Generation and analysis of stable excavation shapes under high rock stresses

Failure criteria based on micromechanical models of splitting failure and shear failure are implemented in boundary element programs to analyze the stability of various two-dimensional excavation shapes under high stress. A technique for modelling the failure processes of progressive spalling, shear, and tensile failure is developed and is used to simulate the formation of stable shapes through progressive failure. The shapes resulting from initially circular openings are strikingly similar to those recognized widely as well-bore breakouts. All the shapes studied lead to the formation of Pointed breakout regions, and these final shapes are stable with respect to splitting, shear, and tensile failure. The size of the failed region is smaller for the case of gradually increasing field stress around a preexisting opening than for an opening made instantaneously in rock with preexisting stress.