The effects of intermediate principle stress on the mechanical behavior of transversely isotropic rocks: Insights from DEM simulations

Abstract

The discrete element method (DEM) is used to study the response of anisotropic rocks under true triaxial testing. Numerical samples of seven different bedding orientations (β = 0 o , 15 o , 30 o , 45 o , 60 o , 75 o , and 90 o ) are created. Six series of test simulations (σ 3 = 0, 10, 30, 50, 70, and 100 MPa) are conducted on each sample, with five different σ 2 values, varied from σ 3 to σ 1 . The effects of anisotropy and intermediate stress on the peak strength, brittle-ductile transition, and degree of anisotropy are subsequently explained through underlying micromechanics. Results show a "fan-shaped" variation of the peak strength with σ 2 , displaying an ascending-then-descending trend. An increasing brittleness with σ 2 is observed at lower confining pressures for all, but medium anisotropy angles. For higher confining pressures, increasing ductility with σ 2 is seen for every anisotropy angle. A U-shaped variation of peak strength with anisotropy angles is noted that flattens under high intermediate stress. Hence, for numerical models of Posidonia shale under normalized σ 2 higher than 0.76, the anisotropy effect is found to be negligible. Micromechanical analyses reveal that the stress asymmetry, suppression of weak plane action as well as the localization and coalescence of microcracks in the intact rock matrix, due to σ 2 , are the contributors towards the obtained trends. Since existing failure criteria do not weigh in these features in geotechnical assessments, this paper helps future studies by providing a deeper understanding of these effects and a comprehensive data set for the analyses of anisotropic rocks under polyaxial stress conditions. K E Y W O R D S discrete element method, intermediate stress, transversely isotropic rock, true triaxial test

Investigation of Mechanical Characterization of Transversely Isotropic Rock Mass Based on Rigid Block Discrete Element Method

“…For numerical simulation, Chen et al proposed the random field simulation using the Karhunen-Loève (K-L) expansion [15]. Faizi et al conducted triaxial numerical simulations based on the discrete element method for different dip angle models and investigated the mechanical behavior of transversely isotropic rock mass [16]. Zhao et al calculated the damage zone at the laminate tip based on the Kachanov equation and discussed the crack connections between the laminate surfaces.…”

“…In addition, using the DEM model allows for a more accurate simulation of crack extension in transversely isotropic rock masses. Adjacent boundaries no longer use the SJ smooth joint model but instead use particle-to-particle contact to transfer forces [16]. By setting different strength parameters for the blocks and joints, we can simulate the columnar transverse isotropic rock properties more realistically.…”

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“…Many rocks exhibit anisotropy in both their mechanical deformation and fluid flow behaviors 1–7 . Failure to consider this aspect could lead to large errors in the prediction of their hydro‐mechanical responses 8–11 .…”

An analytical approach for cased borehole stress calculation in general anisotropic formations

“…Besides these analytical studies, a number of researchers 1,3,5,30 have conducted cased borehole stress analyses using numerical modeling methods, such as finite element method and finite volume method. Other numerical modeling approaches [31][32][33][34][35] applicable to anisotropic material can also be applied to the study of the cased borehole problem. However, there is a lack of rigorous study on the effect of formation anisotropy on cased borehole stress analysis.…”