Brazilian Tensile Strength of Anisotropic Rocks: Review and New Insights

Abstract

Strength anisotropy is one of the most distinct features of anisotropic rocks, and it also normally reveals strong anisotropy in Brazilian test Strength ("BtS"). Theoretical research on the "BtS" of anisotropic rocks is seldom performed, and in particular some significant factors, such as the anisotropic tensile strength of anisotropic rocks, the initial Brazilian disc fracture points, and the stress distribution on the Brazilian disc, are often ignored. The aim of the present paper is to review the state of the art in the experimental studies on the "BtS" of anisotropic rocks since the pioneering work was introduced in 1964, and to propose a novel theoretical method to underpin the failure mechanisms and predict the "BtS" of anisotropic rocks under Brazilian test conditions. The experimental data of Longmaxi Shale-I and Jixi Coal were utilized to verify the proposed method. The results show the predicted "BtS" results show strong agreement with experimental data, the maximum error is only~6.55% for Longmaxi Shale-I and~7.50% for Jixi Coal, and the simulated failure patterns of the Longmaxi Shale-I are also consistent with the test results. For the Longmaxi Shale-I, the Brazilian disc experiences tensile failure of the intact rock when 0 • ≤ β w ≤ 24 • , shear failure along the weakness planes when 24 • ≤ β w ≤ 76 • , and tensile failure along the weakness planes when 76 • ≤ β w ≤ 90 • . For the Jixi Coal, the Brazilian disc experiences tensile failure when 0 • ≤ β w ≤ 23 • or 76 • ≤ β w ≤ 90 • , shear failure along the butt cleats when 23 • ≤ β w ≤ 32 • , and shear failure along the face cleats when 32 • ≤ β w ≤ 76 • . The proposed method can not only be used to predict the "BtS" and underpin the failure mechanisms of anisotropic rocks containing a single group of weakness planes, but can also be generalized for fractured rocks containing multi-groups of weakness planes.

Prediction of Mud Pressures for the Stability of Wellbores Drilled in Transversely Isotropic Rocks

“…Anisotropic rocks exhibit different properties in different directions. Several experimental studies ( [22][23][24][31][32][33][34][35][36][37][38][39][40][41]) indicate that the majority of sedimentary and metamorphic rocks are affected by strength anisotropy related to discontinuities. The results of triaxial tests and direct and indirect tensile tests carried out on these rocks show that strength changes with loading orientation.…”

“…However, the failure pattern in these rocks is quite complex. A review of Brazilian tests on anisotropic rocks performed by Ma et al [24] indicated that 5 types of failure can occur: tensile failure across the weakness planes; tensile failure along the weakness planes; shear failure across the weakness planes; shear failure along the weakness planes; and mixed failure.…”

“…Given the difficulty of describing the complex failure mechanisms in anisotropic rocks, we adopted the Nova and Zaninetti criterion, considering that the uniaxial tensile strength of this criterion is substituted by a phenomenological (apparent) tensile strength, i.e., the "Brazilian test strength" [24].…”

“…According to Ma et al [22][23][24], the stress state in the central region of the disk can be simplified with an element subject to a tensile force (Figure 3). This simplification allowed us to define the tensile strengths reported in Equation (4).…”

“…In order to use the weakness plane model, the Hoek and Brown criterion and the Nova and Zaninetti criterion, we needed the data of triaxial tests and tensile tests carried out on rocks with strength anisotropy at different values of β w . We selected the data of laboratory tests carried out on the following rocks: Martinsburg slate [30], Artificial rock [41], Top seal shale [34], Bossier shale [52], Green River I shale [37], and Longmaxi shale [22][23][24]53]. With a linear regression, we calculated the instantaneous uniaxial compressive strengths C oβw and the instantaneous constants m βw by using the data of the triaxial tests shown in Figure 4.…”