Creep Tests on Salt Samples Performed at Very Small Stresses

: From 1996 to 2021, uniaxial creep tests were performed on salt samples in dead-end drifts of the Varangéville (France) and Altaussee (Austria) mines to take advantage of constant temperature and hygrometry. The applied loads were from 0.05 MPa (relative) to 4.5 MPa, i.e., much smaller than the loads currently applied during standard creep tests performed at the laboratory. Main conclusions are: (1) Steady state is reached after a long period (longer than 8 months). (2) Cumulated transient creep is relatively large (3) Strain rates are faster (than extrapolated from high stresses) by 4-5 orders of magnitude (4) Steady state strain rate is a linear function of the applied stress (approximately) in the σ < 3 MPa domain (5) Strain rate is a decreasing function of grain size (6) The transition between the linear (n = 1) and the non-linear (n = 3 to 5) behavior seems to range between 3 MPa and 4.5 MPa (7) No creep is observed in a very dry environment (8) No threshold for salt creep (or smaller than 0.05 MPa) is observed (9) In the small stress domain, reverse creep is observed. 1. INTRODUCTION It has been suspected for long (Spiers et al., 1990; Urai and Spiers, 2007) that, in the small deviatoric stress domain (σ < 3 MPa), the governing mechanism for salt creep was pressure solution — rather than dislocation creep. A consequence should be that creep rate in this domain is much faster - by several orders of magnitude - than extrapolated from tests performed in the high stress domain. In addition, creep rate should be a decreasing function of grain size; it should be a linear function of the applied stress, and the presence of a small amount of brine at the grains interface should be a necessary condition for active creep. These statements were based on theoretical arguments, geological evidence and the results of tests performed on artificial salt.