Interim summary of Sandia creep experiments on rock salt from the WIPP study area, southeastern New Mexico

Abstract

Triaxial creep experiments on rock salt were carried out in support of the proposed Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico. Results include digital data in condensed data files and numerous graphical comparisons for rock salt from two horizons 2000 to 2100 ft and 2600 to 2800 ft (625 to 650 m; 810 to 875 m) as a function of principal stress difference, temperature and confining pressure. Qualitative examinations showed that the rock salt exhibited all known stages of creep, i.e., primary, secondary and tertiary creep followed by creep fracture. Preliminary evaluations of axial strain time records during primary creep (epsilon/sub 1/, t) are discussed assuming the applicability of so-called power law creep but without implying that the conventional power relationship is a constitutive equation. The primary creep data of this study appear to be independent of pressure within fairly wide experimental scatter. Estimates of steady state creep rates of interest to the WIPP vary from epsilon/sub 1/ < 10/sup -10/ s/sup -1/ to approximately 10/sup -8/ s/sup -1/. Tertiary creep and creep fracture were observed at ambient temperature and at 100/sup 0/C. Creep fracture was not observed in a limited number of experiments at 200/sup 0/C and is deemed unlikely except, possibly, at very low confining pressure, sigma/sub 3/ less than or equal to 100 psi (0.7 MPa). To predict creep fracture, the use of an empirical procedure was suggested which correlates permanent time-dependent strains (damage) with limiting damage which is established from complete quasi-static stress-strain curves including pre- and post-failure records. 19 figures.