• Tunnel and Underground Space
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• v.25 no.3
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• pp.231-243
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• 2015

In this study, both in-situ stress measurements and a lot of laboratory rock tests were conducted at a metal mine in Jeongseon, Korea. The stress ratio obtained from in-situ stress measurements showed a tendency to decrease according to depth below surface and its average value was 1.10. The mechanical properties such as unit weight, absorption ratio, porosity, elastic wave velocity, uniaxial compressive strength, Young's modulus, Poisson's ratio, tensile strength, shore hardness, friction angle and cohesion were investigated for the four different rocks mainly distributed at a studied mine, which were dolomite, felsite, granite and magnetite. The mechanical properties of the four different rocks were compared by means of statistical analyses, whereupon the felsite and the granite turned out to have more strength characteristics than the magnetite. The correlation of mechanical properties was also investigated, whereupon a few results against the general correlation were found out. The failure criteria of the four different rocks were finally discussed by means of both Mohr-Coulomb criterion and Hoek-Brown criterion.

• Journal of the Korean Geotechnical Society
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• v.30 no.1
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• pp.49-63
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• 2014

Bearing capacity factor $N_{\gamma}$ and shape factor were studied for rigid strip and circular footings with a rough base on sand by numerical modelling considering the effect of dilation angle. The numerical model was developed with an explicit finite difference code. Loading procedures and interpretation methods were devised in order to shorten the running time while eliminating the exaggeration of the reaction caused by the explicit scheme. Using the Mohr-Coulomb plasticity model with associated (${\psi}={\phi}$) and nonassociated (${\psi}$ < ${\phi}$) flow-rules, the bearing capacity factor $N_{\gamma}$ was evaluated for various combinations of internal friction angles and dilation angles. Bearing capacity factor decreased as the dilation angle was reduced from the associated condition. An equation applicable to typical sands was proposed to evaluate the relative bearing capacity for the nonassociated condition compared to the associated condition on which most bearing capacity factor equations are based. The shape factor for the circular footing varied substantially when the plane-strain effect was taken into account for the strip footing. The numerical results of this study showed closer trends with the previous experimental results when the internal friction angle was increased for the strip footing. Discussions are made on the reason that previous equations for the shape factor give different results and recommendations are made for the appropriate design shape factor.

• Journal of the Korean Geotechnical Society
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• v.30 no.3
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• pp.29-46
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• 2014

For circular rigid footings with a rough base on sand, combined vertical - horizontal loading capacity was studied by three-dimensional numerical modelling. A numerical model was implemented to simulate the swipe loading and the probe loading methods and an interpretation procedure was devised in order to eliminate the numerical error from the restricted mesh density. Using the Mohr-Coulomb plasticity model, the effect of friction angle was studied under the associated flow-rule condition. The swipe loading method, which is efficient in that the interaction diagram can be drawn with smaller number of analyses, was confirmed to give similar results with the probe loading method, which follows closely the load-paths applied to real structures. For circular footings with a rough base, the interaction diagram for combined vertical (V) - horizontal (H) loading and the inclination factor were barely affected by the friction angle. It was found that the inclination factors for strip and rectangular footings are applicable to circular footings. For high H/V ratios, the results by numerical modelling of this study were smaller than the results of previous studies. Discussions are made on the factors affecting the numerical results and the areas for further researches.

• Journal of the Korean Geotechnical Society
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• v.19 no.2
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• pp.227-236
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• 2003

The evaluation of the reliable strength and deformation characteristics of weathered granite masses is very important for the design of geotechnical structure under working stress conditions. Various types of laboratory test such as triaxial compression test can be performed to determine the strength parameters. However, it is very difficult to obtain the representative undisturbed samples on the site and also the rock specimen cannot represent rock mass including discontinuities, fracture zone, etc. This study aims to investigate the strength and deformation characteristics of granite masses corresponding to its weathering and develop a practical strength parameter evaluation method using the results of PMT. To predict weathering intensity and strength parameters of the weathered granite masess in the field, various laboratory tests and in-situ tests including field triaxial test and PMT are carried out. Based on the results of weathering index tests, the classification method is proposed to identify the weathering degree in three groups for the weathered granite masses. Using the analytical method based on the Mohr-Coulomb failure criteria and the cavity expansion theory, the strength parameters of rock masses were evaluated from the results of PMT. It shows that weathering intensity increases with decreasing the strength parameters exponentially. The strength parameters evaluated with the results of PM almost coincide with the results of field triaxial test.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.480-493
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• 2021

Since the generalized Hoek-Brown criterion (GHB) provides an efficient way of identifying its strength parameter values with the consideration of in-situ rock mass condition via Geological Strength Index (GSI), this criterion is recognized as one of the standard rock mass failure criteria in rock mechanics community. However, the nonlinear form of the GHB criterion makes its mathematical treatment inconvenient and limits the scope of its application. As an effort to overcome this disadvantage of the GHB criterion, the explicit approximate analytical equations for the minimum principal stress, which is associated with the maximum principal stress at failure, are formulated based on the Taylor polynomial approximation of the original GHB criterion. The accuracy of the derived approximate formula for the minimum principal stress is verified by comparing the resulting approximate minimum principal stress with the numerically calculated exact values. To provide an application example of the approximate formulation, the equivalent friction angle and cohesion for the expected plastic zone around a circular tunnel in a GHB rock mass are calculated by incorporating the formula for the approximate minimum principal stress. It is found that the simultaneous consideration of the values of mi, GSI and far-field stress is important for the accurate calculation of equivalent Mohr-Coulomb parameter values of the plastic zone.

• 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.

• Tribology and Lubricants
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• v.23 no.5
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• pp.219-227
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• 2007

The dynamic performance of air foil bearings relies on a coupling between a thin air film and an elastic foil structure. A number of successful analytical techniques to predict dynamic performance have been developed. However, the evaluation of its dynamic characteristic is still not enough because of the mechanical complexity of the foil structure and strong nonlinear behavior of friction force. This work presents a nonlinear transient analysis method to predict dynamic performance of foil bearings. In this method, time dependent Reynolds equation is used to calculate pressure distribution and a finite element method is used to model the bump foil structure. The analysis is treated with a direct implicit integration technique that can handle nonlinear problems and the stick-slip algorithm is used to consider friction force. Using this method the response to the mass unbalance excitation is investigated for various design parameters and operating conditions. The results of analysis show that foil bearing is very effective on the restriction of vibration at the resonance frequency compared to the rigid surface bearings and the effectiveness depends on the operating conditions, static load and a amount of mass unbalance. In addition, there exist optimum values of friction coefficient, bump foil stiffness and number of circumferential slit with regards to minimizing dynamic response at the resonance frequency. These optimum values are system dependent.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.12
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• pp.1080-1085
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• 2012

In this paper a sharp-edged type damping orifice for an aircraft door damper were designed, where the dynamic viscosity of working fluid were assumed to change up to 400cSt. The discharge coefficient of the damping orifice were investigated by CFD analyses and experiments. In particular, the influences of orifice diameter, edge angle, flow direction and the Reynolds number were taken into consideration. Based on this, it has been deduced how high Coulomb friction forces of damper seals is to be allowed to meet the performance criterion with respect to the orifice size.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.2
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• pp.458-466
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• 1998

In this work, a frequency domain method is tested numerically and experimentally to improve nonlinear model parameters using the frequency response function at the nonlinear element connected point of structure. This method extends the force-state mapping technique, which fits the nonlinear element forces with time domain response data, into frequency domain manipulations. The force-state mapping method in the time domain has limitations when applying to complex real structures because it needd a time domain lumped parameter model. On the other hand, the frequency domain method is relatively easily applicable to a complex real structure having nonlinear elements since it uses the frequency response function of each substurcture. Since this mehtod is performed in frequency domain, the number of equations required to identify the unknown parameters can be easily increased as many as it needed, just by not only varying excitation amplitude bot also selecting excitation frequency domain method has some advantages over the classical force-state mapping technique in the number of data points needed in curve fit and the sensitivity to response noise.

• Transactions of Materials Processing
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• v.12 no.1
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• pp.34-42
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• 2003

This paper presents a rigid-plastic finite element method to handle the frictional contact problem between two deformable bodies experiencing large deformation. The variational formulation combined with incremental quasi-static model is employed for treating the contact boundary condition. The frictional behavior of the model obeys Coulomb's law of friction. The proposed contact algorithms are classified into two categories, one for searching contacting nodes and the other for calculating contact forces at the contact surface. A slave node and master contact segment are defined using the geometric condition of finite elements on the contact interface. The penalty parameter is used to limit the penetration between contacting bodies, and the finite elements are coupled with contact boundary elements.us gates and cavity thicknesses. Through this study we have observed that the jetting is related to the die swell of material. This means that the jotting is strongly affected by the elastic flow property rather than the viscous flow property in viscoelastic characteristics of molten polymer. Different resins have different elastic properties, and elastic flow behavior depends on the shear rate of flow, i.e. injection speed. Large die swell would eliminate jetting however, the retardation of die swell would stimulate jetting. In the point of mole design, reducing the thickness ratio of cavity to gate can reduce or eliminate jetting regardless of amount of elasticity of polymer melt.