Experiment on the strength characteristics and failure modes of granite with pre-existing cracks

Rock masses are typically anisotropic discontinuous materials composed of joints, fractures, and interlayers. Many instability and failure cases in geotechnical engineering have been induced by the expansion and transfixion of cracks in the rock masses. The mechanical properties and fracture characteristics of joints usually determine the bearing capacities and fracture modes of rocks. As such, the investigation of the crack initiation and expansion law, strength, and deformation characteristics of fractured rock masses has great significance. In this study, the rock samples investigated were taken from deep zones in the Sanshandao gold mine, where the surrounding rock masses are fractured and the maintenance costs of deep tunnels are very high. To investigate the mechanical behaviors of fractured rock masses, uniaxial compression experiments were conducted on granite samples with pre-existing cracks. First, cracks were generated in cylindrical rock samples by a water-jet cutter. Then, the rock samples with cracks and intact samples were compressively tested using a rigid testing machine (GAW2000) to determine their strength characteristics, crack initiation rules, and failure modes. The experimental results show that the uniaxial compressive strength (UCS) values of granite samples with pre-existing cracks are lower than those of intact samples, and the extent of the UCS reduction is closely related to β, i.e., the angle between the pre-existing crack and the direction of the external load. When β is 75°, the UCS values of the samples are at their minimum, and the reduction rate reaches 84.5%. These experimental results are in good agreement with the numerical solution of the maximum distortion energy criterion. Pre-existing cracks change the failure modes of rocks. With an increase in the dip angle of a pre-existing crack, the crack initiation angle increases monotonically, and the failure mode of the rock sample changes from shear failure to tensile failure. The mechanical properties and crack initiation characteristics of real-fracture rock samples can more accurately reveal the strength characteristics and failure modes of fissured granite, and thereby provide a scientific basis for the support of fractured rock masses and geotechnical engineering design.