method is evaluated for verifying bolt-loosening detection in a lab-scale girder connection. The accuracy of the RCNN-based bolt detector and HLT-based bolt angle estimator are examined with respect to various perspective distortions.

• Structural Engineering and Mechanics
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• v.70 no.1
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• pp.13-31
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• 2019

A nonlinear finite element model (FEM) using ATENA-3D software to simulate the axially compressive behavior of circular steel tube confined concrete (CSTCC) columns infilled with ultra high performance concrete (UHPC) was presented in this paper. Some modifications to the material type "CC3DNonlinCementitious2User" of UHPC without and with the incorporation of steel fibers (UHPFRC) in compression and tension were adopted in FEM. The predictions of utimate strength and axial load versus axial strain curves obtained from FEM were in a good agreement with the test results of eighteen tested columns. Based on the results of FEM, the load distribution on the steel tube and the concrete core was derived for each modeled column. Furthermore, the effect of bonding between the steel tube and the concrete core was clarified by the change of friction coefficient in the material type "CC3DInterface" in FEM. The numerical results revealed that the increase in the friction coefficient leads to a greater contribution from the steel tube, a decrease in the ultimate load and an increase in the magnitude of the loss of load capacity. By comparing the results of FEM with experimental results, the appropriate friction coefficient between the steel tube and the concrete core was defined as 0.3 to 0.6. In addition to the numerical evaluation, eighteen analytical models for confined concrete in the literature were used to predict the peak confined strength to assess their suitability. To cope with CSTCC stub and intermediate columns, the equations for estimating the lateral confining stress and the equations for considering the slenderness in the selected models were proposed. It was found that all selected models except for EC2 (2004) gave a very good prediction. Among them, the model of Bing et al. (2001) was the best predictor.

• Structural Engineering and Mechanics
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• v.70 no.4
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• pp.479-497
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• 2019

In the current code design, the use of a uniform internal pressure coefficient of cooling towers as internal suction cannot reflect the 3D characteristics of flow field inside the tower body with different ventilation rate of shutters. Moreover, extreme weather such as heavy rain also has a direct impact on aerodynamic force on the internal surface and changes the turbulence effect of pulsating wind. In this study, the world's tallest cooling tower under construction, which stands 210m, is taken as the research object. The algorithm for two-way coupling between wind and rain is adopted. Simulation of wind field and raindrops is performed iteratively using continuous phase and discrete phase models, respectively, under the general principles of computational fluid dynamics (CFD). Firstly, the rule of influence of 9 combinations of wind speed and rainfall intensity on the volume of wind-driven rain, additional action force of raindrops and equivalent internal pressure coefficient of the tower body is analyzed. The combination of wind velocity and rainfall intensity that is most unfavorable to the cooling tower in terms of distribution of internal pressure coefficient is identified. On this basis, the wind/rain loads, distribution of aerodynamic force and working mechanism of internal pressures of the cooling tower under the most unfavorable working condition are compared between the four ventilation rates of shutters (0%, 15%, 30% and 100%). The results show that the amount of raindrops captured by the internal surface of the tower decreases as the wind velocity increases, and increases along with the rainfall intensity and ventilation rate of the shutters. The maximum value of rain-induced pressure coefficient is 0.013. The research findings lay the basis for determining the precise values of internal surface loads of cooling tower under extreme weather conditions.

• Structural Engineering and Mechanics
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• v.78 no.5
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• pp.611-622
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• 2021

Composite material-due to low density-causes weight savings, which results in lower fuel consumption of transport vehicles. The aim of the research was to change the existing base-plate of the aluminum airplane container with the composite sandwich plate in order to reduce the weight of the containers of cargo aircrafts. The newly constructed sandwich plate consists of aluminum honeycomb core and composite face-sheets. The face-sheets consist of glass or carbon or hybrid fiber layers. The orientations of the fibers in the face-sheets were 0°, 90° and ±45°. Multi-objective optimization method was elaborated for the newly constructed sandwich plates. Based on the design aim, the importance of the objective functions (weight and cost of sandwich plates) was the same (50%). During the optimization nine design constraints were considered: stiffness, deflection, facing stress, core shear stress, skin stress, plate buckling, shear crimping, skin wrinkling, intracell buckling. The design variables were core thickness and number of layers of the face-sheets. During the optimization both the Weighted Normalized Method of the Excel Solver and the Genetic Algorithm Solver of Matlab software were applied. The mechanical properties of composite face-sheets were calculated by Laminator software according to the Classical Lamination Plate Theory and Tsai-Hill failure criteria. The main added-value of the study is that the multi-objective optimization method was elaborated for the newly constructed sandwich structures. It was confirmed that the optimal new composite sandwich construction-due to weight savings and lower fuel consumption of cargo aircrafts - is more advantageous than conventional all-aluminum container.

• Tunnel and Underground Space
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• v.31 no.4
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• pp.211-220
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• 2021

Tunnel collapse often occurs during deep underground tunneling (> 40 m depth) in South Korea. Natural cavities as well as water supply pipes, sewer pipes, electric power cables, artificial cavities created by subway construction are complexly distributed in the artificial ground in the shallow depths of the urban area. For deep tunnel excavation, it is necessary to understand the properties of the ground which is characterized by porous elements and various geological structures, and their influence on the stability of the ground. This study analyzed geological factors for risk assessment in deep excavation in South Korea based on domestic and overseas case study. As a result, a total of 7 categories and 38 factors were derived. Factors with high weights were fault and fault clay, differential stress, rock type, groundwater and mud inrush, uniaxial compressive strength, cross-sectional area of tunnel, overburden thickness, karst and valley terrain, fold, limestone alternation, fluctuation of groundwater table, tunnel depth, dyke, RQD, joint characteristics, anisotropy, rockburst and so forth.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.440-459
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• 2021

Roadheaders have begun to be adopted in Korean tunneling sites. The performance prediction models proposed by the manufacturer are used by Korean construction companies. The models use UCS (uniaxial compressive strength) value to predict the net cutting rate, but the rock specimens conducted for the uniaxial compression test have 1.0 of the diameter to length ratio. It has been reported that the specimen shape generally influences the rock strength. The previous references studying the shape effect were cited, and the UCS data of Korean rocks are also updated to analyze the shape effect on UCS. The cause of effect was discussed by previous theory. The change amount of UCS values of Korean rocks was estimated by the data, and the modified prediction model for NCR was finally suggested.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.428-439
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• 2021

A spray-on membrane is a material composed of a polymer, and is a spray-type material that is expected to be able to replace materials such as existing shotcrete or sheet membrane for support or waterproofing purposes. In the previous studies, it is expected that the thickness of the support material such as shotcrete can be reduced if the spray-on membrane is additionally installed on the existing cement-based support materials. In this study, a three-point bending test was performed by a spray-on membrane on the high-performance shotcrete on the outside, and comparison was made between the case where high-performance shotcrete and a spray-on membrane were installed. As a result of comparing the values calculated through the standard test and the real-size bending test, there was no significant difference in terms of flexural strength, but it was found that there was a difference in flexural toughness.

• Advances in concrete construction
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• v.13 no.1
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• pp.83-99
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• 2022

The two, NBD and SCB tests using gypsum circular discs each containing a single notch have been experimentally accomplished in a rock mechanics laboratory. These specimens have also been numerically modelled by a two-dimensional particle flow which is based on Discrete Element Method (DEM). Each testing specimen had a thickness of 5 cm with 10 cm in diameter. The specimens' lengths varied as 2, 3, and 4 cm; and the specimens' notch angles varied as 0°, 45° and 90°. Similar semi-circular gypsum specimens were also prepared each contained one edge notch with angles 0° or 45°. The uniaxial testing machine was used to perform the experimental tests for both NBD and SCB gypsum specimens. At the same time, the numerical simulation of these tests were performed by PFC2D. The experimental results showed that the failure mechanism of rocks is mainly affected by the orientations of joints with respect to the loading directions. The failure mechanism and fracturing patterns of the gypsum specimens are directly related to the final failure loading. It has been shown that the number of induced tensile cracks showing the specimens' tensile behavior, and increases by decreasing the length and angle of joints. It should be noted that the fracture toughness of rocks' specimens obtained by NBD tests was higher than that of the SCB tests. The fracture toughness of rocks usually increases with the increasing of joints' angles but increasing the joints' lengths do not change the fracture toughness. The numerical solutions and the experimental results for both NDB and SCB tests give nearly similar fracture patterns during the loading process.