A series of uniaxial compression tests is carried out on the rock-like brittle specimens with pre-existing three-dimensional (3D) cracks. Acoustic emission (AE) monitoring technique and computerized tomography (CT) scan are used to investigate characteristics of crack propagation. It is found that the uniaxial compressive strength

crack initiation stress and displacement decrease with the increase of the crack depth in the specimens. The propagation of shear cracks and secondary cracks is the key factor to control the failure of specimens with a deep crack. The main failure mode is shear failure accompanied with transverse fractures. The initiation of Mode-I can leads to occurrence of turning points on stress-strain curves while the AE energy reaches a peak value. After crack initiation

the AE energy drops to original level

and remains constant. The AE energy of Mode-II crack initiation is 8-10 times higher than that of Mode-I. The wing cracks in the loading process forms an anti-symmetrical shape with the crack center line being the symmetry axis. The initiation angle of the wing crack increases gradually with the crack spreading into the specimen. The propagation shape of wing cracks is leaf-like with an anti-symmetrical structure. A large amount of shear cracks and secondary cracks are observed in rock-like specimens. Shear crack plane is initiated towards the extension direction of pre-existing cracks until the pre-existing crack is terminated.