Study on mesoscopic failure characteristics of layered red shale

To study the mesoscopic failure characteristics of the layered red shale, uniaxial mechanical experiments were carried out by applying DSTD-1000 electro-hydraulic servo rigid pressure on the red shale with different bedding angles. Meanwhile, SEM and XRD experiments were implemented on debris to obtain its mechanical parameters, failure characteristics, microstructure, and relative content of components. Based on this, models of the red shale with different bedding angles were constructed using the discrete element PFC^3D for the uniaxial compression experiments to study the mesoscopic failure characteristics of red shale. The results show that: (1) When the bedding angle θ=30°, 45°, 60° and 75°, the samples fail along the bedding direction to produce a slip surface. (2) The crack evolution process of samples with different bedding angles shows the changes in the slow growth stage, accelerated growth stage, and stable stage. When θ=45°, 60° and 75°, more cracks are generated in the unit strain in the crack evolution process, which leads to the rapid slip failure of the sample along the weak plane of the bedding. (3) The stereographic projection of meso-cracks and the rock fabric diagram show that: the micro-cracks tend to be more evenly distributed, mainly parallel to or sub-parallel to the loading direction when θ=0°, 75° and 90°, showing strong anisotropy after loading failure. When 15°≤θ≤60°, the micro-crack tendency gradually parallels to the bedding direction. (4) The crack initiation stress threshold solved by the elastic modulus method considering the crack evolution characteristics is 36.6%‒60.3% of the peak strength of each bedding, and the crack damage stress threshold is 75.1%‒90.4% of the peak strength of each bedding, which is consistent with the threshold range obtained by a large number of physical experiments. It is indicated that the method has certain applicability for solution of the crack stress threshold.