Takeshi Kodaka

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.

Shear Banding in a Sedimentary Soft Mudstone Subjected to Plane Strain Compression

A plane strain compression (PSC) testing system to observe shear banding in softrock was developed. The deformation characteristics of shear band in a sedimentary soft mudstone were evaluated by locally measuring axial and lateral deformations of specimens in drained PSC tests. Shear deformation and dilatancy of shear band that occurred between the peak stress state and the start of the residual stress state were on the order of 1 and 0.5 mm. Dilatancy of the shear band continued during the residual stress state, resulting in a high-residual friction angle mobilized along the shear band. Shear band deformation characteristics of two other sedimentary softrocks were obtained from axial strains measured locally in triaxial compression tests based on the stress-state dilatancy relationship obtained from the PSC tests.

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.

Seepage Failure Analyses of Sandy Ground Using a Liquefaction Analysis Method Based on Finite Deformation Theory

Most of seepage failure problems can be classified as liquefaction problem, because effective stresses of sand deposits near/at failure state statically come close to zero. It is, therefore, difficult to predict the seepage failure using conventional deformation analysis methods for saturated soil. In the present paper, a newly developed liquefaction analysis method, which is based on the finite deformation theory, is applied to seepage failure problems. Firstly, 1-D unsteady seepage problem is discussed. Numerical solutions under various initial and boundary conditions are compared with analytical solutions based on the one-dimensional elastic consolidation theory. Secondary, 2-D classical seepage failure problem of horizontal ground with an embedded sheet pile is discussed. It is found that a proposed analysis method can quantitatively explain the distributions of the pore water pressure and the mean effective stress in sand deposit during the unsteady seepage flow until failure, but we need further study to accurately predict the deformation.

Elasto-viscoplastic finite element study of the effect of degradation on bearing capacity of footing on clay ground

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

Effects of Dry Density and Water Content on Mechanical Properties of Sand-Bentonite Buffer Material

Mechanical properties of the buffer material of deep geological repository are influenced by various factors. The engineered barrier system of a deep geological repository is subjected to local groundwater flow after it is decommissioned. The changes in water content of buffer material could affect deformation and suction properties. In this study, the influence of water content and dry density on the mechanical properties of sand-bentonite buffer material were investigated. The triaxial compression tests were performed on sand-bentonite specimens of 1400, 1600 and 1800 kg/m3 of dry density and 6, 12 and 18% of water content. The volumetric strains of specimens were evaluated using a newly built double-cell type triaxial testing apparatus. Total suction of specimens was measured using the chiller-mirror hygrometer technique. Total suction was measured on identical specimens prepared for suction measurement and triaxial compression tests. The results indicate that water content reduces deviator stress while dry density increases it. The results also suggest that water content changes strain-softening behaviour of relatively less saturated specimens into strain-hardening behaviour when water content is high. A high confining pressure (of 1.0 MPa) inclines towards strain-hardening behaviour than a small confining pressure (of 0.1 MPa). Water content also increases strain-hardening behaviour. In contrast, dry density reduces strain-hardening behaviour, particularly under a small confining pressure. The results also indicate that dry density reduces the magnitude of volumetric expansion. A high confining pressure encourages volumetric expansion. While water content decreases frictional behavior, it increases cohesive behaviour under a relatively low dry density (of 1400 and 1600 kg/m3). In contrasts, water content reduces cohesive behaviour under a high dry density (of 1800 kg/m3). Thus, a micro-scale analysis would produce more insights on this. While water content has huge influence on total suction, both dry density and confining pressure have no effects on it.

Cyclic Shear Strength of Clay under Simple Shear Condition

The aim of the present study is to investigate the cyclic shear behavior of clay, in particular the simple shear conditions. In general, cyclic triaxial tests are performed to evaluate the deformation and the strength characteristics of clay. However, the direction of cyclic loading in triaxial tests is different from the direction of cyclic shear loading in the natural ground. In the present study, therefore, a cyclic simple shear test apparatus has been newly developed and various cases of cyclic simple shear tests have been performed using reconstituted clay and intact natural clay. From the test results, the cyclic shear behavior of clay, which is useful for constitutive modeling, could be precisely obtained. Furthermore, it has been found that the cyclic shear strength of clay can be determined with only the vertical effective stress.