Rock Salt - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 117

(5.5)

which is an invariant of the deviatoric stress tensor

(5.6)

and describes the deviation from the hydrostatic stress state σ 1 = σ 2 = σ 3 .

Figure 5.2 Constitutive behavior of rock salt

In the compaction zone only elastic strains and primary and secondary creep strains oc-
cur. During initial loading the so-called “dilatancy boundary” forms the upper limit of the
compaction zone (Spiers et al. 1988, Ratigan et al. 1991, Hunsche 1992b, Van Sambeek et
al. 1993). Stress states located within the damage zone are associated with tertiary creep
and the forming of microcracks. Thus, tertiary creep leads to gradual damage of rock salt.
The curve representing the peak strength, also referred to as the “short-time failure
boundary”, forms the upper limit of the damage zone. Results of compression tests
indicate that the peak strength of rock salt is dependent on the applied rate of loading.
High strain rates and stress rates lead to an increase in the peak strength. At very small
strain rates and stress rates as well as at constant loading (creep) ductile failure occurs,