Publications

Estimating the extent of the disturbed rock zone around a WIPP disposal room

Herrick, Courtney G.; Park, B.Y.; Lee, Moo Y.; Holcomb, David J.

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. Copyright 2009 ARMA, American Rock Mechanics Association.