Abstract: Trident cracks are the ubiquitous form of rock mass defect in nature, it has a significant influence on mechanical properties of rock mass. Uniaxial compression tests were conducted on cement mortar samples containing prefabricated trident cracks. The crack initiation, propagation and coalescence processes were recorded by a camera at the same time. The strain fields of the specimens were obtained by the digital image correlation (DIC) method. Combined with the PFC2D software, we studied the strength characteristics, crack modes and crack propagation rules of specimens under different angles of ? and ?. The results show that trident cracks have an obvious weakening effect on uniaxial compressive strength of specimens. When ? is constant at 120°, the sample reaches the maximum compressive strength when ? is 30°. When ? is constant at 90°, the compressive strength of the sample decreases first and then increases with increasing ?, and the maximum compressive strength is achieved when ? is 45°. There are three types of cracks generated in the specimens, including tensile crack (mode I), shear crack (mode II) and mixed crack (mode III). These three types of cracks usually initiate from the tip of crack. The mode I crack propagates along the loading direction normally and usually does not extend to the sample boundary, but the mode II and mode III cracks normally propagate to the sample boundary at a certain angle to the loading direction. By analysing the geometry of cracks and the composition of microcracks that further leads to macrocracks, it is known that the failure of the specimen containing trident cracks under uniaxial compression is tensile failure. The strain fields obtained by the DIC show that when the load reaches a certain stage, stress concentration occurs at the tip of a crack, then microcracks begin to develop and form micro-cracked areas, and finally, macro-cracks are generated. Through the analysis of the principal strain and shear strain fields, it is found that the failure of specimens is tensile failure, and the shear strain has little influence on the crack propagation process.

Key words: rock, trident cracks, uniaxial compression, particle flow code (PFC), digital image correlation