• The Journal of Engineering Geology
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• v.29 no.3
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• pp.237-249
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• 2019

The roughness changes on a fracture surface were analyzed via a multi-stage compression test under high temperatures to assess how the cracks in a rock mass affect groundwater movement. The analyzed samples consist of coarse granitic rocks from approximately 40 and 270 m depth, and fine granitic rocks from 500 m depth. The compression test was conducted on $20{\times}40{\times}5mm$ samples using a loading system where the pressure increases in 10 MPa increments to 120 MPa. A high-resolution 3D confocal laser scanning microscope (CLSM) was used to observe the surface changes, including the roughness changes, at each pressure step. The roughness change was calculated based on the roughness factor. The experimental results indicate that the roughness of the fracture surface varies with rock type under the stepwise pressure conditions. These data provide a basis for predicting groundwater flow along rock fractures.

• Journal of Soil and Groundwater Environment
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• v.10 no.6
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• pp.32-44
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• 2005

314 pumping test data of the National Groundwater Monitoring Wells (NGMWs) are analyzed to present statistical properties of fractured-rock and alluvial aquifers of Korea such as distribution of hydraulic conductivity, empirical relations between transmissivity and specific capacity, and time-drawdown patterns of pumping and recovery test. The mean hydraulic conductivity of alluvial aquifers (1.26 m/day) is 17 times greater than that of fractured-rock aquifers (0.076 m/day). Hydraulic conductivity of fracture-rock aquifers ranges in value over 4 orders of magnitude which coincide with representative values of fractured crystalline rocks and shows distinctive differences among rock types with the lowest values for metamorphic rocks and the highest values for sedimentary rocks. In consideration of the estimated transmissivity with some simplifying assumptions, it Is likely that $32\%$ of groundwater flow for NGMWs would occur through fractured-rock aquifers and $68\%$ through alluvial aquifers. Based on 314 pairs of data, empirical relations between transmissivity and specific capacity are presented for both fractured-rock and alluvial aquifers. Depending on time-drawdown patterns during pumping and recovery test, NGMWs are classified into $4\~5$ types. Most of NCMWs $(83.7\%)$ exhibit the recharge boundary type, which call be attributed to sources of water supply such as streams adjacent to the pumping well, the vertical groundwater flux between fractured-rock and the alluvial aquifers, and the delayed yield associated with gravity drainage occurring in unconfined aquifers.

• The Journal of Engineering Geology
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• v.4 no.2
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• pp.153-168
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• 1994

The status of groundwater development in Chungnam was studied with geological characteristics according to the measured data of Korean Rural Development Corporation. The data of 212 survey wells were used for the relation between catchment area and water discharge, and the data of 344 development wells for the relationships between well depth and discharge, between casing depth and discharge, between rock type and discharge, and the relation with lineaments density. The relationship between the catchment area and discharge does not show any special trend, and it is understood that groundwater of hard rock mass is not so much influenced by the surface catchment area. The relationship between well depth and discharge shows two different trends; discharge increasing with depth for alluvial groundwater, but no certain trend between depth and discharge for groundwater of hard rock zone. Discharge increases linearly with the casing depth, and it is reliable because the casing was installed in the weathered zone against well destruction. Generally the rock type does not show any difference of discharge, but the crystalline rocks such as granite and gneiss yield a little more discharge than the more porous rocks such as sedimentary rock or schist. It suggests that the effect of fracture zone is a major governing factor. In Hongsong and Puyo, there are similar in rock type and casing depth, but the big difference in average discharge. The big discharge of Hongsong is concordant with the higher intersection density and longer length of lineament in Hongsong than those of Puyo. Therefore the groundwater development strategy should be focused on the micro topography analysis and geophysical survey for the understanding of the fracture zone rather than catchment area or rock type.

• Tunnel and Underground Space
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• v.31 no.4
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• pp.270-288
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• 2021

We proposed a numerical method to simulate the hydro-mechanical behavior of rock fracture using a grain-based distinct element model (GBDEM) in the paper. As a part of DECOVALEX-2023 Task G, we verified the method via benchmarks with analytical solutions. DECOVALEX-2023 Task G aims to develop a numerical method to estimate the coupled thermo-hydro-mechanical processes within the crystalline rock fracture network. We represented the rock sample as a group of tetrahedral grains and calculated the interaction of the grains and their interfaces using 3DEC. The micro-parameters of the grains and interfaces were determined by a new methodology based on an equivalent continuum approach. In benchmark modeling, a single fracture embedded in the rock was examined for the effects of fracture inclination and roughness, the boundary stress condition and the applied pressure. The simulation results showed that the developed numerical model reasonably reproduced the fracture slip induced by boundary stress condition, the fracture opening induced by fluid injection, the stress distribution variation with fracture inclination, and the fracture roughness effect. In addition, the fracture displacements associated with the opening and slip showed good agreement with the analytical solutions. We expect the numerical model to be enhanced by continuing collaboration and interaction with other research teams of DECOVALEX-2023 Task G and validated in further study experiments.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.593-609
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• 2021

We proposed a numerical method for the thermo-mechanical behavior of rock fracture using a grain-based distinct element model (GBDEM) and simulated thermally induced fracture slip. The present study is the benchmark simulation performed as part of DECOVALEX-2023 Task G, which aims to develop a numerical method to estimate the coupled thermo-hydro-mechanical processes within the crystalline rock fracture network. We represented the rock sample as an assembly of Voronoi grains and calculated the interaction of the grains (blocks) and their interfaces (contacts) using a distinct element code, 3DEC. Based on an equivalent continuum approach, the micro-parameters of grains and contacts were determined to reproduce rock as an elastic material. Then, the behavior of the fracture embedded in the rock was characterized by the contacts with Coulomb shear strength and tensile strength. In the benchmark simulation, we quantitatively examined the effects of the boundary stress and thermal stress due to heat conduction on fracture behavior, focusing on the mechanism of thermally induced fracture slip. The simulation results showed that the developed numerical model reasonably reproduced the thermal expansion and thermal stress increment, the fracture stress and displacement and the effect of boundary condition. We expect the numerical model to be enhanced by continuing collaboration and interaction with other research teams of DECOVALEX-2023 Task G and validated in further study experiments.

• Journal of Korean Tunnelling and Underground Space Association
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• v.3 no.2
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• pp.33-56
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• 2001

It is well known fact that all rocks exhibit brittle properties and time depends strain properties (creep). An understanding of the time dependent deformation behaviour of rocks is believed to be essential in the field of civil and tunnelling. The rock and concrete creep in various forms of loading conditions and physical environment are reviewed. A comparison of creep behaviour between rocks and concrete is provided, in order to bring two existing relatively independent methods of predicting creep strain closer together. It was felt that the physical process in the creep of rocks would be similar to the process in creep of concrete. Since experiments and observations have shown that non-elastic (creep) mechanical behaviour of all crystalline solids (i.e., concrete, rocks, ceramics and refractories) and single materials have a common base. Also a comparison of the results for the accepted methods of estimating creep in rocks and concrete under - multiaxial loading was attempted to extend the knowledge of deformational characteristics of these two materials.

• The Journal of Engineering Geology
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• v.1 no.1
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• pp.137-145
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• 1991

Groundwater flow in fractured rocks generates many challenging problems to scientist and engineers in the projects related to oil and geothermal reservoirs, subsurface contaminations and underground openings. To circumvent these problems, the numerical simulation of groundwater system is used as an established tool in these days. Discrete modelling approach emphasizes geometric parameters, aperture and transport properties of fracture. On the other hand, continuum modelling approach uses the parameters formulated in a way of average hydraulic property. In recent years, the results of field observations from underground opening indicate that groundwater in rock mass flows in a channel form. The channel flow is postulated as the result of the combined effects of geometric pattern and aperture variation.

• Korean Journal of Materials Research
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• v.33 no.10
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• pp.385-392
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• 2023

A solution combustion process for the synthesis of hollandite (BaAl₂Ti₆O₁₆) powders is described. SYNROC (synthetic rock) consists of four main titanate phases: perovskite, zirconolite, hollandite and rutile. Hollandite is one of the crystalline host matrices used for the disposal of high-level radioactive wastes because it immobilizes Sr and Lns elements by forming solid solutions. The solution combustion synthesis, which is a self-sustaining oxi-reduction reaction between a nitrate and organic fuel, generates an exothermic reaction and that heat converts the precursors into their corresponding oxide products in air. The process has high energy efficiency, fast heating rates, short reaction times, and high compositional homogeneity. To confirm the combustion synthesis reaction, FT-IR analysis was conducted using glycine with a carboxyl group and an amine as fuel to observe its bonding with metal element in the nitrate. TG-DTA, X-ray diffraction analysis, SEM and EDS were performed to confirm the formed phases and morphology. Powders with an uncontrolled shape were obtained through a general oxide-route process, confirming hollandite powders with micro-sized soft agglomerates consisting of nano-sized primary particles can be prepared using these methods.

• Tunnel and Underground Space
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• v.10 no.4
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• pp.553-558
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• 2000

Since 1977, KAERI has conducted the fundamental R&D on the permanent disposal of potential HLW repository in Korea. The first ten year project is divided into three short-term phase studies. The first phase study which shall be finished in March of 2000, has the prime target to develop the disposal concept of HLW. Throughout this study the preliminary and generic disposal repository system has been introduced. The potential repository is proposed to be emplaced into crystalline rocks which is the most common rock types in Korea. The proposed depth of the repository is between 300 to 700 meter. The numerical code, MASCOT-K was developed to asserts the long term safety of the proposed repository concept. Based on this conceptual design preliminary safely assessment was performed. Results show that for the given disposal system the potential radioactive release it well below the regulatory limit.

• Tunnel and Underground Space
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• v.11 no.2
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• pp.132-145
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• 2001

The purpose and need of the study is to quantify the advantage or disadvantage of the environmental friendliness of the partitioning of nuclear fuel cycle. To this end, a preliminary study on the quantitative effect of the partition on the permanent disposal of spent PWR and CANDU fuel (HLW) was carried out. Before any analysis, the so-called reference radionuclide release scenario from a potential repository embedded into a crystalline rock was developed. Firstly, the feature, event and processes (FEPs) which lead to the release of nuclides from waste disposed of in a repository and the transport to and through the biosphere were identified. Based on the selected FEPs, the ‘Well Scenario’which might be the worst case scenario was set up. For the given scenario, annual individual doses to a local resident exposed to radioactive hazard were estimated and compared to that from direct disposal. Even though partitioning and transmutation could be an ideal solution to reduce the inventory which eventually decreases the release time as well as the peaks in the annual dose and also minimize the repository area through the proper handling of nuclides, it should overcome major disadvantages such as echnical issues on the partitioning and transmutation system, cost, and public acceptance, and environment friendly issues. In this regard, some relevant issues are also discussed to show the direction for further studies.