In the paper, based on a modified elastoplastic model with Matsuoka-Nakai failure criterion, two-dimensional (2D) and three-dimensional (3D) soil-water coupled finite element analyses are conducted to investigate a large-scale failure in a soft-rock slope due to heavy rain. Since the failure of the slope was a typical three-dimensional event, it is necessary to estimate the accuracy of 2D analysis that can be easily and commonly used in geotechnical engineering. The characteristics of slope failure, such as the development of shear strain, the deformation of ground, the propagation of shear band and the progressive failure are discussed in detail with 2D and 3D analyses. The differences between 2D and 3D analyses are carefully investigated. It is found that both 2D and 3D soil-water coupled analyses based on the modified elastoplastic model can simulate the progressive failure of a slope.

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In the geotechnical engineering design, rigid plastic analysis is usually used to estimate a factor of safety or an ultimate capacity. Although based on a simple assumption of rigid-plastic material behaviour, limit analysis has a rigorous theoretical background called limit theorems. The implementation of limit analysis to finite element method is recognised as rigid-plastic finite element method. The author recently proposed a new formulation of hybrid type rigid-plastic finite element method based on the interior point method, named primal-dual rigid-plastic finite element method (PDRPFEM). In this paper, characteristics of primal-dual rigid-plastic finite element method are illustrated in contrast to the ordinary rigid-plastic finite element method based on the upper bound theorem. Advantages of the primal-dual rigid-plastic finite element method in the numerical calculations are also explained. In addition to this, as a real rigid-plastic boundary value problem, bearing capacity problems of surface uniform loading on weightless Tresca material (c, φ = 0) are solved by the primal-dual rigid-plastic finite element method. Numerical solutions are compared to the analytical solutions to investigate numerical accuracies of the primal-dual rigid-plastic finite element method.

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

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Underground excavation can induce significant deformation of pre-existing joints in rock mass. Due to the sliding and opening of the joints, consequent changes in the flow characteristics of a jointed rock mass would be anticipated. In the present work, the finite element excavation analysis for a jointed rock mass and the finite element flow and transport analysis using the discrete joint network model are employed and combined so as to make it possible to evaluate excavation induced changes in flow properties of a jointed rock mass. A separate computational module connecting these two codes is incorporated to modify initially imposed transmissivities into the excavation induced ones considering the deformation of joints during an excavation. The effect of excavation on the flow properties is evaluated by including these excavation induced transmissivity changes of individual joints. The excavation induced transmissivities are calculated using the analytical approximation solution developed for flow through a single rock joint. The calculation of the excavation induced transmissivity involves the distributions of shear displacements and normal stresses around the excavation, and explicitly takes into account of the effect of surface geometric roughness. A test simulation for a deep underground repository is performed to demonstrate the typical results predicted in the proposed approach.

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In this paper we will firstly consider the range of validity of Taylor's “stress-dilatancy” hypothesis. From this study we conclude that Taylor's rule is strictly valid only for coaxial deformations. For smooth non-coaxial deformations Taylor's rule must be modified according to the rule proposed by Gutierrez and Ishihara (2000), which includes a “non-coaxiality” correction factor. Strictly speaking the stress-dilatancy rule breaks down if in a process an abrupt rotation of principal axes is imposed, as this is the case at the onset of localization. During this phase the corresponding stress function decreases monotonously, whereas the dilatancy oscillates between large negative and large positive values. Finally it is shown that in globally undrained, displacement controlled tests this incipient post-failure contractancy to dilatancy oscillation should result in a dynamic instability.

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The roles of dilatancy and fabric on the behaviour of granular materials are both numerically and experimentally explored for the study of material instability and failure. This investigation has two basic ingredients : namely a stress dilatancy model with microstructural information embedded through a fabric tensor, and an experimental rendition of force transmission and structure in an assembly of 2-D photoelastic disks. In order to highlight material instability, model simulations of sand behaviour are carried out in axi-symmetric stress conditions along proportional strain paths with varying degrees of controlled dilation (or compaction) including isochoric deformations as a special case. It is shown that sand, otherwise stable under isochoric (undrained) conditions, can actually succumb to an instability or a liquefaction behaviour under other loading paths. This suggests that flow type of failures in soils may not be necessarily restricted to the classic saturated loose sand case in undrained conditions, but could manifest itself under other conditions as well.

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The inverse analysis method, used to accurately predict the two-dimensional deformation behavior of soft grounds, is discussed in this study. To ensure safety when constructing on a soft ground, in-situ observations are usually made. An inverse analysis is then effective for identifying the in-situ parameters of the ground and for predicting future deformation based on the parameters. The settlement, the lateral displacement, and the pore water pressure are measured during the construction of the structure. It is generally difficult to predict the lateral displacement. In this research, therefore, the cross anisotropy (transverse isotropy) of the ground is introduced to overcome the difficulty of predicting the lateral displacement. Furthermore, a simplified hyperbolic model is introduced to simulate the nonlinear shear behavior. The model is convenient for use with the inverse analysis, since it does not require many parameters. The measured pore water pressure is seldom used in the inverse analysis, because information on the pore water pressure is not required, from a mathematical standpoint, in order to identify the consolidation parameters. The effectiveness of applying the measured pore water pressure for the prediction of future consolidation behavior, is clarified in this study. As a result, the hyperbolic nonlinear model and the assumed anisotropy were found to be useful in predicting the future deformation behavior of clay. Furthermore, the pore water pressure measurement was proved to be effective for the predictions in this study.

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In this paper, the mechanism of fault pressurization in rapid slip events is analyzed on the basis of a complete characterization of the thermo-poro-mechanical behavior of a clayey gouge extracted at 760 m depth in Aigion fault in the active seismic zone of the Gulf of Corinth, Greece. It is shown that the thermally collapsible character of this clayey gouge can be responsible for a dramatic reduction of effective stress and a full fluidization of the material. Moreover a back analysis of an existing slip surface inside the clayey core of the fault shows that this failure plane is compatible with principal stress directions locally parallel and perpendicular to the fault axis.

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Experimental evidence shows that different preparation methods produce sand samples with distinctly different stress-strain response. This paper explores and draws conclusions from the measured differences on the monotonic undrained triaxial response of Toyoura sand, prepared by different methods at the same values of void ratio and initial effective stresses. This is achieved by comparing data and simulations performed with a recently developed plasticity constitutive model, which accounts for the effect of inherent fabric anisotropy on the mechanical response. The inherent fabric anisotropy is represented by a second order symmetric fabric tensor, and its effect on the response at different loading directions is expressed by an appropriate dependence of certain constitutive ingredients on a joint isotropic invariant of the loading direction and the fabric tensor. Use of this constitutive scheme to simulate the aforementioned data on Toyoura sand exploits the fact that the preparation method affects both the dilatancy and the hardening response in a systematic manner. Under the premise that these effects are due to the different inherent fabric created by the preparation method, it follows that the foregoing simulations do not require changes in constitutive equations or entirely different sets of model constants. On the contrary, only model constants related to this inherent fabric anisotropy scheme need readjustment, providing insight to how the sample preparation method affects the response and what can be done to model it.

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In general, soft sedimentary rocks exhibit strain-softening and time-dependent behavior. In the present paper, an elasto-viscoplastic constitutive model for soft rocks is proposed, that can universally describe time-dependent behavior such as creep, stress relaxation and strain rate sensitivity by extending Adachi and Oka's elasto-viscoplastic model for frozen sand. It is found that the proposed model can be applied to the time-dependent behavior of a soft tuffaceous rock called Tomuro Stone under the strain rate constant triaxial compression, drained triaxial creep and undrained triaxial stress relaxation.

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Changes in the instantaneous shear modulus associated with the non-coaxiality of NC clay materials are presented. A series of hollow cylindrical shear tests was performed to measure the changes in the instantaneous shear modulus with shear history. After each normally consolidated clay specimen was compressively sheared until a prescribed shear level, it was continuously torsionally sheared under a constant volume to measure the instantaneous shear modulus. Coaxial constitutive relations, such as these of the conventional Cam-clay type of models, predict that the instantaneous shear modulus obtained, does not change regardless of the compressive shear history. On the contrary, it is known that non-coaxial constitutive relations theoretically bring about changes in the instantaneous shear modulus in relation to the compressive shear history. The test results supported the predictions made by the non-coaxial constitutive models. Finally, the proper formulation for the non-coaxiality between the stress and the incremental plastic strain is discussed.

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A formulation of the Element-Free Galerkin Method (EFG method), i.e., one of the mesh-free/meshless methods developed in the field of computational mechanics for solving partial differential equations, is furnished for consolidation within finite strain and its validity for application to soil-water coupled problems is examined through a numerical analysis. The numerical strategy is constructed to solve a set of governing equations, e.g., the equilibrium for the nominal stress rate and the continuity of pore water, and the numerical discretization of the weak form of the governing equations leads to an updated Lagrangian scheme. The accuracy of the proposed numerical strategy is examined through an analysis of unconfined compression tests and simple shear tests under undrained and plane strain conditions through a comparison of stress paths integrated directly from the Cam-clay model within the framework of finite strain. It is also revealed that the particular type of weight function to be adopted in the moving least-squares (MLS) approximation, even in the same order, can determine the resultant shape functions of the EFG method for both the displacement and the pore water pressure field such that they are smoother than those of the usual FEM. The functions are advantageous in that they avoid spatial instability in the numerical solutions for pore water pressure under undrained conditions appearing in saturated soil column tests, where the shape function of the pore water pressure in the conventional FEM computation is adopted as a lower order than that of the displacement to remedy this type of numerical difficulty. To emphasize the applicability and the feasibility of the mesh-free computation, the consolidation phenomena are demonstrated in the analysis of a punch problem for a soft soil foundation which has stress singularity under both ends of a rigid loading platen for the same problem which Yatomi et al. (1989) solved with FEM.

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A numerical method is developed for the prediction of large deformations associated with a geomaterial flow during the occurrences of such geo-disasters as landslides and slope failures. The geomaterial is modeled as a viscous fluid, where a Bingham type constitutive model is proposed based on Coulomb's failure criterion and the viscosity is derived from the cohesion and friction angle. Numerical experiments on a 2D gravitational flow of a geomaterial column show that the constitutive model performs well. For solving Navier-Stokes's equations, Constrained Interpolated Profile (CIP) scheme is utilized, which is able to efficiently treat solid, liquid and gas together and has been successfully applied to problems in fluid dynamics. To prevent numerical instabilities due to a large Bingham viscosity, an implicit calculation procedure is proposed to calculate the viscous forces in the CIP. The method is finally used to simulate an earthquake-induced landslide that took place on May 26th, 2003 in the Northern part of Miyagi Prefecture in Japan. Results from soil-slump tests on the samples from the landslide site are used for determining the parameters in the simulation. Good agreements between the calculated and the observed results are obtained.

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The geotechnical properties of granular soils can be improved using compaction piles. However, the compaction effect and the extent of the densification are not yet clear. Well controlled in situ tests are necessary to investigate this problem. Further insight about the densification process can also be gained by performing numerical analysis using a constitutive model that takes into account the effect of density and confining pressure on the behavior of soils. This paper presents numerical analyses in which a recently proposed model, named subloading t_ij, is used to simulate these effects. The construction process of sand compaction piles involves two stages : the metal casing driving and the sand pile compaction. The process is simulated sequentially and the relative influence of each stage is quantified for different initial ground density conditions. The extent and effectiveness of the densification are also quantified. The results of these predictions are qualitatively compared with the results of field tests carried out before and after the execution of compaction piles in an artificial loose sand deposit. There is good overall agreement between the numerical predictions and the test results, showing that a combination of these two investigation approaches is a way forward to better understanding the densification process. However, this can only be achieved if the constitutive model can properly account for the change in soil behavior with densification.

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This paper presents a summary of recent work on cemented soils at Milan University of Technology (Politecnico). Oedometric and triaxial tests have been performed on lightly bonded soils of medium to very high porosity. Soils tested vary from a rather conventional silica sand-lime mixture to more unusual materials, including expanded clay aggregates, fragmented marine shells or stabilized metallurgical residues. A simple but powerful elasto-plastic bonded soil model is employed to select testing procedures and interpret the results obtained. Both experimental results and model simulations are employed here to illustrate and explore the onset of compaction bands, a new form of localization previously observed in rocks but whose appearance is here first signaled for bonded soils.

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