A Very Slow Creep Test on an Avery Island Salt Sample

The applied deviatoric stress during most creep tests performed on salt samples is in the 3.5–20 MPa range. However, the stresses actually experienced in the vicinity of a salt cavern are much smaller. Any extrapolation is difficult to vindicate, as the dominant micro-mechanisms are strongly suspected to be very different in the low-stress and medium-stress domains. To answer this concern, a very slow creep test was performed on an Avery Island salt sample. To minimize the influence of even the smallest of temperature deviations during the test, the testing apparatus was placed in a remote gallery of the Varangéville salt mine, taking advantage of the very stable temperature conditions offered in an underground environment. The test was performed in multiple stages and lasted 42 months. The successive loads of 0.1, 0.2, 0.3, and 0.5 MPa were applied. Measured steady-state strain rates were of the order of 10⁻¹² s⁻¹, which are significantly faster than that extrapolated from creep tests performed at loads ranging between 3.5 and 20 MPa.