ABSTRACT
The dilation limit can be determined by employing the Constant Mean Stress (CMS) test in which the salt specimen is initially subjected to a prescribed hydrostatic confining pressure at a constant temperature. The deviatoric stress is then applied by simultaneously changing the axial stress and confining pressure at rates that produce no change in the mean stress imposed on the specimen. Under triaxial compression conditions, the axial stress is increased at twice the rate at which the confining pressure is decreased. For triaxial extension conditions, the loading conditions are opposite. Throughout the duration of the test, axial and radial displacements of the specimen are recorded and, thus, the volumetric strain is computed and monitored. The test is terminated when either the specimen fails or one of the principal stresses reaches a machine limit. While applying the deviatoric stress, the volumetric strain will remain zero or will exhibit some initial shear-enhanced compaction until dilation occurs, which causes the volumetric strain to increase (i.e. volume expansion of the specimen caused by microfracturing), as illustrated in Figure 2. The deviatoric stress at
1 INTRODUCTION
The three geomechanical criteria that typically govern underground salt designs in both mines and caverns are (1) creep, (2) dilation, and (3) tension. It is preferred to design the mine or cavern such that stress conditions within the salt are below a threshold where salt dilation is likely to occur. Stress conditions that exceed the salt’s dilation limit will cause the salt to spall and eventually fail. Figure 1 illustrates salt specimens that were subjected to increased stress conditions above their dilation limits. As observed from Figure 1, the cylindrical specimens react to stress by deforming in an outward-bowing fashion. Under increasing
the point where the volumetric strain begins to increase is termed the dilation stress.
