Measurements of elastic wave velocities parallel and perpendicular to the rib (wall) of a drift at the Waste Isolation Pilot Plant (WIPP) were made to map out the extent of the zone disturbed by the excavation and time-dependent processes in the 10+ years since the drift was excavated. Understanding the development of the Disturbed Rock Zone (DRZ) is relevanto the eventual sealing of the WIPP. A DRZ was found that extended in as much as two meters from the drift, with the extent being a function of distance from the back and floor of the drift. Observedecreases in elastic wave velocity near the free surface exceeded 20%, implying high
instantaneous and/or time-dependent damage.
There is a need to understand the spatial and temporal development of the disturbed rock zone (DRZ) around openings at the Waste Isolation Pilot Plant (WIPP). Possible leakage paths around seals, effects on closure rates and safety issues related to deterioration of the salt mechanical properties are issues affected by the DRZ. A drift constructed about 10 years ago as an access to Room Q provided an opportunity to conduct geophysical investigations of the damage that has developed in the salt for a 2 dimensional configuration removed from the disturbances of other excavations. As part of a suite of measurements designed to study the hydrological and mechanical properties of the salt around the drift, an array of ultrasonic transducer emplacement holes was drilled in the rib to a depth of about 6 meters. The mature DRZ is the macroscopic manifestation of grain-scale and larger cracks formed as a result of the instantaneous shear stress around the newly-created opening, and as a result of creep-related failures over a period of years. Velocity of elastic waves in the salt is a function of crack density. The design of the hole array and the transducer assemblies allows the measurement of elastic wave velocities along vertical and horizontal paths tangential to the drift ribs and, within the same hole, perpendicular to the rib. Orthogonal paths are necessary in order to detect the anisotropy of the cracking in the DRZ. Measurements were done using ?Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94A18500. paths in the rib near the back, floor and mid-height of the rib, allowing us to detecthe effect of the varying stress state on the development of the DRZ in these
Previous work at the WIPP, (Munson et al., 1995, Holcomb, 1999, and Hardy and Holcomb, 2000) delineated the DRZ as a function of depth around a vertical shaft and near a concrete seal. Using results published by Brodsky (1990, 1995), changes in velocity can be related to the damage, due to microcracking, required to produce the observed changes. The significance of the results lies in the length of time the DRZ has had to develop (approximately 10 years), the geometrical simplicity of the drift which eases the task of relating the results to models, and the hydrological measurements to be made in conjunction with the velocity and attenuation measurements.