木元 小百合

Recently, a possibility of a new technique of CO2 storage in the ground, namely, CO2 storage as hydrate, has been under discussion. In the present study, the mechanical behavior of CO2 hydrate bearing sediments was investigated to provide the fundamental mechanical properties of CO2 hydrate sediments. Especially, time dependent behavior was focused on and a series of undrained triaxial tests were conducted using high pressure and low temperature triaxial apparatus. From the results, it is found that hydrate-bearing sand specimen exhibits acceleration creep under undrained condition while sand specimen does not. In addition, an elasto-viscoplastic model considering the effect of hydrate saturation is adopted to reproduce the test results.

For partially saturated soils, several stress variables have been used such as the net stress, Bishop stress, generalized Bishop stress and skeleton stress have been used. For elastic behavior of water saturated geomaterials, Biot developed a theory of porous media which is equivalent to the elastic water saturated model with the effective stress with the so-called Biot constant. On the other hand, for inelastic response, the applicability of the effective stress with Biot constant has not yet well been studied. In this study, the authors present the formulation of two phase and three-phase geomaterials based on the continuum thermodynamics with internal variables. It has been found that the effective stress with Biot coefficient is obtained for elastic two-phase materials, and the skeleton stress with Biot constant can be derived for three-phase materials such as partially saturated soil.

A large number of constitutive models for geomaterials, such as soils and rocks, have been proposed over the last three decades. Those models have been implemented into computer codes and have been successfully used to solve practical engineering problems particularly under monotonic loading conditions. Compared with the models for monotonic loadings, more improvements for cyclic models are necessary in order to obtain more accurate predictions for the dynamic behavior of geomaterials, e.g., the behavior during earthquakes. A cyclic elastoplastic model has been developed in this study for sandy soils it is based on the kinematical hardening rule with a yield function that includes the changes in the stress ratio and the mean effective stress considering the degradation of the yield surface. From a simulation with the present model, it has been found that strong non-associativity leads to a large decrease in the mean effective stress during cyclic deformations under undrained conditions, while the model with the associated flow rule does not. This result is quite important because the mean effective stress becomes almost zero at the state of full liquefaction. Compared with the experimental results, the model can accurately reproduce the cyclic behavior of soil.

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 185, depending on the cell pressure. Finally, it was found that the soil material with a highly compressible pore fluid, such as air, must be analyzed with the multi-phase porous mixture theory. However, Terzaghi effective stress is practically applicable when the compressibilities of both the soil particles and the pore fluid are small enough compared to that of the soil skeleton.

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.

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.

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 pore spaces seem to play a role of supporting the soil skeleton and it acts as additional sand particles which enhances the dilation behaviour of the sandy sediments.

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.

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.

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. Hydrate dissociation behaviour is sensitive to changes in the pore pressure during earthquakes. Methane hydrate dissociation temporarily became active in some areas because of the main motion of the earthquake, then methane hydrate dissociation brought about an increase in the average pressure of the fluids during the earthquake. And, it was this increase in average pore pressure that finally caused the methane hydrate dissociation to cease during the earthquake. Copyright (c) 2016 John Wiley & Sons, Ltd.

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.

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.

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.

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. These results are consistent with those obtained from the linear stability analysis.

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 that in the case of the higher water content, the settlement of the crown is large mainly due to the volume compression underneath the crown, while the small confining pressure at the toe and near the slope surface induces large shear deformation with volume expansion. (C) 2015 The Japanese Geotechnical Society. Production and hosting by Elsevier Boy. All rights reserved.

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.

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 motion during strong earthquakes.

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.

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.

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.

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.

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.

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.

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.