Effect Of Inherent Anisotropy Research Articles

Effect of inherent anisotropy on the strength of natural granite residual soil under generalized stress paths

Effect of inherent anisotropy on the strength of natural granite residual soil under generalized stress paths

An evolution law for fabric anisotropy and its application in micromechanical modelling of granular materials

Micromechanical studies of granular materials have demonstrated the importance of their microstructure to their behaviour. This microstructure is often characterized by fabric tensors. Experimental and computational studies have shown that the fabric can change significantly during deformation. Therefore, the evolution of fabric is important to constitutive modelling. Current fabric evolution laws for granular materials have generally been developed for continuum-mechanical models, and use a loading index multiplier associated with a yield surface. Such evolution laws can not be employed with micromechanical models that do not involve an explicit macro-scale yield surface.This study develops an evolution law for fabric anisotropy, based on observations from experiments and Discrete Element Method simulations from literature. The proposed evolution law considers the effects of inherent anisotropy, void ratio, stress ratio, loading direction and intermediate principal stress ratio. In the critical state, the value of the fabric anisotropy depends only on the Lode angle. The predicted evolution of fabric anisotropy is in good agreement with results of Discrete Element Method simulations, showing both hardening and softening behaviour and describing the influence of the initial void ratio.The proposed evolution law can be embedded into micromechanics-based constitutive relations as well as conventional continuum-mechanical models. As an example, a well-established micromechanical model (in which the fabric is considered as constant) has been extended by accounting for the variations in fabric, in combination with the proposed fabric evolution law. The performance of this enhanced micromechanical model has been demonstrated by a comparison between the predicted behaviour and experimental results from literature for Toyoura sand under various loading conditions.

Effects of inherent anisotropy on G0 of unsaturated sand

Effects of inherent anisotropy on G0 of unsaturated sand

Modeling Quasi-Elastic Properties of Sand while considering Inherent and Stress-Induced Anisotropies

The hypoelastic model (HongNam and Koseki, 2005)−which can describe inherent and stress-induced anisotropies in the quasielastic properties of sand subjected to cyclic triaxial and torsional loadings−was modified to more rationally evaluate the elastic components of sand in the general three-dimensional stress state. In the new simulation, the compliance matrix that links the stress and elastic strain increments in the principal stress axes was formulated symmetrically, as required by the law of thermodynamics. Comparisons between the simulation results and experimental data showed that the modified model could effectively simulate the experimental data values. Note that differences between the simulated values of compliance matrix [M] in the material axes obtained by both models were generally small within the employed stress paths, except the M26 and M62 values. The effect of inherent anisotropy on quasi-elastic components of the compliance matrix in the material axes was generally small at low shear stress ratio values (τzθ/σ′θ) during torsional loading from the isotropic stress state, and increased at larger shear stress ratio values, especially during torsional loading from the anisotropic stress state.

Constitutive modeling of principal stress rotation by considering inherent and induced anisotropy of soils

In order to simulate the soil response during principal stress rotation, anisotropic unified hardening (UH) model is developed within the framework of elastoplastic theory. Without introducing any additional mechanism to display the role of stress rotation specifically, this model achieves the simulation by considering the material anisotropy. The effect of inherent anisotropy is reflected using the anisotropic transformed stress method, but a new formula for the stress mapping is adopted to keep the mean stress unchanged. Analysis indicates that from the view of the transformed stress tensor, the anisotropic soil is subjected to loading during pure rotation of principal stress axes, so that plastic strains can be calculated. To represent the induced anisotropy, a fabric evolution law is proposed based on laboratory and numerical test results. At the critical state, the fabric tensor reaches a stable value determined by the stress state, while the critical state line is unique in the plane of void ratio versus mean stress. The anisotropic UH model has concise formulation and explicit elastoplastic flexibility matrix and can provide reasonable predictions for the deformation of anisotropic soils when principal stresses rotate.

Effect of Inherent Anisotropy on the Behavior of Fine-Grained Cohesive Soils

Study on shear strength and compressibility characteristics of soils considering anisotropy phenomenon is essential to the accurate design of structure foundations and stability analysis of earth structures. In this research, a series of triaxial and oedometer tests were used to study the mechanical behavior of a fine-grained cohesive soil considering the effect of inherent anisotropy on the behavior. For this purpose, a special soil sampler was made and different directions were selected to collect the samples from the site. All triaxial samples were isotropically consolidated under the effective stresses of 200, 300 and 500 kPa, and loaded at the rate of 0.05 mm/min. The results showed that the shear strength and compressibility of the soil intensively depend on the sampling direction. With an increase in the anisotropic angle, the shear strength values decreased and the settlement values increased. However, at the anisotropic angle of 72°, the shear strength and settlement have minimum and maximum values, respectively. In addition, a review of failure surface illustrates that as the angle between the failure surface and the bedding direction decreases, the failure surface tends to be closer to soil stratification and it causes an increment in pore water pressure and reduction in shear strength. In general, samples cored perpendicular to the bedding direction showed dilative behavior, whereas the other ones exhibited contractive behavior.

Fabric rates of elliptical particle assembly in monotonic and cyclic simple shear tests: a numerical study

This paper aims to investigate the evolutions of microscopic structures of elliptical particle assemblies in both monotonic and cyclic constant volume simple shear tests using the discrete element method. Microscopic structures, such as particle orientations, contact normals and contact forces, were obtained from the simulations. Elliptical particles with the same aspect ratio (1.4 and 1.7 respectively for the two specimens) were generated with random particle directions, compacted in layers, and then precompressed to a low pressure one-dimensionally to produce an inherently anisotropic specimen. The specimens were sheared in two perpendicular directions (shear mode I and II) in a strain-rate controlled way so that the effects of inherent anisotropy can be examined. The anisotropy of particle orientation increases and the principal direction of particle orientation rotates with the shearing of the specimen in the monotonic tests. The shear mode can affect the way fabric anisotropy rate of particle orientation responds to shear strain as a result of the initial anisotropy. The particle aspect ratio exhibits quantitative influence on some fabric rates, including particle orientation, contact normal and sliding contact normal. The fabric rates of contact normal, sliding contact normal, contact force, strong and weak contact forces fluctuate dramatically around zero after the shear strain exceeds 4 % in the monotonic tests and throughout the cyclic tests. Fabric rates of contact normals and forces are much larger than that of particle orientation. The particle orientation based fabric tensor is harder to evolve than the contact normal or contact force based because the reorientation of particles is more difficult than that of contacts.

Effect of loading induced anisotropy on the shear behavior of rough interfaces

We have utilized a statistical method to model the shear behavior of rough contacts. In contrast to the traditional statistical methods, which only describe the asperity height and curvature distributions, we have introduced a contact orientation distribution in our analysis. We have also incorporated asperity contact sliding and developed an incremental scheme for computing the stress–displacement relationship. Using this enhanced model, we can demarcate the boundary between elastically deforming and sliding asperity contact orientations under given loading. Consequently, we can describe the shear-normal coupling as well as the effects of inherent anisotropy and the induced anisotropy of the sheared interfaces.

Effect of inherent anisotropy on acceleration wave speeds in hypoplasticity

Generally, soils possess an inherent fabric with transverse isotropy on the bedding plane. As a consequence, the deformation-stress characteristics show a dependence on the loading direction. Anisotropy is characterized by a vector normal to the bedding plane. In this paper we investigate the speeds of acceleration waves in an inherently anisotropic hypoplastic material. For the incrementally nonlinear hypoplastic constitutive equation the wave speed spectrum is continuous in contrast to discrete spectra in incrementally linear constitutive models. The so-called flutter instability arises when the equation for the wave speeds has complex solutions. In the principal stress space, a surface of first occurrence of flutter instability can be identified by analogy with the failure surface defined by a vanishing stress rate. We analyse the influence of the anisotropy on these surfaces and the change in their shape and symmetries depending on the bedding angle.

Quasi-Elastic Deformation Properties of Toyoura Sand in Cyclic Triaxial and Torsional Loadings

ABSTRACT Quasi-elastic deformation properties of dry, dense Toyoura sand at very small strain level were investigated by conducting a series of triaxial and torsional shear tests on hollow cylindrical specimens. Strains were measured locally by a newly developed pin-typed local deformation transducer. In the tests, effect of end-restraint at the top cap and pedestal on small strain shear modulus that was measured externally by applying small unload /reload cycles in the torsional direction was found to be significant. A new hypoelastic model was proposed to simulate the inherent and stress-induced anisotropies of quasi-elastic deformation properties of sand, considering rotation of principal stress axes from material axes. Good agreements between experimental and simulation data were observed. In the simulation, effect of inherent anisotropy on the quasi-elastic deformation properties of Toyoura sand was found to be small.

Effect of Anisotropic Yielding on the Flow Liquefaction of Loose Sand

In very loose sand, the ratio Mp of shear stress to mean normal stress at the peak point of the undrained effective stress path (UESP) is very close to the stress ratio Mat the peak point of the capped yield surface. Stress ratios Mp can therefore be used in constructing yield surfaces of sands. These stress ratios have also been used in the past in evaluating flow potential of loose sand. Application of Mp for these purposes requires that factors affecting this stress ratio, and quantitative relationships for the variation of Mp with these factors be determined. In this paper, effects of the intermediate principal stress and direction of loading on Mp are investigated, and models are developed by which these effects can be quantified. It is shown that variations of Mp with these factors are similar to the variations of yielding stresses obtained from stress-strain data. Yield surfaces obtained from the variation of Mp indicated a strong dependency of yielding stresses on inherent anisotropy. Data examined in this paper also suggest that the effects of inherent anisotropy on yielding stresses are controlled primarily by the relative magnitudes of the normal stresses applied in the principal directions of material anisotropy.

Undrained Deformation Characteristics of Loose Sand Under Three-Dimensional Stress Conditions

Using a true triaxial shear test apparatus, a series of static loading tests were performed in undrained conditions on saturated cubical sand specimens. The three principal stresses were programmed to produce radial stress paths with different directions on the octahedral plane in the three-dimensional stress space. The radial stress paths used consisted of 13 stress paths radiating outwards from the center of the octahedral plane with an angle interval of 15 degrees. As it had been observed previously in drained tests, the effects of inherent anisotropy were noted in the deformation and the pore water pressure buildup characteristics. It was observed that the specimen prepared by vertical deposition exhibited stronger resistance to deformation when loaded vertically in the triaxial stress state of compression than when loaded horizontally in the triaxial stress state of extension. The specimens subjected to loads intermediate between triaxial compression and extension exhibited intermediate resistance.