Discontinuity frequency is one of the fundamental measures of the degree of fracturing in a rock mass. Frequency can be expressed in terms of the number of discontinuities that are observed or predicted to occur in a unit volume, a unit area or a unit length of a sample from a given rock mass. The aim of this chapter is to examine the property of discontinuity frequency from a theoretical and a practical point of view. Section 4.2 presents a brief analysis of volumetric and areal frequency. This section is followed by a discussion of linear frequency and how this property is influenced by the number and orientation of discontinuity sets and by the orientation of the sampling line. Section 4.4 contains a detailed investigation of the anisotropy, and the associated extreme values, of linear discontinuity frequency in a rock mass containing several discontinuity sets. The chapter concludes with a discussion of the probabilistic implications of random discontinuity occurrence.
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