Toshihisa Adachi

A strain localization analysis using a viscoplastic softening model for clay

Strain localization has become an attractive subject in geomechanics during the past decade. Shear bands are well known to develop in clay specimens during the straining process. Strain localization is closely related to plastic instability. In the present paper, a non-linear instability condition for the viscoplastic strain softening model during the creep process is firstly obtained. It is found that the proposed viscoplastic model is capable of describing plastic instability. Secondly, a two-dimensional linear instability analysis is performed and the preferred orientation for the growth of fluctuation and the instability condition are derived. It is worth noting that the two instability conditions are equivalent. Finally, the behavior of the clay is numerically analyzed in undrained plane-strain compression tests by the finite element method, considering a transport of pore water in the material at a quasi-static strain rate. The numerical results show that the model can predict strain localization phenomena, such as shear banding. From the numerical calculations, the effects of strain rate and permeability are discussed.

Experimental study on the distribution of earth pressure and surface settlement through three-dimensional trapdoor tests

In order to clarify the mechanical behavior in tunnel excavations, the authors develop and perform three-dimensional trapdoor experiments. The influence of the excavations is considered in the experiments. Based on the results, the distribution of earth pressure is found to greatly affect the process of the excavations. It can be confirmed, therefore, that a higher level of earth pressure is working on a ground where loosened conditions occur due to previously lowered trapdoors.

Instability of gradient dependent elasto-viscoplasticity for clay

A gradient-dependent viscoplastic constitutive model for water saturated clay is proposed to describe the strain localization phenomena and pattern formation during deformation. Second- and fourth-order gradients of volumetric viscoplastic strain are introduced into the constitutive equations to account for the non-local effects due to the motion of microstructures. A linear perturbation analysis is applied to this model. The instability of the government equations (i.e. the constitutive equations and the equations of motion for the clay skeleton and pore water) is discussed for both the one-dimensional and the two-dimensional situations. In addition, issues concerned with the formulation of boundary value problems by finite element analysis in relation to the formulation and the boundary conditions are presented.

Instability of an elasto-viscoplastic constitutive model for clay and strain localization

Abstract A viscoplastic strain softening model is derived and the instability of the model during the creep process is discussed. It is found that the proposed viscoplastic model is capable of describing plastic instability. The applicability of the model is evaluated with the results of experiments on Osaka alluvial clay. Finally, we numerically analyze the behavior of the clay in undrained plane strain compression tests by the finite element method, considering a transport of pore water in the material at a quasi-static strain rate. The numerical results show that the model can predict strain localization phenomena, such as shear banding.

Laboratory model tests and FE analyses on tunneling in the unconsolidated ground with inclined layers

Tunneling model tests are performed in an unconsolidated ground with inclined artificial layers. In order to simulate the model ground with inclined layers, aluminum rods and aluminum blocks are used. Three kinds of formations are introduced for the inclined-layer ground, namely, the 30° formation simulates a ground with lowly inclined layers, the 60° formation with highly inclined layers and the 90° formation simulates a ground with vertical layers. Tunneling process is simulated with a two-dimensional trapdoor apparatus. The 60° formation shows the most significant feature on the non-symmetrical distribution of the earth pressure. For this formation, the outer right part shows a greater concentration of earth pressure than the outer left part of the trapdoor. Simultaneously, the upper left part, for which lateral earth pressure is reduced, shows greater loads than the upper right part of the trapdoor. A numerical simulation by the FE analysis is conducted, using joint elements, to portray the discontinuous behaviors of the model ground, in order to verify the experimental results. By calculating the distributions of earth pressure and surface profiles with the trapdoor displacement, it is confirmed that the calculated results can reasonably portray the experimental results within lower displacements of approximately 1.00 mm.

Instability of Gradient Dependent Viscoplastic Model for Clay Saturated With Water and FEM Analysis

For the last two decade, several constitutive models have been proposed for geomaterials and applied to practical problems. There remains, however, still some outstanding issues to be resolved. One of them is the formation of shear band before and after failure. This problem is strongly connected to the strain localization phenomenon. Recently, the importance of simulation of post localization regime has been pointed out. For this problem an interesting approach has been proposed by Aifantis and co-workers based on the introduction of higher order strain gradients into constitutive equation. The purpose of this paper is to propose a strain gradient dependent elastoviscoplastic model for clay and study the instability condition of the constitutive theory. The model is then implemented into FEM code to simulate the clay behavior under plane strain condition by considering the transport of pore water based on Biot`s theory of solid-fluid mixture. 22 refs., 7 figs.

A Constitutive Model for Frozen Sand

An elasto-viscoplastic constitutive model for frozen sand is proposed based on the elasto-viscoplasticity theory incorporating the new time measure. The proposed model can describe a number of features of the mechanical behavior of the medium, such as rate sensitivity and strain softening under the triaxial compression test loading conditions. The effects of temperature, ambient pressure and the concentration of soil particles are also discussed.

A finite element analysis of strain localization for soft rock using a constitutive equation with strain softening

SummaryIt is well known that deformation and/or strain of geological materials localize when they come close to the failure state. In the present: study, we try to analyze the deformation problem using a constitutive relation with strain hardening and strain softening. The constitutive model of a soft rock and overconsolidated clay using an elasto-plastic constitutive theory with memory was originally developed by Adachi and Oka. This type of formulation is shown to be easily applied to analyze the material behavior of strain softening because there is a similarity to that in viscoplasticity. Using the proposed model, the loss of uniqueness of the solution to the initial value problem can be avoided and a special or complicated numerical technique, e.g., an arc length method, does not need to be used. When we use constitutive equations with strain softening in a finite element analysis, there is a problem of strong mesh size dependency of numerical results. To remedy the mesh size dependency, we generalize the Adachi-Oka model based on the concept of non-localization by Bazant. We apply the proposed constitutive model to the behavior of a sedimentary soft rock in the drained triaxial compression test. It is found that mesh size dependency becomes smaller using the non-localization of the constitutive model.ÜbersichtBekanntlich findet bei geologischen Stoffen in der Nähe des Versagenszustandes eine Lokalisierung der Verformung statt. Die Analyse dieses Problems wird in diesem Beitrag auf der Grundlage eines Stoffansatzes mit Ver- und Entfestigung unternommen. Entwickelt wurde das zugrunde gelegte elastischplastische Stoffmodell mit Gedächtnis von Adachi und Oka für weiches Gestein und übermäßig verdichteten Ton. Wegen der Ähnlichkeit zur Viskoplastizität läßt sich diese Formulierung des Stoffgesetzes leicht auf die Analyse des Verhaltens von entfestigendem Material anwenden, da der Eindeutigkeitsverlust der Lösung des Anfangswertproblems vermieden wird und besondere Rechenverfahren wie etwa die Bogenlängenmethode nicht benötigt werden. Bei der Benutzung von Stoffgesetzen in einer Finite-Element-Rechnung hängen die Ergebnisse stark von der Netzeinteilung ab. Um dies abzustellen, wird das Modell von Adachi und Oka auf der Grundlage von Bazants Konzept der Nichtlokalisierung verallgemeinert. Anwendungsbeispiel ist das Verhalten von weichem Gestein im drainierten Triaxial-Test. Es zeigt sich, daß mit dem Konzept der Nichtlokalisierung im Stoffmodell der Einfluß der Netzeinteilung geringer wird.

Soil-water coupling analysis of progressive failure in cuts with a strain softening model

In this paper, based on an elasto-plastic model with strain hardening and strain softening(Oka and Adachi[5]), a finite element analysis of soil-water coupling problem is conducted to investigate the progressive failure of an imaginary cut. The mechanical behavior of a cut, such as the change of excess pore-water pressure, the redistribution of stress in ground due to strain softening, the propagation of shear band and the progressive failure are discussed in detail. It is found that a soil-water coupling analysis based on an elasto-plastic model with strain softening can simulate the progressive failure of a cut.

GRADIENT DEPENDENT VISCOPLASTIC MODEL FOR CLAY AND NUMERICAL EXPERIMENTS BY FEM

ABSTRACT A gradient dependent viscoplastic constitutive model for water saturated clay is proposed. A linear perturbation stability analysis is applied to this model. We disccussed the instability of this model and the application to a boundary value problem. Finally, we carried out a numerical experiment by using a finite element method.