Rock Salt - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 123

The strain rates arising after tensile failure are described by the fl ow rule (5.21) setting

(5.26)

(5.27)

σ t is the tensile strength of rock salt, which can be assumed to be at the most 2 MPa.
The residual tensile strength can be considered as zero.
The WBI model for rock salt thus requires a total of 19 characteristic parameters. How-
ever, not every problem requires the determination of all the parameters. As already
mentioned, stress levels smaller than the yield stress only lead to elastic as well as pri-
mary and secondary creep deformations. In such cases the stress-strain behavior of the
rock salt is completely described by the parameters E, η s and n. In long-

term studies, the deformations due to secondary creep are often large compared with
the elastic and the primary creep deformations because the latter are time-limited. The
elastic and the primary creep deformations may then be neglected, so that only the two
parameters

ν , E p , η p , m,

η s and n are necessary to describe the stress-strain behavior.
In Germany, rock salt is used for underground storage of gases and liquids and is
planned to be used for underground storage of nuclear waste. In this context fi ve other
constitutive models for rock salt were recently developed in Germany. In all of these
models the temperature dependence of creep is accounted for because it plays an im-
portant role, particularly for nuclear waste disposal. In the following, reference to these
models will be given for further study of the interested reader.
The visco-elasto-plastic model of Minkley & Mühlbauer (2007) is a further develop-
ment of the stress-strain law developed by Minkley et al. (2001). In this model, besides
the three phases of creep, failure and post-failure behavior, also a reduction of the elas-
tic constants with the degree of damage is taken into account.

The model of Günther & Salzer (2007) is an extension of the stress-strain law developed
by Salzer et al. (1998). It consists of a strain-hardening approach, taking into account
the three phases of creep, failure, post-failure behavior and a reduction of the elastic
constants with the degree of damage.
The composite dilatancy model (CDM) developed by Hampel & Schulze (2007) at
and in cooperation with the Federal Institute for Geosciences and Natural Resources
(BGR) in Hannover is a micro-structural model based on the composite models for
transient creep (Weidinger et al. 1998) and steady-state creep (Hampel et al. 1998) and
is extended with respect to dilatancy, failure and post-failure behavior. This model also
describes the infl uence of humidity on creep according to Hunsche & Schulze (2002).
The multi-mechanism deformation-coupled fracture model (MDCF model) was devel-
oped at the Institute for Underground Construction (IUB) of the University of Hannover
(Rokahr et al. 2004). It is partly a micro-structural model based on the Munson-Dawson
models (Munson & Dawson 1979, Munson & Dawson 1984) and their extension with
respect to rock salt healing by Chan et al. (1995) and Chan et al. (1998). The stress-strain
law developed by Hauck (2001) is also incorporated in this model. The MDCF model
simulates the three phases of creep, failure, post-failure behavior and healing.