Triaxial Stresses

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Bezalel C Haimson - One of the best experts on this subject based on the ideXlab platform.

  • failure characteristics of two porous sandstones subjected to true Triaxial Stresses applied through a novel loading path
    Journal of Geophysical Research, 2017
    Co-Authors: John W Rudnicki, Bezalel C Haimson
    Abstract:

    We performed an extensive suite of true Triaxial experiments in two porous sandstones, Bentheim (porosity ≈ 24%) and Coconino (17%). The experiments were conducted using a novel loading path, which maintains constant Lode angle (Θ) throughout the test. This path enabled the examination of the effects of Lode angle and mean stress on failure (σoct,f). Our tests covered σ3 magnitudes between 0 and 150 MPa and of Θ at −30° (axisymmetric extension), −16°, 0°, +11°, +21°, and +30° (axisymmetric compression). Test results revealed the respective contribution of each of the two stress invariants to failure stress, failure plane angle, and failure mode. In both sandstones, the shear stress required for failure increases with mean stress but decreases with Θ when shear failure mode dominates. However, the dependence of failure stress on mean stress and Θ is reversed when the compactive failure mode is in control. The compactive failure mode was evident in Bentheim sandstone when compaction bands were observed under high mean stress. The Coconino sandstone did not reach the compactive failure regime within the maximum confinement applied. The failure plane angle monotonically decreases with increasing mean stress and Θ. For Coconino sandstone, failure plane angle varies between 80° and 50° for σoct,f between 50 and 450 MPa whereas it drops to 0° as σoct,f, approaches 250 MPa in Bentheim sandstone. We employed the bifurcation theory to relate the stress conditions at failure to the development of failure planes. The theory is in qualitative agreement with the experimental data.

  • failure characteristics of two porous sandstones subjected to true Triaxial Stresses
    Journal of Geophysical Research, 2016
    Co-Authors: Bezalel C Haimson
    Abstract:

    We conducted an extensive suite of true Triaxial experiments in two porous sandstones, Bentheim (porosity ≈ 24%) and Coconino (17.5%). Our experiments demonstrate that failure of both sandstones is not only a function of σ3 but also of σ2. For a given σ3, σ1 at failure (σ1,peak) increases as σ2 is raised above σ3 between tests. The σ1,peak reaches a peak as σ2 is about halfway between σ3 and σ1 and then gradually decreases such that when σ2 ≈ σ1,peak, it approaches its initial magnitude when σ2 = σ3. For a constant σ3, failure-plane angle increases with σ2 by a maximum of less than 10° as σ2 rises from σ2 = σ3 to σ2 = σ1,peak. The effect of σ2 on both failure level and failure-plane angle is stronger in the lower-porosity Coconino sandstone than in the Bentheim sandstone. The σ2 dependence of failure mode in the Bentheim is different than Coconino over the same σ3 range. Both sandstones failed dilatantly at low σ3 magnitudes. However, at high σ3 (100–120 MPa), Bentheim sandstone developed shear-enhanced compaction bands, followed by pure compaction bands at σ3 = 150 MPa. Compaction bands were not observed in the Coconino. Microscopic observations via SEM reveal that tensile microcracking is dominant when shear banding occurs (under low σ3), while pervasive grain crushing and pore collapse inside compaction bands are observed at high σ3.

Richeng Liu - One of the best experts on this subject based on the ideXlab platform.

Junlong Shang - One of the best experts on this subject based on the ideXlab platform.

  • rupture of veined granite in polyaxial compression insights from three dimensional discrete element method modeling
    Journal of Geophysical Research, 2020
    Co-Authors: Junlong Shang
    Abstract:

    Granite formations have been frequently involved in subsurface energy‐related activities such as radioactive waste disposal, oil, and gas storage. This is principally because of their mechanical stability, low permeability, and high corrosion resistance. These favorable properties, however, can be compromised by the addition of mineral veins, which tend to occur ubiquitously in upper crustal rock formations. Evaluation of the impact of veins on the integrity and rupture characteristics of granite, especially under true Triaxial Stresses, is therefore important yet currently underemphasized. This study examines the rupture of veined granite in polyaxial compression via a discrete element method model. In the model with soft veins, the rupture is localized along the fabricated inclined veins (45° relative to the horizontal with strike running in the σ2 direction) under low confining Stresses (σ2 < 67 MPa); in contrast, a combined rupture of veins and granite matrix is observed when σ2 is increased to 141.6 MPa. Shear sliding along the inclined veins is revealed by examining the displacement field. Shear‐induced volumetric dilation is suspected in the soft‐veined models in relatively low confining Stresses (σ2 < 67 MPa) with sliding and dilation behavior apparently suppressed at σ2 = 141.6 MPa. Hard veins impede local rupture, resulting in conjugate shear bands. The well‐recognized σ2 effect is observed for the hard‐veined models, while no pronounced σ2 effect is noticed for the soft‐veined models. This study also reveals that vein thickness has a negligible impact on rupture characteristics, which is however profoundly affected by vein orientation.

Hongwen Jing - One of the best experts on this subject based on the ideXlab platform.

Kittitep Fuenkajorn - One of the best experts on this subject based on the ideXlab platform.

  • Shear strengths of sandstone fractures under true Triaxial Stresses
    Journal of Structural Geology, 2013
    Co-Authors: Piyanat Kapang, Chaowarin Walsri, Tanapol Sriapai, Kittitep Fuenkajorn
    Abstract:

    Abstract True Triaxial shear tests have been performed to determine the peak shear strengths of tension-induced fractures in three Thai sandstones. A polyaxial load frame is used to apply mutually perpendicular lateral Stresses ( σ p and σ o ) to the 76 × 76 × 126 mm rectangular block specimens. The normal to the fracture plane makes an angle of 59.1° with the axial (major principal) stress. Results indicate that the lateral stress that is parallel to the fracture plane ( σ p ) can significantly reduce the peak shear strength of the fractures. Under the same normal stress ( σ n ) the fractures under high σ p dilate more than those under low σ p . According to the Coulomb criterion, the friction angle decreases exponentially with increasing σ p / σ o ratio and the cohesion decreases with increasing σ p . The lateral stress σ p has insignificant effect on the basic friction angle of the smooth saw-cut surfaces. The fracture shear strengths under σ p  = 0 correlate well with those obtained from the direct shear tests. It is postulated that when the fractures are confined laterally by σ p , their asperities are strained into the aperture, and are sheared off more easily compared to those under unconfined condition.

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