• The Journal of Engineering Geology
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• v.25 no.1
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• pp.93-102
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• 2015

The characteristics of frozen soils are one of most important factors for foundation design in cold region. The objective of this study is to evaluate the shear strength and stiffness of frozen soils according to the confining conditions during the freezing and shearing phase. A direct shear box is constructed for the frozen specimens and bender elements are mounted on the wall of the shear box to measure shear wave velocities. Specimens are prepared by mixing sand and silt with a silt fraction of 30% in weight and the degree of saturation of 10%, giving a relative density of 60% for all tests. The temperature of the specimens in the freezer is allowed to fall below -5℃, and then direct shear tests are performed. A series of vertical stresses are applied during the freezing and shearing phase. Shear stress, vertical displacement, and shear wave along the horizontal displacement are measured. Experimental results show that in all the tests, shear strength increases with increasing vertical stress applied during the freezing and shearing phases. The magnitude of the increase in shear strength with increasing vertical stress during shearing under fixed vertical stress in the frozen state is smaller than the magnitude of the increase in vertical stress during freezing and shearing. In addition, the change in shear wave velocities varies with the position of the bender elements. In the case of shear waves passing through the shear plane, the shear wave velocities decrease with increasing horizontal displacement. This study provides an evaluation of the properties of shear strength and stiffness of frozen soils under varied confining condition.

• Journal of Ocean Engineering and Technology
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• v.15 no.4
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• pp.60-65
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• 2001

Thermal spray coating is formed by a process in which melted particles flying with high speed towards substrate, then crash and spread on the substrate surface cooled and solidified in a very short time, Stacking of the particles makes coating. In this study, the exfoliation of $Al_2$O$_3$ and Ni-4.5wt.%Al thermally sprayed coating which were deposited by an atmospheric plasma spray apparatus are investigated using an AE method. A tensile test is conducted on notch specimens in a stress range below the elastic limit of substrate. The wave forms of AE generated from the three coating specimens can be classified by FFT analysis into two types which low frequency(type I waveform is considered to corresponds exfoliation of coating layers and type II waveform corresponds the plastic deformation of notch tip or the resultant fracture of coating. The fracture of the coating layers can estimate by AE event and amplitude, because AE features increase when the deformation generates.

• Transactions of the Korean Society of Automotive Engineers
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• v.7 no.6
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• pp.258-269
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• 1999

It is important to evaluate fatigue damage of in-service material in respect to assure safety and remaining fatigue life in structure and mechanical components under cyclic load . Fatigue damage is represented by mathematical modelling with crack growth rate da/dN and cycle ration N/Nf and is detected by X-ray diffraction and ultrasonic wave method etc. But this is estimated generally by single parameter but influenced by many test conditions The characteristics of it indicates fatigue damage has complex fracture mechanism. Therefore, in this study we propose that back-propagation neural networks on the basis of ration of X-ray half-value breath B/Bo, fractal dimension Df and fracture mechanical parameters can construct artificial intelligent networks estimating crack growth rate da/dN and cycle ratio N/Nf without regard to stress amplitude Δ $\sigma$.

• Proceedings of the KIPE Conference
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• 1998.10a
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• pp.1030-1035
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• 1998

This paper describes a new dc-ac inverter system, which for achieving sinusoidal ac waveform make use of parallel loaded frequency resonant inverter consisting of full bridge. Each one of the pair of switches in the inverter is driven to synchronous output frequency and the other is driven to PWM signal with resonant frequency proportional to magnitude of sine wave. Since current through switches is always zero at its turn-on in proposed inverter, low stress and low switching loss is achieved. The theoretical analysis is proved through the experimental test.

• The Journal of Engineering Geology
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• v.24 no.1
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• pp.111-122
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• 2014

For accurate laboratory evaluations of soil deposits, it is essential that the samples are undisturbed. An artificial ground-freezing system is the one of the most effective methods for obtaining undisturbed samples from sand deposits. The objective of this study is to estimate the shear strengths and the characteristics of elastic waves of frozen-thawed and unfrozen specimens through the undrained triaxial compression test. For the experiments, Jumunjin standard sands are used to prepare frozen and unfrozen specimens with similar relative densities (60% and 80%). The water pluviation method is used to simulate the fully saturated condition under the groundwater table. When thawing the frozen specimens, the temperature is measured every minute. After the specimens are completely thawed, undrained triaxial compression tests are conducted using the same procedures as for the unfrozen specimens. During the triaxial tests (saturation, consolidation, and shear phase), compressional and shear waves are measured. The results show that the freeze-thaw process has minor effects on the peak deviatoric stress and shear strength values, and that the process does not affect the internal friction angle. The compressional wave velocity increases with increasing B-value to 1800 m/s in the saturation phase, but tends to remain constant in the process of consolidation and shearing. The shear wave velocity decreases with increasing B-value in the process of saturation, but changes velocity in accordance with the change in effective stress in the processes of consolidation and shearing. The compressional wave velocity has similar values regardless of the freeze-thaw process, but values of shear wave velocity are slighly lower in frozen-thawed specimens than in unfrozen specimens. This study is a preliminary experiment for estimating the shear strength and characteristics of elastic wave velocity in undisturbed frozen specimens that have been obtained using the artificial ground-freezing method.

• Journal of Korean Tunnelling and Underground Space Association
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• v.17 no.6
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• pp.695-702
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• 2015

Recently, the improvement of mechanical and theoretical issues in geo-centrifuge test enhances the applicability and accuracy of the test. Geo-centrifuge test is appropriate to simulate the behaviors of underground structures like tunnel, since tunnel interacts with the soil and/or rock around it and the test can embody the in-situ stress conditions effectively. In this study, the seismic behaviors of twin tunnel were analyzed based on geo-centrifuge test. Flexible segment to mitigate seismic acceleration were implemented in the model with thin and thick thickness. Based on the test results, it was found that flexible segment can decrease the peak acceleration generally, however, thin flexible segment was not able to reduce peak acceleration in short-period seismic wave. Thick flexible segment was more effective in case of high bedrock acceleration condition. Additionally, 3-dimensional numerical analysis was performed to verify the characteristics of seismic behavior and the effect of flexible segment. Consequently, the numerical analysis result showed good agreement with the test result.

• Journal of the Korean Geotechnical Society
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• v.16 no.3
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• pp.131-144
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• 2000

본 연구는 비검측공 시험법으로서 널리 사용되고 있는 충격반향기법(impact-echo test)의 적용서을 수치해석 및 실내 실험을 통하여 분석해 보았다. 즉, 결함이 없는 말뚝과 결함이 있는 말뚝에 대하여 1차원 및 2차원 축대칭 유한요소해석을 실시하였으며, 또한 모노캐스트라고 하는 일종의 플라스틱 원형 봉의 말뚝에 축대창 공극, 비축대칭 공극, 병목부 및 단면 확대부와 같은 결함을 각각 크기와 깊이를 변화시켜 제작한 후 공기 중과 지반 내부에서 충격반향기법 실험을 수행하였다. 실험결과 충격반향기법의 말뚝 결함 탐지능력은 수치해석에서 얻은 결과와 함께 결함의 크기와 위치에 영향을 받는 것으로 나타났으며 결함의 크기가 커질수록 탐지의 정확도가 향상되는 것을 알 수 있었다. 결함의 상대면적이 말뚝 단면적의 50% 이상이면 충격반향기법에 의하여 결함의 위치를 파악할 수 있는 것으로 나타났으며, 공기 중 보다 지반에 근입 된 말뚝의 경우가 더욱 명확한 신호를 제공해주는 것으로 나타났다. 그리고 시간영역의 신호가 주파수 영역의 신호 보다 말뚝의 결함 크기에 더 민감히 반응하므로 주파수 영역에서 탐지할 수 없는 작은 크기의 결함을 시간 영역에서는 탐지할 수 있는 것으로 나타났다.

• Earthquakes and Structures
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• v.17 no.3
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• pp.245-256
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• 2019

An experimental study was conducted to evaluate the dynamic behavior of a free-standing frame equipped with a movable base system using cast iron and mortar as the bearing materials. The preliminary friction test indicated that a graphite layer developed on the interface and exhibited stable friction behavior. The friction coefficient ranged from 0.33 to 0.36 when the applied normal compression stress ranged from 2.6 to 5.2 MPa. The effect of the variation of normal compression stress would be small. Shaking table tests on the free-standing frame showed that rock, slide, and rock-slide responses occurred. The cumulative slide distance reached 381 mm under JMA Kobe wave excitation; however, only a few cyclic slides occurred at the same locations along the moving track. Most surfaces sustained single slides. Similar results can be observed in other shaking conditions. The insufficient cyclic sliding and significant rocking resulted in a few graphite layers on the mortar surfaces. Friction coefficients were generally similar to those obtained in the preliminary friction tests; however, the values fluctuated when the rocking became significant. The collisions due to rocking caused strong horizontal acceleration responses and resulted in high friction coefficient. In addition, the strong horizontal acceleration responses caused by the collisions made the freestanding specimen unable to reduce the input horizontal acceleration notably, even when slippage occurred. Compared with the counterpart fixed-base specimen, the specimen equipped with the iron-mortar base could reduce the horizontal acceleration amplification response and the structural deformation, whereas the vertical acceleration response was doubled due to collisions from rocking.

• Journal of the Korean Geotechnical Society
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• v.25 no.6
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• pp.17-29
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• 2009

There has been an increase in the investigation of deep sea sediments with a consequent increase in the amount of energy required to undertake these investigations. The geotechnical characteristics of Ulleung Basin sediment are explored by using depressurized specimens following methane production tests carried out on pressured core samples obtained at 2,100 m water depth and 110 m below sea floor. Geotechnical index tests, X-ray diffraction, and scanning electron microscope are conducted to identify the geotechnical index parameters, clay mineralogy, chemical composition, and microstructure of the sediments. Compressibility, and elastic and electromagnetic wave parameters are investigated for two samples by using a multi sensing instrumented oedometer cell. The strength chatracteristics are obtained by the direct shear tests. The dominant clay minerals are mostly kaolinite, illite, chlorite, and calcite. The SEM shows a well-developed flocculated structure of the microfossil. Void ratio, electrical resistivity, real permittivity, conductivity, and shear wave velocity show bi-linear behavior with the effective vertical stress: as the vertical effective stress increases. The friction angle obtained by the direct shear test is about $21^{\circ}$, which is similar to the value observed in the Ulleung Basin sediments. This study shows that the understanding of the behavior acting on the diatomaceous marine sediment is important because it often maintains the useful energy resources such as gas hydrate and so will be the new engineering field in the next generation.

• Journal of the Korean Geotechnical Society
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• v.26 no.2
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• pp.5-14
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• 2010

In this study, cyclic triaxial tests were performed under anisotropically consolidated condition by using irregular earthquake loading to consider in-situ condition and seismic wave. Jumunjin sand with a relative density 50 percent was used in the tests. The consolidation pressure ratio (K) was changed from 0.5 to 1.0. The Ofunato and Hachinohe wave were applied as irregular earthquake loadings and liquefaction resistance strengths of each specimen were estimated from the excess pore water pressure (EPWP) ratio. As a results of the cyclic triaxial tests, EPWP ratio increased with increased K value. It shows that isotropically consolidated sand is more susceptible to liquefaction than anisotropically consolidated sand under equal confining pressure and dynamic loadings. From the test results, the relationship between K and EPWP ratio normalized by effective confining pressure and deviator stress was proposed. And a new factor which corrects the liquefaction resistance strength for the in-situ stress condition is proposed.