Numerical damage model for rock based on microcrack growth, interaction, and coalescence

Rock deformation and failure in brittle rocks subjected to compressive stresses occurs by the progressive damage of the material, as cracks initiate and grow on the small scale, and coalesce to form large-scale fractures and faults. Micromechanical models based on fracture mechanics theory have been developed by many researchers for the progressive damage due to crack growth, interaction, and coalescence. In this paper it is shown how these micromechanical models can be used to predict nonlinear rock behaviour such as strain hardening and softening, dilatation, σ₂ sensitivity, rate dependence, and creep. Also, these micromechanical models have been implemented into a two-dimensional finite element model. In each of the elements, the model considers the growth and interaction of microcracks, and under the appropriate circumstances, the coalescence of microcracks into large-scale splitting or shear fractures. Stress-induced anisotropy due to preferential growth of the microcracks in each of the elements is considered. The damage model has been used to simulate the progressive breakout that occurs around boreholes subjected to compressive stresses. The breakout initiates at the boundary of the hole and progresses inward, finally resulting in a stable breakout shape. The results of this analysis are in agreement with both experimental and numerical results.