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 (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.

中文翻译：

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中间主应力对横观各向同性岩石力学行为的影响：DEM模拟的启示

离散元方法（DEM）用于研究真实三​​轴试验下各向异性岩石的响应。创建了七个不同铺垫方向（β = 0 ^o，15 ^o，30 ^o，45 ^o，60 ^o，75 ^o和90 ^o）的数值样本。六个系列试验模拟的（σ ₃ = 0，10，30，50，70，和100兆帕）在每个样品上进行，用五个不同的σ _2倍的值，从改变σ ₃至σ ₁。各向异性和中间应力对峰值强度，脆性-韧性转变和各向异性程度的影响随后通过基本的微观力学进行了解释。结果显示的峰强度与一个“扇形”变化σ ₂，显示的上升-则-降的趋势。随着增加的脆性σ ₂在所有较低围压下，但介质各向异性的角度观察。对于更高的围压下，随着延展性σ ₂看出对于每个各向异性角度。注意到峰值强度随各向异性角的AU形变化在高中间应力下变平。因此，对于波西多尼亚页岩的数值模型下归一化σ ₂高于0.76时，发现各向异性效应可忽略不计。微机械分析表明，应力不对称，弱面作用的抑制以及微裂纹在完整岩石基质的定位和聚结，由于σ ₂，是向所获得的趋势贡献者。由于现有的破坏准则并未在岩土评估中考虑这些特征，因此本文通过提供对这些效应的更深入了解以及用于多轴应力条件下各向异性岩石分析的综合数据集，为将来的研究提供了帮助。