RATDAMPER - A Numerical Model for Coupling Mechanical and Hydrological Properties within the Disturbed Rock Zone at the Waste Isolation Pilot Plant Page: 1 of 10
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RATDAMPER - A Numerical Model for Coupling Mechanical and
Hydrological Properties within the Disturbed Rock Zone at the Waste
Isolation Pilot Plant
Sandia National Laboratories, Carlsbad, New Mexico, USA
T/,1G. (YSl 6)
9A i/i/a a
RESPEC Incorporated, Rapid City, South Dakota, USA
Bundesanstalt fur Geowissenshaften und Rohstoffe (BGR), Germany
ABSTRACT: A numerical model for predicting damage and permeability in the disturbed rock zone (DRZ)
has been developed. The semi-empirical model predicts damage based on a function of stress tensor invari-
ants. For a wide class of problems hydrologic/mechanical coupling is necessary for proper analysis. The
RATDAMPER model incorporates dilatant volumetric strain and permeability. The RATDAMPER model has
been implemented in a weakly coupled code, which combines a finite element structural code and a finite dif-
ference multi-phase fluid flow code. Using the development of inelastic volumetric strain, a value of perme-
ability can be assigned. This flexibility allows empirical permeability functional relationships to be evaluated.
Sandia National Laboratories (SNL), as the scientific
advisor to the Department of Energy (DOE) for the
Waste Isolation Pilot Plant (WIPP), is interested in
the movement of fluids (e.g., gas and liquid) into and
out of the repository horizon and potentially though
the panel closures, and sealed vertical shaft systems.
The host rock consists of bedded halite, polyhalite,
and anhydrite with minor clay seams. Disposal re-
gions are approximately 650m below ground sur-
face, and are mined from the bedded salt of the
Salado Formation. Creep closure is a significant
technical component of the disposal areas as this
phenomenon closes repository rooms, consolidates
waste and backfill, thereby influencing performance
assessment and seal system designs. The disturbed
rock zone (DRZ), formed as a result of damage
caused during deformation into excavated openings,
causes a measurable change in mechanical and hy-
drologic properties. For a wide class of problems
hydrologic/mechanical coupling is necessary for
The RATDAMPER model has been developed to
predict damage and permeability in the DRZ. The
semi-empirical model predicts damage based on a
function of stress tensor invariants and incorporating
dilatant volumetric strain. Laboratory tests using ni-
trogen gas as the permeant have been conducted to
determine permeability of WIPP salt deformed to
various levels of damage (Pfeifle 1995, Pfeifle 1998,
Pfeifle 1999). From these laboratory tests, several
relationships coupling permeability to the develop-
ment of inelastic strain have been formulated. Most
of the available data focus on small volumetric
strains (less than 0.5%). Pfeifle (1999) has recently
completed tests that provide additional data for
larger volumetric strains ranging from 1 to 2.5%. To
avoid function averaging, an approach based on a
scalar measure of damage predicts permeability from
dilatant behavior observed in laboratory experi-
ments. The damage factor, or a measure of dilatancy,
D, is computed as a function of the second invariant
of the deviator stress tensor J2, and the first invariant
of the stress tensor, Ii, (Van Sambeek et al. 1993)
D(0.27.1 , )'
The use of a stress tensor invariant ratio to measure
dilatancy has been previously investigated using
WIPP and other rock salt types (Van Sambeek et al.
1993). The scalar quantity, D, gives a measure of
maximum damage that occurs after excavation and
does not predict damage evolution.
The RATDAMPER model has been implemented
in a weakly coupled code that combines a finite ele-
ment structural code and a finite difference multi-
phase fluid flow code (Statham et al. 1999). Due to
complex processes of viscoplastic creep, stress state
redistribution and pore pressure effects, this engi-
neering approach is considered an appropriate start-
ing analysis to model the empirical relation between
permeability and volumetric strain. Any viable
structural code with a constitutive material law
simulating rock salt deformation processes might be
used to determine the state of stress. Using devel-
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RATH,JONATHAN S.; PFEIFLE,T.W. & HUNSCHE,U. RATDAMPER - A Numerical Model for Coupling Mechanical and Hydrological Properties within the Disturbed Rock Zone at the Waste Isolation Pilot Plant, article, November 27, 2000; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc716973/m1/1/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.