Yosuke Higo

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.

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.

Observation of Microstructural Changes and Strain Localization of Unsaturated Sands Using Microfocus X-ray CT

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.

Quantitative Observation of Strain Localisation in a Partially Saturated Triaxial Specimen Using Microfocus X-ray CT with Image Analysis

Strain localisation behaviour in a partially saturated dense sand specimen during triaxial compression has been observed by microfocus X-ray CT and image analyses of CT images. Digital image correlation (DIC) has provided displacement and strain field in the specimen in a macroscopic point of view. In the meantime, trinarization by a region growing segmentation technique has given local void ratio and degree of saturation microscopically. Through these image analyses of CT images, the progressive strain localisation behaviour and microstructural changes in the shear band have been quantitatively studied.

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.