3D anisotropic microcracking mechanisms of shale subjected to direct shear loading: A numerical insight

The 3D anisotropic shear mechanical characteristics of shale are complex and insufficiently studied. Insight into the microcracking mechanisms, a series of direct shear tests of shale with various bedding orientations were simulated based on Particle Flow Code (PFC). The numerical calibration was conducted based on the experimental results of direct shear tests with three typical bedding orientations and showed good consistency with the experimental results. In the previous studies about the anisotropy in shear strength of shale, the bedding angle only varied from 0(degrees) to 90(degrees), which was not well considered even for two dimensions. In this work, a series of 3D bedding orientations were considered comprehensively. According to the evolution of microcracks, the elastic stage, crack initiation stage and crack coalescence stage were divided. Except for the short-transverse orientation, shear mode microcracks of bedding plane always occur first, but the penetration failure is formed due to the coalescence of matrix-based microcracks. The shear strength of shale is proportional to the percentage of matrix-based microcracks. The effects of 3D bedding orientation on shear strength and microcracking characteristics were significant. The reasons for the anisotropy in shear strength can be concluded: (1) different bedding orientation results in different percentages of each type of microcracks; (2) different normal stresses acting on the bedding planes lead to different apparent shear strengths of the bedding planes. In addition, the shear strength of shale and its anisotropy can be influenced by the difference in stiffness between the bedding plane and matrix, which has not been noticed before.