The influence of a grain-shape fabric on the mechanical behaviour of rock salt: Results from deformation experiments

In order to assess the effect of shape preferred orientation (SPO) of halite grains on deformation of rock salt, ductile plane strain deformation experiments have been conducted. Natural halite cubes with and without an SPO from the Asse mine of northern Germany were deformed at 345 °C, atmospheric pressure and a strain rate of ~10−7 s−1. The SPO was oriented at different angles (ß = 0°, 45° and 90°) to the main shortening direction, Z. The sample is confined in the Y direction and can freely move to the principal stretching direction, X. Most of the strain was accommodated by dislocation creep. The qualitative analysis of the stress-strain data shows the clear influence of the SPO on the mechanical data. At a shortening strain of ca. −10%, the samples with β = 0° and 45° are weaker than the samples without an SPO. In contrast, the sample with β = 90° is stronger than the samples without an SPO. A new SPO is formed in all samples with the long axis of the grain shape fabric being subparallel to the principal stretching axis, X. At these larger shortening strains, the samples show similar strength, with an exception of the 90° oriented sample that is still stronger than the samples without an SPO. Even though the experimental set up has to be improved to obtain quantitative results on the influence of SPO on mechanical strength of rock salt, the results show that the SPO of rock salt likely has to be taken into account when building a repository for nuclear waste and could have an effect on the stability of caverns.