DEM investigation on the mechanical behavior of mudstone in the hollow cylinder torsional shear test

The discrete element method (DEM) has been used to investigate the mechanical and failure behavior and the associated micro-cracking process of mudstone under the hollow cylinder torsional test by using a commercial DEM code PFC3D. Micro-parameters of the numerical model were first calibrated to the experimental results of the uniaxial compression tests and the uniaxial tensile tests. Based on the calibration results, numerical hollow cylindrical torsional shear tests were carried out under three typical stress conditions, i.e. triaxial compression, torsional compression and pure torsional shear. The numerical results show that the mechanical behavior of mudstone, in terms of rock strength, micro-crack initiation and propagation, and failure mode, can be significantly affected by principal stress rotation, which is in line with previous experimental results. The analysis of the micro-crack process confirms that the principal stress rotation plays an important role in the initiation and propagation of micro-cracks. It is shown that the microcracks induced by tensile stress occur in a preferential direction parallel to the major principal stress sigma 1, while the orientation distribution of the shear micro-cracks is related to the shear bands. Furthermore, the initial micro-crack stress is found to decrease after introducing the principal stress rotation, which provides a new interpretation of degradation of strength and permeability of mudstone under principal stress rotation. Three typical failure modes of specimens in different loading states are reproduced in this study, and the microscopic mechanism is also discussed. Based on this study, it is found that ignoring the principal stress rotation may overrate the stability and tightness of underground storage in bedded rock salt caverns.