High compressive stresses near the wall of underground openings produce a brittle scaling off fracture, generally localized in two opposite zones. In the elastic theory, the change introduced by this scaling off in the shape of the opening increases the stress concentration factor at the end of the overbreak and the phenomenon might extend itself. In the contrary, a lot of observations show the stabilization of the shape, which may be explained by a ductile behaviour, related to the decrease of the curve radius. As the scaling off is increased by the use of excavating methods which include smooth cutting out, it may be useful to design an overbreak at such a place that the scaling off will be neutralized.

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