ABSTRACT ABSTRACT: The discrepancies between predicted and measured WIPP in situ Room D closures are markedly reduced through the use of a Tresca flow potential, an improved small strain constitutive model, an improved set of material parameters, and a modified stratigraphy.
1 INTRODUCTION
The mission of the Waste Isolation Pilot Plant (WIPP) Project in S.E. New Mexico is to develop the technology for disposal in bedded salt deposits of the radioactive Transuranic (TRU) waste forms generated by the U.S. defense programs. In the existing regulatory context, a major requirement is to assure that the public safety is preserved. This requires, in part, the development of technology for the prediction, far into the future, of the structural creep response of salt. In the early development of the structural prediction technology, it was decided to rely where possible on first principles or, where that was impossible, on laboratory empirical data as the proper basis for the technology. Back calculations based on in situ measurements would not be acceptable. Thus, results from the WIPP in situ structural experiments would serve solely for validating the independently developed prediction technology. Morgan et al. [1985] were the first to compare an independent prediction calculation to early WIPP South Drift in situ room closure measurements. This calculation used the previously determined reference steady state creep "law", material properties, and stratigraphy as given by Krieg [1984]. It was clear that the closure prediction based on these reference inputs was a factor of three less in magnitude and rate than the measured values. Later, Munson and Fossum [1986] noted that three fundamental aspects of the calculation method needed to be reevaluated: (1) the stress generalization or flow potential, (2) the constitutive model, and (3) the evaluation of the material parameters. They investigated, in a preliminary way, the influence of two common flow potentials and the result of including the previously discarded laboratory loadingstrains into the creep database. These preliminary results were promising, but indicated that more detailed studies would be necessary.
2 REEVALUATION STUDIES
All the reevaluation studies of three of the previously noted major aspects, as well as a reevaluation of the stratigraphy of the WIPP underground will be summarized in this paper. A more complete report of the detailed studies is given by Munson et al. [1989].
2.1 Flow potential investigation
Classical experimental studies on the flow potential for the yielding behavior of metals suggested, although not definitively, a yon Mises, or octahedral, criterion. Typically, the yon Mises criterion was incorporated into the numerical structural codes, because of these classical studies of flow potential, and also in part because the yon Mises is mathematically simpler to use than other flow potential forms. Comparable studies on the creep of metals, primarily of aluminum alloys, even though the studies extended over many years and involved several investigators, were less conclusive. Nevertheless, the commonly used yon Mises flow criterion was assumed for most creep calculations for salt. However, there were no earlier laboratory studies of salt creep to indicate the proper choice of flow rule.