It seems likely that improvements in drilling mechanics will result from an understanding of the stress and strain fields in the rock. Along with this an understanding of rock fracture under the appropriate conditions is required. The stress fields in drilling are characterized by high spatial gradients of stress, and the stress gradients are known to interact with rock fracture properties. The present investigation is concerned with quantifying the stress gradient effect on fracture. Laboratory tests were performed on a variety of specimen configurations for three brittle rocks. A very strong effect of stress gradient was found, indicating an apparent elevation of fracture strength with increasing stress gradient. A correlation was shown with a critically stressed distance concept.

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