Stress-Strain Behavior of Jointed Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 41

is called the “elasticity matrix”.
As an example, Fig. 3.2 shows the axial and lateral strains

ε z , respectively, of a
granite specimen during loading and unloading, measured under uniaxial normal stress
σ z (Walsh 1965c). The stress-strain curves are both more or less linear and reversible up
to high stress levels. Deviations from linearity at low stress levels can be attributed to
the closing of cracks that are pre-existing in the specimen. The sliding between contact-
ing crack surfaces at the beginning of unloading creates a slight hysteresis loop. From
a practical point of view, however, these deviations from a linear elastic stress-strain
behavior in most rocks can be neglected.

ε x and

Figure 3.2 Axial strain
ε z and lateral strain ε x for

Westerly Granite under
uniaxial compression
during loading and
unloading (Walsh 1965c)

For intact rocks with planar grain structure, the assumption of isotropic elastic behavior
usually represents an inadmissible simplifi cation. From experience, intact rocks with pla-
nar grain structure such as schist, slate and some argillaceous rocks often exhibit a signifi -
cantly lower Young's modulus perpendicular to the structure planes than parallel to them
(Figs. 2.5 - 2.7 and 2.16 - 2.18). The elastic behavior of such rocks is therefore normally