Engineering Rock Mechanics

96 Fractures and hemispherical projection

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Figure 7.7 Important fracture characteristics for engineering rock mechanics.

One of the most important characteristics is the extent or persistence
of the fractures because this governs whether the rock mass can act as
a series of discrete blocks and whether water can travel through the
fracture array. However, the estimation of fracture persistence can be
subject to severe measurement bias; in particular, the distribution of
tracelengths of fractures intersected by a scanline is not the same as
the actual population of fracture tracelengths. As a scanline traverses a
rock mass, it is more likely to sample fractures with longer tracelengths
than with shorter tracelengths. If the fracture traces in Fig. 7.3 have the
same location and orientation but are limited in their extents, a fracture
array such as that shown in Fig. 7.8 might result. It is evident from
this array that a borehole or scanline is more likely to intersect the
longer traces than the shorter ones, with the result that the actual and
sampled distributions of tracelengths are different, because the longer
tracelengths are preferentially sampled.
If the actual distribution of tracelengths is negative exponential, the
sampled distribution of tracelengths (of the fractures intersected along a
scanline) will be of a different form, as indicated in Fig. 7.9. Note that the
greatest discrepancy occurs for a tracelength close to zero, because the
scanline will miss almost all of them. Needless to say, sampling surveys
should take the sampling errors into account 6.
In this chapter, we will concentrate on the fracture frequency and
geometrical aspects of fractures. The mechanical properties of fractures
are included in Chapter 8.

in Rock Masses. Int. J. Rock Mech. Min. Sci. Geornech. Abstr., 15, 6, 319-368. The diagram
of the ten fracture characteristics in Fig. 7.7 was first published in Hudson J. A. (1989)
Rock Mechanics Principles in Engineering Practice’. CIRIA Report, Buttenvorths, London,
72 PP.
References to appropriate software packages for this are included in Section 7.3.