Creep behaviour of natural rock salt and its description with the composite model

Natural rock salt in deep underground structures is planned to act as the host material for the permanent storage of radioactive and toxic wastes. Dimensioning and safety analysis of such repositories require a model which allows to predict the creep behaviour of rock salt on the basis of the microstructure and the physical mechanisms of deformation. Already before testing natural rock salt contains a subgrain structure. During deformation under constant stress the creep rate changes until a steady state is reached. The same holds for the microstructural parameters subgrain size and spacing of dislocations inside subgrains. The composite model of plastic deformation takes the observed heterogeneity of the dislocation structure explicitly into account. Using this model in combination with appropriate kinetic laws for dislocation movement, transient as well as steady-state creep of rock salt can be calculated.