RATDAMPER - A Numerical Model for Coupling Mechanical and Hydrological Properties within the Disturbed Rock Zone at the Waste Isolation Pilot Plant Page: 4 of 10
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opment of inelastic volumetric strain, a value of
permeability can be assigned. This flexibility allows
empirical permeability functional relationships to be
evaluated. EGE vED
tOV 30 2
The coupling of mechanical and hyological effects
can be quantified according to the principle of effec-
og= p1+ u-, (2)
Where uTis the stress tensor caused by the weight of
the overburden (essentially constant), p is the pres-
sure of the repository pore fluid, I is the identity ten-
sor, and c- is the effective stress tensor that is ap-
plied to the matrix. The movement of fluids is
directly affected by the hydrologic properties while
fluid movement (and resulting fluid pore pressure)
directly affects the mechanical processes of salt
damage, creep, and healing. Therefore, coupling of
fluid flow and rock mechanics would improve the
analysis of the movement of fluids in the DRZ sur-
rounding the repository.
3 DRZ BEHAVIOR AT WIPP
The degree of rock damage in the DRZ immediately
surrounding the excavation is of major importance to
the analysis of fluid flow near the repository.
Stresses imposed on the host rock during under-
ground excavation, as well as the natural creep of the
rock salt into the excavated openings, will cause
fracturing in the vicinity of the host rock/repository
interface, that will alter the hydrologic properties of
the fluid flow system. This DRZ may be hydrologi-
cally characterized as having an increased porosity
and increased permeability to gas and liquid.
As the underground excavations at WIPP age,
large-scale fracturing will develop in the surround-
ing rock salt, usually becoming visible after several
years. The process of fracturing starts immediately
after excavation, when deviatoric stresses are at a
maximum. Fracturing in the vicinity of the salt
rock/repository interface initiates soon after mining
begins. Micro-fractures develop at the grain level in
response to deviatoric stress changes. Over time,
changes in the state of stress resulting from creep
displacements promote the coalescence of micro-
fractures that become readily visible. A localized,
intense fractured pattern forms, leading to larger but
fewer fracture zones with preferred orientations
controlled by both applied stresses and local stratig-
raphy (Kranz 1983).
Under certain conditions, such as construction of
a rigid bulkhead, stresses in the salt will tend toward
equilibrium. As stress differences decrease, salt
fractures would heal.
Inherent to the fracture process is dilatant behav-
ior of the rock mass that results in increased poros-
ity, permeability and a decrease in pore pressure,
which leads to an increase in effective stress. Dila-
tancy, or the volumetric expansion of the rock vol-
ume, is therefore a measure of the degree of fractur-
ing and damage in the DRZ. Since the mechanical
fracturing processes are time-dependent, it logically
follows that the hydrological properties are also
Different levels of fracturing will have varying
consequences on the mechanical and hydrological
performance of underground openings at WIPP. The
scale of the fracturing affects both the porosity and
the permeability of the surrounding rock. Micro-
fractures significantly increase the permeability of
the surrounding rock mass. Subsequent creep alters
the state of stress, which continues to change the di-
lation and permeability in the rock salt.
As fracturing occurs the salt grains will move
with respect to each other which, on average, causes
rock mass volume expansion. These fractures may
heal if the fractured mass is reloaded along the hy-
drostatic axis (Costin & Wawersik 1980, Brodsky
1990). This is particularly important in rock salt for
which fractures can be healed, as the state of stress
tries to reach equilibrium. Healing may also occur
during deviatoric states of stress, but this is a result
of the normal stress components, and not the devia-
toric stress itself (Stormont 1995).
Permeability and change in permeability are ma-
jor parameters in characterizing the DRZ. Perme-
ability is a function of fracturing in a rock salt mass,
assuming the salt grains are impermeable. The sim-
plest measure of fracturing and damage is the dila-
tancy of the rock mass, which is treated as a scalar.
Therefore permeability can be related to the me-
chanical measurements through dilatancy.
4 PREVIOUS DILATANCY MODELING AT
The relationship between dilatancy and permeability
in rock salt has been derived from constant confining
stress tests on WIPP rock salt (Pfeifle et al. 1998). In
these tests, nitrogen gas was used to determine the
permeability of a damaged WIPP rock specimen.
Stormont (1990) reported the results of a series of
compression tests on WIPP salt. During these tests,
the permeability of the samples was measured after
being subjected to deviatoric loading. Pfeifle (1995)
conducted triaxial stress tests on WIPP salt. These
triaxial stress tests were run at varying confining
pressures, o3, and constant stress difference, d 6(1.0
< 63 < 2.5 MPa, and d- = 25 MPa). These stress
tests were terminated after reaching axial strains of
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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/m1/4/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.