Dimensioning Principles in Potash and Salt: Stability and Integrity

The paper describes the principal geomechanical approaches to mine dimensioning in salt and potash mining, focusing on stability of the mining system and integrity of the hydraulic barrier. Several common dimensioning are subjected to a comparative analysis. We identify geomechanical discontinuum models as essential physical ingredients for examining the collapse of working fields in potash mining. The basic mechanisms rely on the softening behaviour of salt rocks and the interfaces. A visco-elasto-plastic material model with strain softening, dilatancy and creep describes the time-dependent softening behaviour of the salt pillars, while a shear model with velocity-dependent adhesive friction with shear displacement-dependent softening is used for bedding planes and discontinuities. Pillar stability critically depends on the shear conditions of the bedding planes to the overlying and underlying beds, which provide the necessary confining pressure for the pillar core, but can fail dynamically, leading to large-scale field collapses. We further discuss the integrity conditions for the hydraulic barrier, most notably the minimal stress criterion, the violation of which leads to pressure-driven percolation as the mechanism of fluid transport and hence barrier failure. We present a number of examples where violation of the minimal stress criterion has led to mine floodings.