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The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy research and development facility which is intended to demonstrate the safe geologic disposal of transuranic wastes. The mechanical behaviour of crushed rock salt is of interest to the WIPP Project because the mined WIPP salt is a candidate material for use as backfill around the waste packages and in the underground openings during and after the operational phase. It is anticipated that in response to the convergence of the mine openings, the crushed salt will compact sufficiently to serve as an effective component in WIPP seal systems. In this paper, the model for isostatic hot-pressing which was developed by Helle et al. is applied to the problem of compaction of both pure NaCl and natural (WIPP) salt to develop a model capable of predicting densification rates as a function of temperature and applied pressure. Calculations for pure dry NaCl suggest that under conditions in situ, the contribution to densification by diffusional processes is negligible; all compaction will occur by dislocation creep. Assuming then that compaction will occur only by dislocation processes, the model is extended to nominally dry, WIPP rock salt. Model predictions are shown to agree favourably with available test data, and the kinds of laboratory tests that may be done to validate the model are discussed. Hot-pressing mechanism maps for dry WIPP salt under anticipated repository conditions indicate that fractional densities in excess of 0.95 are attained in about 30 yr. At this density, connected porosity, and hence permeability, diminish rapidly, and the recompacted salt can begin to serve as an effective seal material. This prediction underestimates consolidation rates in situ, because moist salt will compact even more rapidly. It will be shown how the model may be extended to include the effects of brine on densification rates. |
