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Fifteen triaxial compression creep tests were performed on clean and argillaceous salt from the Waste Isolation Pilot Plant (WIPP). The temperatures in the tests were either 25°C or 100°C while the stress difference ranged from 3.5 MPa to 21.0 MPa. In ali tests, the confining pressure was 15 MPa. Test duration ranged from 23 to 613 days with an average duration of 300 days. The results of the creep tests supplemented earlier testing and were used to estimate two parameters in the Modified Munson-Dawson constitutive law for the creep behavior of salt. The two parameters determined from each test were the steady-state strain rate and the transient strain limit. These estimates we_e combined with parameter estimates deTLerminedfrom previous testing to study the dependence of both transient and steady-state creep deformation on stress difference. The exponents on stress difference determined in this study were found to be consistent with revised estimates of the exponents reported by other investigator,;. • This report was preparedby RFJSPECInc. underContract No. 05-7502 with SandiaNational Laboratories.

SUMMARYTwo simple constitutive laws appropriate for elastic-plastic deformation up to peak load of low porosity rocks are presented. Both laws account for mean stress dependence of yield hardening between yield and peak strength and non-associative plastic straining. The two constitutive laws are based on the Mohr-Coulomb and the Mises-Schleicher yield criteria. The yield criteria are matched at axisymmetric compression and the relationships among the constitutive parameters are given. The parameters are then evaluated for two evaporite rocks anhydrite and polyhalite based on axisymmetric compression data. Comparison of predicted stress-strain laws with those measured in the laboratory show that these laws can be used for engineering analysis.

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