1 ABSTRACT
The safe geologic isolation of nuclear wastes in an underground repository will require sealing access shafts and drifts for long periods. Crushed salt backfill is being investigated as a potential backfill and seal material through laboratory testing to determine how fundamental properties such as permeability, porosity and creep rate are reduced by pressure and time through consolidation. The test program reported in this paper consisted of four consolidation tests using crushed salt obtained from the Waste Isolation Pilot Plant and the Avery Island Mine. Tests with one- or two-month durations were conducted on samples with maximum particle sizes of 1, 10, and 20 mm, with initial porosities ranging from 26 to 36%, moisture contents of zero and 2%, and initial permeabilities from 103 to 105 md. The tests were performed at ambient temperature and confining pressures ranging from 0.34 MPa to 17 MPa. The most significant observation from the tests was the influence of moisture on changes in permeability, porosity and volumetric creep strain rate. The final permeability and porosity of one moist sample were reduced after one month to about 10>-5 md and 5%, respectively, compared to about 102 md and 14 to 19% for the dry samples. In addition, the consolidation rate for the moist sample was more rapid at comparable porosities. In all of the tests, the volumetric creep strain rate tanged from 10-8 to 10-6 (sec) -1, and did not achieve steady state values after 1 to 2 months of load application.
2 INTRODUCTION
Permeability as a function of porosity
Bulk modulus as a function of porosity
Volumetric creep strain rate properties as a function of time, porosity and stress.
Current schematic designs for seals for a nuclear waste repository in salt include the use of crushed salt produced during repository mining operations as backfill in the tunnels and shafts (Kelsall et al., 1984). If effective within reasonable time periods (tens to hundreds of years), consolidation of the crushed salt may result in encapsulation of the wastes in an essentially impermeable salt monolith, thus reducing the long-term requirements for other parts of the seal system, and enhancing the general confidence regarding the waste isolation capabilities of the site as a whole. Current knowledge regarding the crushed salt consolidation process has been reviewed previously by Kelsall et al. (1984) and IT Corporation (1984). These reports reviewed the properties (bulk modulus, consolidation rate, etc.) of crushed salt as determined from previous laboratory testing. This paper presents results from a six-month laboratory test program of crushed salt consolidation (Kelsall et al., 1984). The properties investigated include:
Tests were conducted on bedded salt from the Waste Isolation Pilot Plant (WIPP) site in New Mexico and on dome salt from Avery Island, Louisiana. All tests were conducted at room temperature (approximately 21 °C).
3 CURRENT LABORATORY INVESTIGATION
The objective of the crushed salt testing reported in this paper was to obtain stress-strain, strain-time, volumetric creep strain rate- time, and permeability-porosity relations for crushed salt consolidated under hydrostatic confining pressures at ambient temperature.
