A design approach using isothermal intact saltrock steady-state creep behavior has developed from laboratory testing, field monitoring, and numerical modelling. Creep data at various stress states can be contoured with respect to strain rate on a stress space plot. The contours flatten towards a viscoplastic criterion (not a brittle or Bingham limit), which we call the Prandtl Limit. It is not practical to test at the low strain rates below the limit, but instrumented virgin ground openings allow insitu confirmation of the Prandtl Limit and measurement of the Prandtl Strain Rate, both fundamental parameters. Brittle failure tests give a classical Mohr-Coulomb failure envelope up to confining stress of about 15 MPa, then failure becomes difficult to define, as strain rates are very fast, and strain weakening is not evidenced below 25% strain. The constitutive model has an outer parabolic surface corresponding to peak strength, and an inner cylindrical surface, the Prandtl Limit, which is an activation shear stress. Below the Prandtl Limit, slow deformation processes dominate; above it, strain rate is dependent on normal stress, indicating processes of dilational dislocation movement, crack generation and healing.


Time-dependent deformation of saltrocks is the result of a number of mechanisms, different ones dominating behavior at different stress levels. For example, saltrocks display steady-state creep with no volume change at low shear stresses; at elevated stresses, they behave as Mohr-Coulomb materials, with deformation behavior dominated by crack growth. It is not reasonable that a single formulation would lead to an acceptable constitutive model for salt behavior at all stress levels, and a slightly different approach has been developed with some success in practice. Although field measurements are not unequivocal proof of the approach, they give support to the model within the time frames, stress ranges, and temperature ranges encountered in mining. On the scale of mine openings, saltrocks are considered true steady- state materials because mechanisms exist for the ionic lattice which permit constant deformation rates (Skrotzki and Haasen, 19B4), in contrast to silicates, oxides, sulfides and other covalently bonded minerals at normal temperatures and pressures. Difficulty in predictions of field creep rates and magnitudes purely from current laboratory test approaches suggests that current creep law forms are incomplete (Munson and Fossum, 1986). To rectify this, a different, field-based approach is suggested, using a combination of field and laboratory data. It is not radically different from previously published approaches (Wallner, 1984, Langer, 1984), but there are differences in the approach to quantifying the behavioral parameters.


Laboratorycreeptests: Isothermal constant stress and structure tests are carried out using a stage-loading method, not allowing strain to exceed 7.5-10%. This limit is because of specimen barreling and platen penetration, and because in mining situations, it is rare that an element of saltrock experiences strains larger than 10%. It is essential that comparable processes are studied on one material, not another altered by having undergone excessive deformation and defect annealing.

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