Engineering properties of rock salt and simplified closed-form deformation solution for circular opening in rock salt under the true triaxial stress state

Underground structures and storage caverns are increasingly being constructed in rock salt due to its unique self-healing ability and impervious nature. Rock salts are likely to experience and develop time-dependent behavior/ deformation. Hence, the characterization of rock salt for its properties, including viscosity, are pertinent for design of these structures. In addition, the material in underground openings experiences true-triaxial stresses and hence it is vital to study the behaviour of such structures under true-triaxial loading conditions. This paper presents results of extensive laboratory testing carried out on specimens of Khewra rock salt for its physico-mechanical properties. Physical and engineering properties such as mineralogy, density, sonic wave velocity, resistivity, point load index and shear strength properties were measured. A simplified closed-form solution which deploys three-dimensional strength criterion (Mogi-Coulomb) with Maxwell viscoelastic behaviour is developed to assess the stress and deformation behaviour of circular opening in plane strain condition. The closed-form solution assumes that the material exhibits the Maxwell behavior when the stresses are lower than the yield stress and the material behavior is the combination of Maxwell and Mogi-Coulomb elastoplastic material, when the stresses are equal to the yield stress. In addition, a cylindrical wellbore problem is solved with the proposed closed-form solution and the deformation of the wellbore was estimated for the different time intervals.