Numerical Modeling of Nonhomogeneous Behavior of Structured Soils during Triaxial Tests

The nonhomogeneous behavior of structured soils during triaxial tests has been studied using a finite element model based on the Structured Cam Clay constitutive model with Biot-type consolidation. The effect of inhomogeneities caused by the end restraint is studied by simulating drained triaxial tests for samples with a height to diameter ratio of 2. It was discovered that with the increase in degree of soil structure with respect to the same soil at the reconstituted state, the inhomogeineities caused by the end restraint will increase. By loading the sample at different strain rates and assuming different hydraulic boundary conditions, inhomogeneities caused by partial drainage were investigated. It was found that if drainage is allowed from all faces of the specimen, fully drained tests can be carried out at strain rates about ten times higher than those required when the drainage is allowed only in the vertical direction at the top and bottom of the specimen, confirming the findings of previous studies. Both end restraint and partial drainage can cause bulging of the triaxial specimen around mid-height. Inhomogeneities due to partial drainage influence the stress–strain behavior during destructuring, a characteristic feature of a structured soil. With an increase in the strain rate, the change in voids ratio during destructuration reduces, but, in contrast, the mean effective stress at which destructuration commences was found to increase. It is shown that the stress–strain behavior of the soil calculated for a triaxial specimen with inhomogeneities, based on global measurements of the triaxial response, does not represent the true constitutive behavior of the soil inside the test specimen. For most soils analyzed, the deviatoric stress based on the global measurements is about 25% less than that for the soil inside the test specimen, when the applied axial strain is about 30%. Therefore it can be concluded that the conventional global measurements of the sample response may not accurately reflect the true stress–strain behavior of a structured soil. This finding has major implications for the interpretation of laboratory triaxial tests on structured soils.     

Two-Surface Plasticity Model for Cyclic Undrained Behavior of Clays

Based on a new type of kinematic hardening and the theory of critical state soil mechanics, a two-surface model is herein developed for predicting the undrained behavior of saturated cohesive soils under cyclic loads. The anisotropic hardening rule works in two steps: (1) introducing a new concept, memory center, to take into account the memory of particular loading history; and (2) regulating the movement of the bounding and loading surfaces according to the direction of loading paths in stress space. Conventional triaxial tests have been performed on reconstituted clay samples in the laboratory. The proposed model is verified with respect to the observed behavior of soil samples. It is shown that like a multisurface model, this model can realistically describe some important responses of clays subjected to both monotonic and cyclic loading, while incorporating the memory of particular loading events.     

Sydney Soil Model. I: Theoretical Formulation

In this paper a theoretical study of the behavior of structured soils, including both clays and sands, is presented. A new model, which is referred to as the “Sydney soil model,” is formulated within the framework of critical state soil mechanics. In the proposed model, the mechanical behavior of soil is divided into two parts, that at a reference state and that attributed to the influence of soil structure. The reference state behavior is formulated according to the soil properties at the critical state of deformation, based on the concept of plastic volumetric hardening. The effects of structure are captured in the model by incorporation of the additional voids ratio that arises owing to the presence of soil structure. The formulation is generalized to include both isotropic compression and general shearing. In part?I of this paper, a new theoretical framework for modeling structured soil behavior and the formulation of the proposed Sydney soil model are introduced. In part?II of this paper, the Sydney soil model is employed to simulate the behavior of clays and sands, including calcareous clays and sands subjected to both drained and undrained shearing, and the performance of the model is evaluated.     

Volumetric Deformation of Natural Clays

A theoretical framework to describe the behavior of natural clay is proposed in a new four-dimensional space, consisting of the current stress state, stress history, the current voids ratio, and a measure of the current soil structure. A key assumption of the proposed framework is that both the hardening and the destructuring of natural clay are dependent on plastic volumetric deformation. Two different assumptions about how this destructuring occurs are proposed, based on which two versions of a complete constitutive model have been formulated. The behavior of reconstituted soil can also be simulated by the proposed model as a special case where the structure of soil has no effect on soil deformation. Characteristics of the proposed model are demonstrated through systematic simulations of the influence of soil structure on clay behavior. The simulated behavior of natural clay is compared qualitatively with widely available experimental data. It is seen that the proposed model successfully represents the main features of natural clays with various soil structures.     

Stress Paths in Relation to Deep Excavations

The mechanical behavior of many soils such as stiff clays depends on their current effective-stress states and stress history. For improving design and analysis of soil-structure interaction associated with deep excavations in these soils, it is important to understand effective-stress changes around excavations caused by both horizontal and vertical stress relief. In this paper, total and effective-stress variations adjacent to a diaphragm wall during construction of a 10-m-deep excavation in stiff fissured clay are reported and discussed. Interpreted field stress paths are compared with some relevant laboratory triaxial stress path tests, which simulate the horizontal and vertical stress relief in the field at an appropriate stress level. The interpreted field effective-stress paths in front of the wall are found to be similar to laboratory stress paths determined in undrained extension tests. Field stress paths behind the wall do not correspond particularly well with those from laboratory undrained compression tests, except when the stress state approaches active failure. The conventional undrained assumption does not seem to hold for the soil located immediately behind the wall during a relatively rapid excavation in the stiff clay.     

Experimental Inspiration for Kinematic Hardening Soil Models

Traditional techniques for identifying yielding of soils in the context of classical elastic–plastic soil models are criticized. However, the extended use of such procedures starts to reveal the kinematic nature of the plastic behavior of soils. It is suggested that the experimental determination of stress response envelopes can provide an objective route toward the collection of stress–strain behavior for soils. Stress response envelopes are presented for true triaxial tests on clay and sand: these clearly reveal the kinematic nature of the soil behavior. Response envelopes are presented for different magnitudes of strain probes. As the magnitude of a strain probe increases, the kinematic element of the response decays and the memory for the increasingly distant history is swept out.     

Determining Intrinsic Compressibility of Fine-Grained Soils

Results of laboratory oedometer tests on reconstituted specimens of four clays prepared at different initial water contents, ranging from the liquid limit to 1.75 times the liquid limit, show that the intrinsic compression line may not be “unique” for a given soil. This suggests that the “intrinsic” parameter Iv, which is based on the constants of intrinsic compressibility, e100?, (void ratio corresponding to σv′ = 100?kPa), and Cc?, (e100??e1000?), may in fact not be a truly intrinsic parameter of the soil, but is dependent on sample preparation. The positioning of the normalized compression curve in e–log–σv′ space is significantly influenced by the initial remolding water content, therefore resulting in differing values of e100? for a given soil depending on the initial water content. The influence of initial water content was greater for kaolinitic and illitic clay than for montmorillonitic clay. It is hypothesized that the difference in behavior may be attributed to differences in mineralogy. The results illustrate that caution should be used when comparing tests results from widespread sources and suggest that a standard level of initial water content be used to evaluate the intrinsic compressibility.     

Elastic Anisotropic Viscoplastic Modeling of the Strain-Rate-Dependent Stress–Strain Behavior of K0-Consolidated Natural Marine Clays in Triaxial Shear Tests

This paper presents a new three-dimensional (3D) anisotropic elastic viscoplastic (EVP) model for the time-dependent stress–strain behavior of K0-consolidated marine clays. A nonlinear creep function with a limit for the creep volumetric strain under an isotropic or odometer K0-consolidated stressing condition and a nonsymmetrical elliptical loading locus are incorporated in the 3D anisotropic EVP model. An α-line defines the inclination of the nonsymmetrical elliptical loading locus in the p′-q plane and is commonly used for natural soils. All model parameters are determined from the results of one set of consolidated undrained compression tests and an isotropic consolidation/creep test. With the parameters determined, the 3D anisotropic EVP model is used to simulate the behavior of K0-consolidation tests and the strain-rate-dependent stress–strain behaviors of the K0-consolidated triaxial compression and extension tests on natural Hong Kong marine deposit clay specimens. These triaxial K0-consolidated specimens were sheared at step-changed axial strain rates from +2?to?+0.2, +20, ?2 (unloading) and +2%/h (reloading) for compression tests; or from ?2?to??0.2, ?20, +2 (unloading), and ?2%/h (reloading) for extension tests, all in an undrained condition. The simulation results of all these tests are compared with the test results. The validation and limitations of the model are then evaluated and discussed.     

Constitutive Model of Soil Based on a Dynamical Systems Approach

A soil when sheared ultimately reaches a steady-state condition at which it deforms at a constant shear stress, effective normal stress, and void ratio. Various systems in nature dynamically evolve similarly from some initial condition, to a final steady-state condition. Such systems have been studied using dynamical systems theory. This technical note uses this theory to model monotonic shear of soil as a dynamical system. The principle proposed is simple—the rates of change of the shear stress, effective normal stress, and void ratio are proportional to the applied values of the shear and effective normal stress with the proportionality values decaying with strain until ultimately these proportionality values become zero at the steady-state condition. It provides a well-formed qualitative principle that fits closely the stress-strain-void ratio curves of undrained shear tests on uncemented, resedimented clays at various over consolidated ratios (OCRs), and drained shear tests on sands and silts at various relative densities, for various stress paths including compression, extension from standard triaxial, and true-triaxial tests. For the undrained shear of resedimented clay, these proportionalities and their decay rates vary smoothly with OCR. For drained shear of sand and silt, the model parameters show orderly variation with relative density. Its value lies in that a well-formed qualitative principle derived from the steady-state condition provides an alternate approach to current complex elastoplastic models based on critical state theory.     

Performance of a Three-Dimensional Hvorslev–Modified Cam Clay Model for Overconsolidated Clay

It is well established that critical state soil mechanics provides a useful theoretical framework for constitutive modeling of soil. Most of the critical state models, including the popular modified Cam clay (MCC) model, predict soil behavior in the subcritical region fairly well. However, the predictions for heavily overconsolidated soils, in the supercritical region, are not so satisfactory. Furthermore, the critical state models were developed from triaxial test data and extension of these models into three-dimensional (3D) stress space has not been investigated thoroughly. In the present work, experiments were carried out to obtain stress–strain behavior of overconsolidated soil in triaxial compression, extension, and plane strain conditions. A novel biaxial device has been developed to conduct the plane strain tests. The experimental results were used to formulate Hvorslev–MCC model which has MCC features in the subcritical region and Hvorslev surface in the supercritical region. The model was generalized to 3D stress space using the Mohr–Coulomb failure criterion. A comparison of the model predictions with test results has indicated that the Hvorslev–MCC model performs fairly well up to the peak supercritical point, during which deformations are fairly uniform and the specimens remain reasonably intact. Limitations of this simple model in predicting postpeak localization are also discussed. The model’s predictions for volumetric response in different shear modes seem to agree reasonably well with test results.     

Application of Lade's Criterion to Cam-Clay Model

Lade's criterion is one of the best criteria for describing the shear yield and failure behavior of soils in 3D stresses, and the original Cam-clay model is the most popular and fundamental elastoplastic model for normally consolidated clays. In this paper, a transformed stress tensor is proposed for combining Lade's criterion and the Cam-clay model. The transformed stress is deduced from what makes the curved surface of Lade's criterion become a cone with the axis being the space diagonal, i.e., Lade's criterion becomes the extended Mises type criterion in the transformed principal stress space. The Cam-clay model revised by Lade's criterion is capable of describing the mechanical behavior of soils in general stresses. It is presented that the revised model can simulate well the drained and undrained behavior of soils, not only under triaxial compression conditions, but also under plane strain, true triaxial, and hollow cylinder conditions. The elastoplastic models for soils, in which only the first and second stress invariants are used, can be extended simply to the model, including the third stress invariant by adopting Lade's criterion.     

State Boundary Surfaces for an Aged Compacted Clay

The yielding and the peak strength of an aged compacted clay were studied by conducting a series of suction-controlled triaxial tests. The test results were interpreted using the framework of intrinsic properties of reconstituted soil. The peak strength envelopes of undisturbed samples lie above those of reconstituted samples. The suction provides additional attractive forces to stabilize the soil structure, which result in the augmentation of the yield stress and peak strength envelope. The shear strength is normalized by the equivalent preconsolidation pressure (pe′) and Hvorslev surfaces are identified from undisturbed samples which expand with suction. A single peak strength envelope and Hvorslev surface will be emerged from the saturated and unsaturated (degree of saturation >80%) samples if the shear strength data are presented in terms of the average skeleton stress. The influence of the soil structure on the shear strength of the aged compacted clay may be measured by the ratio of normalized strengths at the intrinsic critical state which is about 1.26     

Failure Criterion for Cross-Anisotropic Soils

Experimental evidence and analyses of results of three-dimensional (3D) tests show that the shape of the failure surface for soils is influenced by the intermediate principal stress, shear banding, and cross anisotropy. Presented here is a formulation of a general 3D failure criterion for cross-anisotropic soils for both nonrotating and rotating stresses. The formulation relates the loading direction to the principal directions of the cross-anisotropic microstructure of the soil. The criterion is based on a function of stress, previously used as the 3D failure criterion for isotropic frictional materials, which is set equal to a scalar that varies over a sphere. The formulation is specialized for true triaxial tests and torsion shear tests and determination of material parameters is demonstrated. The failure criterion for cross-anisotropic soils is compared with experimental results from the literature to show that it is able to capture the conditions obtained in true triaxial tests without stress rotations as well as the conditions in torsion shear tests performed to study effects of stress rotation. Sets of data from some classic true triaxial tests are reinterpreted to show their true cross-anisotropic behavior.     

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ?b, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.     

Effect of Microfabric on Undrained Static and Cyclic Behavior of Kaolin Clay

Microfabric plays an important role in the engineering behavior of soils. Although many studies are available in the literature on the effect of microfabric on the static behavior of soils, the effect on the cyclic behavior is less understood. In the present study, samples with different microfabric were prepared in the laboratory by reconstituting commercially available kaolin clay with different pore fluids under a consolidation pressure of 100?kPa. Consolidated undrained triaxial tests were carried out on these samples under static and cyclic loading conditions. Dispersed samples were found to have monotonic stress-strain behavior with a peak deviatoric stress and higher peak undrained shear strength than the flocculated samples. However, the dispersed samples were found to offer less resistance to cyclic loading. When subjected to cyclic loading, dispersed samples failed within a few cycles under a cyclic stress ratio (defined as the ratio of cyclic deviatoric stress to the undrained shear strength) close to 0.6, whereas in flocculated samples, sudden failure was not observed even at a higher cyclic stress ratio of 0.9, although strains and pore pressures accumulated to higher values. Postcyclic monotonic tests conducted on samples that did not fail under cyclic loading showed an apparent overconsolidation effect caused by cyclic loading in a similar manner, as reported in the literature.     

Yield, Strength, and Critical State Behavior of a Tropical Saturated Soil

The paper reports laboratory investigations carried out on a tropical soil profile to study its compressibility, strength, critical state and limit state conditions, and their variation with depth. The soil profile comprises a reddish lateritic layer (horizon B) underlain by a saprolitic soil (horizon C) from which a number of block samples were taken. A series of isotropic and anisotropic compression tests, and drained and undrained triaxial tests, were conducted on specimens sampled at depths between 1.0 and 7.0 m, and also in the exposed saprolitic soil. Special triaxial tests, with the pore pressure increased to induce failure, were performed to investigate the failure at low stress levels. On this basis a tensile cutoff on the failure envelope was defined. In order to assess the influence of the natural soil structure, drained and undrained triaxial tests were carried out on compacted samples obtained from depths of 1.0 and 5.0 m. Higher strength parameters were measured for the horizon C soil, which is consistent with its lower clay content. A nonlinearity in the critical state line in q:p′ stress space was identified, but linear regression was used to obtain critical state parameters. The limit state curves for soils from horizon B are centered on the hydrostatic axis, but limit state curves for horizon C suggested anisotropic behavior.     

Correlation between Cyclic Resistance and Shear-Wave Velocity for Providence Silts

As an alternative to a field-based liquefaction resistance approach, cyclic triaxial tests with bender elements were used to develop a new correlation between cyclic resistance ratio (CRR) and overburden stress-corrected shear-wave velocity (VS1) for two nonplastic silts obtained from Providence, Rhode Island. Samples of natural nonplastic silt were recovered by block sampling and from geotechnical borings/split-spoon sampling. The data show that the correlation is independent of the soils’ stress history as well as the method used to prepare the silt for cyclic testing. The laboratory results indicate that using the existing field-based CRR-VS1 correlations will significantly overestimate the cyclic resistance of the Providence silts. The strong dependency of the CRR-VS1 curves on soil type also suggests the necessity of developing silt-specific liquefaction resistance curves from laboratory cyclic tests performed on reconstituted samples.     

Behavior of a Loosely Compacted Unsaturated Volcanic Soil

In this paper, the stress-strain relationship and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil (fill) were studied by conducting three series of triaxial stress path tests: (1) consolidated undrained on the saturated fill; (2) constant water content; and (3) a reducing suction under constant deviator stress on the unsaturated fill. The last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes, respectively. It was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear. For isotropically consolidated unsaturated specimens sheared under a constant water content, a hardening stress-strain and a nonlinear shear strength-suction relationship are observed. At relatively high suctions, both angle of friction and apparent cohesion appear to be independent of suction. Volumetric contraction during shear is observed in this series of tests. On the other hand, anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases. This observed volumetric behavior, unlike the shear strength, is stress path-dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils.