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

C. Y. Kwok, K. Duan, S.A. Faizi
International Journal for Numerical and Analytical Methods in Geomechanics
Discrete element method, intermediate stress, Transversely isotropic rock, True Triaxial Test

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 = 0o, 15o, 30o, 45o, 60o, 75o, and 90o) are created. Six series of test simulations (σ3 = 0, 10, 30, 50, 70, and 100 MPa) are conducted on each sample, with five different σ2 values, varied from σ3 to σ1. 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 σ2, displaying an ascending‐then‐descending trend. An increasing brittleness with σ2 is observed at lower confining pressures for all, but medium anisotropy angles. For higher confining pressures, increasing ductility with σ2 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 σ2 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 σ2, 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.

Keywords: discrete element method, intermediate stress, transversely isotropic rock, true triaxial test

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