• Economic and Environmental Geology
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• v.50 no.1
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• pp.61-71
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• 2017

The effects of joint density and size distribution on the hydrogeologic characteristics of jointed rock masses are addressed through numerical experiments based on the 2-D DFN (discrete fracture network) fluid flow analysis. Using two joint sets, a total of 51 2-D joint network system were generated with various joint density and size distribution. Twelve fluid flow directions were chosen every $30^{\circ}$ starting at $0^{\circ}$, and total of 612 $20m{\times}20m$ DFN blocks were prepared to calculate the directional block conductivity. Also, the theoretical block conductivity, principal conductivity tensor and average block conductivity for each generated joint network system were determined. The directional block conductivity and chance for the equivalent continuum behavior of the 2-D DFN system were found to increase with the increase of joint density or size distribution. However, the anisotropy of block hydraulic conductivity increases with the increase of density discrepancy between the joint sets, and the chance for the equivalent continuum behavior were found to decrease. The smaller the intersection angle of the two joint sets, the more the equivalent continuum behavior were affected by the change of joint density and size distribution. Even though the intersection angle is small enough that it is difficult to have equivalent continuum behavior, the chance for anisotropic equivalent continuum behavior increases as joint density or size distribution increases.

• Journal of Korean Tunnelling and Underground Space Association
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• v.3 no.2
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• pp.3-12
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• 2001

This paper presents the proposed methods of DE (distinct element) modelling to estimate the stability of tunnels in jointed rock masses. First, the criterion to select the joint set(s) contributed to the discontinuous behaviour in a tunnel section is proposed. Selected joint set(s) is(are) considered to form the edges of distinct elements (rock blocks) and the others to modify the elastic properties of rock blocks. The complex DE model with the average and the deviation of joint orientation and joint length for each joint set was compared to the simple model with only the average of joint orientation and the assumption that joint length is infinite. As a result, the latter is suitable to the purpose of tunnel design because it can show the consistent behaviour of a jointed rock mass such as the locally discontinuous failure and the global anisotropic behaviour.

• The Journal of Engineering Geology
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• v.20 no.4
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• pp.425-436
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• 2010

We established a stochastic 3-D fracture network system for fractured rock masses located in Il-Gwang Mine, Busan, to explore the relationship between the acidity of mine drainage and fracture geometry. A field scanline survey and borehole image processing were performed to estimate the best probability distributions of fracture geometry parameters. The stochastic 3-D fracture network system constructed for the rock masses was validated and deemed to be successful. The 3-D fracture network model was suitable for developing conceptual ideas on fluid flow in fractures at a field experimental site. An injection well and three observation wells were drilled at the field experimental site to monitor the acidity of mine drainage induced by the injection of fresh water. The field experiment, which was run for 29 days, yielded a significant relationship (with a high coefficient of determination) between the fracture geometry parameters and the acidity of mine drainage. The results show that pH increased with increasing relative frequency of fracture strike, and decreased with increasing fracture density. The concentration of $SO^{2-}_4$ decreased with increasing relative frequency of fracture strike, and increased with increasing fracture density.

• Tunnel and Underground Space
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• v.26 no.6
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• pp.475-483
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• 2016

A numerical analysis model capable of predicting the shape, the size and the potentiality of collapse of tetrahedral blocks considering the persistence obtained from the field survey of joint distribution around the underground excavation surface has been developed. Numerical functions of analyzing both the exposed trace distribution on the excavation surface and the formation of tetrahedral block controlled by the extent of joint surface have been established and linked to the previously developed three dimensional deterministic block analysis model. To illustrate the reliability of advanced numerical model the case of underground excavation in which the collapse of rock block had practically taken place was studied. Representative orientations of joint sets was determined based on the joint distribution pattern observed on the excavation surfaces. The formation of block on the roof of underground opening was analyzed to unveil the potential tetrahedral block the shape of which was very similar to the collapsed rock block. Mechanisms of collapse process has been also analyzed by considering the three dimensional shape of tetrahedral block.

• The Journal of Engineering Geology
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• v.29 no.1
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• pp.1-12
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• 2019

Earthquake can change underground stress condition around the hypocenter and affect the fracture systems of the rocks. In Korea, the M5.8 Gyeongju earthquake on September 12, 2016 and M5.4 Pohang earthquake on November 15, 2017 occurred inside the Yangsan fault zone and possibly affected the fracture systems in the Yangsan fault zone and nearby rock masses. In this study, the characteristics of the fracture system (fracture orientation, number of the fractures, fracture spacing and aperture, dip angle, fracture density along depth, and relative rock strength) of the rocks in the low/intermediate level radioactive waste repository site located in the coastal area of the East Sea are analyzed by the impact of the Gyeongju and Pohang earthquakes using acoustic televiewer data taken from the boreholes at the radioactive waste repository site in 2005 and 2018. As a result of acoustic televiewer logging analysis, the fracture numbers, fracture aperture, and fracture density along depth overall increased in 2018 comparing to those in 2005. This increase tendency may be due to changes in the fracture system due to the impact of the earthquakes, or due to weathering of the wall of the boreholes for a long period longer than 10 years after the installation of the boreholes in 2005. In the borehole KB-14, on the whole, the orientation of the fractures and the average fracture spacing are slightly different between 2005 and 2018, while dip angle and relative rock strength in 2005 and 2018 are similar each other.

• Tunnel and Underground Space
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• v.20 no.3
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• pp.225-232
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• 2010

In this study, a newly modified 3-dimensional strain-dependent hydraulic conductivity modification relation which incorporates the influences of normal deformation and shear dilation is suggested. Since rock mass is simulated as a orthogonally jointed medium, an anisotropic hydraulic conductivity field can be evaluated using that relation. The empirical relationship on the basis of GSI and disturbance factor has been used to estimate the value of a modulus reduction ratio (ratio of rock mass deformation modulus to rock matrix elastic modulus). Principal hydraulic conductivity directions is not generally coincident with the global coordinate due to the inclining of joint and the influence of joint inclination is evaluated under strain rotation. Result shows that change of hydraulic conductivity does decreases with the increase of GSI and disturbance factor has much effects on the hydraulic conductivity of rock mass getting GSI value above 50. It is found that the inclination of joint impacts on the variation of hydraulic conductivity.

• Tunnel and Underground Space
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• v.13 no.2
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• pp.153-168
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• 2003

The objective of this present study is to understand long term(500 years) thermohydromechanical interaction behavior in the vicinity of a repository cavern on the joint location and repository depth variations. The model includes a saturated discontinuous granitic rock mass, PWR spent nuclear fuel in a disposal canister surrounded with compacted bentonite inside a deposition hole, and mixed bentonite backfilled in the rest of the space within a repository cavern. It is assumed that two joint sets exist within the model. Joint set 1 includes joints of 56$^{\circ}$ dip angle, spaced at 20 m, and joint set 2 is in the perpendicular direction to joint set 1 and includes joints of 34$^{\circ}$ dip angle, spaced at 20 m. In order to understand the behavior change on the joint location variations, 5 different models of 500m in depth are analyzed, and additional 3 different models of 1000 m in depth are analyzed to understand the effect of depth variation.

• Tunnel and Underground Space
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• v.16 no.5 s.64
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• pp.394-404
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• 2006

Dynamic behavior of rock structures depends largely on the dynamic characteristics of ground and input earthquake wave. For blocky rocks with intense discontinuities, the mechanical characteristics of blocks and structural and mechanical characteristics of discontinuities affect overall behavior. In this study, UDEC was adopted to evaluate the dynamic behavior of rocks with various structural characteristics. Obtained results were compared to those of $FLAC^{2D}$, a continuum analysis, and the validity of the method was examined for dynamic analysis of discontinuous rocks for earthquake. Analysis considering the discontinuity showed significant changes in structural shape by the influence of joint behavior, and the behavior by continuum analysis was overestimated.

• The Journal of the Petrological Society of Korea
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• v.16 no.3
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• pp.129-143
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• 2007

We have studied orientational characteristics of microcracks in Mesozoic granites and granitic dyke rocks from Seokmo-do, Ganghwa-gun. Microcracks on horizontal surfaces of rock samples from 14 sites were investigated by image processing. Orientations of these microcracks compared with those of 18 sets of joints in Mesozoic granites from Seokmo-do. From the related chart, microcrack sets show strong preferred orientations which obviously are coincident with the direction of vertical common joints. It follows that the formation of macroscopic joints may be the results of further growth and step-wise jointing of pre-existing microcracks. Orientations of microcracks from this result also compared with those of vertical rift and grain planes for Jurassic and Cretaceous granite quarries in Korea. As shown in the distribution chart, the congruence of distribution pattern among microcracks and rift and grain planes suggests that similar microcrack systems probably occur regionally in Jurassic and Cretaceous granites from Korea. In particular, whole domain of the distribution chart was divided into 16 groups in terms of the phases of distribution of microcracks and planes. These microcrack sets in each domains construct complex composite microcrack systems which have formed progressively by different geologic processes and under varying conditions.

• Tunnel and Underground Space
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• v.19 no.1
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• pp.52-63
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• 2009

This is a study on large-scale rock joint roughness measurements using LIDAR (light detection and ranging) and the Split-FX point cloud processing software. The large-scale rock Joint Roughness Coefficient (JRC) is calculated using the maximum amplitude of joint asperities over the profile length on large-scale Joint surfaces of rock. As the profile length increases, JRC decreases due to scale-effects of rock specimens and is non-stationary. Also JRC shows anisotropy depending on the profile direction. The profile direction is measured relative to either dip or strike of the large-scale joint.