Strength anisotropies in mudrocks (original) (raw)
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Strength anisotropy of shales deformed under uppermost crustal conditions
Journal of Geophysical Research: Solid Earth
Conventional triaxial tests were performed on three sets of samples of Tournemire shale along different orientations relative to bedding (0 ∘ , 45 ∘ , and 90 ∘). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 160 MPa, at strain rates ranging between 3 × 10 −7 s −1 and 3 × 10 −5 s −1. This allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure. Results show that the orientation of principal stress relative to bedding plays an important role on the brittle strength, with 45 ∘ orientation being the weakest. We fit our results with a wing crack micromechanical model and an anisotropic fracture toughness. We found low values of internal friction coefficient and apparent friction coefficient in agreement with friction coefficient of clay minerals (between 0.2 and 0.3) and values of K Ic comparable to that already published in the literature. We also showed that strain rate has a strong impact on peak stress and that dilatancy appears right before failure and hence highlighting the importance of plasticity mechanisms. Although brittle failure was systematically observed, stress drops and associated slips were slow and deformation always remained aseismic (no acoustic emission were detected). This confirms that shales are good lithological candidates for shallow crust aseismic creep and slow slip events.
A review of methods, techniques and approaches on investigation of rock anisotropy
An extensive review on the anisotropy of rock samples has been carried out to characterize the velocity and strength behaviors under a variety of geometrical and mechanical conditions. Primarily, the causes and impacts of anisotropy is discussed to further understand the importance of the effect of such material from an engineering point of view. The strength anisotropy is investigated in laboratory using the standard strength testing practices (UCS, Triaxial, direct shear and etc..) to perceive the directional dependence of strength for anisotropic rocks and the velocity anisotropy using the ultrasonic scanning of the samples under destructive tests to evaluate the cracks propagation, density and orientation. Then, thorough literature review is done to highlight the significant observations that have been previously elicited. Furthermore, the mathematical determination methods of the degree of anisotropy are explored. Finally, this paper summarize that the strength and velocity anisotropy might be influenced by almost the same factors; however, the behavior of each anisotropy may not be the same considering the rock matrix and failure criteria.
Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, 2020
Mine slope geotechnical design requires an engineering estimation of the intact rock's uniaxial compressive strength, which is influenced by the anisotropic nature of sedimentary rocks. In order to determine the rock's anisotropic behaviour, numerous rock samples from diamond drilling were tested in the field by using the point load test (PLT) method. The effect of anisotropy on the calculated point load strength (()) from axial and diametral point load testing was investigated. The finding of this paper can be used to select σci input for the Hoek-Brown failure criteria based on low-cost PLT results.
Bulletin of Engineering Geology and the Environment, 2013
A modified empirical criterion is proposed to determine the strength of transversely anisotropic rocks. In this regard, mechanical properties of intact anisotropic slate obtained from three different districts of Iran were taken into consideration. Afterward, triaxial rock strength criterion introduced by Rafiai was modified for transversely anisotropic rocks. The criterion was modified by adding a new parameter for taking the influence of strength anisotropy into consideration. The results obtained have shown that the parameter can be considered as the strength reduction parameter due to rock anisotropy. The modified criterion was compared to the modified Hoek-Brown and Ramamurthy criteria for different anisotropic rocks. It was concluded that the criterion proposed in this paper is a more accurate and precise criterion in predicting the strength of anisotropic rocks.
Determination of deformability and tensile strength of anisotropic rock using Brazilian tests
International Journal of Rock Mechanics and Mining Sciences, 1998
This paper is the first of a series of two papers dealing with the determination of the deformability, tensile strength and fracturing of anisotropic rocks by diametral compression (Brazilian test) of discs of rock. It presents a combination of analytical and experimental methods for determining in the laboratory the elastic constants and the indirect (Brazilian) tensile strength of transversely isotropic rocks, i.e. rocks with one dominant direction of planar anisotropy. A computer program based on the complex variable function method and the generalized reduced gradient method was developed to determine the elastic constants of idealized linearly elastic, homogeneous, transversely isotropic media from the strains measured at the center of discs subjected to diametral loading. The complex variable function method was also used to construct charts for determining the indirect tensile strength of anisotropic media from the failure loads measured during diametral loading. Brazilian tests were conducted on four types of bedded sandstones assumed to be transversely isotropic. Based on strain measurements obtained with 45 ° strain gage rosettes glued at the center of the discs, the five independent elastic constants of the tested rocks could be determined. The elastic constants determined with the Brazilian tests were compared with those obtained from conventional uniaxial compression tests. The indirect (Brazilian) tensile strength of the tested sandstones was found to depend on the angle between the apparent planes of rock anisotropy and the direction of diametral loading.
Small strain stiffness anisotropy of natural sedimentary clays: review and a model
2013
Very small strain stiffness anisotropy of sedimentary clays is investigated. First, a general formulation of transversely isotropic elastic model is summarised, followed by a description of its complete parameter identification using transversal and longitudinal wave velocity measurements. Then, an extensive experimental database from the literature is reviewed. A number of general trends in the anisotropy evolution is identified, based on which a model is developed describing the dependency of the ratio of in-plane and transversal very small strain shear moduli on the stress state and overconsolidation ratio. Subsequently, an empirical relation between the ratios of shear moduli and Young moduli is quantified. The most problematic tends to be the evaluation of Poisson ratios and evolution of stiffness anisotropy under general stress conditions. These issues remain to be investigated experimentally in future work.
Effects of Anisotropy and Saturation on Geomechanical Behavior of Mudstone
Journal of Geophysical Research: Solid Earth, 2019
The abundance of mudstone in Earth's crust and its academic and industry applications has led to advancements in the understanding of mudstone deposition and preservation. However, there are few quantitative geomechanical studies on mudstone. To test correlations between anisotropy, fluid saturation levels, and deformability and strength parameters in mudstone, a suite of indirect tensile and unconfined and triaxial compression tests was completed on samples from the Mancos Shale and the Agrio Formation. Discs or plugs were tested with axial loading parallel or perpendicular to bedding, at various confining pressures, and at saturation levels from 0 to 17%. Tensile strength, compressive strength, Young's Modulus, Secant Modulus, stress drop, and Energy Released exhibit mechanical anisotropy and decrease due to saturation. Tensile strength anisotropy revealed the weakness of heterolithic bedding in mudstone, as tensile fractures commonly propagated along bedding interfaces. Saturation increased mechanical anisotropy for tensile and compressive strength. A compressive strength reduction of 39% to 75% was observed between dry and partially saturated plugs. Lithofacies were used to classify lithologic variations in different types of mudstone. When partially saturated, sandstone-bearing facies revealed a greater reduction in strength and moduli than clay-rich mudstone facies. Empirical relationships between partially saturated and dry sample strength and moduli are proposed, to allow for estimation of deformation and failure behavior under in-situ conditions. The effects that saturation, orientation, and lithofacies have on mechanical behavior are of interest in numerous
On the Determination of Rock Anisotropy for Stress Measurements
2008
The traditional methods of in situ stress determination can be seriously affected by rock anisotropy, which necessitates both development of the corresponding interpretation methods and accurate determination of the elastic characteristics of the anisotropic rocks. In this contribution we consider the method of determination of anisotropic moduli by testing small subcores drilled in different directions. Rock anisotropy is often induced by the presence of joints, foliation, schistosity, bedding or similar features. In the case when the spacing between the theses features exceeds the length of the subcores the results of moduli determination in separate subcores shows considerable variability. A mechanism of this variability lies in the fact that a particular subcore may or may not be intersected by a joint. We show that despite this, the averaging procedure used in the method of moduli reconstruction, developed previously by the authors, can still be successfully applied, since the averaging over the subcores recovers the large-scale moduli. However, the standard deviation associated with the randomness of joint-subcore intersection can in some case be very large (potentially unlimited) which explains the observed variability.