Details of a Researcher - KYOKOWA Hiroyuki

CiNii Books

A simple method to consider density and bonding effects in modeling of geomaterials Reviewed

Teruo Nakai, Mamoru Kikumoto, Hiroyuki Kyokawa, Hassain M. Shahin, Feng Zhang

Springer Series in Geomechanics and Geoengineering 5 91 - 111 2011

Publishing type：Research paper (scientific journal)

A simple method to describe stress-strain behavior of structured soils under normally and over-consolidated states in one-dimensional stress condition is first presented by introducing a state variable to represent the influence of density. To describe the one-dimensional stress-strain behavior of structured soils, attention is focused on the density and the bonding as the main factors that affect a structured soil, because it can be considered that the soil skeleton structure in a state which is looser than that of a normally consolidated soil is formed by bonding effects. The extension from one-dimensional model to three-dimensional model can be done only by defining the yield function using the invariants of modified stress 'tij' instead of one-dimensional stress 'σ' and assuming the flow rule in modified stress space tij. © Springer-Verlag Berlin Heidelberg 2011.

DOI： 10.1007/978-3-642-18284-6_6

A simple elasto-plastic model for unsaturated soils and interpretations of collapse and compaction behaviours Reviewed

M. Kikumoto, H. Kyokawa, T. Nakai, H. M. Shahin

Unsaturated Soils - Proceedings of the 5th International Conference on Unsaturated Soils 2 849 - 855 2011

Publishing type：Research paper (international conference proceedings)

Stress-strain behaviour of unsaturated soils seems rather different from that of saturated ones. It should however be described properly because soil may lie under an unsaturated state in the actual field. In the present study, a model taking account of the necessary characteristics of unsaturated soils is proposed by extending an existing model for saturated ones based on the Bishop's effective stress. In the model, variation in the degree of saturation is estimated by a newly proposed model for water retention curve considering the effects of hydraulic hysteresis and density, and the increase in the degree of saturation is linked with downward movement of normally consolidation line in the mean effective stress vs. void ratio plane in order to describe the typical behaviours of unsaturated soils. In the present paper, mechanisms of consolidation, soaking and compaction behaviours of unsaturated soils are discussed through the simulations by the proposed model. © 2011 Taylor & Francis Group, London.

Elastoplasitc constitutive model for soils considering the temperature dependency Reviewed

Kikumoto Mamoru, Kyokawa Hiroyuki, Nakai Teruo, Shahin Hossain.Md., Ban Akimasa

NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan 60 ( 0 ) 41 - 41 2011

Language：Japanese Publisher：National Committee for IUTAM

Geotechnical and geoenvironmental problems related with thermo-mechanical behaviour of soils have been increased as a result of various interests in geotechnical applications such as nuclear waste disposal, heat storage, development of marine underground resources, high-voltage cables and other geothermal structures. Although several researchers have been conducted coupled thermo-hydro-mechanical (THM) analyses in order to investigate the geothermal behaviour of ground, the THM characteristics of geomaterials have not been fully understood and have not been properly described by a constitutive model for soils. In the present study, the effects of temperature on the stress-strain characteristics of saturated soils are firstly discussed based on the past experimental researches on the THM behaviour and a simple method to describe the temperature-dependent behaviour for various kinds of soils is then proposed in the context of elastoplastic framework.

DOI： 10.11345/japannctam.60.0.41.0

Elastoplastic constitutive model of soil considering the saturation and unsaturation characteristics Reviewed

Kyokawa Hiroyuki, Kikumoto Mamoru, Nakai Teruo, Tatematsu Kazunori, Koike Mana, Tsukamoto Kohei

NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan 60 ( 0 ) 48 - 48 2011

Language：Japanese Publisher：National Committee for IUTAM

Although stress-strain characteristic of unsaturated soil is complicated and rather different from that of saturated soil, it should be described properly by a constitutive model for soils because soil usually stays under an unsaturated condition in actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one applicable to unsaturated soils. The proposed model is formulated using the Bishop's effective stress and the residual strength is, therefore, assumed to be constant. In the proposed model, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric and distortional behaviors of unsaturated soils are properly described. In the present paper, the outline of the proposed model is explained and applicability of the model is discussed through typical results of simulations.

DOI： 10.11345/japannctam.60.0.48.0

Simple modeling of stress-strain relation for unsaturated soil Reviewed

H. Kyokawa, M. Kikumoto, T. Nakai, H. Shahin

Geotechnical Special Publication ( 202 GSP ) 17 - 25 2010

Publishing type：Research paper (international conference proceedings)

Though stress-strain characteristics of unsaturated soils are complicated and rather different from those of saturated soils, it should be described properly by a constitutive model for soils because soil usually stays under an unsaturated condition in actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one applicable to unsaturated soils. The proposed model is formulated using the Bishop's effective stress and the residual strength is, therefore, assumed to be constant. In the proposed model, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric and distortional behaviors of unsaturated soils are properly described. In addition, a simple method to extend classical water retention curves such as van Genuchten's equation to be able to incorporate the influences of suction histories and density is proposed and applied to the proposed model. In the present paper, the outline of the proposed model is explained and applicability of the model is discussed through typical results of simulations. © 2010 ASCE.

DOI： 10.1061/41103(376)3

Description of stress-induced anisotropy of soils by modified stress within an isotropic hardening elastoplastic model Reviewed

KYOKAWA Hiroyuki, KIKUMOTO Mamoru, NAKAI Teruo

Japanese Geotechnical Journal 5 ( 4 ) 533 - 544 2010

Language：Japanese Publisher：The Japanese Geotechnical Society

The various stress change with rotating principal axes act on soil in actual ground. The distribution of the interparticle contact normals, meanwhile, gradually tends to concentrate towards the direction of the major principal stress σ1 when that of anisotropic stress acts on soil skeleton. And then, the stiffness of soil in the direction of major principal stress relatively increases. In ordinary model, such an induced anisotropy is considered by applying kinematic / rotational hardening rule in ordinary stress space. On the other hand, a new method, in which the induced anisotropy of soil is described simple and general isotropic hardening model by applying the modified stress which reflects the fabric change due to the variations of the intermediate principal stress and the stress histories, is developed in this study. The validity of the proposed model is verified by comparison with the experimental results of true triaxial tests on medium dense sand. It is shown that the proposed model, which obeys isotropic hardening rule and follows associated flow rule in tij* space, suitably considers the influence of the intermediate principal stress and the past stress history simultaneously and properly reproduces the test results under various complicated three-dimensional stress path.

DOI： 10.3208/jgs.5.533

A SIMPLE DESCRIPTION OF THE STRESS-STRAIN AND HYDRAULIC BEHAVIOURS OF UNSATURATED SOIL Reviewed

H. Kyokawa, M. Kikumoto, Teruo Nakai

RECENT DEVELOPMENTS OF GEOTECHNICAL ENGINEERING 74 - 81 2010

Language：English Publishing type：Research paper (international conference proceedings) Publisher：JAPANESE GEOTECHNICAL SOC

Stress-strain behaviour of unsaturated soils is rather complex and quite different from that of saturated soils. However, it should be described properly by a constitutive model for soils because soil usually lies under an unsaturated state in the actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one considering the necessary characteristics of unsaturated soils. The proposed model is formulated using the Bishop's effective stress and the residual strength is, therefore, assumed to be constant. In order to describe the behaviour of unsaturated soils, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric behaviours of unsaturated soils can be properly described. In addition, a simple method to extend classical water retention curves such as van Genuchten's equation is proposed in order to incorporate the influences of suction histories and density and it is applied to the proposed model. Because the volumetric behaviours of unsaturated soils such as consolidation and collapse behaviours are mainly focused on in the present paper, a simplified one-dimensional formulation of the elastoplastic constitutive model is explained. The validity of the proposed model is discussed through typical results of simulations.

MODELING OF TIME-DEPENDENT BEHAVIOR OF CLAY IN ONE-DIMENSIONAL CONSOLIDATION Reviewed

H. M. Shahin, T. Nakai, M. Kikumoto, H. Kyokawa, Y. Miyahara

RECENT DEVELOPMENTS OF GEOTECHNICAL ENGINEERING 54 - 61 2010

Language：English Publishing type：Research paper (international conference proceedings) Publisher：JAPANESE GEOTECHNICAL SOC

One-dimensional consolidation tests on clay have been performed using a one-dimensional model which can consider the effects of density, bonding and time. The model can explain strain rate effect, stress relaxation characteristics and creep characteristics of soils comprehensively without fitting the model for a particular phenomenon. The model can also describe other characteristics of soils such as secondary compression, delayed consolidation and consolidation characteristics of naturally deposited soils. Here, consolidation behavior of clay has been investigated varying the density of soil having different states of bonding. Varying the thickness of soil samples, the dependency of the sample height on the consolidation behavior has been checked as well. It is found that the model can explain well the behavior of secondary consolidation of naturally consolidated soil, overconsolidated soil and structured soil.

Elastoplastic modeling of geomaterials considering the influence of density and bonding Reviewed

T. Nakai, H. Kyokawa, M. Kikumoto, H. M. Shahin, F. Zhang

Prediction and Simulation Methods for Geohazard Mitigation 367 - 373 2009.01

Publishing type：Part of collection (book)

© 2009 Taylor & Francis Group, London. To understand easily the method for modeling the behavior of structured soils, one-dimensional description of elastoplastic behavior is firstly shown, and then it is extended to a constitutive model in general three-dimensional stress conditions. To describe the stress-strain behavior of structured soils, attention is focused on the density and the bonding as the main factors that affect a structured soil, because it can be considered that the soil skeleton structure in a state, which is looser than that of a normally consolidated soil, is formed by bonding effects including interlocking between soil particles and others. The extension from one-dimensional model to three-dimensional model can be done only by defining the yield function using the invariants of modified stress ‘tij’ instead of one-dimensional stress ‘σ’ and assuming the flow rule in modified stress space tij.

A unified method to describe the influences of intermediate principal stress and stress history in constitutive modelling Reviewed

M. Kikumoto, T. Nakai, H. Kyokawa, F. Zhang, H. M. Shahin

Geotechnics of Soft Soils - Focus on Ground Improvement - Proceedings of the 2nd International Workshop on Geotechnics of Soft Soils 151 - 157 2009

Publishing type：Research paper (international conference proceedings)

A simple and unified method for considering the influences of the intermediate principal stress and the stress histories on the deformation and the strength of soils, which obeys isotropic hardening rule in modified stress space, is proposed. The concept of the modified stress tij, which was proposed before by our group to describe the influence of the intermediate principal stress, is extended to a new stress tensor tij* to consider the fabric change of soils due to the variations of the intermediate principal stress and the stress histories. The evolution rule of tij* is established referring to past experimental evidences. In this paper, the modified stress tij* is applied to the isotropic hardening elastoplastic model named subloading tij model. The proposed model is verified by comparing the calculated results with the experimental results of true triaxial tests and directional shear tests on medium dense sand. © 2009 Taylor & Francis Group.

New description of stress-induced anisotropy using modified stress Reviewed

M. Kikumoto, T. Nakai, H. Kyokawa

Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering: The Academia and Practice of Geotechnical Engineering 1 550 - 553 2009

Publishing type：Research paper (international conference proceedings)

A simple and comprehensive method for describing the influences of the intermediate principal stress σ 2 and the stress histories on the deformation and the strength of soils, which assumes associated flow rule and obeys isotropic hardening rule in modified stress space, is proposed. The concept of the modified stress t ij, which was proposed to describe the influence of σ 2, is extended to the proposed modified stress tensor t ij* to consider the fabric change of soils due to the stress histories as well as the variation of σ 2. In this paper, the modified stress t ij* is applied to an elastoplastic model named subloading t ij model. The proposed model is verified by comparing the calculated results with the experimental results on sand. © 2009 IOS Press.

DOI： 10.3233/978-1-60750-031-5-550

Modelling the influence of density and bonding on geomaterials Reviewed

T. Nakai, F. Zhang, H. Kyokawa, M. Kikumoto, H. M. Shahin

Geotechnics of Soft Soils - Focus on Ground Improvement - Proceedings of the 2nd International Workshop on Geotechnics of Soft Soils 65 - 76 2009

Publishing type：Research paper (international conference proceedings)

A simple method to describe stress-strain behavior of structured soils under normally and over consolidated states in three-dimensional stress condition is presented. Based on an idea that structured soils can be made due to a kind of bonding effects between soil particles, a simple elastoplastic model is formulated by extending subloading tij model developed before. First, a new interpretation of subloading surface concept for describing the influence of density and/or confining pressure is described. Then, a method to take into consideration the effect of bonding on the soil behavior in constitutive modeling is developed. By introducing this method for considering bonding to the subloading tij model, the stress-strain behavior of structured soils is described. The validity of the proposed model is confirmed by numerical simulation of oedometer tests and undrained shear tests on structured clay. © 2009 Taylor & Francis Group.

地盤材料の諸特性の簡単且つ統一的なモデリング-密度,ボンディング,時間効果特性を中心として- Reviewed

京川裕之

応用力学論文集 12 371 - 382 2009

Publishing type：Research paper (scientific journal)

A method for considering induced anisotropy in an isotropic hardening model Reviewed

Kikumoto Mamoru, Nakai Teruo, Zhang Feng, Kyokawa Hiroyuki, Ishihara Takahiro

NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan 57 ( 0 ) 156 - 156 2008

Language：Japanese Publisher：National Committee for IUTAM

Soil is a kind of material which behaves anisotropically depending on stress histories. This characteristic of soil is usually called "stress-induced anisotropy". Such stress-induced anisotropy of soil is often modeled by applying kinematic / rotational hardening rule. On the other hand, though ordinary models which obey isotropic hardening rule are simpler than kinematic hardening models, they are not incapable of describing the stress-induced anisotropy. Satake, one of the authors and some researchers previously showed the possibility that even isotropic hardening model can describe the stress-induced anisotropy by using fabric tensor which suitably considers the influence of stress histories and reflecting it on the stress variables. It is, however, difficult to quantitatively estimate the variation of the fabric. Therefore, appropriate models formulated with the fabric tensor, which can be applied to the analysis of boundary value problems, have not been developed. Our research group proposed the modified stress tensor tij, by which we can properly consider the influence of intermediate principal stress on the deformation and the strength of soils. In addition to this, it has been noticed by experimental evidence and theoretical consideration that tij is the modified stress which reflects suitably the fabric tensor under monotonic loading paths. A new isotropic hardening model is proposed in which the induced anisotropy is taken into account by extending the concept of the modified stress tensor tij. As the results, this model is able to take into account the influence of various stress histories in general stress conditions. The influence of intermediate principal stress and that of stress histories are inclusively considered by the extended modified stress tij*. The validity of the model is verified by comparing the results calculated by the model with experimental results not only in anisotropic and shear tests on soils with stress histories under triaxial stress conditions but also in cyclic tests with the change of principal stress directions under three different principal stresses.

DOI： 10.11345/japannctam.57.0.156.0

Description of induced anisotropy of soils using isotropic hardening rule in modified stress space Reviewed

M. Kikumoto, H. Kyokawa, T. Nakai, F. Zhang, M. Hinokio

Proceedings of the 10th International Symposium on Numerical Models in Geomechanics NUMOG 10 - Numerical Models in Geomechanics NUMOG 10 85 - 91 2007

Publishing type：Research paper (international conference proceedings)

A simple method for describing the stress-induced anisotropy of soils, which obey isotropic hardening rule in modified stress space, is proposed. The concept of the modified stress tij, by which the influence of the intermediate principal stress on the deformation and the strength of soils is properly considered, has been proposed before. In the present study, this concept is extended and a new stress measure t*ij is incorporated to be able to consider the fabric change of soils due to variation of stress histories, namely, induced anisotropy. The evolution rule of t*ij is provided referring to the existing experimental evidence. Then, the modified stress t*ij is applied to an isotropic hardening elastoplastic model named Subloading tij model as an example. The model is verified by experimental results in isotropic and anisotropic compression and shear tests with different stress histories under triaxial conditions. © 2007 Taylor & Francis Group.

A simple modeling of structured soils Reviewed

T. Nakai, F. Zhang, H. Kyokawa, M. Kikumoto

NEW FRONTIERS IN CHINESE AND JAPANESE GEOTECHNIQUES 435 - 447 2007

Language：English Publishing type：Research paper (international conference proceedings) Publisher：CHINA COMMUNICATIONS PRESS

A simple method to describe stress-strain behavior of structured soils under normally and over consolidated states in general stress systems is presented. Based on an idea that structured soils can be made due to a kind of bonding effects between soil particles, a simple elastoplastic model is formulated by extending subloading t(ij) model (Nakai and Hinokio, 2004). Firstly, a new interpretation of subloading surface concept for describing the influence of density and/or confining pressure is shown. Secondly a method to take into consideration the effect of bonding on the soil behavior in constitutive modeling is developed. By introducing this method for considering bonding to the subloading t(ij) model, the stress-strain behavior of structured soils can be described. The validity of the proposed model is checked through numerical simulation of oedometer tests and undrained shear tests on structured clays. An interpretation for super/subloading surface concept to describe structured soil by Asaoka et al. (1998) is also given, and the similarities and differences between the present model and super/subloading surface model are also discussed in detail.

Isotropic hardening model for soils considering stress-induced anisotropy Reviewed

M. Kikumoto, H. Kyokawa, T. Nakai, F. Zhang, M. Hinokio

NEW FRONTIERS IN CHINESE AND JAPANESE GEOTECHNIQUES 278 - 289 2007

Language：English Publishing type：Research paper (international conference proceedings) Publisher：CHINA COMMUNICATIONS PRESS

A simple method for describing stress-induced anisotropy of soils, which obeys isotropic hardening rule in modified stress space, is proposed. In present study, a modified stress t(ij), by which influence of intermediate principal stress on deformation and strength characteristics of soils is suitably taken into account, is extended to a new stress tensor t(ij)(*) to be able to consider inclusively the fabric change of soils due to the variation of the intermediate principal stress and the stress histories, namely, stress-induced anisotropy. The evolution rule of t(ij)(*) is provided referring to the past experimental and numerical evidences. In this paper, the modified stress t(ij)(*) is applied to an isotropic hardening elastoplastic model named subloading t(ij) model as an example. The proposed model is verified by results of true triaxial tests.

Extension of subloading t<inf>ij</inf> model to structured soils Reviewed

M. Kikumoto, T. Nakai, F. Zhang, M. Hinokio, A. Yagyu, H. Kyokawa

Proceedings of the International Symposium on Geomechanics and Geotechnics of Particulate Media - Geomechanics and Geotechnics of Particulate Media 305 - 311 2006

Publishing type：Research paper (international conference proceedings)

An elastoplastic constitutive model for clay and sand is proposed, which can describe not only the typical behaviors of normally and over consolidated soils under general stress conditions, but also the behaviors of structured soils that can be found in natural deposited ground. In order to describe the behaviors of structured soils, the superloading surface concept originally proposed by Asaoka et al. (1998) is introduced. In the present model, the structured state of soil is simply defined as the difference of void ratio between the structured and the completely remolded soil under the same stress conditions. The proposed model is verified by oedometer tests on manmade structured soils. Moreover, the characteristics of the proposed model are investigated theoretically for drained and undrained triaxial tests. © 2006 Taylor & Francis Group.