Anisotropic mechanical characteristics and energy evolution of artificial columnar jointed rock masses subjected to multi-level cyclic loading

Columnar jointed rock masses (CJRM) are natural geological structures with regular and irregular polygons. Because of the joints throughout, the rock mass mechanical properties are severely degraded. This study focused on the effect of joint dip angle, fatigue effect, and stress level on the anisotropic strength, deformation behavior, energy evolution characteristics, and failure mode of CJRM specimens. Triaxial compression and multi-level cyclic loading tests were performed on artificial CJRM specimens with a similar geological structure under a 7 MPa confining pressure. Energy dispersive spectroscopy (EDS) tests were performed on the hexagonal prism and joint structure to investigate the failure mechanism of the CJRM specimens. The experimental results show that the relationship between the characteristic stress and the joint dip angle is a typical U-shaped pattern. The anisotropy of characteristic stresses decreases gradually under triaxial compression. The fatigue effect increases peak strength's anisotropy and decreases residual strength's anisotropy. The plastic strain and strain energy depend on stress level, fatigue effect, and joint dip angle. Three failure modes were observed. Disintegration and fracture of hexagonal prisms, and joint cracking result in the failure of CJRM specimens. Overall, the fatigue effect increases the number and distribution of cracks in CJRM specimens.