Page 73 - Rock Mechanics For Underground Mining

IMPORTANT GEOMECHANICAL PROPERTIES OF DISCONTINUITIES

Table 3.3 Classification of discontinuity roughness.

Class Description
I rough or irregular, stepped
II smooth, stepped
III slickensided, stepped
IV rough or irregular, undulating
V smooth, undulating
VI slickensided, undulating
VII rough or irregular, planar
VIII smooth, planar
IX slickensided, planar

Figure 3.10 Different scales of dis-
continuity roughness sampled by dif-
ferent scales of shear test. Waviness
can be characterised by the angle i
(after ISRM Commission, 1978a).

(Figure 3.10). Each of these types of roughness may be quantified on an arbitrary
scale of, say, one to five. Descriptive terms may also be used particularly in the
preliminary stages of mapping (e.g. during feasibility studies). For example, the ISRM
Commission (1978a) suggests that the terms listed in Table 3.3 and illustrated in
Figure 3.11 may be used to describe roughness on two scales – the small scale (several
centimetres) and the intermediate scale (several metres). Large-scale waviness may
be superimposed on such small- and intermediate-scale roughness (Figure 3.10).
Aperture is the perpendicular distance separating the adjacent rock walls of an
open discontinuity in which the intervening space is filled with air or water. Aper-
ture is thereby distinguished from the width of a filled discontinuity (Figure 3.12).
Large apertures can result from shear displacement of discontinuities having appre-
ciable roughness, from outwash of filling materials (e.g. clay), from solution or from
extensile opening. In most subsurface rock masses, apertures will be small, probably
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