Design evaluation: Structural calculations for the construction and salt handling shaft and the waste handling shaft at the Waste Isolation Pilot Plant (WIPP)

One fundamental strategy under consideration for sealing the Waste Isolation Pilot Plant (WIPP) shafts involves the consolidation of the salt backfill in specified portions of the shaft. The criterion for effective salt consolidation was defined to occur when the fractional density of the crushed salt reaches 95%. Thus, the shaft is sealed with a backfill material (crushed salt and/or precompacted salt blocks) that eventually develops properties considered favorable for long-term sealing purposes. This report describes the structural analyses performed to address how much deformation will occur in the shaft as a function of time and depth and how long will it take for the backfill to consolidate to 95% relative density. The two-dimensional axisymmetric shaft model was allowed to creep for a period of 30 years before instantaneous backfill emplacement. Results indicate that the backfill will consolidate to 95% density in less than 300 years at the repository horizon for an initial relative density of 0.60. Most of the backfill in the shaft reaches 95% density in less than 1000 years if the backfill is emplaced at an initial relative density of 0.85. Thus, the calculations show that salt seal materials can reconsolidate in the WIPP shafts to densities nearly equal to those for the intact host rock salt in a relatively short time span. 10 refs., 17 figs