木元 小百合

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 bands; nonetheless, the results manifest convergency through the mesh refinement in terms of the overall shape of shear banding and stress-strain relations. Copyright (c) 2013 John Wiley & Sons, Ltd.

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.

Frequent failures of river embankments have occurred in the world due to heavy rains. Heavy rainfalls bring about an increase of the ground water level within the soil as well as a rise in the water level of the rivers. As a result, embankments have been failed due to the rainfall infiltration and the generation of seepage flow. A series of two-dimensional numerical analyses of river embankments are carried out using a 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 larger the velocity of the seepage flow and the larger the deformation on the surface of the river embankments. Additionally, numerical simulations of a field experiment are used to show that, the method adopted here, can effectively be used to study the practical seepage deformation coupled problems on unsaturated soils.

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.

we have analyzed model river embankments on the clayey foundation with different ground water tables using a dynamic liquefaction analysis method. We have found that the effects of the water saturated region in the bank and the duration time of earthquake motion on the deformation behavior of river embankments are important. The results are consistent with the investigation results of the feature of the deformation and failure of the embankments due to the 2011 off the Pacific coast Tohoku Earthquake. © Springer-Verlag Berlin Heidelberg 2013.

A cyclic elasto-viscoplastic constitutive model is proposed for saturated soils based on the nonlinear kinematic hardening rules by considering the structural degradation of soil particles. The model is based on the overstress type elasto-viscoplasticity theory and includes deviatoric and volumetric components of kinematical hardening parameters. The performance of the model is verified through the isotropic cyclic compression test of soft clay samples. The simulated results with both the linear and non-linear kinematical hardening can well simulate the cyclic compression behavior of clay, which indicates the capability of the proposed model to reproduce the cyclic behavior of soft clayey soils. © Springer-Verlag Berlin Heidelberg 2013.

Earthquake-induced failure of unsaturated road embankments has taken place during the past earthquakes. It has been pointed out that water flow or higher water content of road embankments was a possible reason of the damage. In this study, dynamic resistance of unsaturated embankments with and without the seepage flow has been studied through the centrifugal model tests of unsaturated embankment and their numerical simulations by a multi-phase coupled finite element method with an elasto-plastic constitutive model. During the tests, displacement, pore pressures, and acceleration of embankments have been fully monitored. The dynamic behavior of unsaturated embankments with infiltration of pore water has been discussed on the basis of comparison between the experimental and the numerical results. From the present study, we have found that the seepage flow and the high water content extensively affect the dynamic stability of unsaturated road embankments.

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.

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 and hosting by Elsevier B.V. All rights reserved.

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 and hosting by Elsevier B.V. All rights reserved.

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, using a dynamic liquefaction analysis method. From the analysis results, we find the effects of the soil profiles and the duration time of the earthquake motion on the deformation behavior of the river embankments. The results are consistent with those of the investigation of the features of the deformation and the failure of the embankments due to the 2011 off the Pacific Coast of Tohoku Earthquake. (C) 2012 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.

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 employed to verify the assumptions and to calibrate the material properties of the improved layers. The effects of destructuration in the natural ground cases are observed as excess pore pressure build-up after construction and strain localization. The effects of ground improvement techniques are studied through a comparison of the deformation results and the excess pore water pressure responses with the natural ground cases. The numerical results show that it is important to carefully estimate the unequal long-term settlement for the construction of large-scale embankments. (C) 2012. The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.

平成23年（2011年）3月11日に発生した東北地方太平洋沖地震による宮城県北部地域の地盤被害状況を把握するため，地盤工学会東北支部・関西支部合同調査団を組織し，2011年4月5日～10日，29～30日にかけて調査を実施した。本論文では，その後の聞き取り調査なども踏まえ，河川堤防，海岸堤防，港湾施設，道路・鉄道の盛土・切土斜面，液状化や地盤沈降などの被害に関する調査結果を報告する。調査結果の一部は既に地盤工学会誌に速報として報告されているが，本論文ではそこで報告された被害事例も含めて被害対象ごとに整理して述べる。

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, especially under fully undrained conditions, due to the difference in pore pressures. In the cyclic loading tests under fully undrained conditions, the mean skeleton stress decreased due to the increase in air pressure and led to the failure of the specimen in the case of a lower level of initial suction. In addition, the test results exhibited the strain rate effect on the stress-strain behavior during cyclic loading under fully undrained conditions.

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 degradation. Considering the strain-dependent shear modulus, however, larger strain localization and displacement were predicted even in the early stages of loading. © 2011 Taylor &amp Francis.

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.

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'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's theory for a two-phase mixture. As a boundary problem, the triaxial tests were analyzed via finite elements with an updated Lagrangian formulation to simulate strain localization behavior under large deformations. The present study demonstrates that it is indeed possible to successfully simulate the experimentally observed compaction bands in diatomaceous mudstone. Copyright (C) 2010 John Wiley & Sons, Ltd.

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.

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-thermomechanically 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.

Changes in air pressure during deformation are in some cases important for the practical problems of unsaturated soils. 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 occurrence of pore gas pressure as well as pore water pressure. In the present study, we carried out cyclic loading tests on silty soil under fully undrained conditions as well as drained conditions. From the cyclic tests, we have found that the stress-strain behavior of unsaturated silty soil strongly depends on the initial suction, the shear stress level, and the strain rate on the stress-strain behavior of unsaturated silt. In the fully undrained tests, the mean skeleton stress decreases due to the increase of air pressure and the specimen leads to failure although the specimen does not in the drained tests.

The dynamic analysis of strain localization in a water-saturated clay specimen is numerically studied as a three-dimensional problem using a cyclic elasto-viscoplastic constitutive model. The model is derived based on the nonlinear kinematic hardening rules incorporated with the structural degradation of soil particles. In order to appropriately simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a Biot's type two-phase mixture is derived in the framework of the finite deformation theory with updated Lagrangian scheme. The shear band development is shown through the distributions of viscoplastic shear strain, axial strain, and mean effective stress in the specimen. The effect of mesh-size on the shear banding is also investigated. Using the finer meshes causes the strain to localize into a slightly narrower band than the default mesh pattern nonetheless, the results manifest convergence through the mesh refinement in terms of the overall shape of shear banding and stress-strain relations.

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.

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.

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.

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.

The consolidation behavior of Sri Lankan peaty clay is analyzed using an elasto-viscoplastic model. The model can describe the secondary compression behavior as a continuous process and it can also account for the effect of structural degradation on the consolidation analysis. The analysis takes into account all the main features involved in the process of peat consolidation, namely, finite strain, variable permeability, and the secondary compression. The material parameters required for the analysis and the procedures to evaluate them, using both standard laboratory and field tests, are explained. Initially, the model performance is assessed by comparing the predicted and the observed peat consolidation behavior under laboratory conditions. The results indicate that the model is capable of predicting the observed creep settlements and the effect of layer thickness on the settlement analysis of peaty clay. Then, the model is applied to predict the consolidation behavior of peaty clay under different field conditions. In this context, firstly, the one-dimensional field consolidation of peaty clay, brought about by the construction of compacted earth fill, is predicted. Then, the two-dimensional peat foundation response upon embankment loading is simulated. A good agreement is seen in the comparison of the predicted results with the field observations.

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.

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.

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.

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.

The simulations and numerical analyses of triaxial compression tests on soil samples recovered from the East Nankai trough have been performed by the elasto-viscoplastic constitutive equation. And the numerical analyses of the laboratory model tests on ground deformation in methane hydrate production were carried out by finite element method. From the comparison with the experimental results it is proved that the simulations can express well the behavior of experiments and the numerical analysis results can well reproduce the deformation behavior of ground in the model tests. Copyright 2010, Offshore Technology Conference.

In the production process of methane hydrates, there are several geohazardous problems associated with hydrate dissociation and the leakage of methane gas. In addition, some researchers have pointed out that dissociated methane gas could reform hydrates by means of a decrease in temperature during the drawing up of the gas. Hydrate formation could meet with trouble in the production process. In the present study, therefore, we propose a chemo-thermo-mechanically coupled analysis that takes into account the dissociation-formation process which includes the phase changes between hydrates and fluids, the flows of pore water and gas, the deformation behavior of the solid skeleton, and heat transfer. This method is an extension of the previously developed multiphase analysis method for grounds containing methane considering the dissociation of methane gas (Kimoto et al. 2007a). Using the proposed method, we have numerically analyzed the methane hydrate formation process in the ground by decreasing the temperature. It is predicted that the ground will swell due to the hydrate formation.

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.

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.