• Journal of Korean Tunnelling and Underground Space Association
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• v.14 no.4
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• pp.357-374
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• 2012

Considerable uncertainties are included in ground properties used for tunnel designs due to the limited investigation and tests. In this study, a back analysis was performed to find optimal ground properties based on artificial neural network using both face mapping data and convergence measurement data. First of all, the rock class of a study tunnel is determined from face mapping data. Then the possible ranges of ground properties were selected for each rock class through a literature review on the previous studies and utilized to establish more precise learning data. To find an optimal training model, a sensitivity analysis was also conducted by varying the number of hidden layers and the number of nodes more minutely than the previous study. As a result of this study, more accurate ground properties could be obtained. Therefore it was confirmed that the accuracy of the results could be increased by making use of not only convergence measurement data but also face mapping data in tunnel back analyses using artificial neural network. In future, it is expected that the methodology suggested in this study can be used to estimate ground properties more precisely.

• Tunnel and Underground Space
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• v.24 no.1
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• pp.46-54
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• 2014

Direct shear tests have been initiated to understand the characteristics of joints which crucially affect the stability of rock mass. In this research, numerical approach in direct shear tests has been initiated using 3DEC on the basis of 3D distinct element method. Normal loads were altered in four different levels on artificial joint tests depending on the sawtooth angle and strengths on constant normal stress conditions, measuring the peak shear strength according to the direct shear tests under laboratory condition. Also results obtained from mechanical properties through laboratory test were used to perform numerical modeling, and shear strength obtained from the modeling was used to compare with laboratory direct shear test. As a result numerical analysis from distinct element method can simulate well on the shear behavior of rockmass.

• Tunnel and Underground Space
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• v.17 no.2 s.67
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• pp.90-101
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• 2007

With increasing development of underground space, underground pumped storage power plants, which generate power by felling water in upper reservoir to lower reservoir, have been continuously constructed. For efficient and safe design, construction and maintenance or such power plants, it is very important to understand in-situ stress and the mechanical properties of the surrounding rock mass at the design stage. The power plant presented in this paper is under construction at a depth of $320{\sim}375m$. For stability evaluation of the structure, in-situ stress was measured by over-coring method. Also pressurementer test and a series or laboratory tests were performed to obtain the mechanical properties. Numerical analyses were conducted to check the efficiency of designed support patterns. The results showed that unstable areas occurred partly in the numerical model, and therefore supports were additionally applied. Finally complete stability was obtained and the following excavation has been operated successfully until now.

• Economic and Environmental Geology
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• v.54 no.1
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• pp.137-150
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• 2021

The rock-carved standing Buddha triad in Seoak-dong is a large stone Buddha statue of the Unified Silla era (AD 676 to 935) in ancient Korea, built near the top of the southeastern side of mountain Seondosan in Gyeongju, is characterized by its locational importance, the powerful Amitabha and the gentle sculptural technique of the Bodhisattva. In particular, Amitabha Buddha in andesite rock slope with biotite granite pedestal and two Bodhisattva parallel made by alkali granites seems to express the dignity through the color and texture of the stones. In the Amitabha Buddha, deterioration characteristics are accelerating due to the combination of various joint systems, instability of the slopes and relaxation by the root pressure of plants occurring at the top. In addition, physical properties have deteriorated owing to the increase of discontinuous surfaces as joints, cracks and scalings, and the coverage of algae and lichen is also high. Therefore, deterioration degree in Buddha triad is accelerated due to the physical weathering characteristics from natural rock mass and various biological invasion.

• Journal of Korean Tunnelling and Underground Space Association
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• v.12 no.5
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• pp.399-405
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• 2010

The accumulated displacements and fatigues of rock are increased by the stress-hysteresis, induced from repeated frost-thawing. Also the shear strength is decreased by them continuously. The stress-hysteresis is affected by the atmospheric temperature changes, whose behavior is visco-elasticity, usually. Therefore, to do ideal body analysis, Kelvin model could be used to analyze the frost-thawing behavior in winter. In general, rock slope failure occurs by the deterioration of rocks, which is caused by the repetition of freezing-thawing process. In order to keep the safety of such rock mass structures the deterioration process of rock needs to be described quantitatively using some meaningful parameters. In this work, the deterioration process in freezing-thawing cycle of tuff, which is a famous soft porous rock, is investigated through laboratory tests and successfully described as a differential equation for the change of porosity. And then, the deterioration of the mechanical properties of rock, such as Young's modulus and uniaxial compressive strength, are quantitatively described as a function of the porosity.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.4
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• pp.501-518
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• 2019

The rock properties measured under in-situ geological condition can be used to increase the reliability in numerical simulations with regard to the long-term performance of a high-level waste repository. In this study, the change in mechanical properties of KURT (Korea atomic energy research institute Underground Research Tunnel) granite was evaluated under the simulated THM (Thermo-Hydro-Mechanical) coupled condition due to a deep geological formation in the disposal repository. The rock properties such as uniaxial compression strength, indirect tensile strength, elastic modulus and Poisson's ratio were measured under the coupled test conditions (M, HM, TM, THM). It was found that the mechanical properties of KURT granite is more susceptible to the change in saturation rather than temperature within the test condition of this study. The changes in uniaxial compression strength and indirect tensile strength from the rock samples of dried or saturated conditions showed the maximum relative error of about 20% and 13% respectively under the constant temperature condition. Therefore, it is necessary to use the material properties of rock measured under the coupled THM condition as input parameters for the numerical simulation of long-term performance assessment of a disposal repository

• Tunnel and Underground Space
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• v.19 no.3
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• pp.203-212
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• 2009

The prediction of near field behavior around an underground high-level radioactive waste repository is important for the repository design as well as the safety assessment. In this study, a sensitivity analysis for seven parameters consisted of design parameters and material properties was carried out using a three-dimensional finite difference code. From the sensitivity analysis, it was found that the effects of borehole spacing, tunnel spacing, cooling time and rock thermal conductivity were more significant than the other parameters. For getting a statistical distribution of buffer and rock temperatures around the repository, an artificial neural network, backpropagation, was applied. The reliability of the trained neural network was tested with the cases with randomly chosen input parameters. When the parameter variation is within ${\pm}10%$, the prediction from the network was found to be reliable with about a 1% error. It was possible to calculate the temperature distribution for many cases quickly with the trained neural network. The buffer and rock temperatures showed a normal distribution with means of $98^{\circ}C$ and $83.9^{\circ}C$ standard deviations of $3.82^{\circ}C$ and $3.67^{\circ}C$, respectively. Using the neural network, it was also possible to estimate the required change in design parameters for reducing the buffer and rock temperatures for $1^{\circ}C$.

• Tunnel and Underground Space
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• v.23 no.4
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• pp.297-307
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• 2013

In this study, the thermal analysis is carried out for a result of borehole heater test using ABAQUS ver 6.10 based on finite element analysis code. Thermal-mechanical rock properties as determined by laboratory tests before the in situ test and characteristics of the atmosphere at the test section are used as the initial condition. When comparing the results of the in situ test and thermal analysis, the temperature of C3 observation hole that is 0.9 m away from the heater showed very similar patterns and figures (about $1.3^{\circ}C$ difference). But the results of the A and B observation hole showed a significant difference around $15^{\circ}C{\sim}20^{\circ}C$. To find the reason for these results, the over-coring is carried out for the A1 and B1 observation holes. As a result of checking the excavated rock core with the naked eye, there is no problem on the number and position of the sensor as the test plan. However the state of cement injection in the observation hole is poor.

• The Journal of Engineering Geology
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• v.25 no.3
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• pp.357-368
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• 2015

Brittle failure of Mesozoic granite in the Daejeon region is predicted using empirical analysis and numerical modeling techniques. The input parameters selected for these techniques were based on the results of laboratory tests, including damage-controlled tests. Rock masses that were considered to be strong during laboratory testing were assigned to "group A" and those considered to be extremely strong were assigned to "group B". The properties of each group were then used in the analyses. In-situ stress measurements, or the ratio of horizontal to vertical stress (k), were also necessary for the analyses, but no such measurements have been made in the study area. Therefore, k values of 1, 2, and 3 were assumed. In the case of k=1, empirical analysis and numerical modeling show no indication of brittle failure from the surface to1000 m depth. When k=2, brittle failure of the rock mass occurs at depths below 800 m. For k=3, brittle failure occurs at depths below 600 m. Although both the Cohesion Weakening Friction Strengthening (CWFS) and Mohr-Coulomb models were used to predict brittle failure, only the CWFS model performed well in simulating the range and depth of the brittle failure zone.

• Journal of Korean Tunnelling and Underground Space Association
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• v.3 no.3
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• pp.15-29
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• 2001

For the pertinent use of NMT method, both characteristics of joints (JRC, JCS and ${\phi}_r$) and characteristics of rock mass (Q-Value) must be investigated carefully. The main objective of the study presented is to investigate how sensitive the predicted behaviour of an underground excavation is to various realistic assumptions about some input parameter for the jointed rock mass. Joint pattern in the tunnel is predicted by statistical approach (chi-square test). In this paper, sensitivity studies involving in joint characteristics were carried out. The parametric studies involving change in Barton-Bandis joint model have shown that JCS is relatively insensitive to JRC and ${\phi}_r$. An increase in JRC value may not, according to the Barton-Bandis model, necessarily lead to a decrease in displacement. The importance of dilation in predicting the behaviour of a rock mass around an excavation is emphasized from a comparison of the Barton-Bandis joint behaviour model with the Mohr-Coulomb model. The Barton-Bandis model predicted higher stress, which allow for the build-up of stress caused by dilatant behaviour.