Strength Of Intact Rock And Rock Masses

This paper is in two parts. The first part presents an overview of the strength of intact rock. A short commentary on various methods of fitting failure criteria to experimental data follows, it is demonstrated that the method of fitting the criterion to the test data has a major effect on the estimates obtained of the material properties. The results of a recent analysis of a large data base of test results is then presented. This demonstrates that there are inadequacies in the Hoek-Brown empirical failure criterion as currently proposed for intact rock and, by inference, as extended to rock mass strength. The parameters mi and sc are not material properties if the exponent is fixed at 0.5. Published values of mi can be misleading as mi does not appear to be related to rock type. The Hoek-Brown criterion can be generalised by allowing the exponent to vary. This change results in a better model of the experimental data. Analysis of individual data sets indicates that the exponent, a, is a function of mi which is, in turn, closely related to the ratio of sc/st. A regression analyis of the entire data base provides a model to allow the triaxial strength of an intact rock to be estimated from reliable measurement of its uniaxial tensile and compressive strengths. The method proposed is the most accurate of those methods that do not require triaxial testing and is adequate for preliminary analysis. Analysis is presented that shows applying the Hoek-Brown criterion to most rocks results in systematic errors. Simple relationships for triaxial strength that are adequate for slope design are presented. The second part of this paper presents a discussion of the application of the Hoek-Brown criterion to estimating the shear strength of rock masses for slopes.