Experimental study on anisotropic mechanical characteristics of jointed rock masses under unloading condition

The triaxial unloading tests on the precasted rock samples of a single joint with different dip angles were conducted to investigate the anisotropic mechanical behaviour of jointed rock masses under excavation including the stress-strain curves, the deformation and strength characteristics and the failure modes. (1) The stress-strain curves of jointed samples with dip angles of 0°, 30° and 90° had a yield phase, a softening phase and a residual phase upon unloading, while the samples with dip angles of 45° and 60° had a yield phase only. (2) The variation of the deformation moduli versus the angles of joint inclination is in U-shape and the deformation modulus of the sample with the dip angle of 60° is the smallest. The differences of the deformation characteristics of different samples decreased gradually with the increasing of the confining pressure. (3) The compressive strengths of the samples with dip angles of 0°, 30° and 90° were reduced, and those of the samples with dip angles of 45° and 60° were changed little. The cohesion versus the angle of joint inclination is in U-shape. The cohesion of the sample with the dip angle of 60° is also the smallest. The internal angle of friction increased with increase of the angle of joint inclination. (4) The failure modes of the samples with dip angles of 0°, 30° and 90° were all shear failure across the joint surface and were not effected by the joint. The samples with dip angles of 45° and 60° were sliding failure along the joint surface. (5) It was revealed that the unloading mechanical characteristics of the jointed rock masses were controlled by the strengths of both rocks and joint surfaces.