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Experimental Study on the Behavior of Unsaturated Compacted Silt Under Triaxial Compression
Oka, Fusao,Kodaka, Takeshi,Suzuki, Hirotaka,Kim, Young Seok,Nishimatsu, Norisuke,Kimoto, Sayuri Elsevier 2010 Soils and foundations Vol.50 No.1
<P><B>ABSTRACT</B></P> <P>Most of the experimental investigations conducted on unsaturated soil have been performed under a constant air pressure. Changes in air pressure during deformation are in some cases important in practice. For example, in order to explain the stability problems of embankments during earthquakes and seepage flow, and grounds containing gas associated with the dissociation of methane hydrates, it is necessary to consider the interaction between the soil and the pore fluids. In the present study, we carried out fully undrained tests as well as drained tests, namely, constant water and constant air shearing tests. We performed the fully undrained tests using an air-controlled valve to measure the pore air pressure. For the stress variables of the unsaturated soil, skeleton stress values were used to describe the experimental results. From triaxial compression tests on silty soil, we found that the initial suction, the confining pressure, and the strain rate of unsaturated soil strongly influence the stress-strain behavior of unsaturated silt.</P>
Elasto-viscoplastic modeling of the consolidation of Sri Lankan peaty clay
Karunawardena, Asiri,Oka, Fusao,Kimoto, Sayuri Techno-Press 2011 Geomechanics & engineering Vol.3 No.3
The consolidation behavior of Sri Lankan peaty clay is analyzed using an elasto-viscoplastic model. The model can describe the secondary compression behavior as a continuous process and it can also account for the effect of structural degradation on the consolidation analysis. The analysis takes into account all the main features involved in the process of peat consolidation, namely, finite strain, variable permeability, and the secondary compression. The material parameters required for the analysis and the procedures to evaluate them, using both standard laboratory and field tests, are explained. Initially, the model performance is assessed by comparing the predicted and the observed peat consolidation behavior under laboratory conditions. The results indicate that the model is capable of predicting the observed creep settlements and the effect of layer thickness on the settlement analysis of peaty clay. Then, the model is applied to predict the consolidation behavior of peaty clay under different field conditions. In this context, firstly, the one-dimensional field consolidation of peaty clay, brought about by the construction of compacted earth fill, is predicted. Then, the two-dimensional peat foundation response upon embankment loading is simulated. A good agreement is seen in the comparison of the predicted results with the field observations.
Seismic Analysis of Multi Layered Ground Considering Damping Characteristics of Cohesive soil
( Yong-seong Kim ),( Fusao Oka ) 한국농공학회 2007 한국농공학회 학술대회초록집 Vol.2007 No.-
A Cyclic viscoelastic-viscoplastic constitutive model for clay is incorporated into an effective stress based seismic analysis to describe damping characteristics of clay layer to sand layer during earthquake. The seismic response against fore-shocks, main shock as well as after-shocks of 1995 Hyogoken Nambu Earthquake is analyzed in the present study. Acceleration response in both clay layer and just upper liquefiable sand layer are damped due to viscous effect of cohesive soil. In the case of main shock and the following after-shock that occurred within less than 9 days after main event, acceleration responses near at ground surface are damped due to the developed excess pore water pressure, while that at ground surface are amplified in other cases. Using the viscoelastic-viscoplastic model for clay, time history of acceleration response in upper liquefiable sand can be well calculated, in particular in the range of micro-tremor process after main seismic motion.
이충원(Lee Chung Won),히고 요스케(Higo Yosuke),오카 후사오(Oka Fusao) 한국방재학회 2017 한국방재학회논문집 Vol.17 No.2
본 연구에서는 동적 원심모형실험을 이용하여 도로성토의 지진시 안정성에 대한 배수공의 효과를 평가하였다. 이를 위하여 지진을 모사한 동적하중 작용시의 변위, 간극수압 및 가속도를 측정하였으며, 그 결과에 근거하여 배수공이 설치된 불포화 도로성토의 동적거동을 검토하였다. 또한, 화상해석을 통하여 본 도로성토 내의 변형율 분포를 확인하였다. 실험 결과, 설계기준을 만족시키는 최소한의 배수공을 설치한 도로성토에서는 침투수량의 저감이 확인되었으며, 그와 같은 경향은 특히 비탈면 선단부에서 현저하게 나타났다. 이는 도로성토의 골격응력 및 강성의 감소를 방지하며, 이에 따라 동적하중 작용시의 변위, 변형율, 간극수압의 증가량 및 가속도 응답이 작게 평가되었다. 실제 도로성토의 시공시에는 도로성토로의 침투를 충분히 방지할 수 있도록 배수공을 비탈면 선단부에서 천단 저부까지 설치하여야 할 것이다. In this study, the effect of the drain for the dynamic stability of the road embankment has been investigated through the centrifugal model tests. Based on the measurement of displacement, the pore water pressure and the acceleration during dynamic loading, dynamic behavior of the unsaturated road embankments with drain has been examined, and an image analysis has revealed the distributions of strains in this road embankment. From the test results, it is clarified that the drain reduces much of the infiltration of water into the embankment, especially at the toe, and prevents from decrease of skeleton stress and stiffness of embankment. Thus, in the dynamic loading process, the displacements, the strain levels, the increase in pore water pressure and the acceleration responses of the case with drain are much less than those of the case without drain, although the drain used in the present study is minimum within the design concept. In the case of real road-embankment construction, the drain should be placed from the toe to the bottom of the crest enough to prevent from the infiltration of water into the embankment.
김용성 ( Kim Yong-seong ),이달원 ( Lee Dal-won ),오카후사오 ( Oka Fusao ),고다카다케시 ( Kodaka Takeshi ) 한국농공학회 2003 한국농공학회 학술대회초록집 Vol.2003 No.-
In order to apply a cyclic elasto-plastic and a viscoelastic-viscoplastic constitutive model to actual multi-layered ground conditions during large earthquake, numerica simulations were performed by a liquefaction analysis in the present study. From the liquefaction analysis, it was verified that the models can give a good description of the damping characteristics and liquefaction phenomena of ground accurately during large event which induces plastic deformation in large strain range.