Creep behaviour of bischofite, carnallite and mixed bischofite-carnallite-halite salt rock at in situ conditions

To accurately predict cavern convergence and subsidence caused by solution
mining of K- and Mg-bearing salt bodies, a good understanding of the creep behaviour of
bischofite, carnallite and mixed salts rocks is required. We studied the mechanical properties
of these materials aiming to produce flow laws that can be applied at in situ conditions. We
performed triaxial deformation experiments on natural polycrystalline samples of bischofite,
carnallite (starting grain size ~5 mm), and their mixtures with halite, at in situ PT conditions of
40 MPa and 70 °C. All deformation tests were done in strain rate stepping mode, with
intervening stress relaxation to reach low strain rates. We found that carnallite is 4-5 times
stronger than bischofite, and that bischofite-carnallite-halite mixtures are stronger than
carnallite. The constant strain rate parts of the multistep experiments allowed dislocation creep
laws to be defined for bischofite and carnallite at relatively high stress, with a power law stress
exponent n~5. During stress relaxation, n changes to ~1 at a strain rate of ~10-9 s-1. This is
interpreted as reflecting a change from dislocation creep at the faster strain rates to solutionprecipitation behaviour at slow strain rate, mediated by changing grain size.