• Tunnel and Underground Space
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• v.16 no.3 s.62
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• pp.259-276
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• 2006

In rock mass subject to high in-situ stresses, the failure process of rock is dominated by the stress-induced fractures growing parallel to the excavation boundary. When the ratio of in situ stresses compared to rock strength is greater than a certain value, progressive brittle failure which is characterized by popping and spatting of rock debris occurs due to stress concentration. Traditional constitutive model like Mohr-Coulomb usually assume that the normal stress dependent frictional strength component and the cohesion strength component are constant, therefore modelling progressive brittle failure will be very difficult. In this study, a series of numerical analyses were conducted for surrounding rock mass near crude oil storage cavern using CW-FS model which was known to be efficient for modelling brittle failure and the results were compared with those of linear Mohr-Coulomb model. Further analyses were performed by varying plastic shear strain limits on cohesion and internal friction angle to find the proper values which yield the matching result with the observed failure in the oil storage caverns. The obtained results showed that CW-FS model could be a proper method to characterize essential behavior of progressive brittle failure in competent rock mass.

• Journal of the Korean Geotechnical Society
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• v.19 no.1
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• pp.209-220
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• 2003

Modification of general Hoek and Brown criterion is carried out to estimate the shaft resistance of drilled shaft socketed into rock mass. Since the general Hoek-Brown criterion can consider the in-situ state of the rock mass, the proposed method, estimating the unit shaft resistance of drilled shafts based on the Hoek-Brown criterion, has increased flexibility compared to other methods exclusively considering uniaxial compressive strength of intact rocks. The proposed method can form the upper and lower bounds, and most culled data (from 21 pile load tests) from the literature can be found between these two bounds. A comparison between the estimated and observed unit shaft resistances shows quite a good correlation even with crude assumptions for the input parameters. The best-fit line drawn from this analysis shows that at the lower strength of intact rocks (up to 10MPa), Horvath and Kenney's equation shows a good correlation with the measured values, and fur strong rocks Rosenberg and Journeaux's equation provides a close estimation with colleted data. The results of parametric studies for GSI and confining stress show that the normalized unit shaft resistance increases with these two factors. In addition, coefficient of the equational form of the estimation can vary with GSI and confining stresses.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.168-168
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• 2015

자연하천의 주요한 특징 중 하나인 하천의 사행은 직선수로에서 예측되는 유속분포를 왜곡시키며 매우 복잡한 흐름구조를 형성한다. 이는 하상 경계면에서 발생하는 전단응력 분포의 변화를 야기하는데 하상 경계면에서의 전단응력은 다양한 경험적 관계에 의존하는 유사이동의 한계 소류력 산정 및 오염물질 거동해석의 분산계수 산정에 많은 영향을 미치게 된다. 물리적인 관측을 통한 하상 경계면에서의 전단응력의 관측은 다소 제한적이며 많은 비용을 요구한다. 따라서 하상 경계면에서 발생하는 전단응력의 경우 수심의 20% 이하의 연직 유속분포를 벽법칙에 적용하여 추정하는 방법이 주로 이루어지고 있다. 벽법칙을 이용한 하상 경계면의 전단응력을 계산하는 경우 대수중복층의 유속 분포 $u/u^*=(1/{\kappa})ln(zu^*/{\nu})+B$에서 무차원상수 ${\kappa}$와 B의 적절한 추정이 요구되어 진다. 일반적으로 무차원상수 ${\kappa}$와 B는 수리학적으로 매끄러운 벽면에서 대략 ${\kappa}=0.4$, B=5.5로서 경험적으로 이용되고 있다. 본 연구에서는 직선수로 및 다양한 사행수로의 3차원 흐름장 모의를 수행하여 벽법칙의 대수 중복층을 따르는 주흐름 방향 유속의 연직분포를 비교하였다. 수치모의 소프트웨어로서 Linux 기반의 OpenFOAM이 사용되었으며 모델의 검증을 위해 Chang(1971)에 의해 수행 된 사행수로에서의 유속장 관측 결과와 비교하였고 수치모의 결과가 실험 관측치와 잘 일치하는 것으로 판단되었다. 수치모의에 적용 된 사행수로의 형상은 Hey(1976)에 의해 제안 된 사행하천의 지형학적 인자들 간에 관계를 이용하여 사행도 1.03에서 2.42까지 총 7개의 사행수로 지형을 생성하였다. 사행도의 변화에 따라 만곡부 정점에서 대수중복층 구간의 주흐름 방향 유속의 연직분포를 비교한 결과, 본 연구에서 생성 된 모든 사행수로에서 대수중복층 구간의 무차원 유속 $u^+$와 무차원 거리 $z^+$가 로그 분포를 따르는 것으로 나타났으나 경험적으로 사용되었던 무차원상수 B의 경우 사행도가 증가 할수록 대수적으로 감소하는 경향이 나타났다. 본 연구에서는 이러한 관계가 무차원 상수 B값에 미치는 영향을 반영하여 수리학적으로 매끄러운 벽면에서 적용이 가능한 수정된 대수중복층 식을 제시하고자 한다.

• The Journal of Engineering Geology
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• v.2 no.1
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• pp.1-18
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• 1992

Rock mass having discontinuous plane almost appear roughness which have a great effect on shear strength. Rocks of studied object choose granites (15 samples), gneisses (7 samples), and andesites (1 sample). The purpose of this study was to clarify shear strength of discontinuous planes as value of shear strength angle (${\Phi}_p$), critical stress of roughness (${\sigma}_r$) and shear failure strength (${\tau}_o$). 1. Roughness decrease from ${\Phi}_i=38.03^{\circ}$ to $33.21^{\circ}$ that is, friction angle has the highest value at first stage and has the lowest value at the last stage. 2. Critical angle of roughness distribution within $45^{\circ}$ (test max. $angle=43^{\circ}$), JRC(Joint Roughness Coefficient) is less than 14 and lies distribution range of boundary is following: $JRC=-4.63Ln{\sigma}n+5.63$. 3. When the roughness critical stress(${\sigma}_T) is from 0.1 to 3 .56Mpa, shear failure strength of roughness (${\tau}_o$) is from 0.01 to 0.46Mpa, shear strength(${\tau}$) of discontinuous plane is from 3.65 to 39.11 Mpa. If loading is higher than these values, collapse and sliding will occur on the rock mass.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.12 no.5
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• pp.2061-2066
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• 2011

A cycloid speed reducer is one of the rotational speed regulation devices of the machinery. A cycloid speed reducer has an advantage of transmitting high torque, but is known to be unsuitable for high speed rotation. However, it is almost impossible in an analytical method to find a use limit speed when installing such a speed reducer in a 200ton loading transporter. In this research the cycloid reducer was simulated to get its performance depending on friction energy loss in time domain by using by LS-DYNA. The maximum torque of the cycloid speed reducer is 3.5ton-m, so the comparison of analysis results between a case of 60rpm rotation and a case of 162rpm rotation with such a torque showed the following results. In the case of 60rpm rotation, the maximum stress appearing in the RV gear and the pin gear was 463MPa and 507MPa. Lost power due to friction was 50kW; In the case of 162rpm rotation, the maximum stress appearing in the RV gear and the pin gear was 550MPa and 538MPa. Lost power due to friction was 175kW, which was shown to be almost impossible to use.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2002.11a
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• pp.139-145
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• 2002

The flow zone through propeller jets are used in evaluating the environmental and constructional effects of navigation on the waterway. It relies on the characteristics of ships and water depth. A numerical model using the momentum theory of the propeller and Shield's diagram was developed in a restricted waterway. Equations for discharge are presented based on thrust coefficients and propeller speed and are the most accurate means of defining discharge. Approximate methods for discharge are developed based on applied ship's power. Equations for discharge are as a function of applied power, propeller diameter, and ship speed. Water depth of the waterway and draft of the shop are also necessary for the calculation of the grain size of the initial motion. The velocity distribution of discharge from the propeller was simulated by the Gaussian normal distribution function. The shear velocity and shear stress were from the Sternberg's formula. Case studies to show the influence of significant factors on sediment movement induced by the ship's propeller at the channel bottom are presented.

• Journal of Korean Tunnelling and Underground Space Association
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• v.11 no.4
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• pp.327-351
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• 2009

Laboratory tests are performed in this paper to investigate the brittle failure characteristics of over-stressed rocks taken in deep depth. Also, numerical simulation performed using that the so-called CWFS(Cohesion Weakening Frictional Strengthening) model is known to predict brittle failure phenomenon reasonably well. The most typical rock types of Korean peninsula - granite and gneiss - were used for testing. Results of uniaxial compression tests showed that the crack initiation stress was about 41 % to 42% of the uniaxial compressive strength regardless of rock types, where as, the crack damage stress of granite was about 75%, and that of gneiss was about 97%. Through the damage-controlled test, strength parameters of each rock were obtained as a function of damage degree. After the peak, the crack damage stress and the maximum stress were decreased, The cohesion was decreased and the friction angle was increased with increase of rock damage. Before reaching the peak, the elastic modulus was slightly increased, while decreased after the peak. Poisson's ratio was increased as the damage of rock proceeds. Comparison of uniaxial compression tests and damage-controlled tests shows the crack initiation stress estimated from the damage-controlled test fluctuated within the range of crack initiation stress obtained from the uniaxial compression test; the crack damage stress was less than that estimated from the uniaxial compression test. In order to predict the critical depth that brittle failure occurs, numerical simulations using the CWFS model were performed for an example site. Material parameters obtained from the laboratory tests mentioned above were used for CWFS simulation. Comparison between the critical depth predicted from the numerical simulation using the CWFS model and that predicted by using the damage index proposed by Martin et al.(l999), showed that critical depth cannot be reasonably predicted by the currently used damage index except for circular tunnels. A modified damage index was proposed by the author which takes the shape of tunnels other than circular into account.

• Journal of the Korean Geotechnical Society
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• v.22 no.4
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• pp.51-59
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• 2006

In this paper, the effect of variation of density and uniformity coefficient on shear strength was analyzed from the results of large scale shear test. In addition, the friction coefficient at critical state per vertical load was estimated using the equation proposed by Wood (1998). The test sample fur the test was obtained from the local quarry sites. Tests results show that the shear strength of $2.10g/cm^3$ is relatively larger than that of $1.85g/cm^3$ and uniformity coefficient (5.0) has larger shear strength than that in 10.0. In the meantime, the friction coefficient at critical state shows $1.0{\sim}1.6$ according to the test conditions.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.6
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• pp.1417-1431
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• 1994

In this study, shear strength characteristics of the Nak-Dong river sand in Taegu area are investigated by triaxial compression test, considering shear strain control velocity, relative density, and confining pressure. The results from Lade model and Modified Lade model are compared with the measured value in the laboratory test. The results by the change of shear strain control velocity, relative density, and confining pressure are as follows; 1) The stress limit, which can be Coulomb's law about the Nak-Dong river sand, have ranged 120~200 kpa at 0.08%/min and 120~150 kpa at 0.5%/min. These limits are smaller than that of the calcareous sand and the well-graded, quartz sand. 2) The parameters needed to Lade model and Modified Lade model are much affected by the strain control velocity and the relative density. Consequently, in the field, it is important to use parameters aptly after accurately understanding both the loading condition and subsoil condition. 3) Overall, the principal stress ratio obtained from constitutive model equations is not affected by the control velocity, but both the relative density and confining pressure affect the result of constitutive model equations. Consequently, the study on the various conditions about the relative density and confining pressure is needed to accurately predict the stress-strain behavior on the Nak-Dong river sand. 4) For the range of the used confining pressure in the study, the Lade model shows better agreements with the measured value than the Modified Lade model, comparing the measured value with the principal stress ratio at failure and the internal friction angle of failure envelope obtained from the Lade model and Modified Lade model.

• Journal of the Korean Geotechnical Society
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• v.19 no.4
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• pp.265-275
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• 2003

In this study, shaking table tests were conducted to estimate dynamic interface properties between geosynthetics such as geomembrane, geotextile and geosynthetic clay liner. Accelerations of both shaking table and upper box, and relative displacements between geosynthetics under dynamic loading were measured. Also, the influence of normal stress, frequency of excitation and dry/wet conditions were investigated through the analyses of test results. from the test results, it was found that there is a limited acceleration below which dynamic farce can be transmitted between geosynthetics without the loss of horizontal acceleration. Dynamic interface friction angle between geosynthetics could be calculated through the limited acceleration. Relative displacements induced along geosynthetic interfaces under dynamic loading were not consistent depending on the type of interface and test conditions. The maximum slip displacements between geosynthetics are normalized and normalized slip equations were developed for each interface. By using the normalized slip equation, maximum slip displacements for the geosynthetic interface could be predicted for the given base acceleration and frequency of excitation.