• Tunnel and Underground Space
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• v.25 no.5
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• pp.450-461
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• 2015

Extensive numerical experiments have been carried out to investigate effect of block size and fracture geometry on hydraulic characteristics of fractured rock masses based on connected pipe flow in DFN systems. Using two fracture sets, a total of 72 2-D fracture configurations were generated with different combinations of fracture size distribution and deterministic fracture density. The directional block conductivity including the theoretical block conductivity, principal conductivity tensor and average block conductivity for each generated fracture network system were calculated using the 2-D equivalent pipe network method. There exist significant effects of block size, orientation, density and size of fractures in a fractured rock mass on its hydraulic behavior. We have been further verified that it is more difficult to reach the REV size for the fluid flow network with decreasing intersection angle of two fracture sets, fracture plane density and fracture size distribution.

• Explosives and Blasting
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• v.22 no.3
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• pp.1-12
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• 2004

In bench blasting, rock fragmentation is one of the most important factors determining productivity. Rock fragmentation could be affected by various conditions and these were hewn that rock joint conditions and in-situ block sizes were the biggest effect on it. This research is focused on what or how to influence on rock fragmentation according to relation between blasting conditions and the in-situ rock conditions such as rock joint conditions and in-situ block size. Field measurements were carried out in 3 open pit limestone mines, where in-situ rock conditions and blasting conditions were fully investigated. The results show that the parameters interact with blasting conditions complicatedly and especially in-situ block size has bigger effects. Dip direction of major joint set also can affect on fragmentation. Mean fragment size become smallest when dip direction of major joint set is about $30^{\circ}$ with the bench direction. The reason is considered to be come from difference of propagation paths of elastic wave.

• Explosives and Blasting
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• v.28 no.2
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• pp.1-16
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• 2010

This study focuses on the phenomenon that the blast damaged zone developed on rock slope surfaces can be affected by joint characteristics rather than by explosive power when the pre-splitting is applied to excavate a jointed rock slope. The characteristics of rock joints on a slope were investigated and categorized them into 4 cases. Also an image processing system has been used for comparing the distribution pattern of rock blocks. From this investigation, it was found that the rock blocks bigger than 2,000 mm occupied 42% in the case of single joint set and it showed the well efficiency of pre-splitting blast. In cases of 2~3 parallel joint sets and 2~3 intersecting joint sets are developed on rock surfaces, the rock blocks in the range of 1,000~2,000 mm occupied 43.6% and 35.8%, respectively, and it showed that the efficiency of pre-splitting was decreased. When more than 3 joint sets are randomly developed, however, the rock blocks in the range of 250~500 mm occupied 35% and there was no block bigger than 1,000 mm. This denotes that the blasting with pre-splitting was not effective. The numerical analysis using PFC2D showed that the blast damaged zone in a rock mass could be directly influenced by the pre-splitting. It is, therefore, required to investigate the discontinuity pattern on rock surfaces in advance, when the pre-splitting method is applied to excavate jointed rock slopes and to apply a flexible blating design with a consideration of the joint characteristics.

• Explosives and Blasting
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• v.23 no.2
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• pp.37-44
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• 2005

Recent studies reported that natural block size of rock and joint orientation highly affect on rock fragmentation. In this study, blasting test using high strength cement mortar was carried out to verify this fact. The result of this test indicated that fragmentation is influenced by the joint interval, and at same joint interval condition, fragmentation depends on joint orientation. These results are significantly coincident with field investigations.

• Explosives and Blasting
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• v.20 no.1
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• pp.5-14
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• 2002

파쇄 암석의 파쇄도는 발파효율을 나타내는 중요 척도로서 적재와 분쇄 작업에 큰 영향을 미친다. 그러나 현장에 쌓여 있는 발파암 더미로부터 파쇄도를 조사한다는 것은 용이한 작업이 아니다. 본 연구에서는 석산 발파에서 가장 중요한 요소인 천공방식과 암질의 변화가 파쇄도와 발파진동에 미치는 영향을 조사하였다. 파쇄 입도에 영향을 미치는 중요한 변수인 천공방식, 암질등급(RMR), 현지 암반의 블록 크기, 발파진동 등의 영향을 파쇄암의 평균입도(MFS)와 상위 5개의 대괴 평균치$(L_5)$로 나타내었다. 연구결과, 파쇄암의 평균입도는 상위 5개의 대괴 평균치와 선형적인 관계를 보였다. 발파방법과 파쇄도 평가 결과 재래식 발파에서는 파쇄도를 예측할 수 없었고 커다란 옥석이 생성되었으나, 벤치발파에서는 평균파쇄암의 크기가 비교적 양호한 파쇄 상태를 유지하였다. 현지 암반 블록 크기는 평균 파쇄암의 크기와 선형적 관계를 나타내었다. RMR값이 커짐에 따라 발파진동 추정식의 진동상수 K와 감쇠지수 n의 절대값과 평균 파쇄암의 크기, 그리고 상위 대괴의 크기는 대체적으로 증가하는 경향을 나타내었고, 진동상수 K와 감쇠지수 n도 증가하는 경향을 보였다.

• Journal of the Korean Geotechnical Society
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• v.23 no.10
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• pp.97-107
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• 2007

Accurate evaluation of the slope stability by assuming failure block as the entire slope is considered to be apposite for the small scale slope, whereas it is not the case for the large scale slope. Hence, appropriate estimation of a failure block size is required since the safety factor and the joint strength parameters are the function of the failure block size. In this paper, the size of failure block was investigated by generating 3-dimensional rock joint system based on statistical data of joints obtained from research slope, such as joint orientation, spacing and 3-dimensional joint intensity. The result indicates that 33 potential failure blocks exist in research slope, as large as 1.4 meters at least and 38.7 meters at most, and average block height is 15.2 meters. In addition, the data obtained from 3 dimensional joint system were directly applicable to the probability analysis and 2 and 3 dimensional discontinuity analysis.

• The Journal of Engineering Geology
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• v.32 no.1
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• pp.59-72
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• 2022

This study employed a 3-D numerical analysis based on the distinct element method to estimate the strength and deformability of a Cretaceous biotite granitic rock mass at Gijang, Busan, Korea. A workflow was proposed to evaluate the scale effect and the representative elementary volume (REV) of mechanical properties for fractured rock masses. Directional strength and deformability parameters such as block strength, deformation modulus, shear modulus, and bulk modulus were estimated for a discrete fracture network (DFN) in a cubic block the size of the REV. The size of the mechanical REV for fractured rock masses in the study area was determined to be a 15 m cube. The mean block strength and mean deformation modulus of the DFN cube block were found to be 52.8% and 57.7% of the intact rock's strength and Young's modulus, respectively. A constitutive model was derived for the study area that describes the linear-elastic and orthotropic mechanical behavior of the rock mass. The model is expected to help evaluate the stability of tunnels and underground spaces through equivalent continuum analysis.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 2009.03a
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• pp.153-159
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• 2009

불연속변형해석(DDA)은 유한요소법(FEM)과 개별요소법(DEM)의 장점을 모두 가지고 있는 해석법이다. 3차원 불연속변형해석의 안정성과 해석속도는 접촉찾기(Contact Detection) 알고리즘과 벌칙스프링(Penalty spring)을 이용한 접촉처리 알고리즘에 의해 크게 좌우되는데, 블록의 꼭지점(vertex) 간의 접촉이 발생할 경우, 적합한 접촉면을 결정하는 과정에서 많은 해석오류와 시간적 손실이 발생할 가능성이 있다. 본 연구에서는 블록의 꼭지점에 내접하는 구(inscribed sphere)를 삽입하여 가상의 접촉점과 접촉면을 생성하는 알고리즘을 연구하였다. 이로 인해 발생할 수 있는 해석의 오차의 크기를 확인하고, 간단한 불연속 변형해석의 해석 실례에 적용해 보았다.

• The Journal of Engineering Geology
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• v.32 no.1
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• pp.41-57
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• 2022

A workflow is presented to estimate the size of a representative elementary volume and 3-D hydraulic conductivity tensor based on fluid flow analysis for a discrete fracture network (DFN). A case study is considered for a Cretaceous granitic rock mass at Gijang in Busan, Korea. The intensity and size of joints were calibrated using the first invariant of the fracture tensor for the 2-D DFN of the study area. Effective hydraulic apertures were obtained by analyzing the results of field packer tests. The representative elementary volume of the 2-D DFN was determined to be 20 m square by investigating the variations in the directional hydraulic conductivity for blocks of different sizes. The directional hydraulic conductivities calculated from the 2-D DFN exhibited strong anisotropy related to the hydraulic behavior of the study area. The 3-D hydraulic conductivity tensor for the fractured rock mass of the study area was estimated from the directional block conductivities of the 2-D DFN blocks generated for various directions in 3-D. The orientations of the principal components of the 3-D hydraulic conductivity tensor were found to be identical to those of delineated joint sets in the study area.

• Journal of Korean Tunnelling and Underground Space Association
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• v.6 no.4
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• pp.327-343
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• 2004

During tunnel construction, ground failures often occur due to existence of weak zones, such as faults, joints, and cavities, ahead of tunnel face. It is hard to detect effectively weak zones, which can lead underground structure to fail after excavation and before supporting, by using conventional characterization methods. In this study, an enhanced analytical method of predicting weak zones ahead of tunnel face is developed to overcome some problems in the conventional geophysical exploration methods. The analytical method is based on Coulomb's and Gauss' laws with considering the characteristics of electric fields subjected to rock mass. Using the developed method, closed form solutions are obtained to detect a spherical shaped zone and an oriented fault ahead of tunnel face respectively. The analytical results suggest that the presence of weak zones and their sizes, location, and states can be accurately predicted by combining a proper inversion process with resistance measured from several electrodes on the tunnel face. It appears that the skin depth or resistivity in rock mass is affected by the diameter of tunnel face, natural electric potential and noises induced by experimental measurement and spatial distribution of uncertain properties. The developed analytical solution is verified through experimental tests. About 1800 concrete blocks of 5cm by 5cm by 5cm in size are prepared and used to model a joint rock mass around tunnel face. Weak zones are simulated ahead of tunnel face with a material which has relatively higher conductivity than concrete blocks. Experimental results on the model test show a good agreement with analytical results.