Viscoplastic Behavior Of Salt As Measured In The Corejacking Test

An understanding of the creep and yield behavior of salt is required for the rock mechanics design aspects of solution mining. Most previous studies of salt behavior have been performed on small specimens of salt under homogeneous stress conditions; i.e., uniaxial and triaxial tests on specimens <0.002 m3. The corejacking test is a larger scale test (-0.75 m3) with inhomogeneous stress conditions that provides data on the structural response of salt. Interpretation of the responses from eight such corejacking tests provides verification of the requisite constitutive parameters required to predict salt behavior at different levels of stress and temperature. These constitutive parameters are not those studied in the usual laboratory test. Corejacking test results show almost inseparable elastic and plastic closures simultaneous with the corejack pressurization. Very rapid closure (on a relative basis) continues for a period of time after the pressurization. This rapid closure is shown to be both continuing yield deformation and creep deformation. After the period of rapid closure, the closure rate approaches a constant value. Numerical modeling of the corejack tests using a creep law containing both transient and steady-state creep functions does not provide results that agree with the responses measured in the corejack tests. Using an experimental design technique plus additional numerical modeling that includes yielding of the salt, results in identification of the significant parameters which allow agreement. The significance of the results from the study is demonstrated by comparing the postulated stability and creep response of a solution-mined cavern using the previous understanding of salt behavior and the understanding developed from this study.