Failure in brittle rocks is generally related to initiation, propagation and coalescence of cracks. Crack initiation, propagation and stress redistribution induce rock damage around the excavated space eventually leading to breakout, notch formation and abrupt failure. Based on the analytical solution for sliding crack model, crack initiation threshold and comparison between the stress intensity factor at the tip of the preexisting cracks and the fracture toughness, a method is introduced for predicting the unstable zones with potential for crack initiation around underground spaces. The method needs the crack initiation threshold, principal stresses around the space and fracture toughness as main input parameters. This method is then used for predicting the unstable zone around a circular space and rock characteristics are chosen to be equal to the rock mass around a tunnel in URL, Canada. Additionally, the effect of different parameters on the shape and extent of the unstable zone predicted by this method is discussed.


Rocks failure is generally assumed to be the consequence of crack initiation, propagation and coalescence. Crack initiation and coalescence stages are associated with stress levels of crack initiation and crack damage threshold. Due to the stress redistribution induced by excavation of an underground space, the mechanical properties of the rock surrounding the opening are changed. Knowledge of the extent of the excavation damaged zone is important for design and construction of underground excavations. So it is important to have a suitable criterion to predict the onset of this kind of damage. Due to the extensive studies on stability analysis in underground spaces, which are excavated in brittle rocks, the role of crack initiation threshold and crack growth theories are widely considered in recent researches. The relation between crack growth threshold and fracture mechanic theories also studied in literature.

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