Structure of Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 31

Figure 2.32 Spacing and persistence of discontinuities

Persistence
Discontinuities are often interrupted by rock bridges, or terminate against other discon-
tinuities (cf. Fig. 2.15). Such discontinuities, in literature, are also referred to as “imper-
sistent” or “intermittent” discontinuities, in the following are denoted as “non-persis-
tent” discontinuities.
A measure of the degree to which discontinuities persist is the so-called “planar degree
of separation”. According to Pacher (1959), the planar degree of separation κ p is de-

fi ned as the sum of the separated rock areas A i divided by a reference area A (Fig. 2.32,
right):

(2.8)

κ p is an important parameter for the estimation of shear strength of non-persistent
discontinuities (Section 3.3.3).
In a similar way Müller (1974) defi ned the “linear degree of separation”

κ ℓ as a measure

of persistence of discontinuities on a two-dimensional rock exposure:

(2.9)

where ℓ is a reference length along the trace of a discontinuity and ℓ i are the separated
sections of l, also denoted as “trace lengths”.