At the Oak Ridge National Laboratory we are studying salt mine stability to determine the design parameter or the ultimate disposal of high-level radioactive wastes in salt mines. To be able to predict the effects of elevated temperature (due to heat generated by decay of radioactive wastes) on creep and mine stability, an understanding of creep effects at ambient temperature is necessary. S. Serata in 1959, suggested that creep rates (or closure rates) of mined openings always decrease with time.3 In 1959, both ORNL and Serata (then at the University of Texas) installed several vertical and horizontal convergence measuring stations in the Carey Salt Company's mine at Hutchinson Kansas. Since that time, ORNL has added additional stations at the Carey mines in both Hutchinson and Lyons, Kansas. These stations were designed to determine the effects of both age of the opening and pillar stress on creep rates. Data from these stations tended to indicate that Serata's hypothesis of steadily decreasing vertical convergence was correct, at least for the pillar stress levels encountered in the Kansas mines. In March 1963 Serata indicated that he had successfully extrapolated creep data from laboratory pillar models to creep rates in mined openings 4.
L. Obert, of the Applied Physics Laboratory of the U.S. Bureau of Mines, has developed a somewhat different technique for performing creep tests on pillar models. At our request the Applied Physics Laboratory carried out 1000 hour creep tests on model pillars which they machined from 6 in. diam. cores of Lyons mine salt. Steel restraining rings were epoxied around the upper and lower ends of the specimens and then the centers of the specimens were ground out to form the pillars and surrounding roof and floor. The model pillars had a diameter-to-height ratio of 4, and tests were run with average pillar stresses of 4, 5, 6, 7, 8, 10 and 12 thousand psi. Vertical shortening of the pillars, as a function of time and stress, was measured by means of dial gages mounted between the steel restraining rings. Based on the cumulative deformation curves supplied to us by 0bert, we prepared Figure 1 showing the creep rates as a function of time, obtained by taking the tangents to 0bert's curves at the indicated times. The exponent of the stress dependent term is in reasonable agreement with those obtained by the USBM with salt and similar rocks from other localities. The 0.6 slope obtained for the time function appears to be significantly different than the range of 0.8 to 1 (obtained from unpublished data of USBM and S. Serata) for salt from a different locality. However, extrapolation of the above equation out to periods up to 70 years (in excess of 600,000 hrs.) produces predicted creep rates which agree reasonable well with those actually measured in the Hutchinson and Lyons mines, whereas agreement was poor when compared with extrapolation of the data from models made from salt from the other localities.