Characterization of Fracture Source Mechanism for Rock Salt Under Quasi-Static Unconfined Compression Test Using Acoustic Emission

Abstract

The tensile fractures were identified as the dominant fracture mode under quasi-static unconfined compressive load for the high-halite percentage rock salt found in Germany. The observed fracture mechanism is radically different from the reported dominant fracture mode for other natural and synthetic rocks. Rock salt properties are site-specific and depend on the deposit’s age, mineralogical composition, and tectonic history. It poses a fundamental question: Is the observed behavior typical to the tested specimen of rock salt, or is it a general phenomenon typical to all high-halite percentage rock salts? The present study addresses this question by characterizing the fracture source mechanism in the high-halite percentage Khewra rock salt (> 95% halite mineral) under quasi-static unconfined compressive load. The study employs a unique approach to identifying AE events using a detection function. It has the advantage of isolating individual events from the burst of AE events, giving an advantage over the conventional threshold method. The isolated AE events source mechanism was characterized using the AF-RA method. The results from the analysis conclude that 79% of AE events are due to tensile cracks, and 21% of AE events are due to shear cracks for Khewra rock salt. Regarding AE energy, the cumulative relative tensile cracks AE energy is higher than the shear cracks AE energy. The AF-RA data for Khewra rock salt also align with the established dilatancy boundary, suggesting that triaxial test data along with the AF-RA method could be utilized to identify the dilatancy boundary in rock salts. The major conclusion from the present study is that under the quasi-static unconfined compression test, the tensile split is the dominant fracture mode for the Khewra rock salt. The current observation and previously reported works indicate that rock salts having a high-halite percentage have a tensile fracture as a dominating failure mode irrespective of their geological age and tectonic history.