Experiments with rock: remarks on strength and stability issues

In the classical work of Fairhurst and co-workers on machine stiffness, it was shown that the failure response of a rock specimen can be a mixture of material and structural behavior. This is due to the development, around peak stress, of a localized region of microcracking called the intrinsic zone, which may be a property of the rock. The inhomogeneity formed by the intrinsic zone has a fundamental importance for defining the system behavior in terms of material strength and post-peak instability such that size effects appear. In addition, the testing environment, that is, the load frame, with its finite stiffness and load arrangement, has an impact on the overall response. Observations of the intrinsic zone are presented, and the topics of strength and stability are discussed in relation to tensile and compressive testing of rock.