A true triaxial apparatus was designed and fabricated to achieve multi-axial stress states with prismatic specimens. The device, which is a modification of the University of Minnesota plane-strain apparatus, is equipped with piston assemblies that generate the intermediate principal stress. A closed-loop, servo-hydraulic load frame is used to control the major principal stress, while the minor principal stress is developed by fluid pressure. A stress path was developed to perform experiments under various states of stress at constant mean stress. Results from strength testing on Dunnville sandstone under conventional compression and extension are compared with the results performed under multi-axial stress states. Failure data were plotted in principal stress space to investigate the effect of intermediate stress on strength. A plane fitting method was used to obtain the material parameters of two, six-sided Paul–Mohr–Coulomb failure surfaces, which capture the strength characteristics of the sandstone over a range of mean stress.

Pressure decline caused by the extraction of oil from deep sedimentary layers depends on the pore modulus Kpp, a poroelastic parameter that characterizes the effect of pressure change on pore volum...

#2Feitao Zeng(DUT: Dalian University of Technology)H-Index: 4

Last. Yuan Li(USTB: University of Science and Technology Beijing)H-Index: 3

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Abstract A failure criterion typically is phenomenological since few models exist to theoretically derive the mathematical function. Indeed, a successful failure criterion is a generalization of experimental data obtained from strength tests on specimens subjected to known stress states. For isotropic rock that exhibits a pressure dependence on strength, a popular failure criterion is a linear equation in major and minor principal stresses, independent of the intermediate principal stress. A gen...

#1Feitao Zeng(UMN: University of Minnesota)H-Index: 4

#2Yuan Li(USTB: University of Science and Technology Beijing)H-Index: 3

Last. Joseph F. Labuz(UMN: University of Minnesota)H-Index: 32

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AbstractPaul-Mohr-Coulomb (PMC) failure criterion provides enhanced representations of pyramidal failure surfaces, with recognizable material parameters, by considering all three principal stresses...

Last. Bezalel C. Haimson(UW: University of Wisconsin-Madison)H-Index: 31

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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 compr...

Last. Bezalel C. Haimson(UW: University of Wisconsin-Madison)H-Index: 31

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Abstract We summarize two suites of recent true triaxial tests in two porous sandstones, Coconino and Bentheim. One suite of tests conducted under constant σ 2 and σ 3 , which revealed the effect of σ 2 and σ 3 on rock failure. The other suite of tests, performed under constant Lode angle, determined the failure dependencies on mean stress and Lode angle. Building and modifying failure descriptions by Matsuoka - Nakai (MN) and Lade-Duncan (LD), an explicit failure criterion (modified-MNLD) that ...

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 ...

The Christensen criterion, originally introduced in materials science, has a simple mathematical form and uniaxial tensile and compressive strength as the only parameters, making it an attractive candidate for rock engineering purposes. In this study, the applicability of the criterion to rock materials is examined. Explicit equations for application of the criterion under biaxial, triaxial compression, triaxial extension, and polyaxial states of stresses are derived. A comprehensive strength da...

Last. Joseph F. Labuz(UMN: University of Minnesota)H-Index: 32

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The Paul-Mohr-Coulomb failure criterion includes the intermediate principal stress sigma(II) and friction angles at the limiting stress states of sigma(II)= sigma(III) and sigma(II) = sigma(I), where sigma(I) and sigma(III) are major and minor principal stresses. Conventional triaxial compression (sigma(II) = sigma(III)), extension (sigma(II) = sigma(I)), and plane strain (sigma(I) not equal sigma(II) not equal sigma(III)) experiments were performed on dry rock. The failure data were plotted in ...

A plane strain condition for testing rock is developed through passive restraint in the form of a thick-walled cylinder. The so-called biaxial frame generates the intermediate principal stress that imposes a triaxial state of stress on a prismatic specimen. Major and minor principal stresses and corresponding strains are accurately measured, providing data to calculate the elastic (Young’s modulus and Poisson’s ratio), inelastic (dilatancy angle), and strength (friction angle and cohesion) param...

Last. Zhao Pengfei(SWPU: Southwest Petroleum University)H-Index: 3

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Abstract The mechanical strength of shale is highly influenced by anisotropy and hydration, which cannot be ignored for the stability evaluation of the geotechnical structures related to shale after water invasion. In the present work, a modified Hoek-Brown failure criterion for unsaturated intact shale considering the effects of anisotropy and hydration has been developed in order to obtain an accurate strength evaluation for unsaturated intact shale. In this proposed failure criterion (abbrevi...