Discontinuity size

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Although it is relatively easy to specify the size of a discontinuity in terms of its surface area, size is one of the most difficult discontinuity properties to measure accurately. This is because only by completely dismantling a given rock mass is it possible to trace and to measure the complete area of each discontinuity. This has never been done satisfactorily for a rock mass. When studying the size of discontinuities it is desirable also to consider their shape. In a completely blocky rock mass, where all discontinuities terminate at other planar discontinuities, the shapes will take the form of complex polygons whose geometry is governed by the locations of the bounding discontinuities. It was noted in Chapter 5 that discontinuity occurrence is often random, leading to negative exponential distributions in spacing. It is reasonable to suppose, therefore, that the linear dimensions of discontinuities would also be of negative exponential form. Sampling difficulties have so far made it impossible to prove or disprove this hypothesis. In view of these sampling difficulties, a number of workers have adopted the simplifying assumption that discontinuities are circular, to provide a starting point for the analysis of size (Baecher et al., 1977 and Warburton, 1980a).