• Geomechanics and Engineering
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• v.11 no.4
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• pp.571-585
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• 2016

The crest of the Pubugou central core rockfill dam (CCRD) cracked in the first and second impounding periods. To evaluate the safety of the Pubugou CCRD, an inversion analysis of the constitutive model parameters for rockfill materials is performed based on the in situ deformation monitoring data. The aim of this work is to truly reflect the deformation state of the Pubugou CCRD and determine the causes of the dam crest cracks. A novel real-coded genetic algorithm based upon the differences in gene fragments (DGFX) is proposed. It is used in combination with the radial based function neural network (RBFNN) to perform the parameters back analysis. The simulated settlements show good agreements with the monitoring data, illustrating that the back analysis is reasonable and accurate. Furthermore, the deformation gradient of the dam crest has been analysed. The dam crest has a great possibility of cracking due to the uncoordinated deformation, which agrees well with the field investigation. The deformation gradient decreases to the value lower than the critical one and reaches a stable state after the second full reservoir.

• Journal of the Korea Concrete Institute
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• v.21 no.6
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• pp.773-780
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• 2009

Experiments on steel-concrete interface are performed to investigate and determine the mechanical roles and properties of interface parameters. The intrinsic different nature of bonded and unbonded interfaces are addressed based on the experimental observations that were obtained from two types of tests considering bonded and unbonded interfaces. The unbonded tests are performed for the specimens that are in unbonded when the initially bonded specimens are tested first. Four cases of lateral confinements including pure slip, and low and medium levels of lateral pressure are taken into account to investigate the effects of lateral confinements on interface behavior. It is shown that the maximum shear strengths, the levels of residual strengths and the Mode II fracture energy release rates are linearly related to the confinement levels. Based on the experimental evidences obtained from this study, the values of interface parameters required in a steel-concrete interface constitutive model will be presented in the companion paper.

• Nuclear Engineering and Technology
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• v.41 no.9
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• pp.1143-1156
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• 2009

The SPACE code that is based on a multi-dimensional two-fluid, three-field model is under development for licensing purposes of pressurized water reactors in Korea. Among the participating research and industrial organizations, KAERI is in charge of developing the physical models and correlation packages for the constitutive equations. This paper introduces a developed wall-to-fluid heat transfer package for the SPACE code. The wall-to-fluid heat transfer package consists of twelve heat transfer subregions. For each sub-region, the models in the existing safety analysis codes and the leading models in literature have been peer reviewed in order to determine the best models which can easily be applicable to the SPACE code. Hence a wall-to-fluid heat transfer region selection map has been developed according to the non-condensable gas quality, void fraction, degree of subcooling, and wall temperature. Furthermore, a partitioning methodology which can take into account the split heat flux to the continuous liquid, entrained droplet, and vapor fields is proposed to comply fully with the three-field formulation of the SPACE code. The developed wall-to-fluid heat transfer package has been pre-tested by varying the independent parameters within the application range of the selected correlations. The smoothness between two adjacent heat transfer regimes has also been investigated. More detailed verification work on the developed wall-to-fluid heat transfer package will be carried out when the coupling of a hydraulic solver with the constitutive equations is brought to completion.

• Journal of the Korean Geotechnical Society
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• v.23 no.3
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• pp.89-100
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• 2007

In the companion paper, we provided the novel elastic-plastic constitutive model based on the micromechanics theory. Herein, the elastic and elastic-plastic deformation of granular soils is meticulously analyzed. To guarantee high accuracy of the microscopic parameter, the systematic procedure to evaluate the parameters is provided. The analysis of the elastic response during the isotropic and triaxial compression shows that the stress-level dependency of cross-anisotropic elastic moduli is induced by the power relationship of the contact force in the normal contact stiffness, while the evolution of fabric anisotropy is more pronounced during triaxial compression. The micromechanical analysis indicates that the plastic strains are likely to occur at very small strains. The plastic deformation of tangential contacts has an important role in the reduction of soil stiffness during axial loading.

• Journal of the Korean Geosynthetics Society
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• v.19 no.1
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• pp.93-101
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• 2020

In order to predict the nonlinear behaviour of the soil, the elasto-plastic hyperbolic model was selected, which was considered to be relatively simple and highly predictable. The soil parameter determination and the behavior analysis program were developed by formalizing the functions related to the constitutive model. Each soil parameter was determined from the results of the drained triaxial compression tests of Baekma river sand with the change of relative density. The stress-strain behavior was predicted using the soil parameters determined under each condition. As a result, the deviator stress for the axial strain is verified to have a good match between the measured value and predicted value at each relative density. In the relationship between the volumetric stain and the axial strain, when the relative density is loose, the measured value and predicted value tend to match, and when relative density is dense, the predicted value of the volumetric strain appears somewhat smaller than the measured value due to the limitation of the constitutive model.

• Journal of the Korean Geotechnical Society
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• v.19 no.5
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• pp.223-232
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• 2003

In this study, a constitutive model called the disturbed state concept (DSC) was modified to be applied to the interface shear stress-displacement relationship between geosynthetics. The DSC model is comprised of two reference states, namely the relative intact (RI) and the fully adjusted (FA) state, and one function, namely the disturbance function. This model is a unified approach and can allow for various models as an RI state such as elastic-perfectly plastic model, hierarchical model, and so on. In addition, by using this model, the elastic and plastic displacements can be considered simultaneously. Comparisons between the measured data and predicted results through the parameters determined from four sets of large direct shear tests showed good agreements with each other, especially for the smooth geomembrane-involved interface. Although there are slight differences at peak shear strength for textured geomembrane-involved interface, this model can still be useful to predict the position of displacement at peak strength and the large displacement (or residual) shear strength.

• Journal of the Korean Geotechnical Society
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• v.22 no.10
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• pp.173-181
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• 2006

A Two Mobilized-Plane Model is proposed for monotonic and cyclic soil response including liquefaction. This model is based on two mobilized planes: a plane of maximum shear stress, which rotates, and a horizontal plane which is spatially fixed. By controlling two mobilized planes, the model can simulate the principal stress rotation effect associated with simple shear from different $K_0$ states. The proposed model gives a similar skeleton behaviour for soils having the same mean stress, regardless of $K_0$ conditions as observed in laboratory tests. The soil skeleton behaviour observed in cyclic drained simple shear tests, including compaction during unloading and dilation at large strain is captured in the model. Undrained monotonic and cyclic response is predicted by imposing the volumetric constraint of the water on the drained or skeleton behaviour. This constitutive model is incorporated into the dynamic coupled stress-flow finite difference program of FLAC (Fast Lagrangian Analysis of Continua). The model was first calibrated with drained simple shear tests on Fraser River sand, and verified by comparing predicted and measured undrained behaviour of Fraser River sand using the same input parameters.

• Tunnel and Underground Space
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• v.11 no.2
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• pp.109-119
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• 2001

Up to now single large cavern was excavated for each undergroud hydraulic powerhouse in Korea. But the Yangyang underground hydraulic powerhouse consists of two large caverns; a powerhouse cavern and main transformer cavern. In this carte, the structural stability of the caverns, especially the rock pillar formed between two large caverns, should be guaranteed to be sound to make the caverns permanently sustainable. In this research, the Distinct Element Method(DEM) was used to analyze the structural stability of two caverns and the rock pillar. The Barton-Bandis joint model was used as a constitutive model. The moot significant parameters such as in-site stress, JRC of in-situ natural joints, and spatial distribution characteristics of discontinuities were acquired through field investigation. In addition, two different cases; 1) with no support system and 2) with a support system, were analysed to optimize a support system and to investigate reinforcing effects of a support system. The results of analysis horizontal displacement and joint shear displacement proved to be reduced with the support system. The relaxed zone in the rock pilar also proved to be reduced in conjunction with the support system. Having a support system in place provided the fact that the non zero minimum principal stresses were still acting in the rock pillar so that the pillar was not under uniaxial compressive condition but under triaxial compressive condition. The structural stability f an approximately 36 m wide rock pillar between two large caverns was assured with the appropriate support system.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.10
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• pp.1793-1799
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• 2003

An optimal design of a multi-layered plate structure to endure high-velocity impact has been suggested by using size optimization after numerical simulations. The NET2D, a Lagrangian explicit time-integration finite element code for analyzing high-velocity impact, was used to find the parameters for the optimization. Three different materials such as mild steel, aluminum for a multi-layered plate structure and die steel for the pellet, were assumed. In order to consider the effects of strain rate hardening, strain hardening and thermal softening, Johnson-Cook model and Phenomenological Material Model were used as constitutive models for the simulation. It was carried out with several different gaps and thickness of layers to figure out the trend in terms of those parameters' changes under the constraint, which is against complete penetration. Also, the measuring domain has been shrunk with several elements to reduce the analyzing time. The response surface method based on the design of experiments was used as optimization algorithms. The optimized thickness of each layer in which perforation does not occur has been obtained at a constant velocity and a designated total thickness. The result is quite acceptable satisfying both the minimized deformation energy and the weight criteria. Furthermore, a conceptual idea for topology optimization was suggested for the future work.

• Structural Engineering and Mechanics
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• v.74 no.1
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• pp.1-18
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• 2020

For the large deformation of shear walls under vertical and horizontal loads, there are difficulties in obtaining accurate simulation results using the response analysis method, even with fine mesh elements. Furthermore, concrete material nonlinearity, stiffness degradation, concrete cracking and crushing, and steel bar damage may occur during the large deformation of reinforced concrete (RC) shear walls. Matrix operations that are involved in nonlinear analysis using the traditional finite-element method (FEM) may also result in flaws, and may thus lead to serious errors. To solve these problems, a planar four-node element was developed based on vector mechanics. Owing to particle-based formulation along the path element, the method does not require repeated constructions of a global stiffness matrix for the nonlinear behavior of the structure. The nonlinear concrete constitutive model and bilinear steel material model are integrated with the developed element, to ensure that large deformation and damage behavior can be addressed. For verification, simulation analyses were performed to obtain experimental results on an RC shear wall subjected to a monotonically increasing lateral load with a constant vertical load. To appropriately evaluate the parameters, investigations were conducted on the loading speed, meshing dimension, and the damping factor, because vector mechanics is based on the equation of motion. The static problem was then verified to obtain a stable solution by employing a balanced equation of motion. Using the parameters obtained, the simulated pushover response, including the bearing capacity, deformation ability, curvature development, and energy dissipation, were found to be in accordance with the experimental observation. This study demonstrated the potential of the developed planar element for simulating the entire process of large deformation and damage behavior in RC shear walls.