Very slow creep tests on rock samples

Twelve years ago, creep tests at very low deviatoric stress were performed on an Etrez salt sample in the Varangeville Mine. Recently, a new testing campaign was performed on various salt samples to gain further insight on salt behavior. Creep tests are performed under a 0.1 MPa uniaxial loading on rock-salt samples from the Varangeville and Avery Island Mines and under a 0.24 MPa uniaxial loading on a crushed-salt sample. To minimize the effects of temperature variations, testing devices were placed in an underground mine room, where temperature fluctuations are of the order of one-hundredth of a degree Celsius. The mechanical loading is provided by dead weights. The deformations were measured through special displacement sensors with a resolution of 1/80 μm. A typical steady-state strain rate reached after 6 months is −2.4 × 10⁻¹² s⁻¹. The influence of air hygrometry, which is approximately 74% RH in the mine, is smaller than expected.