The disturbed rock zone (DRZ) is an important feature which is evaluated in the Waste Isolation Pilot Plant (WIPP) performance assessment (PA) to predict post-closure repository performance. Mining of a WIPP disposal room disturbs the stress state sufficiently to cause fracturing of the surrounding rock, and this fracturing will alter the mechanical and hydrological properties of the salt. DRZ extent, and permeability, controls the majority of the brine that enters or exits the repository in PA modeling of the undisturbed scenario. Extensive laboratory data from experiments performed on rock salt demonstrate that damage can be modeled in terms of stress invariants. In this paper the DRZ extent is calculated based on a dilatant damage criterion. The calibrated damage factor C in the damage criterion is determined by comparing ultrasonic wave velocity field measurements obtained in the S-90 drift with a numerical analysis that predicts the salt’s behavior. Ultrasonic velocities decrease in the presence of microcracks and loosened grain boundaries associated with salt damage. The most extensive DRZ exists during early times, within the first ten years of mining. The maximum predicted DRZ surrounding a WIPP disposal room is approximately 2.25 m below, 4.75 m above, and 2 m laterally. This paper also presents several lines of evidence, based on previous studies, that support the prediction of DRZ size by applying a WIPP specific damage criterion calibrated using ultrasonic velocity measurements.
The Waste Isolation Pilot Plant (WIPP), located in southeastern New Mexico, is operated by the U.S. Department of Energy (DOE) as the underground disposal facility for defense-related transuranic (TRU) waste. It is located in the Salado bedded salt formation at a depth of about 655 m (Figure 1). Salt at this depth behaves as a viscous material having a stress state that is initially at lithostatic. Mining of the salt disturbs the static equilibrium and induces three deformational processes: (1) elastic deformation, (2) inelastic viscoplastic flow, and (3) inelastic-damage induced flow [1]. Fracturing of the rock surrounding a disposal room changes the mechanical and hydrological properties of the salt. The extent and permeability of this disturbed rock zone (DRZ) controls the majority of the brine that enters or leaves the repository under ‘undisturbed’ conditions (i.e., no human intrusion into the repository). As shown in Figure 1, Location 1 in the S-90 (“South 90”) access drift to Room Q provided an opportunity to conduct geophysical investigations of the extent of salt damage for a nearly perfect two-dimensional configuration isolated from other areas of rock disturbance.
Inelastic-damage induced flow (or simply “damage”), which is manifested by the initiation, growth, coalescence, and healing of microfractures, is the least understood of the three deformational processes mentioned above. Over the years, various methods have been undertaken to measure the spatial and temporal changes during salt damage. This study used ultrasonic velocity measurements obtained from the rib (side) of the S-90 access drift, which was excavated in 1988, to assess the extent of the DRZ [2].
