Sayuri Kimoto

A chemo–thermo–mechanically coupled analysis of ground deformation induced by gas hydrate dissociation

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.

Effect of dilatancy on the strain localization of water-saturated elasto-viscoplastic soil

Abstract 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. 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 permeability prominently affect shear strain localization behavior.

Three-dimensional strain localization of water-saturated clay and numerical simulation using an elasto-viscoplastic model

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 over-consolidation 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, which can consider structural changes, is adopted. The results of the simulation include not only the distribution of shear strain on the surfaces of the specimens, but also the distributions of strain, stress, and pore water pressure inside the specimens. Through a comparison of the experimental results and the simulation results, the mechanisms of strain localization are studied under three-dimensional conditions.

An Elasto-Viscoplastic Model and Multiphase Coupled FE Analysis for Unsaturated Soil

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 theory of porous media. The theory is a generalization of Biots two-phase mixture theory for saturated soil. A soil-water-air three-phase coupled finite element method is developed in the present study using the governing equations for multiphase soil based on the non-linear finite deformation theory. The average skeleton stress is defined as the difference between the total stress and the average pressure of the two fluids and is used in the proposed elasto-viscoplastic constitutive model. A van Genuchten (1980) type of equation is employed as the constitutive equation between the liquid saturation and the suction pressure. Numerical simulations of unexhausted-undrained compression with different strain rates are conducted under plane strain conditions, and the applicability of the proposed method is evaluated with respect to strain localization and the effect of suction.

Cyclic Elastoviscoplastic Constitutive Model for Clay Considering Nonlinear Kinematic Hardening Rules and Structural Degradation

AbstractA 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 proposed model to reproduce the cyclic behavior of soft clayey soils.

A Hydro-Mechanical Coupled Analysis of an Unsaturated River Embankment due to Seepage Flow

In this paper, a soil-water coupled elasto-plastic finite element analysis is applied to the problem of seepage flow by incorporating unsaturated seepage characteristics and assuming the pore air pressure in the unsaturated soil region to be atmospheric pressure. It is shown that the proposed soil deformation–seepage flow coupled analysis method is applicable to safety investigations of river embankments and that the existing evaluation criterion for the seepage failure of river embankments is not always on the safe side.

A Multi-Phase Coupled FE Analysis Using an Elasto-Viscoplastic Model for Unsaturated Soil

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 Biots 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-phase soil based on the non-linear finite deformation theory. The average skeleton stress is defined as the difference between the total stress and the average pressure of the two fluids and is used in the proposed elasto-viscoplastic constitutive model. A van Genuchten (1980) type of equation is employed as the constitutive equation between the liquid saturation and the suction pressure. Numerical simulations of unexhausted-undrained compression are conducted under plane strain conditions, and the applicability of the proposed method is evaluated with respect to strain localization and the effect of suction.

Elasto-viscoplastic modeling of Osaka soft clay considering destructuration and its effect on the consolidation analysis of an embankment

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.

A Finite Element Analysis of the Thermo-Hydro-Mechanically Coupled Problem of a Cohesive Deposit Using a Thermo-Elasto-Viscoplastic Model

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.

An Elasto-Viscoplastic Model for Clay Considering Destructuralization and Prediction of Compaction Bands

Instability is usually considered to be 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 both around the failure state, and away from the failure line. The instability is connected to structural degradation, and formulated as a shrinkage of the overconsolidation boundary surface and static yield surface in the model. One-dimensional consolidation process of clay has been simulated to study the effect of structural degradation on consolidation behavior. 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 inhomogeneity when structural degradation is taken into account. This phenomenon of the compressive strain localization is regarded as a compaction band, which may cause large displacements.