Stress-Strain Behavior of Jointed Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 69

For a two-dimensional, non-persistent discontinuity with open sections Equation (3.72)
can also be applied provided that the shear strength of the discontinuity is isotropic
and the linear degree of separation

κ l is replaced by the planar degree of separation κ p

defi ned in Section 2.7.2:

(3.73)

It has been observed in a large number of direct shear tests carried out on artifi cial open
non-persistent discontinuities produced in specimens composed of both natural rock
and model materials such as gypsum that failure in the material bridges occurs in two
phases: tensile wing cracks were found to appear fi rst at the tips of the open joints fol-
lowed by secondary shear cracks propagating towards the adjacent joints (Lajtai 1969a,
Lajtai 1969b, Savilahti et al. 1990, Jung et al. 1995, Wong et al. 1999, Gehle 2002, Gehle
& Kutter 2003).
As an example, Fig. 3.17 shows these two phases of failure in rock bridges located
between horizontal open cracks in a gypsum specimen (Gehle 2002). Thus, crack for-
mation and propagation in the rock bridges generally occurs neither along the disconti-
nuity's plane nor by means of pure shear failure.

Figure 3.17 Direct shear test on a
gypsum specimen with horizontal
open cracks (Gehle 2002)