Creep behaviour of bischofite, carnallite and mixed bischofite-carnallite-halite salt rock

Many salt deposits consist of bischofi te, carnallite and halite in the form of layers and mixtures. During extraction from the subsurface by solution mining, the material in the undissolved walls will fl ow into the caverns. This may cause surface subsidence. In order to accurately predict the fl ow of wall rock material and related subsidence, good understanding of the creep behaviour of bischofi te, carnallite and mixed salt rocks under in situ conditions is required. The strain rate of such salts in underground mines is normally in the range of 10-8 to 10-15 s-1 (Van Eekelen et al. 1981, Jackson & Talbot 1986) which is a rate that cannot be achieved easily in laboratory scale experiments. However, laboratory experiments can be used to defi ne fl ow laws that allow extrapolation to real in situ conditions. In order to perform such extrapolation in a reliable manner, good understanding of the deformation mechanism of the material is needed,