Influence Of Inherent Anisotropy Research Articles

Investigation on the anisotropy of mechanical properties and brittleness characteristics of deep laminated sandstones

Rock brittleness is a crucial mechanical property and essential for fracability evaluation and fracturing scheme design in unconventional reservoirs. However, the influence of inherent anisotropy on deep laminated sandstone’s mechanical properties and brittleness characteristics is rarely investigated. The energy transformation and damage evolution reflected by complete stress–strain curves are analyzed during the entire process of rock rupture under compressions. A new brittleness index is established based on energy evolution during sandstone failure. Its advantages involve comprehensively considering the energy transformation characteristics at both pre-peak and post-peak stages and the capability to characterize the effect of confining pressure and bedding plane (BP) geometry on sandstone brittleness. The triaxial compression tests on sandstones are conducted to validate the reliability and accuracy of the new brittleness index. Numerical simulations are then performed to further investigate the manner in which BP angle, BP density, and confining pressure control the brittleness anisotropy of deep laminated sandstones based on the finite element method. Then the acoustic emission (AE) characteristics of anisotropic sandstone and correlations between AE mode and brittleness index are discussed. The results indicated that the anisotropy of mechanical properties and brittleness of deep laminated sandstones were significantly affected by BP angle, BP density, and confining pressure. With the increase of BP angle, the brittleness index of deep laminated sandstone decreases first and then increases, showing a U-shape variation law, whose maximum and minimum values are obtained at φ = 0° and φ = 45°, respectively. The AE characteristics were closely related to rock brittleness, which was jointly controlled by BP geometry and confining pressure. The results provide a basis for the brittleness and fracability evaluation and optimum hydraulic fracturing design in deep laminated sandstones.

A Laboratory Study on the Influence of Anisotropy on the Soil Behavior

Anisotropy refers to any directional-dependence on material property. In recent years, the anisotropy of intermediate-strain behavior and shear strength, small-strain stiffness anisotropy has attracted attention. The main aim of this paper is to clarify the influence of inherent anisotropy on the compressibility characteristics and shear strength behavior of fine-grained soils. A special manufactured device has been made in order to prepare and collect the samples in different directions. Then, a series of conventional tests have been carried out on the soil samples consists of unconfined compressive and Oedometer tests. Low plasticity clay has been used with three liquidity indices (0.22, 0.32, and 0.44) in order to achieve the goal of the study. The results have showed that the anisotropy had an impact on the undrained shear strength of cohesive soil and the soil strength can be considered as strength anisotropy. The higher anisotropy angle causes the more reduction in the shear strength. The settlement is affected by anisotropy, increasing of the anisotropy leads to the increase of the settlement, the compression and the swelling indices. The coefficient of consolidation in the vertical direction is lower than the coefficient in inclined direction and the ratio of (ch/cv) tends to increase with anisotropy angle the highest ratio is 1.4.

Influence of inherent anisotropy on the seismic behavior of liquefiable sandy level ground

Inherent anisotropy is a crucial aspect to consider for an improved understanding of the strength and deformation characteristics of granular materials. It has been the focus of intense investigation since the mid-1960s. However, inherent anisotropy’s influence on ground seismic responses, such as liquefaction, has not been extensively studied. In this paper, inherent anisotropy’s influence on ground seismic responses is examined through a series of dynamic centrifuge model tests on liquefiable level sand deposits. During the model setup, five different deposition angles (0, 30, 45, 60, and 90 degrees) were achieved using a specially designed rigid container. The models were exposed to tapered sinusoidal input accelerations and the recorded results were fully investigated. It was found that deposition angle-caused inherent anisotropy significantly influenced the excess pore pressure responses during the shaking and dissipation phases. The amount of excess pore pressure build-up and the high excess pore pressure duration increased with the deposition angle, while the dissipation rate decreased as the deposition angle increased. The inherent anisotropy also influenced liquefaction-induced ground settlement, with volumetric strain increasing along with the deposition angle. With respect to response acceleration, inherent anisotropy’s effects depended on the amount of excess pore pressure build-up (i.e., degree of liquefaction). In view of these results, it was concluded that a sandy ground, deposited at a higher angle (i.e., closer to 90 degrees), is more susceptible to liquefaction and that inherent anisotropy’s influence should be considered when evaluating the liquefaction potential and performing effective stress analyses.

Elastoplastic Model for Clay with Microstructural Consideration

Clay material can be considered as a collection of clusters, which interact with each other mainly through mechanical forces. From this point of view, clay is modeled by analogy to granular material in this paper. An elastoplastic stress-strain relationship for clay is derived by using the granular mechanics approach developed in previous studies for sand. However, unlike sand, clay deformation is generated not only by the mobilizing but also by compressing clusters. Thus, in addition to the Mohr-Coulomb’s plastic shear sliding and a dilatancy type flow rule, a plastic normal deformation has been modeled for two clusters in compression. The overall stress-strain relationship can then be obtained from the mobilization and compressing of clusters through a static hypothesis of the macro-micro relations. The predictions are compared with the experimental results for clay under both drained and undrained triaxial loading conditions. Three different types of clay, including remolded and natural clay, have been selected to evaluate the model’s performance. The comparisons verify that this model is capable of accurately reproducing the overall behavior of clay, which accounts for the influence of key parameters such as void ratio and mean stress. A section of this paper is devoted to show the model’s capability of considering the influence of inherent anisotropy on the stress-strain response under undrained triaxial loading conditions.

Stress Induced and Inherent Anisotropy on Elastic Stiffness of Soft Clays

The influence of anisotropy on the elastic shear modulus of clays was addressed using test results obtained from a series of laboratory tests; i.e., triaxial test and Square oedometer. Under various stress stages imposing under triaxial condition, the deviator stress had an indiscernible influence on the elastic shear modulus. The path of vertically logged elastic shear modulus of clay could reasonably be expressed in terms of mean effective stress. Two pairs of bender elements were installed in the square oedometer so that shear wave velocities of samples which were trimmed parallel or perpendicular to their bedding direction can be measured in the vertical and horizontal directions. For sample trimmed parallel to the bedding direction (conventional one), the horizontal shear wave velocity was larger than the vertical one. The opposite observation, however, was observed for sample trimmed 90 degrees apart. The extents of different in vertically and horizontally logged shear wave velocities from both types of samples were almost similar. This clearly indicated the influence of inherent anisotropy rather than the stress induced. The mean effective stress can well represent the stage variable of elastic shear modulus.

An elasto-plastic model for granular materials with microstructural consideration

In this paper, we have extended the granular mechanics approach to derive an elasto-plastic stress–strain relationship. The deformation of a representative volume of the material is generated by mobilizing particle contacts in all orientations. Thus, the stress–strain relationship can be derived as an average of the mobilization behavior of these local contact planes. The local behavior is assumed to follow a Hertz–Mindlin’s elastic law and a Mohr–Coulomb’s plastic law. Essential features such as continuous displacement field, inter-particle stiffness, and fabric tensor are discussed. The predictions of the derived stress–strain model are compared to experimental results for sand under both drained and undrained triaxial loading conditions. The comparisons demonstrate the ability of this model to reproduce accurately the overall mechanical behavior of granular media and to account for the influence of key parameters such as void ratio and mean stress. A part of this paper is devoted to the study of anisotropic specimens loaded in different directions, which shows the model capability of considering the influence of inherent anisotropy on the stress–strain response under a drained triaxial loading condition.

Induced anisotropy in a sand

Controlled changes of principal stress directions have been applied to dense sand samples undergoing plane strain. The new apparatus and development problems are described before results are presented for a series of tests in which a single sudden rotation of principal stress directions was applied to each sample of dense sand. The magnitudes of these single rota- tions varied from 0° to 90°, individual rotations differing by 5° or 10°. The influence of inherent anisotropy was eliminated by depositing the samples in the direction of the subsequently applied intermediate principal stress (δe2, =O). The results demonstrate that induced anisotropy can have a large effect on magnitudes of strain increments and cause a small and quickly reducing deviation of the principal axes of stress and strain increment. There is a strong indication that for plastic strain no-extension directions define axes of induced anisotropy. Des changements contrôlés de directions de contrainte principale ont été appliqués à des échantillons de sable dense subissant une déformation plane. Les nouveaux appareils et leur constructions sont décrits avant de présenter les résultats d'une série d'essais dans lesquels une seule rotation soudaine de directions de contrainte principale fut appliquée à chaque échantillon de sable dense. L'importance de ces simples rotations variait de 0° à 90°, les rotations individuelles différant de 5° ou 10°. L'influence de I'anisotropie propre fut ˜ĺiminée en déposant les échantillons dans la direction de la contrainte principale intermtdiaire (δe2 = 0) qui allait &citc;tre. Les résultats démontrent que l'anisotropie induite peut avoir un effet considérable sur I'importance de l'accroissement de contrainte et provoquer une petite divergence rapidement réduite des axes principaux d'accroissement de contrainte et de dtformation. 11 y a une forte prtsomption que pour la déformation plastique les directions de non-extension définissent les axes d'anisotropie induite.