Experimental and GBM3D study on the failure mechanical behavior of granite with different grain sizes under conventional triaxial compression

Granite, as a ty pical crystalline rock in the earth's crust, is one of the ideal media in protection works for radioactive waste disposal. Grain size significantly influences the conventional triaxial compression mechanical properties of granite, thus also on the safety and stability of nuclear waste repositories. Thus, triaxial compression tests were performed by combining experiments with GBM3D based on PFC3D to study granite's conventional triaxial mechanical properties in grain size. Furthermore, the flexible boundaries were constructed in the numerical model using wall-zone coupling to apply lateral constraints in the model. The experiment outcomes demonstrate that coarse-grain granite contains more defects than the fine-grain granite. Concerning mechanical properties, fine-grained granite has much better mechanical properties than coarse-grained granite. However, the compressive of the initial defects at high confining pressures narrow this gap. The numerical simulation results show that the GBM3D model based on the flexible boundaries applied to the confining pressure can reasonably simulate the macroscopic mechanical behavior and non-uniform deformation of granite. This method better reflects the inhomogeneous deformation of granite specimens. The larger the grain size, the greater the spatial variation between mineral crystals, the more significant the effect on compressive stress atrophy and the tendency to produce macroscopic shear failure zones. As the confining pressure increases, the damage to the specimen is dominated by P_CT/P_IC leading to P_CS/P_TC transformation.