Effect of the layer orientation on mechanics and energy evolution characteristics of shales under uniaxial loading

The uniaxial compression tests were conducted on the cylindrical shale specimens with bedding plane inclined at 0° and 90° to the axial loading direction, respectively. Effect of the bedding orientation on the mechanical property and energy evolution characteristics of shales was revealed. The failure mechanisms of the specimens with layers in 0° orientation showed splitting failure along weak bedding, while the specimens with layers in 90° orientation were failed by shearing sliding. The values of compressive strength, elastic modulus and shear modulus of samples at 0° were higher than those of samples at 90° and there was little difference of Poisson's ratio between samples at 0° and 90°. The analysis of the stress-strain energy and acoustic emission (AE) energy indicated that the growth rate of absorbed energy density and elastic energy density at 0° was significantly faster than that at 90°, hence their final values at 0° were relatively larger than the latter. Moreover, higher energy release was observed for specimens at 0°. The energy release and rapid growth of energy dissipation also appeared more early at 0°. The stress ratio 63% was a critical point of energy distribution at which differences started to arise between samples at 0° and 90°. These results indicated that the failure of shale at 0° was more violent and devastative than the failure of shale at 90°.