Mechanical Properties Testing of Rock Salt Using an Automated Triaxial Compression Test With a Constant Mean Stress Criterion
Underground salt storage caverns are designed to prevent salt dilation, which can lead to leaks, cavern communication, and cavern instability. The maximum and minimum operating pressures are also usually limited by the dilation strength of the salt. Identifying the dilation point of salt from the results of standard triaxial strength compression tests is not trivial and is prone to uncertainty. To overcome this limitation, the RESPEC Materials Testing Laboratory developed a new test method to determine the dilation strength of rock salt. The new test method uses computer-controlled, servohydraulic testing machines to subject the test specimen to increasing shear stress while maintaining constant mean stress (CMS), and the dilation point is identified when the trend in volumetric strain changes from compaction to expansion. By maintaining CMS while monitoring volumetric strain, the effects of elastic and creep deformations are negated.
An MTS servohydraulic test system is used to perform the tests, and the CMS test protocol illustrates that modern test systems allow a nonstandard load path that can focus on a particular aspect of salt-rock characterization; namely, the onset of dilation. The test can also be used to investigate material anisotropy and determine elastic moduli. Two decades of experience have verified that the CMS test protocol is preferred to the more traditional triaxial test method for identifying the dilation strength of evaporites. The CMS test is the industry standard for determining salt dilation and is used to evaluate cavern stability. The RESPEC laboratory continues to use this test protocol for all rock-salt evaluations, and the standard is recognized by ASTM International Standard Testing Protocol (STP) 1350.
After the CMS procedure had been developed and verified, RESPEC completed research studies over the next 5 years for the US Department of Energy (DOE) and National Energy Technology Laboratory (NETL). The studies were performed to develop and demonstrate a new cavern design criterion based on salt damage (dilation). As part of this research, the RESPEC laboratory designed and validated another CMS test that is performed in triaxial extension instead of triaxial compression. This new, nonlinear RESPEC Dilation (RD) criterion is the standard method used for cavern stability analyses.
For further information on these projects, please see the publicly available references below.
- Mellegard, K. D. and T. W. Pfeifle, 1999. “Laboratory Evaluation of Mechanical Properties of Rock Using an Automated Triaxial Compression Test With a Constant Mean Stress Criterion,” Nondestructive and Automated Testing for Soil and Rock Properties, ASTM STP 1350, W. A. Marr and C. E. Fairhurst (eds.), American Society for Testing and Materials, West Conshohocken, PA, pp. 247–258.
- DeVries, K. L., K. D. Mellegard, and G. D. Callahan, 2002. Salt Damage Criterion Proof-of-Concept Research, RSI-1675, prepared by RESPEC, Rapid City, SD, for United States Department of Energy, National Energy Technology Laboratory, Pittsburgh, PA.
- DeVries, K. L., K. D. Mellegard, G. D. Callahan, and W. M. Goodman, 2005. Cavern Roof Stability for Natural Gas Storage in Bedded Salt, RSI-1829, prepared by RESPEC, Rapid City, SD, for the US Department of Energy, National Energy Technology Laboratory, Pittsburgh, PA.