• Tunnel and Underground Space
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• v.32 no.6
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• pp.518-529
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• 2022

In Task C of the DECOVALEX-2023 project, nine institutes from six nations are developing their numerical codes to simulate thermo-hydro-mechanical coupled behavior for the FE experiment performed at Mont Terri underground rock laboratory, Switzerland. Currently, Step 1 for comparing the simulation results to field data is the ongoing stage, and we used the OGS-FLAC simulator for a series of numerical simulations. As a result, temperature increase depending on the heating hysteresis was well simulated, and saturation variation in the bentonite depending on phase change was observed. However, due to the suction overestimation, relative humidity and temperature change in the bentonite and the pressure variation in the Opalinus clay showed a difference compared to the field data. From the observation, it is confirmed that the effect of the bentonite capillary pressure is dominant to the flow analysis in the disposal system. We further plan to draw improved results considering tunnel support material and accurate initial water pressure distribution. Additionally, the thermal, hydrological, and mechanical anisotropy of the Opalinus clay was well simulated. From the simulation results, we confirmed the applicability of the OGS-FLAC simulator in the disposal system analysis.

• Structural Engineering and Mechanics
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• v.70 no.3
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• pp.351-365
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• 2019

In this paper, free vibration of Cooper-Naghdi micro sandwich cylindrical shell with saturated porous core and reinforced carbon nanotube (CNT) piezoelectric composite face sheets is investigated by using first order shear deformation theory (FSDT) and modified couple stress theory (MCST). The sandwich shell is subjected to magneto-thermo-mechanical loadings with temperature dependent material properties. Energy method and Hamilton's principle are used for deriving of the motion equations. The equations are solved by Navier's method. The results are compared with the obtained results by the other literatures. The effects of various parameters such as saturated porous distribution, geometry parameters, volume fraction and temperature change on the natural frequency of the micro-sandwich cylindrical shell are addressed. The obtained results reveal that the natural frequency of the micro sandwich cylindrical shell increases with increasing of the radius to thickness ratio, Skempton coefficient, the porosity of the core, and decreasing of the length to radius ratio and temperature change.

• Tunnel and Underground Space
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• v.17 no.6
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• pp.464-474
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• 2007

An enhancement in the performance and safety of a high-level waste repository requires a validation of its engineered barrier. An engineering-scale test (named "KENTEX") has been conducted to investigate the thermal-hydro-mechanical behaviors in the engineered barrier of the Korean reference disposal system The validation test started on May 31, 2005 and is still under operation. The experimental data obtained allowed a preliminary and qualitative interpretation of the thermal-hydro-mechanical behaviors in the bentonite blocks. The temperature was higher as it became closer to the heater, while it became lower as it was farther away from the heater. The water content had a higher value in the part close to the hydration surface than that in the heater part. The relative humidity data suggested that a hydration of the bentonite blocks might occur by different drying-wetting processes, depending on their position. The total pressure was continuously increased by the evolution of the saturation front in the bentonite blocks and thereby the swelling pressure. Near the heater region, there was also a significant contribution of the thermal expansion of bentonite and the vapor pressure in the pores of the bentonite blocks.

• Tunnel and Underground Space
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• v.31 no.4
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• pp.221-242
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• 2021

A numerical study of the performance assesment of coupled thermo-hydro-mechanical (THM) processes in improved Korean reference disposal system (KRS⁺) for high-level radioactive waste is conducted using TOUGH2-MP/FLAC3D simulator. Decay heat from high-level radioactive waste increases the temperature of the repository, and it decreases as decay heat is reduced. The maximum temperature of the repository is below a maximum temperature criterion of 100℃. Saturation of bentonite buffer adjacent to the canister is initially reduced due to pore water evaporation induced by temperature increase. Bentonite buffer is saturated 250 years after the disposal of high-level radioactive waste by inflow of groundwater from the surrounding rock mass. Initial saturation of rock mass decreases as groundwater in rock mass is moved to bentnonite buffer by suction, but rock mass is saturated after inflow of groundwater from the far-field area. Stress changes at rock mass are compared to the Mohr-Coulomb failure criterion and the spalling strength in order to investigate the potential rock failure by thermal stress and swelling pressure. Additional simulations are conducted with the reduced spacing of deposition holes. The maximum temperature of bentonite buffer exceeds 100℃ as deposition hole spacing is smaller than 5.5 m. However, temperature of about 56.1% volume of bentonite buffer is below 90℃. The methodology of numerical modeling used in this study can be applied to the performance assessment of coupled THM processes for high-level radioactive waste repositories with various input parameters and geological conditions such as site-specific stress models and geothermal gradients.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.21 no.1
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• pp.23-41
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• 2023

Coupled thermo-hydraulic-mechanical (THM) processes are essential for the long-term performance of deep geological disposal of high-level radioactive waste. In this study, a numerical sensitivity analysis was performed to analyze the effect of rock properties on THM responses after the execution of the heater test at the Kamaishi mine in Japan. The TOUGH-FLAC simulator was applied for the numerical simulation assuming a continuum model for coupled THM analysis. The rock properties included in the sensitivity study were the Young's modulus, permeability, thermal conductivity, and thermal expansion coefficients of crystalline rock, rock salt, and clay. The responses, i.e., temperature, water content, displacement, and stress, were measured at monitoring points in the buffer and near-field rock mass during the simulations. The thermal conductivity had an overarching impact on THM responses. The influence of Young's modulus was evident in the mechanical behavior, whereas that of permeability was noticed through the change in the temperature and water content. The difference in the THM responses of the three rock type models implies the importance of the appropriate characterization of rock mass properties with regard to the performance assessment of the deep geological disposal of high-level radioactive waste.

• Steel and Composite Structures
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• v.33 no.4
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• pp.509-523
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• 2019

In the present work, the buckling analysis of micro sandwich plate with an isotropic/orthotropic cores and piezoelectric/polymeric nanocomposite face sheets is studied. In this research, two cases for core of micro sandwich plate is considered that involve five isotropic Devineycell materials (H30, H45, H60, H100 and H200) and an orthotropic material also two cases for facesheets of micro sandwich plate is illustrated that include piezoelectric layers reinforced by carbon and boron-nitride nanotubes and polymeric matrix reinforced by carbon nanotubes under temperature-dependent and hydro material properties on the elastic foundations. The first order shear deformation theory (FSDT) is adopted to model micro sandwich plate and to apply size dependent effects from modified strain gradient theory. The governing equations are derived using the minimum total potential energy principle and then solved by analytical method. Also, the effects of different parameters such as size dependent, side ratio, volume fraction, various material properties for cores and facesheets and temperature and humidity changes on the dimensionless critical buckling load are investigated. It is shown from the results that the dimensionless critical buckling load for boron nitride nanotube is lower than that of for carbon nanotube. It is illustrated that the dimensionless critical buckling load for Devineycell H200 is highest and lowest for H30. Also, the obtained results for micro sandwich plate with piezoelectric facesheets reinforced by carbon nanotubes (case b) is higher than other states (cases a and c).The results of this research can be used in aircraft, automotive, shipbuilding industries and biomedicine.

• Journal of the Korean Geotechnical Society
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• v.29 no.1
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• pp.81-92
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• 2013

This paper presents the results of a numerical investigation into the behavior of retaining wall subject to cycles of freezing and thawing due to seasonal temperature change. The thermo-hydro-mechanical coupled finite element modeling strategy was first established to simulate the wall behavior. A series of finite element analyses were then performed on a range of conditions representing seasonal temperature change characteristics. The results indicated that the average freezing temperature and the number of cycles of freezing and thawing were the primary influencing factors for the wall behavior. Also revealed was that the duration of freezing period does not significantly affect the wall displacement and the lateral earth pressure, and that the earth pressure on the wall does not significantly change due to the freezing and thawing action suggesting that the increase in the wall displacement during the freezing and thawing action may be attributed to degradation of backfill due to the freezing and thawing action.

• Tunnel and Underground Space
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• v.17 no.1 s.66
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• pp.32-42
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• 2007

Severe damage can occur around deposition holes due to complex interaction of thermo-hydro-mechanical (THM) loading during the long term operation of high level radioactive waste repository. Many candidate sites for repository are located in crystalline rock mass, therefore mechanism of damage follows the form of brittle fracture and failure. This paper briefly introduces major outcomes from 15 years international collaborative project, DECOVALEX, and presents major study results for current ongoing benchmark test study from DECOVALEX-THMC, to evaluate the effect of THM loading to rock mass in excavation damaged zone (EDZ) near deposition holes. Through benchmark test model by simplifying THM loading to boundary loading obtained numerical results are compared, and discrete fracture interaction after up to 1 million years operation is discussed.

• Tunnel and Underground Space
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• v.9 no.1
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• pp.1-11
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• 1999

A finite element code was developed to analyze coupled thermo-hydro-mechanical phenomena. This code is based on the finite element formulation provided by Noorishad et al. (1984) and Joint behavior was simulated Goodman's joint constitutive model. The developed code was applied for T-H-M coupling analysis for two kinds of shaft models, with a joint or without a joint respectively. For a model without a joint, temperature increased from the shaft wall to outward evidently. The radial displacement showed opposite directions of outward and inward at some distance from shaft wall. For a model with a joint, closure of joint was found due to thermal expansion. The temperature distribution along a joint showed relatively lower than that of rock matrix because of low thermal conductivity and high specific heat of water. And it could be concluded that effects of thermal flow to joint were more than that of hydraulic flow in a rock mass.

• Tunnel and Underground Space
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• v.33 no.1
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• pp.10-28
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• 2023

Long-term shear behavior of the rock discontinuities should be analyzed and its stability should be evaluated to ensure the long-term stability of a high-level radioactive waste disposal repository. The long-term shear behavior of the discontinuities can be modeled with creep and RSF models. The shear creep test, velocity step test, and slide-hold-slide test can be performed to determine their model parameters or analyze the shear behavior by experiments under various conditions. Testing apparatuses for direct shear, triaxial compression, and biaxial shear were mainly used and improved to reproduce the thermo-hydro-mechanical conditions of local bedrock, and it was confirmed that the shear behavior could vary. In order to design a high-level radioactive waste disposal site in Korea, the long-term behavior of rock discontinuities should be investigated in consideration of rock types, thermo-hydro-mechanical conditions, metamorphism, and restoration of shear resistance.