Rock Salt - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 121

in which

The yield function Ff t and the plastic potential Q t are defi ned as

(5.18)

with

(5.19)

and

(5.20)

Thus, fi ve parameters are required to describe tertiary creep: the viscosity η t , the ini-

tial uniaxial yield stress σ f0 , the fl ow angle φ f , the strain-softening modulus M and the

dilatancy angle σ eff plane F t = 0 forms the dilatancy boundary (Fig. 5.2).

The yield function Ff t contains a variable yield stress

ψ . In the σ m -

σ f , which is a function of the tertiary

ε v t < 0)
occurs due to the formation of microcracks. This is associated with a decrease in the yield
stress to values of

ε v t . As soon as the initial yield stress

volumetric strain σ f0 is exceeded, dilatancy (

ε v t and by the magnitude of the strain- σ f < σ f0 which is controlled by

softening modulus M. Thus, σ f represents a measure for damage of the intact rock, that is,

the dilatancy boundary of the damaged rock salt is lower than in the undisturbed state
( σ f0 ). As a consequence, the position of the dilatancy boundary is a function of the

tertiary volumetric strain

σ f =

ε v t that has already taken place and therewith a measure of

damage of intact rock. If σ f = 0, the residual strength is reached.

The viscoplastic strain rates arising after creep failure are described by the following
fl ow rule:

(5.21)

where the yield functions Ff f and F f * are defi ned as with Ff t :

(5.22)