Stress-Strain Behavior of Jointed Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 58

3.3 Discontinuities

3.3.1 Types of Discontinuities
Discontinuities have a considerable influence on the deformability and strength of a
rock mass. Fig. 3.10 shows some types of discontinuities that are frequently encoun-
tered in nature. Their appearance is described in qualitative terms given in Section 2.7.2
(Fig. 2.31). With regard to the stress-displacement behavior, which will be treated in the
following sections, an estimation of the degree of persistence of a discontinuity is also
of considerable importance (for a definition, see Section 2.7.2 and Fig. 2.32).

Figure 3.10 Appearance of discontinuities (Wittke 1990)

3.3.2 Stress-Displacement Behavior of Discontinuities Loaded
by a Normal Compressive Stress
Largely closed discontinuities do not deform under a compressive stress applied normal
to their surfaces regardless of their surface properties, that is, if they are planar, uneven,
smooth or rough, and if they are persistent or non-persistent.
Discontinuities with rough or uneven surfaces are normally not fully closed. As a re-
sult, an applied normal compressive stress σ n causes localized stress concentrations and

plastic zones in the neighboring rock mass (Fig. 3.11, upper). The resulting irreversible
deformations lead to a partial closure of the discontinuity. Experimental studies have
shown that the relationship between the applied normal compressive stress σ n and the

mean closure δ n,D of such a discontinuity is nonlinear (Goodman 1976, Bandis 1980,

Witherspoon et al. 1980, Bandis et al. 1983, Swan 1983, Malama & Kulatilake 2003).