Mechanisms of dilatancy in rock salt at the grain-scale and implications for the dilatancy boundary

This paper presents an experimental investigation of the dilatancy boundary in rock salt based on triaxial compression tests (room temperature 10(-5) s(-1)) of naturally wet and dried Leinesteinsalz samples. The experiments indicate that dilatancy in the Leinesteinsalz is observed at confining pressures <= 15 MPa, while at higher confining pressure mechanical behavior is non-dilatant. Two microphysical models were constructed based on mechanisms based on (1) intergranular slip and tensile failure and (2) the opening of voids due to plastic strain misfits between individual grains. A comparison of the experimental data with the first model indicates that slip on grain boundaries can only occur when the friction on the grain boundaries is lowered by the presence of fluid inclusions. The absence of any significant difference between dilatancy in dry and wet Leinesteinsalz eliminates the possibility that dilatancy is controlled by intergranular slip. The second model shows the typical type of behavior observed in the experiment with initial compaction and a later switch to dilatancy as axial strain progresses but fails to show a switch to compaction-only behavior at high confinement. Hence, dilatancy in rock salt is likely to be controlled by the opening of voids related to misfit strains, but the model needs to be modified.