論文

公開件数: 134 件
No. 掲載種別 単著・共著区分 タイトル 著者 誌名 出版者 巻号頁 出版日 ISSN DOI URL 概要
1 研究論文(学術雑誌)

Numerical modelling of internal erosion during hydrate dissociation based on multiphase mixture theory
T. Akaki,S. Kimoto
Int J Numer Anal Methods Geomech.

44, 327-350
2020

10.1002/nag.3023


2 研究論文(学術雑誌)

An international code comparison study on coupled thermal, hydrologic and geomechanical processes of natural gas hydrate-bearing sediments
M.D. White,T.J. Kneafsey,Y. Seol,W.F. Waite,S. Uchida,J.S. Lin,E.M. Myshakin,X. Gai,S. Gupta,M.T. Reagan,A.F. Queiruga,S. Kimoto,R.C. Baker,R. Boswell,J. Ciferno,T. Collett,J. Choi,S. Dai,M. De La Fuente,P. Fu,T. Fujii,C.G. Intihar,J. Jang,X. Ju,J. Kang,J.H. Kim,J.T. Kim,S.J. Kim,C. Koh,Y. Konno,K. Kumagai,J.Y. Lee,W.S. Lee,L. Lei,F. Liu,H. Luo,G.J. Moridis,J. Morris,M. Nole,S. Otsuki,M. Sanchez,S. Shang,C. Shin,H.S. Shin,K. Soga,X. Sun,S. Suzuki,N. Tenma,T. Xu,K. Yamamoto,J. Yoneda,C.M. Yonkofski,H.C. Yoon,K. You,Y. Yuan,L. Zerpa,M. Zyrianova
Marine and Petroleum Geology
Elsevier BV
120, 104566-104566
2020/10
0264-8172
10.1016/j.marpetgeo.2020.104566


3 研究論文(学術雑誌)

Formulation of a constitutive model for three-phase porous materials and the skeleton stress
Fusao Oka,Sayuri Kimoto
Soils and Foundations
Elsevier BV
60/ 4, 1001-1010
2020/08
0038-0806
10.1016/j.sandf.2020.05.003


4 研究論文(国際会議プロシーディングス)

Numerical simulation of seepage failure by upward flow considering internal erosion
Kimoto, S,Akaki, T,Kodama, H
Japanese Geotechnical Society Special Publication

7/ 2, 654-659
2019

10.3208/jgssp.v07.100


5 研究論文(学術雑誌)

CO2ハイドレート含有砂のひずみ速度依存性と弾粘塑性構成式の適用
岩井裕正,小西陽太,木元小百合
土木学会論文集C(地圏工学)

75/ 3, 273-287
2019

10.2208/jscejge.75.273


6 研究論文(学術雑誌)

A computational model for dynamic strain localization in unsaturated elasto-viscoplastic soils
Oka, F,Shahbodagh Khan,Kimoto, S
Int J Numer Anal Methods Geomech.

43, 138-165
2019

10.1002/nag.2857


7 研究論文(国際会議プロシーディングス)

Undrained triaxial creep test of carbon dioxide hydrate-bearing sand
Yoshimoto, M. and Kimoto, S.
AGU Fall Meeting 2019, San Francisco, USA, 9-13 December 2019 (Abstract)


2019/12




8 研究論文(国際会議プロシーディングス)

Numerical Analysis of Methane Hydrate Dissociation considering Sand Production based on Multiphase Mixture Theory
Kimoto, S., Takubo, G., Akaki, T.
AGU Fall Meeting 2019, San Francisco, USA, 9-13 December 2019 (Poster)


2019/12




9 研究論文(学術雑誌)

CO2ハイドレート含有砂試料のクリープ挙動と弾粘塑性構成式による再現
木元小百合,吉本将基,田窪尭
材料

68/ 11, 812-817
2019/11




10 研究論文(国際会議プロシーディングス)

3Dynamic analysis of partially saturated river embankment considering liquefaction using a multiphase coupled FEM analysis method
Kimoto, S,Yui, H,Oka, F
Proc. 16th Asian Regional Conference on Soil Mechanics and Geotechnical Engineeirng

JGS-008
2019/10




11 研究論文(学術雑誌)

Large Deformation and Failure Analysis of River Embankments Subjected to Seismic Loading
Babak Shahbodagh,Hamidreza Sadeghi,Sayuri Kimoto,Fusao Oka
Acta Geotechnica


2019/07

10.1007/s11440-019-00861-3


12 (MISC)総説・解説(学術雑誌)

南海トラフ巨大地震による大阪府域の液状化危険度
木元小百合
鑑定おおさか, 公益社団法人 大阪府不動産鑑定士協会

52, 2-7
2019/03




13 研究論文(学術雑誌)

Rate effect on the stress-strain relations of synthetic carbon dioxide hydrate-bearing sand and its dissociation test by thermal stimulation
Iwai, H,Konishi, Y,Saimyou, K,Kimoto, S,Oka, F
Soils and Foundations

58/ 5
2018

10.1016/j.sandf.2018.05.007


14 研究論文(学術雑誌)

Numerical Analysis of Damaged River Embankment during the 2011 Tohoku Earthquake Using a Multiphase-Coupled FEM Analysis Method
Chen, L,Kimoto, S
Geofluids


2018

10.1155/2018/5191647


15 研究論文(国際会議プロシーディングス)

Time dependent behavior of CO2 hydrate-bearing sediments and its modelling by an elasto-viscoplastic model
Kimoto, S,Konishi, Y,Iwai,H. Wu,Yuguang,Yoshimoto, M,Takubo, G
Int. Sym. on Energy Geotechnics, SEG-2018, 26-28 September, Lausanne, Switzerland, Digital Proceedings


2018/09




16 研究論文(学術雑誌)

A cyclic elastoplastic constitutive model and effect of non-associativity on the response of liquefiable sandy soils
Fusao Oka,Sayuri Kimoto
Acta Geotechnica
Springer Verlag
13/ 6, 1-15
2018/06/18
1861-1133
10.1007/s11440-018-0659-0


17 研究論文(国際会議プロシーディングス)

Effect of the non-associativity of plasticity model on the cyclic behavior of geomaterials
Fusao Oka,Sayuri Kimoto
Key Engineering Materials
Trans Tech Publications Ltd
725, 322-327
2017
1013-9826
10.4028/www.scientific.net/KEM.725.322

The present paper proposed a cyclic plasticity model with a non-associativity parameter, i.e., the model includes non-associative flow rule and associative one. In the present model, the non-associative parameter controls the non-associativity. The model is derived based on the non-linear kinematical hardening rule with two hardening parameters for both the volumetric and deviatoric strains. From the simulation by the present model, we have found the strong non-associativity leads to the large decrease in the mean effective stress, i.e. almost zero mean effective stress during the cyclic deformations under undrained conditions while the model with associated flow rule is not.
18 研究論文(国際会議プロシーディングス)

Undrained Triaxial Compression Tests on Artificial CO2-hydrate-bearing Sand Specimens
Hiromasa Iwai,Yota Konishi,Sayuri Kimoto
Energy Procedia
Elsevier Ltd
114, 3175-3184
2017
1876-6102
10.1016/j.egypro.2017.03.1446

Global warming has been recognized as one of the severe issues of global scale which should be addressed immediately. CO2 capture and storage technologies have been believed to be an expected countermeasure for reduction of the greenhouse gas released by plants and industrial activities. Recently, a new method for CO2 capture and storage has been proposed by some researchers, that is, CO2 sequestration in the form of CO2-hydrate under the seafloor. In this method, CO2 is injected into the sandy layer of seabed ground where the CO2-hydrate can exist stably. In order to achieve the safe and sustainable sequestration, it is essential to characterize the mechanical response such as stress-strain relationships or deformations of CO2-hydrate-bearing sandy sediments. In the present study, we have conducted a series of undrained triaxial compression tests on artificial CO2-hydrate-bearing sand specimens. From the experiment results, it is found that both stiffness and shear strength of CO2-hydrate-bearing sediments increase as increases in content of CO2-hydrates, and excess pore water pressure is reduced so that negative excess pore water pressure can be observed. The CO2-hydrates in the
19 研究論文(国際会議プロシーディングス)

Cyclic elastoplastic constitutive model for soils based on non-linear kinematical hardening rule with volumetric and deviatoric kinematical parameters
Fusao Oka,Sayuri Kimoto
Springer Series in Geomechanics and Geoengineering
Springer Verlag
191289, 81-87
2017
1866-8763
10.1007/978-3-319-56397-8_10

The aim of this paper is to present cyclic constitutive models for sandy and clayey soils based on the kinematical hardening rule with the strain-degradation, and the applicability of the models to the practically important problem such as the of the levee during earthquakes. For sandy soils, we have been developed a cyclic elasto-plastic model based on the kinematical hardening rule with two yield surfaces for the change of the stress ratio and the mean effective stress considering the degradation of the yield surface. From the simulation by the present model, we have found the strong non-associativity leads to the large decrease in the mean effective stress during the cyclic deformations under undrained conditions while the model with associated flow rule is not. This result is quite important because the mean effective stress becomes almost zero at the full liquefaction state. We have found that the model can well reproduce the cyclic behavior of soil.
20 研究論文(国際会議プロシーディングス)

A numerical model of internal erosion for multiphase geomaterials
Sayuri Kimoto,Toshifumi Akaki,Benjamin Loret,Fusao Oka
Springer Series in Geomechanics and Geoengineering
Springer Verlag
191289, 125-131
2017
1866-8763
10.1007/978-3-319-56397-8_16

Internal erosion is the detachment of fine soil particles due to seepage flow, with ensuing increasing porosity, and transport of these particles out of the soil mass. In the present study, firstly we have formulated the constitutive equations of the internal erosion, that is, the erosion criteria and the rate equation of the mass transfer. The driving force for erosion is assumed to be given by the interaction term, i.e. relative velocity between two phases in the equation of motions for the two-phase mixture. Then, field equations to simulate hydro-mechanical behavior due to the internal erosion were derived in the framework of multiphase mixture theory. In addition, laboratory erosion tests using gap-graded sandy soil are simulated by the proposed model and the validity are discussed with respect to the rate of eroded soil mass and the particle size distribution after the erosion test.
21 研究論文(学術雑誌)

The effective stress concept and the evaluation of changes in pore air pressure under jacketed isotropic compression tests for dry sand based on the 2-phase porous theory
Sayuri Kimoto,Fusao Oka,Yasuhiro Morimoto
INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS
WILEY
41/ 18, 1894-1907
2017/12
0363-9061
10.1002/nag.2705

The effective stress concept for solid-fluid 2-phase media was revisited in this work. In particular, the effects of the compressibility of both the pore fluid and the soil particles were studied under 3 different conditions, i.e., undrained, drained, and unjacketed conditions based on a Biot-type theory for 2-phase porous media. It was confirmed that Terzaghi effective stress holds at the moment when soil grains are assumed to be incompressible and when the compressibility of the pore fluid is small enough compared to that of the soil skeleton. Then, isotropic compression tests for dry sand under undrained conditions were conducted within the triaxial apparatus in which the changes in the pore air pressure could be measured. The ratio of the increment in the cell pressure to the increment in the pore air pressure, m, corresponds to the inverse of the B value by Bishop and was obtained during the step loading of the cell pressure. In addition, the m values were evaluated by comparing them with theoretically obtained values based on the solid-fluid 2-phase mixture theory. The experimental m values were close to the theoretical values, as they were in the range of approximately 40 to
22 研究論文(国際会議プロシーディングス)

Dynamic Analysis of River Embankment Considering Soil Improvement Using a Finite Deformation FE analysis Method
Kimoto, S,Matsuoka, H,Oka, F
15th IACMAG, 19-23 October 2017, Wuhan, China


2017/10




23 研究論文(国際会議プロシーディングス)

Numerical study on unfavorable influences of methane hydrate production on dynamic behavior of seabed sediments during an earthquake
Akaki, T,Kimoto, S
19th Int. Conf. on Soil Mechanics and Geotechnical Engineering, Seoul, Korea, Sep. 17-22


2017/09




24 研究論文(国際会議プロシーディングス)

Behavior of unsaturated sandy soil during triaxial compression tests under fully undrained conditions and its modelling
Kimoto, S,Ishikawa, R,Akaki, T
19th Int. Conf. on Soil Mechanics and Geotechnical Engineering, Seoul, Korea, Sep. 17-22


2017/09




25 研究論文(学術雑誌)

New Evaluation Method for Liquefaction of Ground Using Dynamic Liquefaction Analysis Method and Its Application (vol 16, 2017)
Fusao Oka,Hirokazu Yui,Sayuri Kimoto,Teppei Kinugawa
INTERNATIONAL JOURNAL OF GEOMECHANICS
ASCE-AMER SOC CIVIL ENGINEERS
17/ 3
2017/03
1532-3641
10.1061/(ASCE)GM.1943-5622.0000882


26 研究論文(学術雑誌)

Erratum for "new evaluation method for liquefaction of ground using dynamic liquefaction analysis method and its application" by fusao oka, hirokazu yui, sayuri kimoto, and teppei kinugawa [Int. J. Geomech, 08216004] DOI: 10.1061/(ASCE)GM.1943-5622.000055
Fusao Oka,Hirokazu Yui,Sayuri Kimoto,Teppei Kinugawa
International Journal of Geomechanics
American Society of Civil Engineers (ASCE)
17/ 3
2017/03/01
1532-3641
10.1061/(ASCE)GM.1943-5622.0000882

The following corrections should be made to the original paper: In Eq. (2), subscript "L" is missing for the safety factor for liquefaction. The corrected equation is PL = ∫ 20 0 F(10 - 0.5z)dz (2) F = (1 - FL)
FL &lt
1.0 = 0
FL ≥ 1.0.
27 研究論文(学術雑誌)

Chemo-thermo-mechanically coupled seismic analysis of methane hydrate-bearing sediments during a predicted Nankai Trough Earthquake
T. Akaki,S. Kimoto,F. Oka
INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS
WILEY-BLACKWELL
40/ 16, 2207-2237
2016/11
0363-9061
10.1002/nag.2527

In the present study, we have developed a numerical method which can simulate the dynamic behaviour of a seabed ground during gas production from methane hydrate-bearing sediments. The proposed method can describe the chemo-thermo-mechanical-seismic coupled behaviours, such as phase changes from hydrates to water and gas, temperature changes and ground deformation related to the flow of pore fluids during earthquakes. In the first part of the present study, the governing equations for the proposed method and its discretization are presented. Then, numerical analyses are performed for hydrate-bearing sediments in order to investigate the dynamic behaviour during gas production. The geological conditions and the material parameters are determined using the data of the seabed ground at Daini-Atsumi knoll, Eastern Nankai Trough, Japan, where the first offshore production test of methane hydrates was conducted. A predicted earthquake at the site is used in the analyses. Regarding the seismic response to the earthquake which occur during gas production process, the wave profiles of horizontal acceleration and horizontal velocity were not extensively affected by the gas production. Hydrat
28 研究論文(学術雑誌)

New Evaluation Method for Liquefaction of Ground Using Dynamic Liquefaction Analysis Method and Its Application
Fusao Oka,Hirokazu Yui,Sayuri Kimoto,Teppei Kinugawa
INTERNATIONAL JOURNAL OF GEOMECHANICS
ASCE-AMER SOC CIVIL ENGINEERS
16/ 5
2016/10
1532-3641
10.1061/(ASCE)GM.1943-5622.0000557

Nankai megathrust earthquakes are expected to occur with a return period of approximately 90-200 years along the fault of the Nankai Trough. Because they are interplate earthquakes, it is predicted that they will cause heavy liquefaction damage as a result of the long duration of their ground motion, especially around bay areas with soft deposits. In this study, a numerical analysis was performed for the ground in Osaka City with a hypothetical earthquake in the Nankai Trough region. In the numerical simulation of the dynamic behavior, a liquefaction analysis program was used. To quantify the degree of liquefaction at certain points, an index, named the liquefaction risk index (LRI), was defined and obtained by the weighted integration of the effective stress-decreasing ratio (ESDR) with respect to depth. Moreover, the LRI values were evaluated by data from the 1995 Hyogoken-Nanbu earthquake. It was found that the obtained LRI values were greater than 6.0 for the points at which heavy liquefaction damage had occurred. From the numerical results, the risk of liquefaction was evaluated with these LRI values.
29 研究論文(学術雑誌)

A large-scale excavation in soft Holocene deposit and its elasto-viscoplastic analysis
Fusao Oka,Naoki Takada,Kazuyuki Shimono,Yosuke Higo,Sayuri Kimoto
ACTA GEOTECHNICA
SPRINGER HEIDELBERG
11/ 3, 625-642
2016/06
1861-1125
10.1007/s11440-016-0445-9

In the present study, the large-scale excavation in the construction is numerically back-analyzed using a soil-water-coupled finite element method with an elasto-viscoplastic model which considers the strain-induced degradation. The measurements of the deformation have been performed during the construction of a new railway station in Osaka, Japan, in which a large and deep excavation has been successfully carried out using a special deep mixing type of soil improvement method with earth retaining walls through the thick Holocene Osaka Umeda clay deposit. A comparison between the numerical results and the measurements of the excavation at Osaka shows that the simulation method can reproduce the overall deformation of the soft ground and the earth retaining walls including the time-dependent behaviour during the excavation and a deep mixing soil improvement method as an additional technique for stability are effective.
30 研究論文(学術雑誌)

多相連成解析手法に基づく地盤の内部浸食の数値解析
赤木 俊文,青田 周平,木元 小百合,岡 二三生
土木学会論文集A2(応用力学)
公益社団法人 土木学会
72/ 2
2016/01
2185-4661
10.2208/jscejam.72.I_75


31 研究論文(学術雑誌)

化学-熱-力学連成動的解析手法を用いたガス生産過程におけるメタンハイドレート含有地盤の地震時挙動の検討
赤木 俊文,木元 小百合
計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science
日本計算工学会
21
2016/01




32 研究論文(国際会議プロシーディングス)

Development of a temperature and pressure controlled triaxial apparatus and dissociation tests of carbon dioxide hydrate containing soils
Hiromasa Iwai,Kazuki Saimyou,Sayuri Kimoto,Fusao Oka
15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, ARC 2015: New Innovations and Sustainability
Asian Regional Conference on Soil Mechanics and Geotechnical Engineering
518-521
2015

10.3208/jgssp.JPN-143

Methane hydrates are viewed as not only a new energy resource but also a trigger of marine geohazard such as large deformations and landslides of seabed ground. Many researcher have showed that methane hydrate containing sand specimens become stronger than that of without hydrates, however, few study have been performed to investigate the mechanical properties subjected to the hydrate dissociation. The main purpose of our study is to investigate behaviors of the deformation associated with dissociation of the hydrates. In this study, we prepared carbon dioxide hydrate containing specimens and conducted a series of dissociation tests using a heating method under undrained conditions. These experiments require the high pressure and the low temperature conditions to form the carbon dioxide, and we have developed a new triaxial apparatus. The dissociation results indicates that even a small percentage of hydrate saturation produced the large pore gas pressure and the sequential reduction of the effective confining pressure due to the hydrate dissociation, and extensive strain was observed.
33 研究論文(国際会議プロシーディングス)

Dynamic analysis of hydrate-bearing seabed sediments considering methane gas production induced by depressurization
T. Akaki,S. Kimoto,F. Oka
15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, ARC 2015: New Innovations and Sustainability
Asian Regional Conference on Soil Mechanics and Geotechnical Engineering
676-680
2015

10.3208/jgssp.JPN-134

Understanding the mechanical behavior of hydrate-bearing sediments during earthquakes is important for the safe and long term gas production from methane hydrates. Additionally, long term and widespread gas production may have effects on the mechanical properties of the seabed ground. In the present study we have analyzed the dynamic behavior of hydrate-bearing sediments considering gas production by the depressurization method. Firstly, we simulated gas production process using a chemo-thermo-mechanically coupled method for different levels of depressurizing pressure. Then, we conducted dynamic analyses by using the results of the production analyses as the initial conditions in order to consider the effects of gas production on dynamic behavior of the sediments. We have modeled the hydrate bearing-sediments as a multiphase mixture composed of gas, water, soil, and hydrates. For the constitutive model for soils, we used a cyclic elasto-viscoplastic model for clayey soils with nonlinear kinematic hardening. A predicted earthquake motion at Kumanonada along the Nankai trough was input. From the numerical results, we have found that the production of natural gas may affect the ground
34 研究論文(国際会議プロシーディングス)

Instability of dissociation process of methane hydrate bearing soil
S. Kimoto,H. Iwai,T. Akaki,F. Oka
Springer Series in Geomechanics and Geoengineering
Springer Verlag
none, 245-251
2015
1866-8763
10.1007/978-3-319-13506-9_35

Methane hydrates which exist in both the permafrost sediments and the deep seabed ground are viewed as a new energy resource. It is, however, well known that dissociation process of gas hydrate such as methane hydrate may lead to instability such as large deformation, uncontrollable gas production etc. In the present study, a linear instability analysis was performed in order to investigate which variable has a significant effect on the onset of the instability of methane hydrate bearing geo-materials subjected to methane hydrate dissociation. Stability analysis shows that the onset of the growing instability of the material system mainly depends on the hydrate dissociation rate and the strain hardening-softening parameters. In addition, the stability depends on the wave number of the fluctuation. For larger value of the wave number of the fluctuation, the magnitude of viscoplastic parameter affects the instability, as well as the hardening-softening parameter.
35 研究論文(国際会議プロシーディングス)

Dynamic analysis of strain localization in water-saturated elasto-viscoplastic material
F. Oka,B. Shahbodagh Khan,M. Mirjalili,S. Kimoto
Springer Series in Geomechanics and Geoengineering
Springer Verlag
none, 355-360
2015
1866-8763
10.1007/978-3-319-13506-9_51

Although strain localization has been studied for the last four decades, there are still many problems, in particular for the dynamic localization problem. In the present study, we have numerically analyzed a dynamic strain localization problem for the water-saturated elasto-viscoplastic constitutive material. For the material model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil skeleton are considered. In order to numerically simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a two-phase mixture using the updated Lagrangian method is adopted. The shear band development is discussed through the distributions of the viscoplastic shear strain in a material.
36 研究論文(国際会議プロシーディングス)

Dynamic behavior of hydrate-bearing sediments during earthquakes
S. Kimoto,T. Akaki,T. Kitano,H. Iwai,F. Oka
Computer Methods and Recent Advances in Geomechanics
CRC PRESS-TAYLOR & FRANCIS GROUP
1669-1674
2015



In the present study, we have analyzed the dynamic behavior of hydrate-bearing sediments considering the production of methane gas by the depressurization method. Firstly, we simulated the production process of methane hydrates by depressurization method by using a chemo-thermo-mechanically coupled method for different levels of depressurizing pressure. Then, we conducted dynamic analyses by using the results of the production analyses as the initial conditions in order to see the effects of the production of methane gas to the mechanical behavior of the sediments during earthquake. We have modeled the hydrate bearing-sediments as a multiphase mixture composed of gas, water, soil, and hydrates. For the constitutive model for soils, we used a cyclic elasto-viscoplastic model for clayey soils with nonlinear kinematic hardening. A predicted earthquake motion at Kumanonada along the Nankai trough was input. From the numerical simulations, we have found that the production of natural gas may affect the ground behavior during strong earthquakes.
37 研究論文(国際会議プロシーディングス)

Numerical evaluation of the deformation of earth retaining wall reinforced by soil buttress method during the excavation in soft soil
N. Takada,K. Simono,F. Oka,S. Kimoto,Y. Higo
Computer Methods and Recent Advances in Geomechanics
CRC PRESS-TAYLOR & FRANCIS GROUP
1037-1042
2015



Large scale excavation work has been done through the thick Holocene Osaka soft clay deposit in Osaka which is associated with the renewal of railway station. Soil buttress method using in the construction is an effective ground improvement method for excavation of soft soil. In the present study, using a water-soil coupled finite element method with elasto-viscoplastic model which considers degradation associated with structural changes, the effectiveness of the soil buttress was studied. From the numerical study, we have found that the soil buttress method is an effective method to reduce the deformation of the earth retaining wall as well as the surrounding ground during the excavation.
38 研究論文(国際会議プロシーディングス)

Numerical simulation of landslide due to excavation of soft rock using an elasto-viscoplastic water-soil coupled FEM
T. Takyu,S. Kimoto,F. Oka
Computer Methods and Recent Advances in Geomechanics
CRC PRESS-TAYLOR & FRANCIS GROUP
1095-1100
2015



In this study, we carried out numerical analysis of actual landslides in soft rocks induced by the excavation by water-soil coupled finite element method using an elasto-viscoplastic constitutive model. The model can express a strain softening characteristic and the time dependent behaviors such as creep of soft rock. After the landslide, the underground water drainage works were constructed, and the landslide has been stable as of today. Therefore we studied the effect of imposed lowering of the water table under the ground at a preliminary study. Numerical analyses show the underground water drainage methods are effective countermeasures for the landslide.
39 研究論文(国際会議プロシーディングス)

A numerical simulation of seismic behavior of highway embankments considering seepage flow
R. Kato,F. Oka,S. Kimoto
Computer Methods and Recent Advances in Geomechanics
CRC PRESS-TAYLOR & FRANCIS GROUP
755-760
2015



Recently, it is often observed that damage due to earthquake has been affected by the infiltration of water induced by heavy rains and typhoons. Therefore, it is necessary to consider the seepage flow for the dynamic deformation analysis. In the present study, a water-soil coupled elasto-plastic finite element analysis method incorporating unsaturated soil-water characteristics is adopted, and we analyzed the behavior of a highway embankment under different groundwater conditions during the 2007 Noto Hanto Earthquake. From the comparison between numerical and actual results, it can be concluded that the proposed method is applicable for the numerical modeling of embankment with various water level.
40 研究論文(国際会議プロシーディングス)

Dynamic analysis of river embankments during earthquakes using a finite deformation FE analysis method
H. Sadeghi,S. Kimoto,F. Oka,B. Shahbodagh
Computer Methods and Recent Advances in Geomechanics
CRC PRESS-TAYLOR & FRANCIS GROUP
637-642
2015



In this paper, the dynamic behavior and failure pattern of a river embankments lying over liquefiable soil analyzed using a three-phase coupled FEM finite deformation analysis considering liquefaction and cyclic elasto-plastic and elasto-viscoplastic constitutive models are discussed. The models have been subjected to both interplate and intraplate earthquakes, namely 1995 Kobe and 2011 Tohoku earthquakes. Results show that the modes and levels of damage are sensitive to both earthquake type and subsoil formation. While a short duration earthquake such as Kobe induces less damage, the case subjected to the 2011 Tohoku earthquake with long duration undergoes excessive settlements and more complex failure mode. It is observed that when part of the embankment body is liquefied, its deformation-failure pattern will be different from the common sliding failure modes assumed in the current slope stability analysis.
41 研究論文(学術雑誌)

Cyclic Elastoviscoplastic Constitutive Model for Clay Considering Nonlinear Kinematic Hardening Rules and Structural Degradation
Sayuri Kimoto,Babak Shahbodagh Khan,Mojtaba Mirjalili,Fusao Oka
INTERNATIONAL JOURNAL OF GEOMECHANICS
ASCE-AMER SOC CIVIL ENGINEERS
15/ 5
2015/10
1532-3641
10.1061/(ASCE)GM.1943-5622.0000327

A cyclic elastoviscoplastic constitutive model for clayey soils is proposed based on the nonlinear kinematic hardening rules and considering the structural degradation. The performance of the model is verified through the undrained triaxial test simulation of soft clay samples under cyclic and monotonic loading conditions and the cyclic compression test. The simulated results are compared with the experimental data through stress-strain relations and stress paths. The simulated results have shown a good agreement with the experimental data, which indicates the capability of the proposedmodel to reproduce the cyclic behavior of soft clayey soils. (C) 2014 American Society of Civil Engineers.
42 (MISC)総説・解説(学術雑誌)

液状化解析プログラムLIQCAを用いた地盤地震応答解析
岡二三生,渦岡良介,木元小百合,立石章,加藤亮輔,足立有史
地盤工学会誌

63/ 10, 12-15
2015/10




43 研究論文(学術雑誌)

Stability Analysis of Methane Hydrate-Bearing Soils Considering Dissociation
Hiromasa Iwai,Sayuri Kimoto,Toshifumi Akaki,Fusao Oka
ENERGIES
MDPI AG
8/ 6, 5381-5412
2015/06
1996-1073
10.3390/en8065381

It is well known that the methane hydrate dissociation process may lead to unstable behavior such as large ground deformations, uncontrollable gas production, etc. A linear instability analysis was performed in order to investigate which variables have a significant effect on the onset of the instability behavior of methane hydrate-bearing soils subjected to dissociation. In the analysis a simplified viscoplastic constitutive equation is used for the soil sediment. The stability analysis shows that the onset of instability of the material system mainly depends on the strain hardening-softening parameter, the degree of strain, and the permeability for water and gas. Then, we conducted a numerical analysis of gas hydrate-bearing soil considering hydrate dissociation in order to investigate the effect of the parameters on the system. The simulation method used in the present study can describe the chemo-thermo-mechanically coupled behaviors such as phase changes from hydrates to water and gas, temperature changes and ground deformation. From the numerical results, we found that basically the larger the permeability for water and gas is, the more stable the simulation results are. Thes
44 研究論文(学術雑誌)

Instability of Dissociation Process of Methane Hydrate Bearing Soil, Bifurcation and Degradation of Geomaterials in the New Millennium, Bifurcation and Degradation of Geomaterials in the New Millennium
Sayuri Kimoto,Hiromasa Iwai,Toshifumi Akaki,Fusao Oka
and Degradation of Geomaterials in the New Millennium, Proc. of the 10th International Workshop on Bifurcation and Degradation in Geomaterials, Hong Kong, May 28-30, 2014, Kam-Tim Chau and Jidong Zhao (Eds),
Springer

2015/05




45 研究論文(学術雑誌)

ガスハイドレート含有地盤材料への弾粘塑性構成式の適用
木元 小百合,岩井 裕正,最明 和樹,進藤 拓人,岡 二三生
材料
The Society of Materials Science, Japan
64/ 4, 317-322
2015/04/01
1880-7488
10.2472/jsms.64.317

Methane hydrates are ice-like materials composed of methane and water which naturally exist in the permafrost sediments and the deep seabed ground. They are presently viewed as a potential energy resource for the 21st century because an amount of methane gas is trapped in hydrate reservoirs around Nankai trough and Japan sea. Recently, the world's first offshore production test from methane hydrates bearing sediments by the de-pressurization method have been conducted off the coasts of Atsumi and Shima peninsula in Japan. In the present study, an elasto-viscoplastic model for soil is applied to the triaxial test results of the hydrate-bearing sandy soils as well as the pure hydrate samples. In the model, softening due to structural degradation and hydrate bonding effect are taken into consideration. Simulation results show the applicability of the model to the hydrate-bearing soil as well as the pure methane hydrate.
46 研究論文(学術雑誌)

Study of dynamic stability of unsaturated embankments with different water contents by centrifugal model tests
Yosuke Higo,Chung-Won Lee,Tatsuya Doi,Teppei Kinugawa,Makoto Kimura,Sayuri Kimoto,Fusao Oka
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
55/ 1, 112-126
2015/02
0038-0806
10.1016/j.sandf.2014.12.009

It has been pointed out that the damage to unsaturated embankments caused by earthquakes is attributed to the high water content brought about by the seepage of underground water and/or rainfall infiltration. It is important to study the effects of the water content on the dynamic stability and deformation mode of unsaturated embankments in order to develop a proper design scheme, including effective reinforcements, for preventing severe damage, This paper presents a series of dynamic centrifugal model tests with different water contents to investigate the effect of the water content on the deformation and failure behaviors of unsaturated embankments. By measuring the displacement, the pore water pressure and the acceleration during dynamic loading, as well as the initial suction level, the dynamic behavior of unsaturated embankments with an approximately optimum water content, a higher than optimum water content, and a lower than optimum water content, are discussed. In addition, an image analysis reveals the displacement field and the distribution of strain in the embankment, by which the defortnation mode of the embankment with the higher water content is clarified. It is found
47 (MISC)総説・解説(学術雑誌)

メタンハイドレート生産時の地盤変形シミュレーション
木元小百合
地盤工学会誌

63/ 2, 22-25
2015/02




48 研究論文(学術雑誌)

液状化解析プログラムLIQCAを用いた地盤地震応答解析 (特集テーマ 地盤地震応答解析の最前線)
岡 二三生,渦岡 良介,木元 小百合,立石 章,加藤 亮輔,足立 有史
地盤工学会誌 : 土と基礎
公益社団法人地盤工学会
63/ 10
2015/01
1882-7276



49 研究論文(学術雑誌)

メタンハイドレート生産時の地盤変形シミュレーション (特集テーマ 深海底地盤工学 : メタンハイドレート)
木元 小百合
地盤工学会誌 : 土と基礎
公益社団法人地盤工学会
63/ 2
2015/01
1882-7276



50 研究論文(学術雑誌)

メタンハイドレート産出過程に発生した地震に対する海底地盤の動的応答の数値解析
赤木 俊文,木元 小百合
計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science
日本計算工学会
20
2015/01




51 研究論文(学術雑誌)

メタンハイドレート生産時の地盤変形シミュレーション(<特集>深海底地盤工学-メタンハイドレート-)
木元 小百合
地盤工学会誌
社団法人地盤工学会
63/ 2
2015/01
1882-7276



52 研究論文(学術雑誌)

液状化解析プログラムLIQCAを用いた地盤地震応答解析(<特集>地盤地震応答解析の最前線)
岡 二三生,渦岡 良介,木元 小百合,立石 章,加藤 亮輔,足立 有史
地盤工学会誌
公益社団法人地盤工学会
63/ 10
2015/01
1882-7276



53 研究論文(学術雑誌)

不飽和砂質土の三軸圧縮および等方圧縮時の挙動と空気-水-土連成解析法によるシミュレーション
赤木 俊文,森本 恭弘,石川 椋,木元 小百合
第59回地盤工学シンポジウム, 2014年11月27-28日, 長野


2014/12




54 研究論文(学術雑誌)

CO2ハイドレート含有地盤模擬供試体の非排気-非排水条件における分解時変形挙動に関する研究
岩井 裕正,最明 和樹,木元 小百合,岡 二三生
第6回メタンハイドレート総合シンポジウム, CSMH-6 2014, 2014年12月3 -4日


2014/12




55 研究論文(学術雑誌)

メタンガス生産の影響を考慮したメタンハイドレート含有海底地盤の動的解析
赤木 俊文,木元 小百合,岡 二三生
第6回メタンハイドレート総合シンポジウム, CSMH-6 2014, 2014年12月3 -4日


2014/12




56 研究論文(学術雑誌)

南海トラフ巨大地震を想定した大阪市域における液状化解析
木元 小百合,由井 洋和,松岡 浩志,岡 二三生
GeoKansai Geo-Symposium 2014, 地下水盤環境・防災計測技術に関するシンポジウム, 平成26年11月


2014/11




57 研究論文(学術雑誌)

analysis and numerical simulation of the dissociation process of methane hydrate bearing soil
Hiromasa Iwai,Sayuri Kimoto,Toshifumi Akaki,Fusao Oka
Proc. of the 14th Int. Conf. of International Association for Computer Methods and Recent Advances in Geomechanics, Kyoto, Japan, 22-25 September
CRC Press

2014/09




58 研究論文(学術雑誌)

Dynamic analysis of strain localization in water-saturated clay using a cyclic elasto-viscoplastic model
B. Shahbodagh Khan,M. Mirjalili,S. Kimoto,F. Oka
INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS
WILEY-BLACKWELL
38/ 8, 771-793
2014/06
0363-9061
10.1002/nag.2221

A computational framework is presented for dynamic strain localization and deformation analyses of water-saturated clay by using a cyclic elasto-viscoplastic constitutive model. In the model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil particles are considered. In order to appropriately simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a two-phase mixture is derived in the updated Lagrangian framework. The shear band development is shown through the distributions of viscoplastic shear strain, the axial strain, the mean effective stress, and the pore water pressure in a normally consolidated clay specimen. From the local stress-strain relations, more brittleness is found inside the shear bands than outside of them. The effects of partially drained conditions and mesh-size dependency on the shear banding are also investigated. The effect of a partially drained boundary is found to be insignificant on the dynamic shear band propagation because of the rapid rate of applied loading and low permeability of the clay. Using the finer mesh results in slightly narrower shear
59 研究論文(国際会議プロシーディングス)

Multiphase coupled numerical simulation of the rainfall infiltration into unsaturated embankments
S. Kimoto,E. F. Garcia,F. Oka
Poromechanics V - Proceedings of the 5th Biot Conference on Poromechanics

2103-2112
2013

10.1061/9780784412992.248

In order to study the effect of the water infiltration due to the rise of the water level of the rivers on the river dikes, a series of two-dimensional numerical analyses of experiments are carried out using a multi-phase seepage-deformation coupled method for unsaturated soil. The mechanism of the surface deformation and the strain localization on these soil structures are discussed mainly with respect to the water permeability of the soils. Results obtained by the simulations show that the deformation of the embankments significantly depends on the water permeability of the soil and it is localized on the slope surface at the river sides. The larger the saturated water permeability of the soil, the velocity of the seepage flow and the deformation on the surface of the river embankments are larger. The method adopted here, can effectively be used to study the practical seepage deformation coupled problems on unsaturated soils. © 2013 American Society of Civil Engineers.
60 研究論文(学術雑誌)

Investigation of localized deformation in partially saturated sand under triaxial compression using microfocus X-ray CT with digital image correlation
Yosuke Higo,Fusao Oka,Tomohiro Sato,Yoshiki Matsushima,Sayuri Kimoto
Soils and Foundations
Japanese Geotechnical Society
53/ 2, 181-198
2013
0038-0806
10.1016/j.sandf.2013.02.001

In this paper, localized deformation in partially saturated sand was investigated quantitatively using microfocus X-ray computed tomography (CT) and an image analysis of the CT images. Triaxial compression tests on a partially saturated dense Toyoura sand specimen were carried out under a low confining pressure and under drained conditions for both air and water. The development of localized deformation was observed macroscopically using microfocus X-ray CT, and the displacement field over the entire specimen was quantified by an image analysis of the CT images with the digital image correlation (DIC) technique. The progressive development of shear bands is discussed with reference to these images. In addition, the region of localization was observed microscopically by partial CT scanning on a micron scale with high spatial resolution. Changes in the particulate structures are also discussed herein. The DIC image analysis of the partial CT images provided a microscopic displacement field and indicated that very fine localized shear deformation developed before the shear bands had become visible in the macroscopic investigation. © 2013 The Japanese Geotechnical Society. Production
61 研究論文(学術雑誌)

Investigation of localized deformation in partially saturated sand under triaxial compression using microfocus X-ray CT with digital image correlation
Yosuke Higo,Fusao Oka,Tomohiro Sato,Yoshiki Matsushima,Sayuri Kimoto
Soils and Foundations
Japanese Geotechnical Society
53/ 2, 181-198
2013
0038-0806
10.1016/j.sandf.2013.02.001

In this paper, localized deformation in partially saturated sand was investigated quantitatively using microfocus X-ray computed tomography (CT) and an image analysis of the CT images. Triaxial compression tests on a partially saturated dense Toyoura sand specimen were carried out under a low confining pressure and under drained conditions for both air and water. The development of localized deformation was observed macroscopically using microfocus X-ray CT, and the displacement field over the entire specimen was quantified by an image analysis of the CT images with the digital image correlation (DIC) technique. The progressive development of shear bands is discussed with reference to these images. In addition, the region of localization was observed microscopically by partial CT scanning on a micron scale with high spatial resolution. Changes in the particulate structures are also discussed herein. The DIC image analysis of the partial CT images provided a microscopic displacement field and indicated that very fine localized shear deformation developed before the shear bands had become visible in the macroscopic investigation. © 2013 The Japanese Geotechnical Society. Production
62 (MISC)研究発表要旨(全国大会,その他学術会議)

グラベルドレーン工法の有効性とマクロエレメントの検証のための3次元有効応力解析
立石章,岡二三生,渦岡良介,木元小百合,宇野浩樹
地盤工学研究発表会発表講演集(CD-ROM)

48th, ROMBUNNO.884
2013




63 研究論文(学術雑誌)

Simulation of long-term consolidation behavior of soft sensitive clay using an elasto-viscoplastic constitutive model
M. R. Karim,F. Oka,K. Krabbenhoft,S. Leroueil,S. Kimoto
International Journal for Numerical and Analytical Methods in Geomechanics
WILEY-BLACKWELL
37/ 16, 2801-2824
2013/11
0363-9061
10.1002/nag.2165

The phenomenon of excess pore water pressure increase or stagnation and continuing large ground deformation in soft sensitive clay following the completion of construction of embankment is simulated for a case study at Saint Alban, Quebec, Canada. The present model employs an updated Lagrangian finite element framework and is combined with an automatic time increment selection scheme. The simulation based on an elasto-viscoplastic constitutive model considers soil-structure degradation effect. It is shown that without consideration for the microstructural degradation effect, it is not possible to reproduce the field responses of soft sensitive clay even during the construction of the embankment. When the soil-structure degradation effect is considered, the present model can offer reasonably accurate prediction for the consolidation behavior of soft sensitive clay, including the so-called anomalous pore water pressure generation and continuing large deformation even after the end of construction, which has been posing numerous uncertainties on the long-term performance of earth structures. © 2013 John Wiley &amp
Sons, Ltd.
64 研究論文(学術雑誌)

河川堤防における電磁波・電磁法による内部構造探査事例
山田 茂治,岡 二三生,木元 小百合
基礎工

41/ 9
2013/09




65 研究論文(学術雑誌)

河川堤防における電磁波・電磁法による内部構造探査事例 (特集 最近の地盤探査とその応用)
山田 茂治,岡 二三生,木元 小百合
基礎工 = The foundation engineering & equipment, monthly : 土木・建築基礎工事と機材の専門誌
総合土木研究所
41/ 9
2013/01
0285-5356



66

Visualization of Strain Localization and Microstructures in Soils during Deformation Using Microfocus X-Ray CT
Y. Higo,F. Oka,S. Kimoto,T. Sanagawa,M. Sawada,T. Sato,Y. Matsushima
Advances in Computed Tomography for Geomaterials: GeoX 2010
John Wiley and Sons
43-51
2013/01/21

10.1002/9781118557723.ch5

It is well known that strain localization is an important issue for the onset of failure problems. In order to clarify the mechanism of failure, it is necessary to visualize the strain localization and microstructure changes in detail. The aim of this paper is to observe strain localization behavior and microstructures of soils during deformation process using a microfocus X-ray CT. Strain localization of unsaturated Toyoura sand specimen and Maruyamagawa clay specimen during compression tests have been observed and discussed. In addition, microstructures of sands have been visualized by partial CT scan. In the partial CT images, we can clearly visualize each soil particle distinguished from the others. © ISTE Ltd 2010.
67 研究論文(学術雑誌)

堤防内部構造調査における電磁波速度構造調査の適用について
山田 茂治,岡 二三生,木元 小百合,肥後 陽介
地盤の環境・計測シンポジウム2012論文集


2012/12




68 研究論文(学術雑誌)

Damage patterns of river embankments due to the 2011 off the Pacific Coast of Tohoku Earthquake and a numerical modeling of the deformation of river embankments with a clayey subsoil layer
F. Oka,P. Tsai,S. Kimoto,R. Kato
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
52/ 5, 890-909
2012/10
0038-0806
10.1016/j.sandf.2012.11.010

Due to the 2011 off the Pacific Coast of Tohoku Earthquake, which had a magnitude of 9.0, many soil-made infrastructures, such as river dikes, road embankments, railway foundations and coastal dikes, were damaged. The river dikes and their related structures were damaged at 2115 sites throughout the Tohoku and Kanto areas, including Iwate, Miyagi, Fukushima, Ibaraki and Saitama Prefectures, as well as the Tokyo Metropolitan District. In the first part of the present paper, the main patterns of the damaged river embankments are presented and reviewed based on the in situ research by the authors, MLIT (Ministry of Land, Infrastructure, Transport and Tourism) and JICE (Japan Institute of Construction Engineering). The main causes of the damage were (1) liquefaction of the foundation ground, (2) liquefaction of the soil in the river embankments due to the water-saturated region above the ground level, and (3) the long duration of the earthquake, the enormity of fault zone and the magnitude of the quake. In the second part of the paper, we analyze model river embankments on a foundation ground with various soil profiles, including a clayey soil layer and various ground water tables, usi
69 研究論文(学術雑誌)

Long-term consolidation analysis of a large-scale embankment construction on soft clay deposits using an elasto-viscoplastic model
Mojtaba Mirjalili,Sayuri Kimoto,Fusao Oka,Tatsuo Hattori
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
52/ 1, 18-37
2012/02
0038-0806
10.1016/j.sandf.2012.01.010

In this paper, the consolidation analysis of a large-scale embankment construction in Osaka City is presented, where a conventional levee with a height of about 8 m has been extended to a super levee with a total width of 215 m. The ground consists of alluvial sandy layers and soft clay deposits, which have been locally improved by several methods, including deep mixing beneath the conventional levee and the combination of sand drains and sand compaction piles under the extended back slope. A long-term consolidation analysis of this super levee construction is carried out using an elasto-viscoplastic constitutive model. The layered construction procedure is applied to properly simulate the construction sequence of the super levee. The effects of the structural degradation and the strain dependency of the shear modulus, as two aspects of destructuration in clay materials, are studied in terms of the consolidation behavior for the unimproved case. For the improved case, the analysis is implemented by including the ground-improved zones in the finite element simulation. The field observation data obtained during the preloading process, before the construction of the super levee, are e
70 研究論文(学術雑誌)

動的問題(土構造物)(1)(6.地盤と構造物,総括,<特集>第47回地盤工学会研究発表会)
木元 小百合
地盤工学会誌
社団法人地盤工学会
60/ 12
2012/01
1882-7276



71 研究論文(学術雑誌)

東北地方太平洋沖地震被害調査報告 —宮城県北部—
岡 二三生,吉田 信之,甲斐 誠士,飛田 哲男,肥後 陽介,鳥居 宣之,鏡原 聖史,中西 典明,木元 小百合,山川 優樹,東瀬 康孝,渦岡 良介,京谷 孝史
地盤工学ジャーナル

7/ 1
2012/01
1880-6341
10.3208/jgs.7.37
URL

72 研究論文(国際会議プロシーディングス)

A chemo-thermo-mechanically coupled behavior during gas hydrate dissociation and its numerical analysis
S. Kimoto,F. Oka,Y. Miki,T. Fukuda,H. Iwai
Springer Series in Geomechanics and Geoengineering
Springer Verlag
203129, 85-91
2011
1866-8763
10.1007/978-94-007-1421-2_11

Gas hydrates, especially methane hydrates are viewed as a potential energy resource since a large amount of methane gas is trapped mainly within ocean sediments and regions of permafrost. In the present study, gas production process by heating-depressurizing method was simulated. The simulation was conducted for the model with inclined seabed ground with hydrate bearing layer in order to investigate the mechanical behavior during dissociation. The method has been developed based on the chemo-thermo-mechanically coupled analysis, taking into account of the phase changes from solids to fluids, that is, water and gas, the flow of fluids, heat transfer, and the ground deformation (Kimoto et al. 2010). As for the constitutive model for hydrate-bearing sediments, an extended elasto-viscoplastic model for unsaturated soils considering the effect of hydrate bonding is used.
73 研究論文(国際会議プロシーディングス)

Instability of Unsaturated Soil During the Water Infiltration
F. Oka,E. Garcia,S. Kimoto
ADVANCES IN BIFURCATION AND DEGRADATION IN GEOMATERIALS
SPRINGER-VERLAG BERLIN
293-299
2011
1866-8755


It is known that the unstable behavior of unsaturated soil includes collape behavior which is due to the decrease of suction as well as the shear failure. In addition, we often meet the numerical instability for the simulation of unsaturated soil during the wetting process. In the present study, we have performed a onedimensional linear instability analysis for the air-water-soil coupled three-phase viscoplastic material. From the instability analysis of the viscoplastic unsaturated soil subjected to the water infiltration, we have found that the specific moisture capacity, the matric suction and the hardening parameter affect the instability. Then, we have performed numerical simulations of the behavior of unsaturated soil during the water infiltration using a multi-phase coupled elasto-viscoplastic finite element analysis method. The conditions under which the numerical instability occurs during the simulation agree well with the results by linear instability analysis.
74 研究論文(国際会議プロシーディングス)

Observation of Microstructural Changes and Strain Localization of Unsaturated Sands Using Microfocus X-ray CT
Y. Higo,F. Oka,S. Kimoto,T. Sanagawa,Y. Matsuhima
ADVANCES IN BIFURCATION AND DEGRADATION IN GEOMATERIALS
SPRINGER-VERLAG BERLIN
37-43
2011
1866-8755
10.1007/978-94-007-1421-2_5

It is known that unsaturated soil exhibits more brittle failure due to the collapse of the water meniscus, caused by shearing or the infiltration of water, than saturated soil. The aim of this paper is to observe the strain localization behavior and the microstructural changes in partially saturated soil during the deformation process using microfocus X-ray CT. The strain localization of fully saturated, partially saturated, and air-dried sand specimens during triaxial compression tests is observed and discussed. In addition, the microstructures in the shear bands of partially saturated specimen are discussed.
75 研究論文(国際会議プロシーディングス)

A Chemo-Thermo-Mechanically Coupled Analysis of Ground Deformation Induced by Methane Hydrate Dissociation
Sayuri Kimoto,Fusao Oka,Tomohiko Fushita
BIFURCATIONS, INSTABILITIES AND DEGRADATIONS IN GEOMATERIALS
SPRINGER-VERLAG BERLIN
145-165
2011
1866-8755


In the present paper, we have numerically analyzed the dissociation process of seabed ground and predicted the deformation of hydrate-bearing sediments. The simulation is conducted using the chemo-thermo-mechanically coupled model that takes into account the phase changes between hydrates and fluids during dissociation, the deformation behavior of the solid skeleton, and heat transfer simultaneously (Kimoto et al. 2007a). In addition, the dependency of the permeability coefficients for water and gas on hydrate saturation is introduced in the present analysis. From the analytical results, it has been found that ground deformation is induced by the dissipation and generation of water and gas and by a reduction of soil strength during the dissociation process.
76 研究論文(学術雑誌)

Elasto-viscoplastic modeling of the consolidation of Sri Lankan peaty clay
Asiri Karunawardena,Fusao Oka,Sayuri Kimoto
Geomechanics and Engineering
Techno Press
3/ 3, 233-254
2011
2092-6219
10.12989/gae.2011.3.3.233

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
77

Observation of compaction bands in diatomaceous mudstone by PTV and μX-CT
F. Oka,Y. Higo,S. Kimoto,K. Nakashima,H. Ohta,T. Sanagawa
14th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering


2011




78 研究論文(学術雑誌)

電磁波速度構造調査による地下水調査事例
山田 茂治,北川 善治,岡 二三生,木元 小百合,肥後 陽介
地盤の環境・計測シンポジウム2011論文集


2011/12




79 研究論文(学術雑誌)

MONOTONIC AND CYCLIC BEHAVIOR OF UNSATURATED SANDY SOIL UNDER DRAINED AND FULLY UNDRAINED CONDITIONS
Sayuri Kimoto,Fusao Oka,Junya Fukutani,Taichi Yabuki,Kousuke Nakashima
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
51/ 4, 663-681
2011/08
0038-0806
10.3208/sandf.51.663

Changes in air pressure during monotonic and cyclic loading are in some cases important for the behavior of unsaturated soil. For example, in order to investigate the stability of embankments and slope failure during earthquakes, it is necessary to consider the effect of the pore air or the pore gas pressure as well as the pore water pressure and the interaction between the soil and the pore fluids. In the present study, we carried out a series of monotonic and cyclic loading tests on sandy soil used for the improvement of river embankments. The effects of the initial suction, the confining pressure, and the degree of compaction under fully undrained conditions, namely, constant water and constant air shearing tests, as well as under drained conditions for both air and water, were studied. For the stress variables of the unsaturated soil, the skeleton stress was used to describe the experimental results and was defined as the difference between the total stress tensor and the average pore pressure of water and gas (Oka et al., 2010). From the monotonic and cyclic test results, we found that the stress-strain behavior of unsaturated sandy soil strongly depends on the initial suction
80 研究論文(学術雑誌)

Elasto-viscoplastic modeling of Osaka soft clay considering destructuration and its effect on the consolidation analysis of an embankment
M. Mirjalili,S. Kimoto,F. Oka,Y. Higo
Geomechanics and Geoengineering

6/ 2, 69-89
2011/06
1748-6025
10.1080/17486025.2010.531148

A numerical modeling of Osaka soft clay was carried out using an elasto-viscoplastic constitutive model. The effect of destructuration, demonstrated by the shrinkage of the yield and the overconsolidation boundary surfaces and the strain-dependent elastic shear modulus, were studied through a comparison of the simulations with the experimental results of undrained triaxial compression tests. Although consideration of the structural degradation in the modeling of soft soil behavior leads to a substantial improvement, in terms of strain softening and post-peak responses, the strain-dependent shear modulus was introduced to reproduce more precise behavior, particularly before the peak stress. In order to evaluate the effect of these two aspects in a boundary value problem, a two-dimensional consolidation analysis of an embankment construction on a soft clay layer was conducted for three different cases. The deformations and the excess pore pressure responses for each case were presented and discussed. The strain localization, the consequent large ground displacement, and the temporary increase in pore pressure during the consolidation were observed in the cases with structural degrada
81 研究論文(学術雑誌)

Numerical analysis of a one-dimensional infiltration problem in unsaturated soil by a seepage-deformation coupled method
E. Garcia,F. Oka,S. Kimoto
INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS
WILEY-BLACKWELL
35/ 5, 544-568
2011/04
0363-9061
10.1002/nag.908

A multiphase coupled elasto-viscoplastic finite element analysis formulation, based on the theory of porous media, is used to describe the rainfall infiltration process into a one-dimensional soil column. Using this framework, we have numerically analyzed the generation of pore water pressure and deformations when rainfall is applied to the soil. A parametric study, including rainfall intensity, soil-water characteristic curves, and permeability, is carried out to observe their influence on the changes in pore water pressure and volumetric strain. From the numerical results, it is shown that the generation of pore water pressure and volumetric strain is mainly controlled by material parameters alpha and n' that describe the soil-water characteristic curve. A comparison with the laboratory results shows that the proposed method can describe very well the characteristics observed during the experiments of one-dimensional water infiltration into a layered unsaturated soil column. Copyright (C) 2010 John Wiley & Sons, Ltd.
82 研究論文(学術雑誌)

An elasto-viscoplastic model for diatomaceous mudstone and numerical simulation of compaction bands
F. Oka,S. Kimoto,Y. Higo,H. Ohta,T. Sanagawa,T. Kodaka
INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS
JOHN WILEY & SONS LTD
35/ 2, 244-263
2011/02
0363-9061
10.1002/nag.987

It is well known that highly porous rocks under relatively high confining pressure succumb to volumetric compression with the emergence of the so-called compaction bands. These normally occur as perpendicular or very slightly inclined deformation bands with respect to the direction of the most compressive principal stress. An experimental study of diatomaceous mudstone, a highly structured and porous soft rock, was conducted to demonstrate the existence of compaction bands in laboratory tests. In these tests, the local strain field on the specimen&apos;s face is determined by means of image analysis. The main objective of this paper is the numerical simulation of compaction bands in diatomaceous mudstone. First, an elasto-viscoplastic model considering microstructural degradation is used to simulate the behavior of diatomaceous mudstone at the element test level. It has been found that such a model can accurately reproduce the stress strain and dilatancy responses of diatomaceous mudstone. Then, a numerical analysis of a series of triaxial compression tests under drained conditions was carried out using the elasto-viscoplastic model within the framework of Biot&apos;s theory for a
83 研究論文(学術雑誌)

STUDY OF STRAIN LOCALIZATION AND MICROSTRUCTURAL CHANGES IN PARTIALLY SATURATED SAND DURING TRIAXIAL TESTS USING MICROFOCUS X-RAY CT
Yosuke Higo,Fusao Oka,Sayuri Kimoto,Taisuke Sanagawa,Yoshiki Matsushima
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
51/ 1, 95-111
2011/02
0038-0806
10.3208/sandf.51.95

It is well known that strain localization and microstructural changes are important issues in the onset of failure problems. In particular, unsaturated soil exhibits more brittle failure due to the collapse of the water meniscus, caused by shearing or the infiltration of water, than saturated soil. The aim of this paper is to observe the strain localization behavior and the microstructural changes in partially saturated soil during the deformation process using microfocus Xray CT. The micro focus X-ray CT system employed in this study has a very high spatial resolution of 5 mu m, which is enough to visualize the sand particles and the other particles individually. In addition, X-ray CT scans can be performed under triaxial conditions. The strain localization of fully saturated, partially saturated, and air-dried sand specimens during triaxial compression tests is observed and discussed. The microstructure of unsaturated soil, consisting of soil particles, pore water, and pore air, is successfully observed in partial CT scans. Through a comparison of the microstructures in the shear bands and in the initial state, the microstructural changes are discussed.
84

Elasto-viscoplastic constitutive modeling of the swelling process of unsaturated clay
F. Oka,S. Kimoto,Y. Higo,H. Feng
UNSATURATED SOILS: THEORETICAL AND NUMERICAL ADVANCES IN UNSATURATED SOIL MECHANICS
CRC PRESS-TAYLOR & FRANCIS GROUP
507-514
2010



An elasto-viscoplastic model for unsaturated expansive soil is developed. An evolutional equation is adopted to describe the absorption of water into the interlayer of clay platelets. In addition, the internal compaction effect caused by swelling of clay unit is expressed with the expansion of the overconsolidation boundary surface and the static yield surface. Based on the model, a one-dimensional finite element analysis is conducted to study the development of swelling pressure. Comparing the experimental results and the simulated results, it is found that the proposed model can reproduce the effects of dry density and the initial water content on the swelling behavior.
85 研究論文(学術雑誌)

Instability analysis and simulation of water infiltration into an unsaturated elasto-viscoplastic material
E. Garcia,F. Oka,S. Kimoto
INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES
PERGAMON-ELSEVIER SCIENCE LTD
47/ 25-26, 3519-3536
2010/12
0020-7683
10.1016/j.ijsolstr.2010.08.027

A linear instability analysis was performed in order to investigate which variables have a significant effect on the onset of the instability of an unsaturated viscoplastic material subjected to water infiltration. It was found that the onset of the growing instability of the material system mainly depends on the specific moisture capacity, the suction, and the hardening parameter. Then, in order to simulate the water infiltration process of a one-dimensional unsaturated soil column, a multiphase coupled elasto-viscoplastic finite element analysis was performed based on the theory of porous media. The results of the numerical simulations are discussed with respect to the effect of the specific moisture capacity and the initial suction on the development of volumetric strain. We found that rapid transitions from unsaturated to saturated states and higher levels of initial suction lead to the contractive behavior of the material and instability. The instability detected by the numerical results is consistent with the theoretical results obtained through the linear instability analysis. (C) 2010 Elsevier Ltd. All rights reserved.
86 研究論文(学術雑誌)

A COUPLED MPM-FDM ANALYSIS METHOD FOR MULTI-PHASE ELASTO-PLASTIC SOILS
Yosuke Higo,Fusao Oka,Sayuri Kimoto,Yuichi Morinaka,Yusuke Goto,Zhen Chen
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
50/ 4, 515-532
2010/08
0038-0806
10.3208/sandf.50.515

The Material Point Method (MPM), as proposed by Sulsky et al. (1994), has been developed to simulate large deformations and failure evolution involving different material phases in a single computational domain. A continuum body is divided into a finite number of subregions represented by Lagrangian material points, while the governing equations are formulated and solved with the Eulerian grid. Since this grid can be chosen arbitrarily, mesh tangling does not appear in the MPM. To design a simple but robust spatial discretization procedure, the MPM is coupled with the finite difference method (FDM) in the present study for simulating fully and partially saturated elasto-plastic soil responses based on the simplified three-phase method. Governing equations for the soil skeleton and the pore fluid are discretized by the MPM; and FDM, respectively. Soil-water coupled analyses for fully saturated soils and seepage-deformation coupled analyses for unsaturated soils are performed, and the potential of the proposed method is demonstrated via numerical examples.
87 研究論文(学術雑誌)

A SEEPAGE-DEFORMATION COUPLED ANALYSIS OF AN UNSATURATED RIVER EMBANKMENT USING A MULTIPHASE ELASTO-VISCOPLASTIC THEORY
Fusao Oka,Sayuri Kimoto,Naoaki Takada,Hitoshi Gotoh,Yosuke Higo
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
50/ 4, 483-494
2010/08
0038-0806
10.3208/sandf.50.483

A multiphase deformation analysis of a river embankment was carried out using an air-soil-water coupled finite element method capable of considering unsaturated seepage flow. A numerical model for unsaturated soil was constructed based on the mixture theory and an elasto-viscoplastic constitutive model. The theory used in the analysis is a generalization of Biot's two-phase mixture theory for saturated soil. An air-soil-water coupled finite element method was developed using the governing equations for three-phase soil based on the nonlinear finite deformation theory, i.e., the updated Lagrangian method. Two-dimensional numerical analyses of the river embankment under seepage conditions were conducted, and the deformation associated with the seepage flow was studied. We have found that the occurrence of large deformations corresponds to the large values of the hydraulic gradients at the toe of the embankment, and that the overflow of river water makes the embankment more unstable. It has been confirmed that seepage-deformation coupled three-phase behavior can be simulated well with the proposed method.
88 研究論文(学術雑誌)

A chemo–thermo–mechanically coupled analysis of ground deformation induced by gas hydrate dissociation
Kimoto, S,Oka, F,Fushita, T
International Journal of Mechanical Sciences
PERGAMON-ELSEVIER SCIENCE LTD
52/ 2, 365-376
2010/02
0020-7403
10.1016/j.ijmecsci.2009.10.008

We present a numerical analysis of gas hydrate dissociation in hydrate-bearing sediments in the seabed ground. Behavior of multiphase materials has been described within the framework of a macroscopic continuum approach through the use of the theory of porous media. The proposed simulation method has been developed based on chemo-thermo-mechanically coupled analysis, taking into account phase changes from solids to fluids, that is, water and gas, flow of water and gas, heat transfer, and ground deformation. From the numerical results, it has been found that ground deformation is induced by generation and dissipation of water and gas, and by reduction of soil strength due to the loss of hydrates. (C) 2009 Elsevier Ltd. All rights reserved.
89 研究論文(学術雑誌)

EXPERIMENTAL STUDY ON THE BEHAVIOR OF UNSATURATED COMPACTED SILT UNDER TRIAXIAL COMPRESSION
Fusao Oka,Takeshi Kodaka,Hirotaka Suzuki,Young Seok Kim,Norisuke Nishimatsu,Sayuri Kimoto
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
50/ 1, 27-44
2010/02
0038-0806
10.3208/sandf.50.27

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.
90 研究論文(国際会議プロシーディングス)

A seepage-deformation coupled analysis method for unsaturated river embankments
R. Kato,S. Sunami,F. Oka,S. Kimoto,T. Kodaka
PREDICTION AND SIMULATION METHODS FOR GEOHAZARD MITIGATION
CRC PRESS-TAYLOR & FRANCIS GROUP
401-407
2009



In the present paper, a water-soil coupled elasto-plastic finite element analysis method is proposed for unsaturated river embankments by incorporating unsaturated soil-water characteristics. The simplified three-phase method is developed based on the multi-phase porous theory by assuming that the compressibility of pore air is very high. Using the proposed method, the behavior of a river embankment, consisting of sandy soil in different conditions, is numerically analyzed while the river water level is high. From the numerical results, the seepage-deformation characteristics of the river embankment and the effects of permeability and the initial saturation are clarified.
91 研究論文(国際会議プロシーディングス)

Effect of Poisson's Ratio on the Dynamic Instability of Water-Saturated Elasto-Plastic Soil
F. Oka,A. P. Heitor,N. Kita,S. Kimoto
PORO-MECHANICS IV
DESTECH PUBLICATIONS, INC
1103-+
2009



One of the authors has developed a liquefaction analysis method based on a Biot type of two-phase mixture theory and a cyclic elasto-plastic constitutive model (Oka et a]. 1999). When the authors applied the liquefaction method to a soil ground with a smaller Poisson's ratio, instability was experienced during the numerical analysis. In the present study, we have investigated the instability of a water-saturated elasto-plastic material based on a linear perturbation analysis. The stability analysis shows that a material with a smaller Poisson's ratio may exhibit instability, i.e., a growth in small fluctuations. In addition, we have shown some numerical examples of the dynamic response of a water-saturated ground with different Poisson's ratios.
92 研究論文(国際会議プロシーディングス)

An MPM-FDM coupled simulation method for analyzing partially-saturated soil responses
Y. Higo,S. Kimoto,F. Oka,Y. Morinaka,Y. Goto,Z. Chen
PREDICTION AND SIMULATION METHODS FOR GEOHAZARD MITIGATION
CRC PRESS-TAYLOR & FRANCIS GROUP
219-+
2009



The Material Point Method (MPM), as proposed by Sulsky et al. (1994), has been used to simulate large deformations of materials. A continuum body is divided into a finite number of subregions represented by Lagrangian material points, while the governing equations are formulated and solved with the Eulerian grid. Since this grid can be chosen arbitrarily, mesh tangling does not appear in the MPM. In the present study, the MPM is coupled with the finite difference method (FDM) for simulating unsaturated soil responses based on the simplified three-phase method. The soil skeleton and the pore fluid are discretized by the MPM and FDM, respectively. Seepage and deformation coupled analyses for unsaturated soils are performed, and the potential of the proposed method is demonstrated via example problems.
93

Numerical modeling of swelling and the osmotic consolidation of bentonite
F. Oka,H. Feng,S. Kimoto,A.J. Tinet,Y. Higo
Computational Geomechanics, COMGEO I - Proceedings of the 1st International Symposium on Computational Geomechanics

532-543
2009




94

A coupled material point/finite difference scheme for model-based simulation of fluid-saturated porous media
Y. Higo,Y. Goto,S. Kimoto,F. Oka,Z. Chen
Computational Geomechanics, COMGEO I - Proceedings of the 1st International Symposium on Computational Geomechanics

447-456
2009




95

A multiphase elasto-viscoplastic analysis of an unsaturated river embankment associated with seepage flow
F. Oka,S. Kimoto,N. Takada,Y. Higo
PREDICTION AND SIMULATION METHODS FOR GEOHAZARD MITIGATION
CRC PRESS-TAYLOR & FRANCIS GROUP
127-132
2009



In the present study, a multiphase deformation analysis of a river embankment has been carried out using an air-soil-water coupled finite element method considering the unsaturated seepage flow. A numerical model for unsaturated soil is constructed based on the mixture theory and an elasto-viscoplastic constitutive model. The theory used in the analysis is a generalization of Biot's two-phase mixture theory for saturated soil. An air-soil-water coupled finite element method is developed using the governing equations for three-phase soil based on the nonlinear finite deformation theory, i.e., the updated Lagrangian method. Two-dimensional numerical analyses of the river embankment under seepage conditions have been conducted, and the deformation associated with the seepage flow has been studied. From the numerica/11 methods, it has been found that the seepage-deformation coupled three-phase behavior can be simulated well with by the proposed method.
96 研究論文(学術雑誌)

不飽和浸透-変形連成解析手法と河川堤防への適用
加藤 亮輔,岡 二三生,木元 小百合,小高 猛司,角南 進
土木学会論文集C

65/ 1
2009/02

10.2208/jscejc.65.226


97 研究論文(学術雑誌)

An elasto-viscoplastic numerical analysis of the swelling process of unsaturated bentonite
Oka, F,Feng, H,Kimoto, S,Higo, Y
Int. J. for Numerical and Analytical Methods in Geomechanics

11, 369-376
2008




98 研究論文(学術雑誌)

弾粘塑性構成式と南海トラフ海底地盤で採取された土への適用
岡二三生,木元小百合
材料,

57/ 3, 237-242
2008




99 研究論文(国際会議プロシーディングス)

Oka, F., Kimoto, S., Kato, R., Sunami, S., Kodaka, T.
Unsaturated River,Embankment due to,Seepage Flow,Overflow
Proc. 12th IACMAG, edited by D.N.Singh, Goa, India, Oct. 1-6

2029-2041
2008




100 研究論文(国際会議プロシーディングス)

Elasto-viscoplastic Numerical Analysis of a Deep Excavation in an Osaka Soft Clay Deposit Using the Open-cut Method
Oka, F,Higo, Y,Kimoto, S,Nakano, M,Mukai,H. Kunotsubo, H,Izumitani, T,Takeda, S,Amano, K,Nagaya, J
Proc. 12th IACMAG, edited by D.N.Singh, Goa, India, Oct. 1-6

4709-4715
2008




101 研究論文(国際会議プロシーディングス)

Multi-phase coupled elasto-viscoplastic analysis of unsaturated soil slope during seepage flow
Oka, F,Kimoto, S,Yamazaki, S,Kato, R
Proc. of XXII ICTAM, SM08, 25-29 August 2008, Adelaide, Australia


2008




102 研究論文(国際会議プロシーディングス)

不飽和河川堤防の多相浸透―変形連成解析
岡二三生,山崎真也,木元小百合,肥後陽介,森中雄一
第57回理論応用力学講演会講演論文集、NCTAM2008

347-348
2008




103 研究論文(国際会議プロシーディングス)

A numerical simulation of triaxial tests of unsaturated soil at constant water and air content by using an elasto-viscoplastic model
F. Oka,H. Feng,S. Kimoto,T. Kodaka,H. Suzuki
UNSATURATED SOILS: ADVANCES IN GEO-ENGINEERING
CRC PRESS-TAYLOR & FRANCIS GROUP
735-741
2008



It is known that air can be trapped in some parts of embankments during heavy rain or overflow. In this case, air pressure, as well as water pressure, may change under partially drained conditions. However, most laboratory test programs have been conducted under constant air pressure conditions. In this paper, a numerical model for unsaturated soils based on the mixture theory and an elasto-viscoplastic constitutive model is presented. The collapse behavior, due to a decrease in suction, is expressed by the shrinkage of the overconsolidation boundary surface, the static yield surface, and the viscoplastic potential surface. The theory used in the analysis is a generalization of Biot's two-phase mixture theory for saturated soil. A soil-water-air coupled finite element method is developed in the present study using the governing equations for multiphase soil based on the nonlinear finite deformation theory. Three-dimensional numerical analyses at constant water and constant air content are conducted and the applicability of the proposed method is confirmed. The performance of the model is examined with reference to triaxial compression tests preformed on unsaturated soil at constant
104 研究論文(国際会議プロシーディングス)

Higo, Y., Oka, F., Kimoto, S., Sato, T., Sawada, M. and Sanagawa, T.
Higo, Y,Oka, F,Kimoto, S,Sato, T,Sawada, M,Sanagawa, T
The Twenty-First KKCNN Symposium on Civil Engineering, October 27–28, 2008, Singapore


2008




105 研究論文(学術雑誌)

An elasto-viscoplastic constitutive model and its application to the sample obtained from the seabed ground at Nankai trough
Fusao Oka,Sayuri Kimoto
Zairyo/Journal of the Society of Materials Science, Japan

57/ 3, 237-242
2008/03
0514-5163
10.2472/jsms.57.237

Methane hydrates are presently viewed as a potential energy resource for the 21st century because an amount of methane gas is trapped in hydrate reservoirs around Nankai trough. Geotechnical survey has been conducted around Nankai trough in order to obtain background information of the Methane hydrate layers. In the present study, an elaso-viscoplastic model for soil is applied to the triaxial test results of the samples from Nankai trough. In the model, softening due to structural degradation of the soil, and kinematic hardening are considered. Simulation results shows the applicability of the model to the deep seabed soil. © 2008 The Society of Materials Science.
106 研究論文(国際会議プロシーディングス)

Numerical simulation of the ground deformation due to methane hydrate dissociation by a chemo-thermo-mechanically coupled analysis
S. Kimoto,F. Oka,T. Fushita,M. Fujiwaki
NUMERICAL MODELS IN GEOMECHANICS: NUMOG X
TAYLOR & FRANCIS LTD
303-309
2007



In order to predict ground deformation due to the dissociation of methane hydrates, we have developed a simulation method based on the chemo-thermo-mechanically coupled analysis. With this method, the phase change from hydrates to fluids, the flows of pore water and gas, the mechanical behavior of the solid skeleton, and heat transfer can be simultaneously solved. The numerical method is based on the finite element method using an updated Lagrangian formulation. Applying the proposed method, we have numerically analyzed the dissociation process for the heating method. It has been predicted that ground deformation is caused by the generation of water and gas during the dissociation.
107 研究論文(国際会議プロシーディングス)

Simulation of triaxial compression tests on soil samples obtained from seabed ground in deep sea by elasto-viscoplastic constitutive equation
E. Ogisako,S. Nishio,A. Denda,F. Oka,S. Kimoto
PROCEEDINGS OF THE SEVENTH (2007) ISOPE OCEAN MINING (& GAS HYDRATES) SYMPOSIUM
INTERNATIONAL SOCIETY OFFSHORE& POLAR ENGINEERS
63-+
2007



The simulations and numerical analyses of triaxial compression tests on soil samples obtained from seabed ground in deep sea were performed by elasto-viscoplastic constitutive equation. From the results it is proved that simulation can express very well the experimental results. And from the results of three dimensional finite element analyses it is found that the shear strain and volumetric strain are distributed nonuniformly in the test specimen.
108 研究論文(学術雑誌)

A chemo-thermo-mechanically coupled numerical simulation of the subsurface ground deformations due to methane hydrate dissociation
Sayuri Kimoto,Fusao Oka,Tomolliko Fushita,Masaya Fujiwaki
COMPUTERS AND GEOTECHNICS
ELSEVIER SCI LTD
34/ 4, 216-228
2007
0266-352X
10.1016/j.compgeo.2007.02.006

In order to predict ground deformations due to the dissociation of methane hydrates, we have developed a simulation method based on a chemo-thermo-mechanically coupled analysis. Within this method, the phase change from hydrates to fluids, the flow of pore water and gas, the mechanical behavior of the solid skeleton, and heat transfer can all be simultaneously solved. The numerical method is based on the finite element method using an updated Lagrangian formulation. Applying the proposed framework, we have numerically analyzed the dissociation process that occurs in the heating and depressurizing methods of natural gas production. It has been predicted that ground deformation is caused by the generation of water and gas during the dissociation process. (c) 2007 Elsevier Ltd. All rights reserved.
109 研究論文(国際会議プロシーディングス)

Three –dimensional strain localization of clay with different shapes of specimens
Higo,Y,Oka, F,Kimoto, S,Kodaka, T,Adachi, T
Proc. 13th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, Allied Publishers Private Limited, ASSMGE, Vol.1

3-6
2007




110 研究論文(国際会議プロシーディングス)

A hydro-mechanical coupled analysis of the deformation of an unsaturated river embankment due to seepage flow and overflow
Oka, F,Kimoto, S,Gotoh, H,Kato, R,Sunami, S,Kodaka, T
Proc. APCOM’07 in conjunction with EPMESCXI, Dec.3-6, 2007, Kyoto Japan


2007




111 研究論文(国際会議プロシーディングス)

Numerical analysis of deformation of methane hydrated contained soil due to the dissociation of gas hydrate
Sayuri Kimoto,Fusao Oka,Masaya Fujiwaki,Yuji Fujita
BIFURCATIONS, INSTABILITIES, DEGRADATION IN GEOMECHANICS
SPRINGER-VERLAG BERLIN
361-+
2007



We have developed a simulation method to predict the ground deformation due to the dissociation of methane hydrate. In the dissociation process, the phase change from solid to fluids leads to the change in partial stresses in the porous media, which will cause the ground deformation. The simulations are based on the chemo-thermo-mechanical coupled finite element analysis, in which the phase change, the flow of pore fluids, the mechanical behavior of solid skeleton, and heat transfer are simultaneously solved. We treat the ground as unsaturated soils, and apply an elasto-viscoplastic constitutive model to the soil skeleton. Using the proposed method, we have numerically analyzed the dissociation process for heating methods. Ground deformation has been predicted which is caused by water and gas generation during the dissociation.
112 研究論文(国際会議プロシーディングス)

A finite element analysis of the thermo-hydro-mechanically coupled problem of a cohesive deposit using a thermo-elasto-viscoplastic model
Sayuri Kimoto,Fusao Oka,YoungSeok Kim,Naoaki Takada,Yosuke Higo
ENGINEERING PLASTICITY AND ITS APPLICATIONS FROM NANOSCALE TO MACROSCALE, PTS 1 AND 2
TRANS TECH PUBLICATIONS LTD
340-341, 1291-+
2007
1013-9826


We propose a thermo-hydro-mechanically coupled finite element analysis method for clay with a thermo-elasto-viscoplastic model. The volume changes in soil particles and pore fluids are introduced into the analysis method. The instability of the problem is studied and a numerical simulation of the thermal consolidation is presented using the newly developed analysis method. it was confirmed that the analysis method can reproduce the thermal consolidation phenomenon well.
113 研究論文(学術雑誌)

Elasto-viscoplastic finite element study of the effect of degradation on bearing capacity of footing on clay ground
S. Boonlert,F. Oka,S. Kimoto,T. Kodaka,Y. Higo
Geomechanics and Geoengineering

2/ 4, 235-251
2007/12
1748-6025
10.1080/17486020701669058

The bearing capacity of footing has been studied by both conventional and numerical methods by many researchers. However, degradation of the microstructure of material, that is, a change in the microstructure of the soil, has not been adequately taken into account. Degradation of microstructure causes strain softening of materials and it leads to strain localization such as shear bands and slip bands. From an engineering point of view the strain localization is crucial because it is a precursor of failure. In the present study, finite element analyses of the bearing capacity of a shallow foundation on homogeneous and inhomogeneous saturated clay strata have been conducted using an elasto-viscoplastic soil constitutive model of microstructure change. A series of analyses of footing on clay deposit with different microstructure parameters have been carried out. Numerical results show that strain localization can be predicted during the loading of rigid footing on highly structured soil and strain localization affects the footing-soil interaction. The effects of footing roughness on the failure mechanism are also discussed in the study.
114 (MISC)総説・解説(学術雑誌)

メタンハイドレート分解時の地盤変形シミュレーション
木元小百合,岡二三生
土と基礎

55/ 10, 13-16
2007/10




115 研究論文(学術雑誌)

Effects of sample shape on the strain localization of water-saturated clay
Takeshi Kodaka,Yosuke Higo,Sayuri Kimoto,Fusao Oka
INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS
JOHN WILEY & SONS LTD
31/ 3, 483-521
2007/03
0363-9061
10.1002/nag.585

Strain localization is an important geotechnical problem related to large deformations and the onset of failure, such as slope failure. It is necessary, therefore, to clarify the mechanisms of the strain localization of geomaterials in order to predict large deformations of the ground. For the last two decades, many researchers have studied the strain localization of geomaterials through both experimental and numerical works. Most of the works, however, particularly the numerical studies, have been treated as two-dimensional plane strain problems for the sake of simplicity, even though the actual phenomena are generally three dimensional. In order to understand the deformation, the failure, and the strain localization of clay under three-dimensional conditions, triaxial tests on clay and their numerical simulation are performed in the present study. In particular, focus is mainly placed on the effects of sample shape on the localization behaviour of normally consolidated and overconsolidated clays. A series of undrained triaxial compression tests, using rectangular clay specimens with different shapes and strain rates, is conducted. Localized shear strain distributions are successf
116 研究論文(国際会議プロシーディングス)

An elasto-viscoplastic model and multiphase coupled FE analysis for unsaturated soil
F. Oka,T. Kodaka,S. Kimoto,Y. S. Kim,N. Yamasaki
Geotechnical Special Publication

2/ 147, 2039-2050
2006
0895-0563
10.1061/40802(189)172

Rate sensitivity is an important characteristic of geomaterials for both saturated and unsaturated soils. However, many constitutive models for unsaturated soil have been constructed within the framework of the rate independent theory. The present study addresses an elasto-viscoplastic constitutive model which considers the effect of suction for unsaturated clayey soil and a soil-water-air three-phase coupled analysis using the elasto-viscoplastic model. The proposed constitutive model adopts the average skeleton stress for the effective stress from the viewpoint of the mixture theory. Hence, it has become possible to construct a model for unsaturated soil starting with a model for saturated soil by substituting the average skeleton stress for the effective stress and introducing the suction effect into the constitutive model. Furthermore, the collapse behavior, which is brought about by a decrease in suction, is described by the shrinkage of the overconsolidation boundary surface, the static yield surface, and the viscoplastic potential surface. A numerical analysis for multiphase materials is conducted within the framework of a continuum mechanics approach through the use of the
117 研究論文(学術雑誌)

水-土連成数値解析法による洪水・地震時の土構造物の変形・破壊予測
岡二三生,小高猛司,木元小百合,加藤亮輔
第55回理論応用力学講演会, 京大会館, 1月24日


2006




118 研究論文(国際会議プロシーディングス)

A multi-phase coupled FE analysis using an elasto-viscoplastic model for unsaturated soil
Fusao Oka,Takeshi Kodaka,Sayuri Kimoto,Young Seok Kim,Nobuhiro Yamasaki
Geotechnical Special Publication

156, 124-131
2006
0895-0563
10.1061/40870(216)10

The present study addresses an elasto-viscoplastic constitutive model which considers the effect of suction in unsaturated clayey soil and a soil-water-air three-phase coupled analysis using the elasto-viscoplastic model. The proposed constitutive model adopts the average skeleton stress for the effective stress from mixture theory. Hence, it has become possible to construct a model for unsaturated soils starting with a model for a saturated soil by substituting the average skeleton stress for the effective stress and introducing for the suction effect into the constitutive model. Furthermore, the collapse behavior, which is brought about by a decrease in suction can be described by the shrinkage of the overconsolidation boundary surface, the static yield surface, and the viscoplastic potential surface. A numerical analysis for multiphase materials is conducted within the framework of a continuum mechanics approach through the use of the theory of porous media. The theory is a generalization of Biot's two-phase mixture theory for saturated soil. A soil-water-air three-phase coupled finite element method has been developed in the present study using the governing equations for multi
119 研究論文(国際会議プロシーディングス)

Hydro-Mechanical Coupled Analysis of Unsaturated River Embankment due to Seepage Flow
Oka, F,Kodaka, T,Kimoto, S,Kato, R,Sunami, S
Key Engineering Materials, 2007, Vols. 340-341

1223-1230
2006/09




120 研究論文(学術雑誌)

Three-dimensional strain localization of water-saturated clay and numerical simulation using an elasto-viscoplastic model
Y. Higo,F. Oka,T. Kodaka,S. Kimoto
PHILOSOPHICAL MAGAZINE
TAYLOR & FRANCIS LTD
86/ 21-22, 3205-3240
2006/07
1478-6435
10.1080/14786430500321203

Since strain localization is a precursor of failure, it is an important subject to address in the field of geomechanics. Strain localization has been analysed for geomaterials by several researchers. Many of the studies, however, treated the problems brought about by strain localization as two-dimensional problems, although the phenomena are generally three-dimensional. In the present study, undrained triaxial compression tests using rectangular specimens and their numerical simulation are conducted in order to investigate the strain localization behaviour of geomaterials under three-dimensional conditions. In the experiments, both normally consolidated and over-consolidated clay samples are tested with different strain rates. Using the distribution of shear strain obtained by an image analysis of digital photographs taken during deformation, the effects of the strain rates, the dilation, and the overconsolidation on strain localization are studied in detail. The analysis method used in the numerical simulation is a coupled fluid-structure finite element method. The method is based on the finite deformation theory, in which an elasto-viscoplastic model for water-saturated clay, whi
121 研究論文(国際会議プロシーディングス)

Strain localization of rectangular clay specimen under undrained triaxial compression conditions
F. Oka,T. Kodaka,S. Kimoto,T. Ichinose,Y. Higo
PROCEEDINGS OF THE 16TH INTERNATIONAL CONFERENCE ON SOIL MECHANICS AND GEOTECHNICAL ENGINEERING, VOLS 1-5
MILLPRESS SCIENCE PUBLISHERS
2, 841-844
2005



In the present study, in order to grasp deformation and failure behaviors of clay under 3-D condition as well as large deformation and strain localization, a series of triaxial compression tests using rectangular clay specimens are performed. Various patterns of strain localization are successfully observed by an image analysis. 3-D bifurcation phenomena, e.g. formation and progress of various strain localization patterns, failure with buckling, unstable behavior in the stress-strain relations, can be also observed in the preset tests. Then, a 3-D finite element simulation using an elasto-viscoplastic model for a saturated clay is carried out. The simulated results can well explain the observed strain localization patterns of rectanguler clay sepcimens.
122 研究論文(国際会議プロシーディングス)

Calibration of elasto-viscoplastic models for cohesive soils
Oka, F,Kimoto, S
Proc. 11th Int. Conference of IACMAG

449-456
2005




123 研究論文(国際会議プロシーディングス)

Numerical simulation of the triaxial compression behaviour of unsaturated silt using an elasto-viscoplastic model
Kim, Y,Kimoto, S,Oka, F,Kodaka, T
Proc. 11th Int. Conference of IACMAG

361-368
2005




124 研究論文(国際会議プロシーディングス)

A finite element analysis of the deformation behavior of a multiphase seabed ground due to the dissociation of natural gas hydrates
Oka, F,Kimoto, S,Kodaka, T,Takada, N,Fujita, Y,Higo, Y
Proc. 11th Int. Conference of IACMAG

127-134
2005




125 研究論文(国際会議プロシーディングス)

An elasto-viscoplastic model for clay considering destructuralization and prediction of compaction bands
Oka, F,Kimoto, S
Geomechanics, Testing, Modeling and Simulation, Geotechnical Special Publication No. 143, ASCE, J.A. Yamamuro, J.Koseki editors

71-80
2005




126

堤防の内部構造調査における各種物理探査手法の適用性の検討
岡 二三生,小高 猛司,木元小百合,芝田 弘一,服部 浩二,北川 義治,山田 茂治
地盤の環境・計測技術に関するシンポジウム2005 論文集

49-52
2005




127 研究論文(学術雑誌)

An elasto-viscoplastic model for clay considering destructuralization and consolidation analysis of unstable behavior
S Kimoto,F Oka
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
45/ 2, 29-42
2005/04
0038-0806
10.3208/sandf.45.2_29

Instability is usually considered as a problem of shear failure. Unstable behavior is also observed during the consolidation process, whereby the stress paths depart from the failure line. In the present study, an elasto-viscoplastic constitutive model is extended to describe instability of both around the failure state, and away from the failure line. The instability is connected to structural degradation, and formulated as shrinkage of overconsolidation boundary surface and static yield surface in the constitutive model. One-dimensional consolidation process of clay has been simulated to study the effect of structural degradation on behavior during consolidation. The proposed model can effectively reproduce certain types of unstable behavior during consolidation, such as stagnation, or a temporary increase in pore water pressure, and a sudden increase in the settlement rate. Moreover, the distributions of axial strain exhibit apparent strain localization when structural degradation is taken into account. This phenomenon of the compressive strain localization is regarded as compaction bands, which may cause a large displacement.
128 研究論文(学術雑誌)

Strain localization analysis of elasto-viscoplastic soil considering structural degradation
S Kimoto,F Oka,Y Higo
COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING
ELSEVIER SCIENCE SA
193/ 27-29, 2845-2866
2004
0045-7825
10.1016/j.cma.2003.09.017

Natural deposits are affected by chemical bonding or the cementation between soil particles during the sedimentation process. Under the effect called 'aging', aggregations and linkage between assemblages are formed in the soil structure. It is known that the structure affects the characteristics of the strength, the deformation, and the strain localization. Strain localization is generally considered to be important as a presage of failure. It has recently been recognized that strain localization may occur also during compressive deformation, particularly for porous materials with large confining stress, and the localized compressive deformation may cause large displacement. The aim of the present paper is to study the effect of structural degradation on the localization of both shear and compressive strain for water-saturated clay. Firstly, an elasto-viscoplastic constitutive model considering structural changes with the viscoplastic deformation is proposed in the present paper. Structural collapse is expressed as the shrinking of both the overconsolidation boundary surface and the static yield surface with increasing viscoplastic deformation in the model. Secondly, finite element
129 研究論文(国際会議プロシーディングス)

Instability and strain localization of water-saturated clay based on an elasto-viscoplastic model
F Oka,Y Higo,S Kimoto
BIFURCATIONS & INSTABILITIES IN GEOMECHANICS
A A BALKEMA PUBLISHERS
233-246
2003



The aim of the present paper is to study the instability and strain localization of water saturated clay. In particular, effects of dilatancy and strain rate are discussed using an elasto-viscoplastic constitutive model. The model is based on the non-linear kinematic hardening theory and a Chaboche type of viscoplasticity model. The elasto-viscoplastic model for both normally consolidated and overconsolidated clays can address both negative and positive dilatancies. Firstly, the instability of the model under undrained creep conditions is presented in terms of the accelerating creep failure. The analysis shows that clay with positive dilatancy is more unstable than clay with negative dilatancy. Secondly, a finite element analysis of the deformation of water-saturated clay is presented with focus on the numerical results under plane strain conditions. From the present numerical analysis, it is found that both dilatancy and strain rate prominently affect shear strain localization behavior.
130 研究論文(学術雑誌)

弾-熱粘塑性構成式を用いた熱-水-土連成有限要素法と飽和粘土の変形解析
岡二三生,肥後陽介,金榮錫,井村雄一,木元小百合
応用力学論文集, 土木学会

6, 427-436
2003




131 研究論文(学術雑誌)

Step-changed strain rate effect on the stress-strain relations of clay and a constitutive modeling
F Oka,T Kodaka,S Kimoto,S Ishigaki,C Tsuji
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
43/ 4, 189-202
2003/08
0038-0806
10.3208/sandf.43.4_189

Strain rate sensitivity is one of the typical time-dependent behaviors of soil as well as creep and stress relaxation. In particular, it is well known that a unique stress-strain curve exists for each different strain rate in clayey soil: the isotaches characteristics. Originally, the concept of isotaches was proposed in one-dimensional consolidation of clay; however, we can also observe the isotaches characteristics in the stress-strain relation obtained by the triaxial compression test with various strain rates. The purpose of the present study is to reexamine the concept of isotaches through the detailed results of triaxial tests on reconstituted Fukakusa clay and a constitutive modeling. Undrained triaxial tests both of normally consolidated and overconsolidated clay were performed with various constant strain rates and step-changed strain rates to observe the isotaches characteristics in a wide range of axial strain. The results of the step-changed strain rates test show that the phenomenon of isotaches exists in the range of low level strain. It is also seen, however, that this phenomenon is not observed in a range of high level strain, in particular, around the critical stat
132 研究論文(学術雑誌)

Anisotropic behavior of soft sedimentary rock and a constitutive model
F Oka,S Kimoto,H Kobayashi,T Adachi
SOILS AND FOUNDATIONS
JAPANESE GEOTECHNICAL SOC
42/ 5, 59-70
2002/10
0038-0806
10.3208/sandf.42.5_59


133 研究論文(学術雑誌)

Effect of dilatancy on the strain localization of water-saturated elasto-viscoplastic soil
F Oka,Y Higo,S Kimoto
INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES
PERGAMON-ELSEVIER SCIENCE LTD
39/ 13-14, 3625-3647
2002/06
0020-7683
10.1016/S0020-7683(02)00171-3

It is well known that geomaterials such as soils exhibit an increase in volume during shearing deformation, referred to as dilatancy. Dilatancy is a typical property of such granular materials as soils and is closely related to changes in the microstructure. Normally consolidated clay exhibits negative dilatancy or contractancy, namely, a decrease in volume during shearing. On the other hand, overconsolidated clay shows positive dilatancy, namely, an increase in volume during shearing. The aim of the present paper is to study the effects of the microstructure, such as dilatancy and permeability, on the strain localization of water-saturated clay using an elasto-viscoplastic constitutive model. Based on the non-linear kinematic hardening theory and a Chaboche type of viscoplasticity model, an elasto-viscoplastic model for both normally consolidated and overconsolidated clays is proposed; the model can address both negative and positive dilatancies. Firstly, the instability of the model under undrained creep conditions is analyzed in terms of the accelerating creep failure. The analysis shows that clay with positive dilatancy is more unstable than clay with negative dilatancy. Second
134 研究論文(国際会議プロシーディングス)

Effect of dilatancy on strain localization of water saturated cohesive soil based on an elasto-viscoplastic model
F Oka,Y Higo,S Kimoto
COMPUTATIONAL MECHANICS, VOLS 1 AND 2, PROCEEDINGS
ELSEVIER SCIENCE BV
1147-1154
2001



In the present paper, effect of dilatancy on strain localization was numerically studied for dilatant and contractant clays using a newly developed elasto-viscoplastic constitutive model which can be applied to both normally and overconsolidated clays. From the present numerical analysis, it was found that dilatancy characteristics plays an important role in the strain localization behavior.