ABSTRACT
Stress relaxation experiments were carried out in 1-D compaction mode on granular salt with various pore fluid phases at room temperature. Merck salt (99.5% pure NaCl), sieved into different grain size fractions, was used as starting material. Samples were first compacted rapidly to a fixed porosity in the range 30–0.3% and the applied stress was then allowed to relax at fixed crosshead displacement. The effects of water content, applied stress, grain size, and porosity were systematically investigated. In fully dry, argon-flushed samples, the measured relaxation creep rates were insensitive to grain size but very sensitive to applied stress with an apparent power law stress exponent (n) of around 20. However, if the samples were exposed to lab air, the stress exponent dropped sharply (n = 5.1) compared to the dry samples, showing significant weakening by moisture. In samples flooded with NaCl saturated solution, the stress exponent was found to be almost exactly 1 at the lower stresses investigated (<20–15 MPa). The creep rate of samples flooded with brine was sensitive to grain size. The mechanical results together with microstructural analysis showed that the dominant deformation mechanism in the dry, argon-flushed samples was probably dislocation glide whereas it was pressure solution in the brine-flooded samples at stresses <20–15 MPa. However, the relaxation behaviour seen in samples tested with lab air (n = 5.1) is less easily explained. The dominant process is certainly not pure dislocation creep or glide, since it is activated by the access of lab air (moisture). Extensive recrystallization by grain boundary migration is observed in these samples and it must play a role in controlling relaxation and apparent n-value, possibly with a contribution from pressure solution and/or plasticity coupled solution transfer.
