Page 72 - Rock Mechanics For Underground Mining

ROCK MASS STRUCTURE AND CHARACTERISATION

Table 3.1 Classification of discontinuity spacing.

Description Spacing (mm)
extremely close spacing <20
very close spacing 20–60
close spacing 60–200
moderate spacing 200–600
wide spacing 600–2000
very wide spacing 2000–6000
extremely wide spacing >6000

Table 3.2 Classification of discontinuity persistence.

Description Modal trace length (m)
very low persistence <1
low persistence 1–3
medium persistence 3–10
high persistence 10–20
very high persistence 20

Figure 3.9 Illustration of persis-
tence of various sets of discontinuities
(after ISRM Commission, 1978a).

The ISRM Commission (1978a) uses the most common or modal trace lengths of
each set of discontinuities measured on exposures (section 3.4.1) to classify persis-
tence according to Table 3.2.
Roughness is a measure of the inherent surface unevenness and waviness of the
discontinuity relative to its mean plane. The wall roughness of a discontinuity has a
potentially important influence on its shear strength, especially in the case of undis-
placed and interlocked features (e.g. unfilled joints). The importance of roughness
declines with increasing aperture, filling thickness or previous shear displacement.
The important influence of roughness on discontinuity shear strength is discussed in
section 4.7.2.
When the properties of discontinuities are being recorded from observations made
on either drill core or exposures, it is usual to distinguish between small-scale surface
irregularity or unevenness and larger-scale undulations or waviness of the surface
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