• Journal of the Korean Geotechnical Society
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• v.17 no.2
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• pp.13-20
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• 2001

외부하중의 작용방향에 의한 암반의 강도 및 변형이 달라지는 것을 강도 이방성, 또는 변형이방성이라 정의되어 진다. 강도 또는 변형 이방성은 층리진 퇴적압, 박층모양의 변성암, 균일하게 절리가 나있는 암석에서 흔히 볼 수 있다. 이러한 강도이방성에 대한 경험적 및 이론적 관점에서의 파괴 규준은 Jaeger(1960), McLamore와 Gray(1967), Donath(1972), Nova(1980), Hoek과 Brown(1980), Ramamurthy(1985)등 많은 학자들에 의해서 연구되어져 왔다. 본 논문에서는 셰일에 대한 삼축 압축시험을 통하여 층리각도에 따른 강도 이방성을 확인하고, 점착력, 내부 마찰각 및 재료상수의 물성치를 회귀분석을 통하여 구하였다. 또한, 수정된 Hoek과 Brown, Ramamurthy 등의 식을 통해 기존의 파괴규준식 및 수정된 식이 강도이방성적 특성을 나타내는 이 지역 셰일에 대한 적용가능성을 논하였다.

• Tunnel and Underground Space
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• v.21 no.3
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• pp.174-180
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• 2011

An anisotropic version of Mohr-Coulomb failure criterion is proposed in order to provide a strength criterion for transversely isotropic rock. The concept of fabric tensor introduced by Pietruszczak & Mroz (2001) is employed to define the friction angle and cohesion as scalar functions of the fabric tensors. The anisotroy in these two strength parameters are calculated in association with the consideration of the relative rotation between the principal stress coordinate and the principal material triad. The critical plane on which the anisotropic function maximized is found by an optimization technique based on the Lagrange multiplier method. To demonstrate the performance of the anisotropic failure criterion, conventional triaxial tests on the samples having various inclinations of weakness plane are simulated and the resulting triaxial strength and dip angle of failure plane are discussed.

• Tunnel and Underground Space
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• v.13 no.3
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• pp.169-179
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• 2003

Anisotropic shear strength of rock joints is studied based on the artificially fractured specimens using experimental and analytical methods. Series of direct shear tests are performed to obtain the strength, stiffness and friction angle of joints under various low normal stresses and shearing directions. The results of shear strength and stiffness show anisotropic value according to shearing direction under low normal stress specially less than 2.45 MPa. But, the effect of joint roughness on strength decreases with increasing normal stress. To estimate more effectively the peak shear strength under low normal stress, the modified Barton's equation is suggested.

• Tunnel and Underground Space
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• v.10 no.1
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• pp.70-79
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• 2000

We carried out NPT-ensemble (constant-number of particles, pressure, and temperature) Molecular Dynamics (MD) simulations for measuring strength anisotropy under uniaxial compressive stress rotated to the crystallographic axes in $\alpha$-quartz. Uniaxial compressive strengths of a single quartz crystal were measured in directions of the a- and c-axis. Measured uniaxial strength of a single quartz crystal was higher in the direction parallel to the c-axis than that measured in the direction normal to the c-axis. However the reverse was found in calculated uniaxial strengths by MD simulation. The contradictive result of strengths was observed in both cases but was found to be different in origin. Strength anisotropy of defectless $\alpha$-quartz crystal in MD simulation is basically caused by structural difference of quartz. By contrast, anisotropy of measured strength in the uniaxial compression test is related to oriented micro-defects developed during crystal growth.

• Tunnel and Underground Space
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• v.17 no.2 s.67
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• pp.119-127
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• 2007

Based on the critical plane approach, a methodology far predicting the anisotropic strength ot transversely isotropic rock is Proposed. It is assumed that the rock failure is governed by Hoek-Brown failure criterion. In order to establish an anisotropic failure function, Mohr envelope equivalent to the original Hoek-Brown criterion is used and the strength parameters m, s are expressed as scalar functions of orientation. The conjugate gradient method, which is one of the robust optimization techniques, is applied to the failure function for searching the orientation giving the maximum value of the anisotropic function. While most of the existing anisotropic strength models can be applied only when the stress condition is the same as that of conventional triaxial compression test, the proposed model can be applied to the general 3-dimensional stress conditions. Through the simulation of triaxial compression tests for transversely isotropic rock sample, the validity of the proposed method is investigated by comparing the predicted triaxial strengths and inclinations of failure plane.

• Journal of the Korean Geotechnical Society
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• v.24 no.11
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• pp.111-120
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• 2008

The characteristic of induced anisotropy is investigated in this study for the Pohang mudstone involving the cut plane discontinuity. The uniaxial and triaxial compression tests are performed for anisotropic rocks with artificial joint to look into anisotropic strength characteristics. Both the uniaxial compressive strength and triaxial compressive strength show the lowest value at the angle of cut plane, ${\beta}=30^{\circ}$ and the shoulder type of anisotropy is obtained. Anisotropy ratio (Rc) in uniaxial compression measures 9.0, whereas Rc=1.29-1.98 in triaxial compression is appeared. A series of analyses are made with the test results to derive the suitable parameter values when it is applied to the Ramamurthy (1985) failure criterion. The result of uniaxial compression test is analyzed by introducing the n-index into Ramamurthy failure criterion. The result shows that, n=l is suitable for ${\beta}=0^{\circ}{\sim}30^{\circ}$ and n=3 is suitable for ${\beta}=30^{\circ}{\sim}90^{\circ}$. To analyze the result of triaxial compression test by Ramamurthy failure criterion, anisotropy ratio in uniaxial compression test is added to Ramamurthy's equation and material constants are estimated by modified Ramamurthy's equation. When these values are applied back to Ramamurthy failure criterion, the predicted values are well fitted to the test results. And strength anisotropy for failure criteria of Jaeger (1960), McLamore & Gray (1967) and Hoek & Brown (1980) are also investigated.

• Journal of the Korean GEO-environmental Society
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• v.9 no.7
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• pp.45-51
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• 2008

Triaxial compression tests on anisotropic rock specimens are carried out to investigate the failure strength characteristic of anisotropic rocks. The test core specimens were obtained in Daegu region. Test specimens are rock cores with the 7 different angles of bedding plane. The applied confining pressures were 5, 10, 20, 30, 40 MPa, and the rate of displacement was adopted 0.1%/min to fail the specimen within 5-15 min. The results were analyzed by using the failure criteria for anisotropic rocks proposed by Hoek & Brown (1980) and Jaeger (1960). The results of this study are summerised as follows: The results of inherent anisotropy show the shoulder type of anisotropy, and the effect of anisotropy is reduced as the confining pressure increases. The compressive strength of anisotropic rock shows the highest value at the ${\beta}$ (the angle of bedding plane) = $0^{\circ}$ and $90^{\circ}$ and the lowest value at $30^{\circ}$. The Hoek & Brown failure criterion for anisotropic rocks gives a relatively good agreement with the measured strength in all the range of ${\beta}$ angles, but the theory of Jaeger shows a reasonable agreement only in the range of ${\beta}=15^{\circ}$ and $45^{\circ}$.

• Tunnel and Underground Space
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• v.18 no.3
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• pp.194-201
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• 2008

In order to investigate the characteristics in tensile strength of transversely isotropic rock, a new anisotropic tensile failure function was suggested. According to the function, the tensile strength is minimum in the normal direction to a weakness plane and rises exponentially to its maximum on a plane perpendicular to the weakness plane. The anisotropic function is defined in terms of three strength parameters which can be identified trom direct tensile tests of transversely isotropic rocks. By incorporating the suggested function into the critical plane approach, a numerical procedure which enables to search the tensile strength and the direction of critical plane at failure was presented. The validity of the suggested numerical procedure was checked through the simulation of direct tensile tests reported in a literature. The numerical results from the simulation were in good agreements with those from the laboratory tests.

• Journal of the Korean Geotechnical Society
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• v.18 no.5
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• pp.133-141
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• 2002

Isotropic rock and anisotropic rock have different tensile strength which has the greatest influence on rock failure. In this study, elastic modulus of anisotropic rock is obtained through uniaxial compression test, and tensile strength and tension failure behavior are analyzed through indirect tensile strength test. Stress concentration factor of a specimen at the center is obtained from anisotropic elastic modulus and strain by indirect tensile strength test. Theoretical solutions for tensile strength of isotropic and anisotropic rock are compared. Stress concentration factor of anisotropic rock is either higher or lower than isotropic rock depending on the inclination angle of bedding plane. The use of stress concentration factor of isotropic rock resulted in overestimation or underestimation of tensile strength.

• Tunnel and Underground Space
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• v.10 no.2
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• pp.165-172
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• 2000

When tunnels or underground structures are constructed in anisotropic rock mass, designers and constructors have to consider the anisotropic characteristics in rock mass because their physical and mechanical properties are depended on the anisotropic angles(${\beta}$). In this study, therefore, we have first investigated the mechanical behavior of the gneiss specimen from lab. tests, and then have analysed the behavior of specimens for to the transversely isotropic model in elastic medium using the FLAC program. The results of this study were summarized as follows; 1) In the result of the variation tests, in general, the properties of strength were depended on the angle of inclination in spite of the hard rock. And except for the shear strength test, the lowest and peak stress were appeared at 60$^{\circ}$ and 90$^{\circ}$respectively. 2) The results of specimen modeling analysis using FDM well indicated the mechanical behaviors of the specimen of transversely isotropic model.