Cavity expansion of rock and effect of stress state

Cavity expansion tests were simulated using the discrete element method (DEM), where a two-dimensional bonded particle model was validated to investigate the initiation of tensile fracture and breakdown (peak) pressure. The effect of far-field stress, as well as borehole size, on cavity expansion of rock was studied; specimens with three different sizes under various levels of far-field stresses were tested numerically. It is evident that an increase in borehole size results in a lower breakdown pressure. Furthermore, it is illustrated that the breakdown pressure increases with the decrease of the deviatoric component of the stress state, regardless of specimen size. As the far-field stress approaches the hydrostatic condition, the size effect on breakdown pressure reduces. The stress state affects the brittleness of the system, modifying the transition between strength-driven and energy-driven failure.