Carbonaceous slate is one kind of metamorphic rocks with developed foliation, which is frequently encountered during tunnel construction in Western China. The foliation plays a crucial role in the stability of tunnels. For this, we conducted uniaxial compression tests, acoustic emission (AE) monitoring and scanning electron microscope (SEM) tests on carbonaceous slate. The results show that the strength, failure mode, and AE characteristics exhibit marked anisotropy with the angle between the axial and the foliation (beta). As beta increases, the ultrasonic wave velocity decreases monotonically, whereas the uniaxial compressive strength (UCS) displays a distinctive U-shaped trend. The elastic modulus initially decreases and then increases. The cumulative AE counts curve and energy curve show a stepped growth when beta 45 degrees. Upon failure, the energy release accounts for the highest proportion (67%) when beta = 45 degrees, while the proportions in other cases are less than 37%. The maximum percentage (31%) of shear cracks is reported when beta = 60 degrees, which is six times greater than that at beta = 0 degrees. Moreover, Kernel density estimation analysis reveals that the high concentration area with low AF (AE counts/duration time) and high RA (rise time/amplitude) increases initially, and then decreases when beta > 60 degrees. In addition, nine types of cracks and seven modes of failure were identified. The foliation angle has a pronounced impact on shear failure modes in comparison with tensile failure modes. The supports could suffer larger deformation when beta >= 60 degrees compared to other cases. The failure behaviors correspond well with field observations. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

To achieve the loading of the stress path of hard rock, the spherical discrete element model (DEM) and the new flexible membrane technology were utilized to realize the transient loading of three principal stresses with arbitrary magnitudes and orientations. Furthermore, based on the deep tunnel of China Jinping Underground Laboratory II (CJPL-II), the deformation and fracture evolution characteristics of deep hard rock induced by excavation stress path were analyzed, and the mechanisms of transient loading-unloading and stress rotation-induced fractures were revealed from a mesoscopic perspective. The results indicated that the stress-strain curve exhibits different trends and degrees of sudden changes when subjected to transient changes in principal stress, accompanied by sudden changes in strain rate. Stress rotation induces spatially directional deformation, resulting in fractures of different degrees and orientations, and increasing the degree of deformation anisotropy. The correlation between the degree of induced fracture and the unloading magnitude of minimum principal stress, as well as its initial level is significant and positive. The process of mechanical response during transient unloading exhibits clear nonlinearity and directivity. After transient unloading, both the minimum principal stress and minimum principal strain rate decrease sharply and then tend to stabilize. This occurs from the edge to the interior and from the direction of the minimum principal stress to the direction of the maximum principal stress on the epsilon 1-epsilon 3 1-epsilon 3 plane. Transient unloading will induce a tensile stress wave. The ability to induce fractures due to changes in principal stress magnitude, orientation and rotation paths gradually increases. The analysis indicates a positive correlation between the abrupt change amplitude of strain rate and the maximum unloading magnitude, which is determined by the magnitude and rotation of principal stress. A high tensile strain rate is more likely to induce fractures under low minimum principal stress. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).