A large proportion of rock mechanics problems involve the strength and deformability of rock masses. Recognition that these properties differ markedly, to an unknown extent, from the corresponding properties of intact laboratory specimens from the same rock was a main stimulus to Dr. Leopold Mueller when he urged the formation of the International Society for Rock Mechanics (ISRM) in 1962. Dr. Mueller recognized that joints, heterogeneities, and in-situ stresses present in the large scale (i.e., rock mass) situation were key elements influencing the differences between a rock mass and an intact rock specimen. In the forty years since ISRM was founded, impressive advances have been made in the development of numerical modeling procedures which allow explicit consideration of discontinuities, heterogeneities and in-situ stresses - both static and dynamic - but the task of defining appropriate values for deformation properties, especially for discontinuities and heterogeneities, remains. Valuable work has been done, albeit largely empirical, in developing broad rock classification systems which can provide guidance for practical design, but more is now possible. With the availability of numerical codes designed for geotechnical materials and problems, there is now a real opportunity to develop a more fundamental underpinning to these systems [e.g. Fakhimi (1992)]. Although, in the absence of actual data, it is necessary to make numerous assumptions on the constitutive behavior of the component elements (e.g. joints), valuable insight into the mechanics of rock mass deformation, and the critical combination of conditions and variables, can probably be achieved through parametric analysis. Such studies could form very good thesis topics for graduate students - with the classification systems representing the field data for numerical experiments. Currently, it is often assumed that all joints in a rock mass exhibit similar constitutive behavior, even though each set of joints may have formed at different geological times; and there are likely to be differences even between joints in the same set. Such variations can lead to large differences in the overall development of rock mass deformation. In some cases, e.g. where deformation is arrested due to early introduction of stabilizing support, the small variations may be of secondary importance. In situations where large deformations are inevitable, as in bulk caving mining systems, these variations may be decisive in the success or failure of the system. In practical situations, where specific data on strength or deformability is often unavailable, a designer has no alternative but to select the most reasonable values - usually derived either from empirical correlations, such as Deere's (1980) classification between compressive strength and modulus of elasticity for laboratory specimens, or the rock mass deformability estimates of Serafim and Pereira (1983) and Bieniawski (1989), who have established empirical correlations with the Rock Mass Rating (RMR) classification system, Bieniawski (1988). In isotropic rock, the modulus of deformability does not affect the stress distribution; where there are (isotropic) heterogeneities in rock mass properties it is the ratio of the deformabilities, rather than absolute values, that influences stresses.

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